Author name code: demarque ADS astronomy entries on 2022-09-14 author:"Demarque, Pierre" ------------------------------------------------------------------------ Title: Discovery of strong progenitor age dependence of type Ia supernova luminosity standardization process and discordance in cosmology Authors: Lee, Young-Wook; Chung, Chul; Demarque, Pierre; Park, Seunghyun; Son, Junhyuk; Kang, Yijung Bibcode: 2021arXiv210706288L Altcode: Supernova (SN) cosmology is based on the assumption that the width-luminosity relation (WLR) and the color-luminosity relation (CLR) in the type Ia SN luminosity standardization would not vary with progenitor age. Unlike this expectation, recent age datings of stellar populations in host galaxies have shown significant correlations between progenitor age and Hubble residual (HR). It was not clear, however, how this correlation arises from the SN luminosity standardization process, and how this would impact the cosmological result. Here we show that this correlation originates from a strong progenitor age dependence of the WLR and the CLR, in the sense that SNe from younger progenitors are fainter each at given light-curve parameters $x_1$ and $c$. This is reminiscent of Baade's discovery of two Cepheid period-luminosity relations, and, as such, causes a serious systematic bias with redshift in SN cosmology. Other host properties show substantially smaller and insignificant differences in the WLR and CLR for the same dataset. We illustrate that the differences between the high-$z$ and low-$z$ SNe in the WLR and CLR, and in HR after the standardization, are fully comparable to those between the correspondingly young and old SNe at intermediate redshift, indicating that the observed dimming of SNe with redshift is most likely an artifact of over-correction in the luminosity standardization. When this systematic bias with redshift is properly taken into account, there is no or little evidence left for an accelerating universe, posing a serious question to one of the cornerstones of the concordance model. Title: Stellar evolution models with entropy-calibrated mixing-length parameter: application to red giants Authors: Spada, Federico; Demarque, Pierre; Kupka, Friedrich Bibcode: 2021MNRAS.504.3128S Altcode: 2021arXiv210408067S; 2021MNRAS.tmp.1089S We present evolutionary models for solar-like stars with an improved treatment of convection that results in a more accurate estimate of the radius and effective temperature. This is achieved by improving the calibration of the mixing-length parameter, which sets the length scale in the 1D convection model implemented in the stellar evolution code. Our calibration relies on the results of 2D and 3D radiation hydrodynamics simulations of convection to specify the value of the adiabatic specific entropy at the bottom of the convective envelope in stars as a function of their effective temperature, surface gravity, and metallicity. For the first time, this calibration is fully integrated within the flow of a stellar evolution code, with the mixing-length parameter being continuously updated at run-time. This approach replaces the more common, but questionable, procedure of calibrating the length scale parameter on the Sun, and then applying the solar-calibrated value in modelling other stars, regardless of their mass, composition, and evolutionary status. The internal consistency of our current implementation makes it suitable for application to evolved stars, in particular to red giants. We show that the entropy calibrated models yield a revised position of the red giant branch that is in better agreement with observational constraints than that of standard models. Title: Testing the entropy calibration of the radii of cool stars: models of α Centauri A and B Authors: Spada, Federico; Demarque, Pierre Bibcode: 2019MNRAS.489.4712S Altcode: 2019MNRAS.tmp.2125S; 2019arXiv190900701S We present models of α Centauri A and B implementing an entropy calibration of the mixing-length parameter αMLT, recently developed and successfully applied to the Sun (Spada et al. 2018, ApJ, 869, 135). In this technique the value of αMLT in the 1D stellar evolution code is calibrated to match the adiabatic specific entropy derived from 3D radiation-hydrodynamics simulations of stellar convective envelopes, whose effective temperature, surface gravity, and metallicity are selected consistently along the evolutionary track. The customary treatment of convection in stellar evolution models relies on a constant, solar-calibrated αMLT. There is, however, mounting evidence that this procedure does not reproduce the observed radii of cool stars satisfactorily. For instance, modelling α Cen A and B requires an ad hoc tuning of αMLT to distinct, non-solar values. The entropy-calibrated models of α Cen A and B reproduce their observed radii within 1{{ per cent}} (or better) without externally adjusted parameters. The fit is of comparable quality to that of models with freely adjusted αMLT for α Cen B (within 1σ), while it is less satisfactory for α Cen A (within 2.5σ). This level of accuracy is consistent with the intrinsic uncertainties of the method. Our results demonstrate the capability of the entropy calibration method to produce stellar models with radii accurate within 1{{ per cent}}. This is especially relevant in characterizing exoplanet-host stars and their planetary systems accurately. Title: Improved Calibration of the Radii of Cool Stars Based on 3D Simulations of Convection: Implications for the Solar Model Authors: Spada, F.; Demarque, P.; Basu, S.; Tanner, J. D. Bibcode: 2018ApJ...869..135S Altcode: 2018arXiv181101817S Main-sequence, solar-like stars (M ≲ 1.5 M ⊙) have outer convective envelopes that are sufficiently thick to affect significantly their overall structure. The radii of these stars, in particular, are sensitive to the details of inefficient, superadiabatic convection occurring in their outermost layers. The standard treatment of convection in stellar evolution models, based on the mixing-length theory (MLT), provides only a very approximate description of convection in the superadiabatic regime. Moreover, it contains a free parameter, α MLT, whose standard calibration is based on the Sun and is routinely applied to other stars, ignoring the differences in their global parameters (e.g., effective temperature, gravity, chemical composition) and previous evolutionary history. In this paper, we present a calibration of α MLT based on 3D radiation hydrodynamics (RHD) simulations of convection. The value of α MLT is adjusted to match the specific entropy in the deep, adiabatic layers of the convective envelope to the corresponding value obtained from the 3D RHD simulations, as a function of the position of the star in the ({log}g,{log}{T}eff}) plane and its chemical composition. We have constructed a model of the present-day Sun using such entropy-based calibration. We find that its past luminosity evolution is not affected by the entropy calibration. The predicted solar radius, however, exceeds that of the standard model during the past several billion years, resulting in a lower surface temperature. This illustrative calculation also demonstrates the viability of the entropy approach for calibrating the radii of other late-type stars. Title: YaPSI: a new database of evolutionary tracks and isochrones Authors: Spada, F.; Demarque, P.; Kim, Y. -C.; Boyajian, T. S.; Brewer, J. M. Bibcode: 2018IAUS..334..362S Altcode: The Yale-Potsdam Stellar Isochrones (YaPSI) cover the low and intermediate stellar mass regime (0.15 to 5.0 M⊙) for a wide range of solar-scaled chemical compositions (metallicity from -0.5 to +0.3; helium mass fraction from 0.25 to 0.37, assigned independently of each other). The tracks are finely spaced in mass, to allow for accurate interpolation. The models feature state-of-the-art input physics relevant to low-mass stars modeling (surface boundary conditions, equation of state), thus updating the faint end of the Yonsei-Yale (YY) isochrones. Utility codes, such as an isochrone interpolator in age, metallicity and helium content, are also provided. The YaPSI isochrones are in good agreement with the empirical mass-luminosity and mass-radius relations available to date, and provide satisfactory fitting of the color-magnitude diagrams of well-studied open clusters. Title: Final Hubble Space Telescope Astrometry of the Procyon Binary System Authors: Bond, Howard E.; Gilliland, Ronald L.; Schaefer, Gail H.; Barstow, Martin A.; Demarque, Pierre; Holberg, Jay B.; Kozhurina-Platais, Vera Bibcode: 2018RNAAS...2..147B Altcode: 2018RNAAS...2c.147B Not Available

Based on observations with the NASA/ESA Hubble Space Telescope obtained at Space Telescope Science Institute, operated by Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Title: The Sirius System and Its Astrophysical Puzzles: Hubble Space Telescope and Ground-based Astrometry Authors: Bond, Howard E.; Schaefer, Gail H.; Gilliland, Ronald L.; Holberg, Jay B.; Mason, Brian D.; Lindenblad, Irving W.; Seitz-McLeese, Miranda; Arnett, W. David; Demarque, Pierre; Spada, Federico; Young, Patrick A.; Barstow, Martin A.; Burleigh, Matthew R.; Gudehus, Donald Bibcode: 2017ApJ...840...70B Altcode: 2017arXiv170310625B Sirius, the seventh-nearest stellar system, is a visual binary containing the metallic-line A1 V star Sirius A, the brightest star in the sky, orbited in a 50.13 year period by Sirius B, the brightest and nearest white dwarf (WD). Using images obtained over nearly two decades with the Hubble Space Telescope (HST), along with photographic observations covering almost 20 years and nearly 2300 historical measurements dating back to the 19th century, we determine precise orbital elements for the visual binary. Combined with the parallax and the motion of the A component, these elements yield dynamical masses of 2.063+/- 0.023 {M}⊙ and 1.018+/- 0.011 {M}⊙ for Sirius A and B, respectively. Our precise HST astrometry rules out third bodies orbiting either star in the system, down to masses of ∼15-25 {M}{Jup}. The location of Sirius B in the Hertzsprung-Russell diagram is in excellent agreement with theoretical cooling tracks for WDs of its dynamical mass, and implies a cooling age of ∼126 Myr. The position of Sirius B on the mass-radius plane is also consistent with WD theory, assuming a carbon-oxygen core. Including the pre-WD evolutionary timescale of the assumed progenitor, the total age of Sirius B is about 228 ± 10 Myr. We calculated evolutionary tracks for stars with the dynamical mass of Sirius A, using two independent codes. We find it necessary to assume a slightly subsolar metallicity, of about 0.85 {Z}⊙ , to fit its location on the luminosity-radius plane. The age of Sirius A based on these models is about 237-247 Myr, with uncertainties of ±15 Myr, consistent with that of the WD companion. We discuss astrophysical puzzles presented by the Sirius system, including the probability that the two stars must have interacted in the past, even though there is no direct evidence for this and the orbital eccentricity remains high.

Based in part on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, and from the Mikulski Archive for Space Telescopes at STScI, which are operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Title: VizieR Online Data Catalog: Historical and HST Astrometry of Sirius A,B (Bond+, 2017) Authors: Bond, H. E.; Schaefer, G. H.; Gilliland, R. L.; Holberg, J. B.; Mason, B. D.; Lindenblad, I. W.; Seitz-McLeese, M.; Arnett, W. D.; Demarque, P.; Spada, F.; Young, P. A.; Barstow, M. A.; Burleigh, M. R.; Gudehus, D. Bibcode: 2017yCat..18400070B Altcode: We have assembled a compilation of published historical measurements of the position angle (PA) and the angular separation of Sirius B relative to Sirius A. Our tabulation is based on a critical review of measures contained in the Washington Double Star Catalog maintained at the USNO and from our additional literature searches. Notes included in the tabulation give extensive commentary on the historical observations.

Many early publications provided measures averaged over multiple nights or even an entire observing season for the purpose of reducing computational labor in subsequent analyses. With modern computers, there is no need for such averaging, so we opted to present the individual measures whenever available. However, if an observer reported more than one measurement on a given night, we did compute the mean position for that night. If the original publication only reported a mean across several nights, we tabulated that mean as reported.

The visual micrometer observations did not always include a contemporaneous measurement of both the PA and separation. These omissions are listed as -99.0 in the table. The measurement uncertainties were assigned through our orbital fitting method described in the paper. Measurements that were rejected from the orbital solution are identified in the Notes column and are listed with uncertainties of 0.

(3 data files). Title: The Yale-Potsdam Stellar Isochrones Authors: Spada, F.; Demarque, P.; Kim, Y. -C.; Boyajian, T. S.; Brewer, J. M. Bibcode: 2017ApJ...838..161S Altcode: 2017arXiv170303975S We introduce the Yale-Potsdam Stellar Isochrones (YaPSI), a new grid of stellar evolution tracks and isochrones of solar-scaled composition. In an effort to improve the Yonsei-Yale database, special emphasis is placed on the construction of accurate low-mass models ({M}* < 0.6 {M}⊙ ), and in particular on their mass-luminosity and mass-radius relations, both crucial for characterizing exoplanet-host stars, and, in turn, their planetary systems. The YaPSI models cover the mass range 0.15-5.0 {M}⊙ densely enough to permit detailed interpolation in mass, and the metallicity and helium abundance ranges [Fe/H] = -1.5 to +0.3 and Y 0 = 0.25-0.37 are specified independently of each other (i.e., no fixed {{Δ }}Y/{{Δ }}Z relation is assumed). The evolutionary tracks are calculated from the pre-main sequence up to the tip of the red giant branch. The isochrones, with ages between 1 Myr and 20 Gyr, provide UBVRI colors in the Johnson-Cousins system, and JHK colors in the homogenized Bessell & Brett system, derived from two different semi-empirical {T}eff}-color calibrations from the literature. We also provide utility codes, such as an isochrone interpolator, in age, metallicity, and helium content, and an interface of the tracks with an open-source Monte Carlo Markov-Chain tool for the analysis of individual stars. Finally, we present comparisons of the YaPSI models with the best empirical mass-luminosity and mass-radius relations available to date, as well as isochrone fitting of well-studied stellar clusters. Title: HST Observations of Astrophysically Important Visual Binaries Authors: Bond, Howard E.; Barstow, Martin; Burleigh, Matthew; Demarque, Pierre; Gilliland, Ronald L.; Girard, Terrence M.; Gudehus, Donald H.; Holberg, Jay B.; Nelan, Edmund; Schaefer, Gail Bibcode: 2016hst..prop14342B Altcode: We propose to continue our long-term program of astrometry of close visual binaries, with the primary goal of determining purely dynamical masses for 3 important main-sequence stars and 9 white dwarfs (WDs). A secondary aim is to set limits on third bodies in the systems down to planetary mass. Three of our targets are naked-eye stars with much fainter companions that are extremely difficult to image from the ground. Our other 2 targets are double WDs, whose small separations and faintness likewise make them difficult to measure using ground-based techniques. Observations have been completed for a 3rd double WD.

The bright stars, to be imaged with WFC3, are: (1) Procyon (P = 40.83 yr), containing a bright F star and a much fainter WD companion. With the continued monitoring proposed here, we will obtain masses to an accuracy of better than 1%, providing a testbed for theories of both Sun-like stars and WDs. (2) Sirius (P = 50.14 yr), an A-type star also having a faint WD companion, Sirius B, the nearest and brightest of all WDs. (3) Mu Cas (P = 21.08 yr), a nearby metal-deficient G dwarf for which accurate masses will lead to the stars' helium contents, with cosmological implications.

The faint double WDs, to be observed with FGS, are: (1) G 107-70 (P = 18.84 yr), and (2) WD 1818+126 (P = 12.19 yr). Our astrometry of these systems will add 4 accurate masses to the handful of WD masses that are directly known from dynamical measurements. The FGS measurements will also provide precise parallaxes for the systems, a necessary ingredient in the mass determinations. Title: Entropy in Adiabatic Regions of Convection Simulations Authors: Tanner, Joel D.; Basu, Sarbani; Demarque, Pierre Bibcode: 2016ApJ...822L..17T Altcode: 2016arXiv160407437T One of the largest sources of uncertainty in stellar models is caused by the treatment of convection in stellar envelopes. One-dimensional stellar models often make use of the mixing length or equivalent approximations to describe convection, all of which depend on various free parameters. There have been attempts to rectify this by using 3D radiative-hydrodynamic simulations of stellar convection, and in trying to extract an equivalent mixing length from the simulations. In this Letter, we show that the entropy of the deeper, adiabatic layers in these simulations can be expressed as a simple function of {log}g and {log}{T}{{eff}}, which holds potential for calibrating stellar models in a simple and more general manner. Title: Hubble Space Telescope Astrometry of the Procyon System Authors: Bond, Howard E.; Gilliland, Ronald L.; Schaefer, Gail H.; Demarque, Pierre; Girard, Terrence M.; Holberg, Jay B.; Gudehus, Donald; Mason, Brian D.; Kozhurina-Platais, Vera; Burleigh, Matthew R.; Barstow, Martin A.; Nelan, Edmund P. Bibcode: 2015ApJ...813..106B Altcode: 2015arXiv151000485B The nearby star Procyon is a visual binary containing the F5 IV-V subgiant Procyon A, orbited in a 40.84-year period by the faint DQZ white dwarf (WD) Procyon B. Using images obtained over two decades with the Hubble Space Telescope, and historical measurements back to the 19th century, we have determined precise orbital elements. Combined with measurements of the parallax and the motion of the A component, these elements yield dynamical masses of 1.478 ± 0.012 M⊙ and 0.592 ± 0.006 M⊙ for A and B, respectively. The mass of Procyon A agrees well with theoretical predictions based on asteroseismology and its temperature and luminosity. Use of a standard core-overshoot model agrees best for a surprisingly high amount of core overshoot. Under these modeling assumptions, Procyon A’s age is ∼2.7 Gyr. Procyon B’s location in the H-R diagram is in excellent agreement with theoretical cooling tracks for WDs of its dynamical mass. Its position in the mass-radius plane is also consistent with theory, assuming a carbon-oxygen core and a helium-dominated atmosphere. Its progenitor’s mass was 1.9-2.2 M⊙, depending on its amount of core overshoot. Several astrophysical puzzles remain. In the progenitor system, the stars at periastron were separated by only ∼5 AU, which might have led to tidal interactions and even mass transfer; yet there is no direct evidence that these have occurred. Moreover the orbital eccentricity has remained high (∼0.40). The mass of Procyon B is somewhat lower than anticipated from the initial-to-final-mass relation seen in open clusters. The presence of heavy elements in its atmosphere requires ongoing accretion, but the place of origin is uncertain.

Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, and from the Mikulski Archive for Space Telescopes at STScI, which are operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Title: Observations of Binary Stars with the Differential Speckle Survey Instrument. V. Toward an Empirical Metal-Poor Mass-Luminosity Relation Authors: Horch, Elliott P.; van Altena, William F.; Demarque, Pierre; Howell, Steve B.; Everett, Mark E.; Ciardi, David R.; Teske, Johanna K.; Henry, Todd J.; Winters, Jennifer G. Bibcode: 2015AJ....149..151H Altcode: 2015arXiv150300363H In an effort to better understand the details of the stellar structure and evolution of metal-poor stars, the Gemini North telescope was used on two occasions to take speckle imaging data of a sample of known spectroscopic binary stars and other nearby stars in order to search for and resolve close companions. The observations were obtained using the Differential Speckle Survey Instrument, which takes data in two filters simultaneously. The results presented here are of 90 observations of 23 systems in which one or more companions was detected, and six stars where no companion was detected to the limit of the camera capabilities at Gemini. In the case of the binary and multiple stars, these results are then further analyzed to make first orbit determinations in five cases, and orbit refinements in four other cases. The mass information is derived, and since the systems span a range in metallicity, a study is presented that compares our results with the expected trend in total mass as derived from the most recent Yale isochrones as a function of metal abundance. These data suggest that metal-poor main-sequence stars are less massive at a given color than their solar-metallicity analogues in a manner consistent with that predicted from the theory. Title: Stellar diameters and temperatures - VI. High angular resolution measurements of the transiting exoplanet host stars HD 189733 and HD 209458 and implications for models of cool dwarfs Authors: Boyajian, Tabetha; von Braun, Kaspar; Feiden, Gregory A.; Huber, Daniel; Basu, Sarbani; Demarque, Pierre; Fischer, Debra A.; Schaefer, Gail; Mann, Andrew W.; White, Timothy R.; Maestro, Vicente; Brewer, John; Lamell, C. Brooke; Spada, Federico; López-Morales, Mercedes; Ireland, Michael; Farrington, Chris; van Belle, Gerard T.; Kane, Stephen R.; Jones, Jeremy; ten Brummelaar, Theo A.; Ciardi, David R.; McAlister, Harold A.; Ridgway, Stephen; Goldfinger, P. J.; Turner, Nils H.; Sturmann, Laszlo Bibcode: 2015MNRAS.447..846B Altcode: 2015MNRAS.447..850B; 2014arXiv1411.5638B We present direct radii measurements of the well-known transiting exoplanet host stars HD 189733 and HD 209458 using the CHARA Array interferometer. We find the limb-darkened angular diameters to be θLD = 0.3848 ± 0.0055 and 0.2254 ± 0.0072 mas for HD 189733 and HD 209458, respectively. HD 189733 and HD 209458 are currently the only two transiting exoplanet systems where detection of the respective planetary companion's orbital motion from high-resolution spectroscopy has revealed absolute masses for both star and planet. We use our new measurements together with the orbital information from radial velocity and photometric time series data, Hipparcos distances, and newly measured bolometric fluxes to determine the stellar effective temperatures (Teff = 4875 ± 43, 6092 ± 103 K), stellar linear radii (R* = 0.805 ± 0.016, 1.203 ± 0.061 R⊙), mean stellar densities (ρ* = 1.62 ± 0.11, 0.58 ± 0.14 ρ⊙), planetary radii (Rp = 1.216 ± 0.024, 1.451 ± 0.074 RJup), and mean planetary densities (ρp = 0.605 ± 0.029, 0.196 ± 0.033 ρJup) for HD 189733b and HD 209458b, respectively. The stellar parameters for HD 209458, an F9 dwarf, are consistent with indirect estimates derived from spectroscopic and evolutionary modelling. However, we find that models are unable to reproduce the observational results for the K2 dwarf, HD 189733. We show that, for stellar evolutionary models to match the observed stellar properties of HD 189733, adjustments lowering the solar-calibrated mixing-length parameter to αMLT =1.34 need to be employed. Title: Empirically determined properties of the K-dwarf HD 189733 and implications for evolutionary models of low-mass stars Authors: Boyajian, Tabetha S.; von Braun, Kaspar; Feiden, Gregory A.; Huber, Daniel; Basu, Sarbani; Demarque, Pierre; Fischer, Debra; Schaefer, Gail; White, Timothy; Maestro, Vicente; Brewer, John Michael; Lamell, Brooke; Spada, Federico; Mann, Andrew; Lopez-Morales, Mercedes; Ireland, Michael; Farrington, Christopher D.; van Belle, Gerard; Kane, Stephen R.; Jones, Jeremy; Ten Brummelaar, Theo; Ciardi, David R.; McAlister, Harold A.; Ridgway, Stephen T.; goldfinger, PJ Bibcode: 2015AAS...22525703B Altcode: We present direct measurements of the stellar radii and effective temperatures for HD189733 and HD209458 (see poster by von Braun et al.). We use the stellar radius and temperature along with the mass from the Keplerian orbital solution to explore deviations with model predictions. The stellar properties for HD209458, a F9 dwarf, are consistent with indirect estimates derived from spectroscopic and evolutionary modeling. However, we find that models are unable to reproduce the observational results for the K2 dwarf, HD 189733. We show that, for stellar evolutionary models to match the observed stellar properties of HD 189733, adjustments lowering the solar-calibrated mixing length parameter need to be employed. Title: Fundamental Parameters of the Two Hall-of-Famers HD 189733 and HD 209458 Authors: von Braun, Kaspar; Boyajian, Tabetha S.; Feiden, Gregory A.; Huber, Daniel; Basu, Sarbani; Demarque, Pierre; Fischer, Debra; Schaefer, Gail; White, Timothy; Maestro, Vicente; Brewer, John Michael; Lamell, Brooke; Spada, Federico; Mann, Andrew; Lopez-Morales, Mercedes; Ireland, Michael; Farrington, Christopher D.; van Belle, Gerard; Kane, Stephen R.; Jones, Jeremy; Ten Brummelaar, Theo; Ciardi, David R.; McAlister, Harold A.; Ridgway, Stephen T.; Goldfinger, PJ Bibcode: 2015AAS...22525702V Altcode: HD 189733 and HD 209458 are two of the most thoroughly studied exoplanet systems. They also represent the only transiting systems for which spectroscopy studies have produced radial velocity signatures of both the planets and the parent stars, yielding direct mass measurements. We present the results of our interferometric radius measurements and spectrophotometric observations of these two hall-of-fame exoplanet hosts. Our results, combined with trigonometric parallaxes and literature broad-band photometry, yield empirical values for stellar and planetary radii and stellar effective temperatures and luminosities. Using the directly determined component masses from previous studies, we calculate surface gravities and bulk densities for the parent stars and their exoplanets, creating a nearly model-independent set of fundamental astrophysical parameters for two of exoplanet science's most important stepping stones. Title: Modeling Convective Core Overshoot and Diffusion in Procyon Constrained by Asteroseismic Data Authors: Guenther, D. B.; Demarque, P.; Gruberbauer, M. Bibcode: 2014ApJ...787..164G Altcode: 2014arXiv1404.6153G We compare evolved stellar models, which match Procyon's mass and position in the HR diagram, to current ground-based asteroseismic observations. Diffusion of helium and metals along with two conventional core overshoot descriptions and the Kuhfuss nonlocal theory of convection are considered. We establish that one of the two published asteroseismic data reductions for Procyon, which mainly differ in their identification of even versus odd l values, is a significantly more probable and self-consistent match to our models than the other. The most probable models according to our Bayesian analysis have evolved to just short of turnoff, still retaining a hydrogen convective core. Our most probable models include Y and Z diffusion and have conventional core overshoot between 0.9 and 1.5 pressure scale heights, which increases the outer radius of the convective core by between 36% and 43%, respectively. We discuss the significance of this comparatively higher than expected core overshoot amount in terms of internal mixing during evolution. The parameters of our most probable models are similar regardless of whether adiabatic or nonadiabatic model p-mode frequencies are compared to the observations, although, the Bayesian probabilities are greater when the nonadiabatic model frequencies are used. All the most probable models (with or without core overshoot, adiabatic or nonadiabatic model frequencies, diffusion or no diffusion, including priors for the observed HRD location and mass or not) have masses that are within 1σ of the observed mass 1.497 ± 0.037 M ⊙. Title: The Effect of Metallicity-dependent T-τ Relations on Calibrated Stellar Models Authors: Tanner, Joel D.; Basu, Sarbani; Demarque, Pierre Bibcode: 2014ApJ...785L..13T Altcode: 2014arXiv1402.1182T Mixing length theory is the predominant treatment of convection in stellar models today. Usually described by a single free parameter, α, the common practice is to calibrate it using the properties of the Sun, and apply it to all other stellar models as well. Asteroseismic data from Kepler and CoRoT provide precise properties of other stars which can be used to determine α as well, and a recent study of stars in the Kepler field of view found α to vary with metallicity. Interpreting α obtained from calibrated stellar models, however, is complicated by the fact that the value for α depends on the surface boundary condition of the stellar model, or T-τ relation. Calibrated models that use typical T-τ relations, which are static and insensitive to chemical composition, do not include the complete effect of metallicity on α. We use three-dimensional radiation-hydrodynamic simulations to extract metallicity-dependent T-τ relations and use them in calibrated stellar models. We find the previously reported α-metallicity trend to be robust, and not significantly affected by the surface boundary condition of the stellar models. Title: The Chemical Composition Dependence of Stellar Envelope Convection and Overshoot Authors: Tanner, J. D.; Basu, S.; Demarque, P. Bibcode: 2013ASPC..478..377T Altcode: Using a grid of 3D hydrodynamic simulations in the outer layers of convection, we examine how properties of convection and overshoot are affected by the heavy element and helium abundance. We find that simulations with high-Z or low-Y exhibit larger mean and turbulent velocities throughout the superadiabatic region; they also have more overshoot than the low-Z or high-Y ones. Using the turbulent velocity field from simulations, we also find that helium-induced changes to the atmospheric dynamics results in different absolute Doppler shifts of weak spectral lines. Title: Helium-abundance and Other Composition Effects on the Properties of Stellar Surface Convection in Solar-like Main-sequence Stars Authors: Tanner, Joel D.; Basu, Sarbani; Demarque, Pierre Bibcode: 2013ApJ...778..117T Altcode: 2013arXiv1310.0475T We investigate the effect of helium abundance and α-element enhancement on the properties of convection in envelopes of solar-like main-sequence stars using a grid of three-dimensional radiation hydrodynamic simulations. Helium abundance increases the mean molecular weight of the gas and alters opacity by displacing hydrogen. Since the scale of the effect of helium may depend on the metallicity, the grid consists of simulations with three helium abundances (Y = 0.1, 0.2, 0.3), each with two metallicities (Z = 0.001, 0.020). We find that changing the helium mass fraction generally affects structure and convective dynamics in a way opposite to that of metallicity. Furthermore, the effect is considerably smaller than that of metallicity. The signature of helium differs from that of metallicity in the manner in which the photospheric velocity distribution is affected. We also find that helium abundance and surface gravity behave largely in similar ways, but differ in the way they affect the mean molecular weight. A simple model for spectral line formation suggests that the bisectors and absolute Doppler shifts of spectral lines depend on the helium abundance. We look at the effect of α-element enhancement and find that it has a considerably smaller effect on the convective dynamics in the superadiabatic layer compared to that of helium abundance. Title: The Radius Discrepancy in Low-mass Stars: Single versus Binaries Authors: Spada, F.; Demarque, P.; Kim, Y. -C.; Sills, A. Bibcode: 2013ApJ...776...87S Altcode: 2013arXiv1308.5558S A long-standing issue in the theory of low-mass stars is the discrepancy between predicted and observed radii and effective temperatures. In spite of the increasing availability of very precise radius determinations from eclipsing binaries and interferometric measurements of radii of single stars, there is no unanimous consensus on the extent (or even the existence) of the discrepancy and on its connection with other stellar properties (e.g., metallicity, magnetic activity). We investigate the radius discrepancy phenomenon using the best data currently available (accuracy <~ 5%). We have constructed a grid of stellar models covering the entire range of low-mass stars (0.1-1.25 M ⊙) and various choices of the metallicity and mixing length parameter, α. We used an improved version of the Yale Rotational stellar Evolution Code, implementing surface boundary conditions based on the most up-to-date PHOENIX atmosphere models. Our models are in good agreement with others in the literature and improve and extend the low mass end of the Yale-Yonsei isochrones. Our calculations include rotation-related quantities, such as moments of inertia and convective turnover timescales, useful in studies of magnetic activity and rotational evolution of solar-like stars. Consistent with previous works, we find that both binaries and single stars have radii inflated by about 3% with respect to the theoretical models; among binaries, the components of short orbital period systems are found to be the most deviant. We conclude that both binaries and single stars are comparably affected by the radius discrepancy phenomenon. Title: Variation of Stellar Envelope Convection and Overshoot with Metallicity Authors: Tanner, Joel D.; Basu, Sarbani; Demarque, Pierre Bibcode: 2013ApJ...767...78T Altcode: 2013arXiv1302.5707T We examine how metallicity affects convection and overshoot in the superadiabatic layer of main sequence stars. We present results from a grid of three-dimensional radiation hydrodynamic simulations with four metallicities (Z = 0.040, 0.020, 0.010, 0.001), and spanning a range in effective temperature (4950 < T eff < 6230). We show that changing the metallicity alters properties of the convective gas dynamics, and the structure of the superadiabatic layer and atmosphere. Our grid of simulations shows that the amount of superadiabaticity, which tracks the transition from efficient to inefficient convection, is sensitive to changes in metallicity. We find that increasing the metallicity forces the location of the transition region to lower densities and pressures, and results in larger mean and turbulent velocities throughout the superadiabatic region. We also quantify the degree of convective overshoot in the atmosphere, and show that it increases with metallicity as well. Title: Comparing the Effect of Radiative Transfer Schemes on Convection Simulations Authors: Tanner, Joel D.; Basu, Sarbani; Demarque, Pierre Bibcode: 2012ApJ...759..120T Altcode: 2012arXiv1209.3784T We examine the effect of different radiative transfer schemes on the properties of three-dimensional (3D) simulations of near-surface stellar convection in the superadiabatic layer, where energy transport transitions from fully convective to fully radiative. We employ two radiative transfer schemes that fundamentally differ in the way they cover the 3D domain. The first solver approximates domain coverage with moments, while the second solver samples the 3D domain with ray integrations. By comparing simulations that differ only in their respective radiative transfer methods, we are able to isolate the effect that radiative efficiency has on the structure of the superadiabatic layer. We find the simulations to be in good general agreement, but they show distinct differences in the thermal structure in the superadiabatic layer and atmosphere. Title: Modelling fully convective stars in eclipsing binaries: KOI-126 and CM Draconis Authors: Spada, F.; Demarque, P. Bibcode: 2012MNRAS.422.2255S Altcode: 2012arXiv1202.4468S We present models of the components of the systems KOI-126 and CM Draconis, the two eclipsing binary systems known to date to contain stars with masses low enough to have fully convective interiors. We are able to model satisfactorily the system KOI-126, finding consistent solutions for the radii and surface temperatures of all three components, using a solar-like value of the mixing-length parameter α in the convection zone and PHOENIX NextGen 1D model atmospheres for the surface boundary conditions. Depending on the chemical composition, we estimate the age of the system to be in the range 3-5 Gyr. For CM Draconis, on the other hand, we cannot reconcile our models with the observed radii and Teff using the current metal-poor composition estimate based on kinematics. Higher metallicities lessen but do not remove the discrepancy. We then explore the effect of varying the mixing-length parameter α. As previously noted in the literature, a reduced α can be used as a simple measure of the lower convective efficiency due to rotation and induced magnetic fields. Our models show a sensitivity to α (for α < 1.0) sufficient to partially account for the radius discrepancies. It is, however, impossible to reconcile the models with the observations on the basis of the effect of the reduced α alone. We therefore suggest that the combined effects of high metallicity and α reduction could explain the observations of CM Draconis. For example, increasing the metallicity of the system towards super-solar values (i.e. Z= 2 Z⊙) yields an agreement within 2σ with α= 1.0. Title: Modeling Kepler Observations of Solar-like Oscillations in the Red Giant Star HD 186355 Authors: Jiang, C.; Jiang, B. W.; Christensen-Dalsgaard, J.; Bedding, T. R.; Stello, D.; Huber, D.; Frandsen, S.; Kjeldsen, H.; Karoff, C.; Mosser, B.; Demarque, P.; Fanelli, M. N.; Kinemuchi, K.; Mullally, F. Bibcode: 2011ApJ...742..120J Altcode: 2011arXiv1109.0962J We have analyzed oscillations of the red giant star HD 186355 observed by the NASA Kepler satellite. The data consist of the first five quarters of science operations of Kepler, which cover about 13 months. The high-precision time-series data allow us to accurately extract the oscillation frequencies from the power spectrum. We find that the frequency of the maximum oscillation power, νmax, and the mean large frequency separation, Δν, are around 106 and 9.4 μHz, respectively. A regular pattern of radial and non-radial oscillation modes is identified by stacking the power spectra in an echelle diagram. We use the scaling relations of Δν and νmax to estimate the preliminary asteroseismic mass, which is confirmed with the modeling result (M = 1.45 ± 0.05 M ⊙) using the Yale Rotating stellar Evolution Code (YREC7). In addition, we constrain the effective temperature, luminosity, and radius from comparisons between observational constraints and models. A number of mixed l = 1 modes are also detected and taken into account in our model comparisons. We find a mean observational period spacing for these mixed modes of about 58 s, suggesting that this red giant branch star is in the shell hydrogen-burning phase. Title: Variation in convective properties across the HR diagram Authors: Tanner, Joel; Basu, Sarbani; Demarque, Pierre; Robinson, Frank Bibcode: 2011IAUS..271..401T Altcode: We perform 3D radiative hydrodynamic simulations to study convection in low-mass main-sequence stars with the aim of improving stellar models. Comparing models from a 0.90 Msolar evolutionary track with 3D simulations reveals distinct differences between simulations and mixing length theory. The simulations show obvious structural differences throughout the superadiabatic layer where convection is inefficient at transporting energy. The discrepancy between MLT and simulation changes as the star evolves and the dynamical effects of turbulence increase. Further, the simulations reveal a T-tau relation that is sensitive to the strength of the turbulence, which is in contrast to 1D stellar models that use the same T-tau relation across the HR diagram. Title: Evolutionary scenarios and chemical inhomogeneities of extended horizontal branch stars Authors: Esch, Lisa; Demarque, Pierre; Basu, Sarbani Bibcode: 2011JPhCS.271a2039E Altcode: Extended Horizontal Branch (EHB) stars are observed in many globular clusters and as field stars in the Galactic halo. They belong to old stellar populations of the halo and the old disk. Their evolutionary status is unclear, and still a current subject of debate. Current interest in these stars arise from their association with the discoveries of helium abundance inhomogeneities in the globular clusters ω Cen and NGC 2808. The origin of the inhomogeneities is not yet understood, but there are many interpretations.

In order to better understand EHB stars, we explore the evolution of standard blue Horizontal Branch (HB) models using up-to-date physics. We present several grids of post Zero Age Horizontal Branch (post-ZAHB) evolutionary models to include both canonical and non-canonical evolutionary scenarios, as well as to compare models that contain semi-convection to models without semi-convection. We follow the models to the termination of nuclear helium burning. The detailed properties of the models, including shell flashes and breathing pulses, are described. Title: Characterizing Convection in Stellar Atmospheres Authors: Tanner, Joel; Basu, Sarbani; Demarque, Pierre; Robinson, Frank Bibcode: 2011JPhCS.271a2080T Altcode: We perform 3D radiative hydrodynamic simulations to study the properties of convection in the superadiabatic layer of stars. The simulations show differences in both the stratification and turbulent quantities for different types of stars. We extract turbulent pressure and eddy sizes, as well as the T-τ relation for different stars and find that they are sensitive to the energy flux and gravity. We also show that contrary to what is usually assumed in the field of stellar atmospheres, the structure and gas dynamics of simulations of turbulent atmospheres cannot be parameterized with Teff and log(g) alone. Title: Solar-like oscillations in cluster stars Authors: Stello, D.; Basu, S.; Bedding, T. R.; Brogaard, K.; Bruntt, H.; Chaplin, W. J.; Christensen-Dalsgaard, J.; Demarque, P.; Elsworth, Y. P.; García, R. A.; Gilliland, R. L.; Hekker, S.; Huber, D.; Karoff, C.; Kjeldsen, H.; Lebreton, Y.; Mathur, S.; Meibom, S.; Molenda-Żakowicz, J.; Noels, A.; Roxburgh, I. W.; Silva Aguirre, V.; Sterken, C.; Szabó, R. Bibcode: 2010AN....331..985S Altcode: 2010arXiv1007.0009S This article summaries a talk given at the HELAS IV international meeting We present a brief overview of the history of attempts to obtain a clear detection of solar-like oscillations in cluster stars, and discuss the results on the first clear detection, which was made by the Kepler Asteroseismic Science Consortium (KASC) Working Group 2.

Data from Kepler. Title: 2D solar modeling Authors: Ventura, P.; Penza, V.; Li, L.; Sofia, S.; Basu, S.; Demarque, P. Bibcode: 2010Ap&SS.328..295V Altcode: 2009Ap&SS.tmp..271V; 2009Ap&SS.tmp..283V; 2009arXiv0912.4998V Understanding the reasons of the cyclic variation of the total solar irradiance is one of the most challenging targets of modern astrophysics. These studies prove to be essential also for a more climatologic issue, associated to the global warming. Any attempt to determine the solar components of this phenomenon must include the effects of the magnetic field, whose strength and shape in the solar interior are far from being completely known. Modeling the presence and the effects of a magnetic field requires a 2D approach, since the assumption of radial symmetry is too limiting for this topic. We present the structure of a 2D evolution code that was purposely designed for this scope; rotation, magnetic field and turbulence can be taken into account. Some preliminary results are presented and commented. Title: Detection of Solar-like Oscillations from Kepler Photometry of the Open Cluster NGC 6819 Authors: Stello, Dennis; Basu, Sarbani; Bruntt, Hans; Mosser, Benoît; Stevens, Ian R.; Brown, Timothy M.; Christensen-Dalsgaard, Jørgen; Gilliland, Ronald L.; Kjeldsen, Hans; Arentoft, Torben; Ballot, Jérôme; Barban, Caroline; Bedding, Timothy R.; Chaplin, William J.; Elsworth, Yvonne P.; García, Rafael A.; Goupil, Marie-Jo; Hekker, Saskia; Huber, Daniel; Mathur, Savita; Meibom, Søren; Sangaralingam, Vinothini; Baldner, Charles S.; Belkacem, Kevin; Biazzo, Katia; Brogaard, Karsten; Suárez, Juan Carlos; D'Antona, Francesca; Demarque, Pierre; Esch, Lisa; Gai, Ning; Grundahl, Frank; Lebreton, Yveline; Jiang, Biwei; Jevtic, Nada; Karoff, Christoffer; Miglio, Andrea; Molenda-Żakowicz, Joanna; Montalbán, Josefina; Noels, Arlette; Roca Cortés, Teodoro; Roxburgh, Ian W.; Serenelli, Aldo M.; Silva Aguirre, Victor; Sterken, Christiaan; Stine, Peter; Szabó, Robert; Weiss, Achim; Borucki, William J.; Koch, David; Jenkins, Jon M. Bibcode: 2010ApJ...713L.182S Altcode: 2010arXiv1001.0026S Asteroseismology of stars in clusters has been a long-sought goal because the assumption of a common age, distance, and initial chemical composition allows strong tests of the theory of stellar evolution. We report results from the first 34 days of science data from the Kepler Mission for the open cluster NGC 6819—one of the four clusters in the field of view. We obtain the first clear detections of solar-like oscillations in the cluster red giants and are able to measure the large frequency separation, Δν, and the frequency of maximum oscillation power, νmax. We find that the asteroseismic parameters allow us to test cluster membership of the stars, and even with the limited seismic data in hand, we can already identify four possible non-members despite their having a better than 80% membership probability from radial velocity measurements. We are also able to determine the oscillation amplitudes for stars that span about 2 orders of magnitude in luminosity and find good agreement with the prediction that oscillation amplitudes scale as the luminosity to the power of 0.7. These early results demonstrate the unique potential of asteroseismology of the stellar clusters observed by Kepler. Title: Calculation of Solar p-Mode Oscillation Frequency Splittings Based on a Two-Dimensional Solar Model Authors: Li, L.; Basu, S.; Sofia, S.; Demarque, P. Bibcode: 2009ASPC..416..321L Altcode: 2008arXiv0810.4943L We compute the p-mode oscillation frequencies and frequency splittings that arise in a two-dimensional model of the Sun that contains toroidal magnetic fields in its interior. Title: Direct Calculation of Turbulent Dissipation in Convective Zones Authors: Penev, Kaloyan; Barranco, J.; Sasselov, D.; Robinson, F.; Demarque, P. Bibcode: 2009AAS...21361205P Altcode: The current understanding of the turbulent dissipation in stellar convective zones is based on the assumption that the turbulence follows Kolmogorov scaling. This assumption is valid for some cases in which the time frequency of the external shear is high (e.g. solar p-modes). However, for many cases of astrophysical interest (e.g. binary orbits, stellar pulsations e.t.c.) the timescales of interest lie outside the regime of applicability of Kolmogorov scaling.

We present direct calculations of the dissipation efficiency of the turbulent convective flow in this regime using simulations of anelastic convection with external forcing. We show that the effects of the turbulent flow are well represented by an effective viscosity coefficient and we provide the values of the effective viscosity as a function of the perturbation frequency. In addition we justify a perturbative method for finding the effective viscosity (proposed by Goodman and Oh 1997) and apply it to actual simulations of the surface convective zones of low mass stars. Title: Comparing Properties of Convection in the Surface Layers of Stars With Different Masses and Evolutionary States Authors: Robinson, F.; Tanner, J.; Basu, S.; Demarque, P. Bibcode: 2009ASPC..416..357R Altcode: By comparing convective properties computed from simulations derived from a series of G and K type stellar models, we find marked differences between simulations and mixing length theory. These differences increase with the strength of the turbulence (as measured by the size of the turbulent pressure). Two of the most obvious differences are in the run of the superadiabaticity versus depth and in the size of the convective eddies. Provided the turbulence is weak (i.e. the ratio of turbulent pressure to gas pressure is a few percent) the estimate of eddy size used in a stellar model eddy (i.e. αHp) is quite close to the mean size of the simulated eddies. However, as the strength of the turbulence is increased (as happens in more evolved or hotter or more massive stars) the mixing length estimate becomes a very poor approximation to the actual eddy size. We also show results of a comparison study between different simulation codes conducted by an independent collaborator. Despite significant differences in input physics, numerical methods and treatments of radiation, the resolved convective quantities differ by less than 10%. Title: Dissipation Efficiency in Turbulent Convective Zones in Low-Mass Stars Authors: Penev, K.; Sasselov, D.; Robinson, F.; Demarque, P. Bibcode: 2009ApJ...704..930P Altcode: 2008arXiv0810.5145P We extend the analysis of Penev et al. to calculate effective viscosities for the surface convective zones of three main-sequence stars of 0.775 M sun, 0.85 M sun, and the present day Sun. In addition, we also pay careful attention to all normalization factors and assumptions in order to derive actual numerical prescriptions for the effective viscosity as a function of the period and direction of the external shear. Our results are applicable for periods that are too long to correspond to eddies that fall within the inertial subrange of Kolmogorov scaling, but no larger than the convective turnover time, when the assumptions of the calculation break down. We find moderately anisotropic viscosity, scaling linearly with the period of the external perturbation, with its components having magnitudes between three and ten times smaller than the Zahn's prescription. Title: Evolutionary Status of 85 Pegasi Authors: Bach, K.; Lee, J.; Demarque, P.; Kim, Y. -C. Bibcode: 2009ApJ...703..362B Altcode: We have investigated the evolutionary status of the visual binary 85 Peg (HD 224930) using a high-resolution spectroscopic analysis and the astrometric calibration of the stellar parameters. In spite of well-determined stellar parameters from HIPPARCOS astrometry and from spectroscopy, 85 Peg has for a long time revealed peculiar properties when attempts have been made to combine an evolutionary model with observation. There has been a chronic problem of the mass ratio of the two components when comparing 85 Peg's dynamical properties to the photometric magnitude difference. Moreover, 85 Peg has been suspected to have unique characteristics in its chemical composition. In order to determine accurate spectroscopic abundances, we have obtained high-resolution echelle spectra for 85 Peg. From our elemental analysis, we found that 85 Peg is α-enhanced with respect to the scaled solar abundance by a factor of 2. We then produced, within the framework of the standard stellar theory, grids of stellar model using the most recent observational results. To avoid many-fold degeneracy among physical quantities, a statistical minimization test was carried out between theoretical model grids. Enforcing consistency between the modeling and statistical constraints, we derived a reliable set of physical parameters and confirmed the trinarity of the system. In the context of asteroseismology, the theoretical frequency spectrum of 85 Peg was calculated. 85 Peg A is expected to have a first order spacing Δν ~ 165 μHz at the reference frequency ν0 = 790 μHz. Title: Asteroseismology and interferometry of the red giant star ɛ Ophiuchi Authors: Mazumdar, A.; Mérand, A.; Demarque, P.; Kervella, P.; Barban, C.; Baudin, F.; Coudé du Foresto, V.; Farrington, C.; Goldfinger, P. J.; Goupil, M. -J.; Josselin, E.; Kuschnig, R.; McAlister, H. A.; Matthews, J.; Ridgway, S. T.; Sturmann, J.; Sturmann, L.; ten Brummelaar, T. A.; Turner, N. Bibcode: 2009A&A...503..521M Altcode: 2009arXiv0906.3386M The GIII red giant star ɛ Oph has been found to exhibit several modes of oscillation by the MOST mission. We interpret the observed frequencies of oscillation in terms of theoretical radial p-mode frequencies of stellar models. Evolutionary models of this star, in both shell H-burning and core He-burning phases of evolution, are constructed using as constraints a combination of measurements from classical ground-based observations (for luminosity, temperature, and chemical composition) and seismic observations from MOST. Radial frequencies of models in either evolutionary phase can reproduce the observed frequency spectrum of ɛ Oph almost equally well. The best-fit models indicate a mass in the range of 1.85 ± 0.05 {M_⊙} with radius of 10.55 ± 0.15{R_⊙}. We also obtain an independent estimate of the radius of ɛ Oph with highly accurate interferometric observations in the infrared K' band, using the CHARA/FLUOR instrument. The measured limb-darkened disk angular diameter of ɛ Oph is 2.961 ± 0.007 mas. Together with the Hipparcos parallax, this translates into a photospheric radius of R = 10.39 ± 0.07 {R_⊙}. The radius obtained from the asteroseismic analysis matches the interferometric value quite closely even though the radius was not constrained during the modelling. Title: Isochrones for late-type stars Authors: Demarque, Pierre Bibcode: 2009IAUS..258..383D Altcode: A brief summary of the history of stellar evolution theory and the use of isochrones is given. The present state of the subject is summarized. The major uncertainties in isochrone construction are considered: chemical abundances and color calibrations, and the treatment of turbulent convection in stellar interior and atmosphere models. The treatment of convection affects the modeling of stellar interiors principally in two ways: convective core overshoot which increases evolutionary lifetimes, and the depth of convection zones which determines theoretical radii. Turbulence also modifies atmospheric structure and dynamics, and the derivation of stellar abundances. The symbiosis of seismic techniques with increasingly more realistic three-dimensional radiation hydrodynamics simulations is transforming the study of late-type stars. The important case of very low mass stars, which are fully convective, is briefly visited. Title: Two-Dimensional Stellar Evolution Code Including Arbitrary Magnetic Fields. II. Precision Improvement and Inclusion of Turbulence and Rotation Authors: Li, Linghuai; Sofia, Sabatino; Ventura, Paolo; Penza, Valentina; Bi, Shaolan; Basu, Sarbani; Demarque, Pierre Bibcode: 2009ApJS..182..584L Altcode: 2008arXiv0810.4938L In the second paper of this series we pursue two objectives. First, in order to make the code more sensitive to small effects, we remove many approximations made in Paper I. Second, we include turbulence and rotation in the two-dimensional framework. The stellar equilibrium is described by means of a set of five differential equations, with the introduction of a new dependent variable, namely the perturbation to the radial gravity, that is found when the nonradial effects are considered in the solution of the Poisson equation. Following the scheme of the first paper, we write the equations in such a way that the two-dimensional effects can be easily disentangled. The key concept introduced in this series is the equipotential surface. We use the underlying cause-effect relation to develop a recurrence relation to calculate the equipotential surface functions for uniform rotation, differential rotation, rotation-like toroidal magnetic fields, and turbulence. We also develop a more precise code to numerically solve the two-dimensional stellar structure and evolution equations based on the equipotential surface calculations. We have shown that with this formulation we can achieve the precision required by observations by appropriately selecting the convergence criterion. Several examples are presented to show that the method works well. Since we are interested in modeling the effects of a dynamo-type field on the detailed envelope structure and global properties of the Sun, the code has been optimized for short timescales phenomena (down to 1 yr). The time dependence of the code has so far been tested exclusively to address such problems. Title: VizieR Online Data Catalog: Velocities and photometry in NGC 2112 (Carraro+, 2008) Authors: Carraro, G.; Villanova, S.; Demarque, P.; Moni Bidin, C.; McSwain, M. V. Bibcode: 2009yCat..73861625C Altcode: U, B, V and I images centred on NGC 2112 were obtained at the Cerro Tololo Inter-American Observatory 1.0-m telescope, which is operated by the SMARTS1 consortium. The telescope is equipped with a new 4000x4000 CCD camera having a pixel scale of 0.289arcsec/pixel, which allows one to cover a field of 20x20arcmin2. This allows us to cover the entire cluster, which has an estimated diameter of 18 arcmin (Dias et al., 2002, Cat. <VII/229>). Observations were carried out on 2005 November 30.

Medium-resolution spectroscopic observations were carried out on the night of 2006 February 15 (Julian Date 2453783.57006) with the Hydra spectrograph onboard the Wisconsin Indiana Yale NOAO (WIYN) telescope at Kitt Peak National Observatory

Echelle spectrograms of stars 535, 261, 717, 304 and 836 were obtained on 2007 October 29 with the Magellan Inamori Kyocera Echelle (MIKE) spectrograph mounted on the Nasmyth focus of Landon Clay 6.5-m telescope at the Magellan Observatory. Data were obtained with both the blue and red arms.

(1 data file). Title: Dissipation Efficiency of Turbulent Convection Authors: Penev, Kaloyan; Sasselov, D.; Barranco, J.; Demarque, P.; Robinson, F. Bibcode: 2009AAS...21431402P Altcode: The turbulent flow in stellar convective zones is believed to be an important dissipation mechanism for phenomena like tides and stellar pulsations. The current understanding of this dissipation is based on the assumption that the turbulence follows Kolmogorov scaling. This assumption is reasonable for external shear with high time frequency (e.g. solar p-modes). However, for many cases of astrophysical interest (e.g. binary orbits, Cepheid pulsations etc.) the relevant timescales fall outside of the inertial subrange. We present direct calculations of the turbulent dissipation derived from simulations of stratified anelastic convection with external shear built directly into the equations of motion. We show that the observed dissipation is well parametrized as an effective viscosity coefficient and we derive the values of this coefficient as a function of the forcing period and the direction and amplitude of the shear. In addition we justify a perturbative method for finding the effective viscosity (proposed by Gooman and Oh. 1997) and use it to estimate the effective viscosity in the surface convective zones of low mass stars, using fully compressible numerical simulations with realistic physics specific to such stars. Title: New Yonsei-Yale (Y 2) Isochrones and Horizontal-Branch Evolutionary Tracks with Helium Enhancements Authors: Han, S. -I.; Kim, Y. -C.; Lee, Y. -W.; Yi, S. K.; Kim, D. -G.; Demarque, P. Bibcode: 2009gcgg.book...33H Altcode: Recent studies [4,8,7,9,3] suggest that peculiar features observed in the horizontal-branch (HB) and main-squence (MS) of some Globular Clusters (GCs) are naturally reproduced by the presence of super-He-rich populations. However, there are no isochrones and self-consistent HB tracks available in the literature that are based on the up-to-date input physics and wide ranges of He and metal abundances. Here we present new sets of Y 2 isochrones and HB evolutionary tracks with the effects of He enhancements. The most up-to-date input physics are adopted, and the effects of α-enhancements ([α/Fe]=0.3) are also fully taken into account. These isochrones and tracks have been constructed for more realistic evolutionary population syntheses for old stellar populations, such as GCs and elliptical galaxies. The new isochrones and HB tracks well reproduce the observed color magnitude diagrams of Galactic GCs, including peculiar features observed in the HB and MS of ω Centauri and NGC 2808. Title: The Nature of p-Modes and Granulation in Procyon: New MOST Photometry and New Yale Convection Models Authors: Guenther, D. B.; Kallinger, T.; Gruberbauer, M.; Huber, D.; Weiss, W. W.; Kuschnig, R.; Demarque, P.; Robinson, F.; Matthews, J. M.; Moffat, A. F. J.; Rucinski, S. M.; Sasselov, D.; Walker, G. A. H. Bibcode: 2008ApJ...687.1448G Altcode: We present new photometry of Procyon, obtained by MOST during a 38 day run in 2007, and frequency analyses of those data. The long time coverage and low point-to-point scatter of the light curve yield an average noise amplitude of about 1.5-2.0 ppm in the frequency range 500-1500 μHz. This is half the noise level obtained from each of the previous two Procyon campaigns by MOST in 2004 and 2005. The 2007 MOST amplitude spectrum shows some evidence for p-mode signal: excess power centered near 1000 μHz and an autocorrelation signal near 55 μHz (suggestive of a mode spacing around that frequency), both consistent with p-mode model predictions. However, we do not see regularly spaced frequencies aligned in common l-valued ridges in echelle diagrams of the most significant peaks in the spectrum unless we select modes from the spectrum using a priori assumptions. The most significant peaks in the spectrum are scattered by more than ±5 μHz about the predicted l-valued ridges, a value that is consistent with the scatter among individually identified frequencies obtained from ground-based radial velocity (RV) observations. We argue that the observed scatter is intrinsic to the star, due to short lifetimes of the modes and the dynamic structure of Procyon's thin convection zone. We compare the MOST Procyon amplitude and power density spectra with preliminary results of three-dimensional numerical models of convection by the Yale group. These models show that, unlike in the Sun, Procyon's granulation signal in luminosity has a peak coinciding with the expected frequency region for p-modes near 1000 μHz.

Based on data from the MOST satellite, a Canadian Space Agency mission, jointly operated by Dynacon, Inc., the University of Toronto Institute of Aerospace Studies, and the University of British Columbia, with the assistance of the University of Vienna. Title: The Y2 Isochrones Getting an Extra Dimension Authors: Yi, Sukyoung K.; Kim, Yong-Cheol; Demarque, Pierre; Lee, Young-Wook; Han, Sang-Il; Kim, Do Gyun Bibcode: 2008IAUS..252..413Y Altcode: The Yonsei-Yale Isochrones have been widely used since its birth in 2001. We announce a major upgrade mainly making varieties of helium values available. The recent works on the globular clusters with extreme helium abundances have called for such a need. The new version of the Y2 Isochrones are available for [α/Fe] = 0 through 0.6, ΔY/ΔZ = 1.5 through 3.0, and extreme helium abundances (Y = normal 0.05, 0.1, 0.15, 0.2), and for 11 metallicity grids, with full capability of interpolation. The database will be powerful for making population models. Besides, the accuracy of the models on the lower main sequence has been substantially improved. We illustrate the major upgrades and demonstrate the power of the new grids. Title: YREC: the Yale rotating stellar evolution code. Non-rotating version, seismology applications Authors: Demarque, P.; Guenther, D. B.; Li, L. H.; Mazumdar, A.; Straka, C. W. Bibcode: 2008Ap&SS.316...31D Altcode: 2007Ap&SS.tmp..447D; 2007arXiv0710.4003D The stellar evolution code YREC is outlined with emphasis on its applications to helio- and asteroseismology. The procedure for calculating calibrated solar and stellar models is described. Other features of the code such as a non-local treatment of convective core overshoot, and the implementation of a parametrized description of turbulence in stellar models, are considered in some detail. The code has been extensively used for other astrophysical applications, some of which are briefly mentioned at the end of the paper. Title: The old open cluster NGC 2112: updated estimates of fundamental parameters based on a membership analysis† Authors: Carraro, G.; Villanova, S.; Demarque, P.; Moni Bidin, C.; McSwain, M. V. Bibcode: 2008MNRAS.386.1625C Altcode: 2008arXiv0802.3243C; 2008MNRAS.tmp..416C We report on a new, wide-field (20 × 20 arcmin2), multicolour (UBVI), photometric campaign in the area of the nearby old open cluster NGC 2112. At the same time, we provide medium-resolution spectroscopy of 35 (and high-resolution of additional 5) red giant and turn-off stars. This material is analysed with the aim to update the fundamental parameters of this traditionally difficult cluster, which is very sparse and suffers from heavy field star contamination. Among the 40 stars with spectra, we identified 21 bona fide radial velocity members which allow us to put more solid constraints on the cluster's metal abundance, long suggested to be as low as the metallicity of globulars. As indicated earlier by us on a purely photometric basis, the cluster [Fe/H] abundance is slightly supersolar ([Fe/H] = 0.16 +/- 0.03) and close to the Hyades value, as inferred from a detailed abundance analysis of three of the five stars with higher resolution spectra. Abundance ratios are also marginally supersolar.

Based on this result, we revise the properties of NGC 2112 using stellar models from the Padova and Yale-Yonsei groups.

For this metal abundance, we find that the cluster's age, reddening and distance values are 1.8 Gyr, 0.60 mag and 940 pc, respectively. Both the Yale-Yonsei and Padova models predict the same values for the fundamental parameters within the errors.

Overall, NGC 2112 is a typical solar neighbourhood, thin-disc star cluster, sharing the same chemical properties of F-G stars and open clusters close to the Sun.

This investigation outlines the importance of a detailed membership analysis in the study of disc star clusters.

This paper includes data gathered with the 6.5 Magellan Telescopes, located at Las Campanas Observatory, Chile.

The data discussed in this paper will be made available at the WEBDA open cluster data base http://www.univie.ac.at/webda, which is maintained by E. Paunzen and J.-C. Mermilliod. ‡

E-mail: gcarraro@eso.org (GC); sandro.villanova@unipd.it (SV); demarque@astro.yale.edu (PD); mbidin@das.uchile.cl (CMB); mcswain@lehigh.edu(MVM) Title: Effects of Chemical Abundances on the Structure and Dynamics of the Solar Atmosphere Authors: Demarque, Pierre; Basu, S.; Robinson, F.; Hunter, C.; Kallinger, T. Bibcode: 2008AAS...212.1707D Altcode: 2008BAAS...40..211D This paper presents a comparison between two three-dimensional radiative hydrodynamical (3D RHD) simulations of the solar outer layers using the Grevesse & Sauval (1998; GS98) and the Asplund, Grevesse & Sauval (2005; AGS05) mixtures, respectively.

The AGS05 mixture is a revision of the solar abundance mixture using a model atmosphere based on the stratification and dynamics of a solar 3D RHD simulation performed with the GS98 mixture, using the code of Stein & Nordlund (1989, 1998). Our simulations use the same code, and physically realistic treatment of radiative opacities and equation of state as Robinson et al. (2003).

A comparison between our GS98 and AGS05 3D simulations shows differences both in the mean vertical temperature gradient and in the turbulent velocity field in the line forming region of the solar atmosphere. The whole superadiabatic layer (SAL) in the AGS05 simulation, including its peak, is shifted outward with respect to the GS98 SAL. Turbulent velocities, critical for absorption line profile calculations, are larger in the AGS05 simulation by about 10%. Since opacities for the AGS05 mixture are lower than those for the GS98 mixture, the T-τ relations derived from the two simulations are different. At 5000K, τ obtained from the GS98 simulation is about 30% larger than that obtained from the AGS05 simulation.

These results show the importance of preserving self consistency in the chemical abundances between the model atmosphere and the 3D HRD simulation. When deriving the solar abundances, one must iterate the chemical composition in the model atmosphere, and hence in the 3D simulation on which the atmosphere is based.

This research was supported in part by NASA/ATP grant NAG5-13299 (PD and FR) and NSF grant ATM 0348837 to SB. Title: A cgi synthetic CMD calculator for the YY Isochrones Authors: Demarque, P.; Virani, S. N.; Murphy, E. J.; Woo, J. -H.; Kim, Y. -C.; Yi, S. K. Bibcode: 2008arXiv0801.0451D Altcode: We describe a web-based cgi calculator for constructing synthetic color-magnitude diagrams for a simple stellar population (SSP) using the Yonsei-Yale (YY) isochrone data base. This calculator is designed to be used interactively. It creates quick look CMD displays in (B-V) and (V-I) colors. Stochastic effects on the CMDs are included. Output in tabular form is also provided for special purpose displays, or for combining the CMDs of different stellar populations. This research tool has applications in studies of the stellar content of our Galaxy and external systems. It provides an easy way to interpret the CMDs in resolved stellar populations. It offers the means to explore the dependence of the integrated properties of unresolved stellar systems on stellar parameters (ages, chemical composition, binarity) and on the characteristics of their parent population (IMF slope and mass range). Title: Turbulent Convection in Stars: Some Recent Advances Authors: Demarque, P.; Robinson, F. J.; Straka, C. W. Bibcode: 2007ASPC..374...27D Altcode: Advances are discussed in two areas in which improved numerical treatments of radiation hydrodynamics, coupled with advances in helio- and asteroseismology, provide stringent tests of stellar interior models. Stellar evolution data are fundamental building blocks in population studies. The first area is convective core overshoot which affects evolutionary timescales and the evolution of integrated spectral energy distributions in stellar systems. The second area is the treatment of radiation hydrodynamics in the outer layers of cool stars with convection zones, with the aim of removing the shortcomings of the canonical mixing-length theory. A realistic physical description is also needed to calculate p-mode excitation and amplitudes, tidal dissipation in convection zones, and to derive accurate chemical abundances. Title: Surface Convection in Sun and a Population II Star Authors: Jung, Y. K.; Kim, Y. -C.; Robinson, F. J.; Demarque, P.; Chan, K. L. Bibcode: 2007ASPC..362..306J Altcode: Three-dimensional hydrodynamic simulations including radiative energy transfer were performed to study the characteristics of surface convection in Sun and a population II star. To compare convection phenomenon of a Pop II star with that of the Sun, we have constructed a Pop II star whose mass is 0.7 solar mass and metallicity is z = 0.0004. The domain of simulation is a rectangular box containing a convection-radiation transition region with an impenetrable boundary. The mass and the momentum are conserved.

The thermal and dynamic variables of each layer, including overshoot, were explored. We found that the effect of turbulence in a Pop II star is lower than that of Sun. The vertical and horizontal sizes of granules, dominantly affected by the turbulence, are also smaller in a Pop II star. These results indicate that a Pop II star has fewer turbulent structures than Sun because of its higher density and surface gravity. Title: Probing turbulence with seismic observations of α Centauri Authors: Straka, C. W.; Demarque, P.; Robinson, F. J. Bibcode: 2007IAUS..239..388S Altcode: No abstract at ADS Title: Comparing convection in the Sun with Procyon A Authors: Robinson, Frank; Demarque, Pierre Bibcode: 2007IAUS..239..358R Altcode: No abstract at ADS Title: Calibration of Stellar Parameters of 85 Peg System Authors: Bach, Kiehunn; Kim, Yong-Cheol; Demarque, Pierre Bibcode: 2007JASS...24...31B Altcode: We have investigated the evolutionary status of 85 Peg within the framework of standard evolutionary theory. 85 Peg has been known to be a visual and spectroscopic binary system in the solar neighborhood. In spite of the accurate information of the total mass (∼ 1.5 M_{⊙}) and the distance (∼12pc) from the HIPPARCOS upshape parallax, it has been undetermined an individual mass, therefore the evolved status of the system. Moreover, the coupled uncertainties of chemical composition and age, make matters worse in predicting an evolutionary status of the system. Nevertheless, we computed the various possible models for 85 Peg, and then calibrated stellar parameters by adjusting to the recent observational data. Our modelling computation has included recently updated input physics and stellar theory such as opacity, equation of state, and chemical diffusion. Through a statistical assessment, we have derived a confident parameter set as the best solution which minimized χ^2 within the observational error domain. Most of all, we found that 85 Peg is not a binary system but a triple system with an unseen companion 85 Peg B_{b}∼0.16M_{⊙}. The aim of the present paper is (1) to provide a complete modelling of the stellar system based on the evolutionary theory, and (2) to constrain the physical dimensions such as mass, metallicity and age. Title: On Dissipation inside Turbulent Convection Zones from Three-dimensional Simulations of Solar Convection Authors: Penev, Kaloyan; Sasselov, Dimitar; Robinson, Frank; Demarque, Pierre Bibcode: 2007ApJ...655.1166P Altcode: 2006astro.ph..7016P The development of two-dimensional and three-dimensional simulations of solar convection has lead to a picture of convection quite unlike the usually assumed Kolmogorov spectrum turbulent flow. We investigate the impact of this changed structure on the dissipation properties of the convection zone, parameterized by an effective viscosity coefficient. We use an expansion treatment developed by Goodman & Oh, applied to a numerical model of solar convection, to calculate effective viscosity as a function of frequency and compare this to currently existing prescriptions based on the assumption of Kolmogorov turbulence. The results quite closely match a linear scaling with period, even though this same formalism applied to a Kolmogorov spectrum of eddies gives a scaling with a power-law index of 5/3. Title: Short-Period Variables in the Local Group Dwarf Galaxies Tucana and LGS3 Authors: Bernard, E. J.; Monelli, M.; Gallart, C.; Aparicio, A.; Bertelli, G.; Cassisi, S.; Cole, A. A.; Demarque, P.; Dolphin, A. E.; Drozdovsky, I.; Ferguson, H. C.; Hidalgo, S.; Mateo, M.; Mayer, L.; Navarro, J.; Pont, F.; Skillman, E. D.; Stetson, P. B.; Tolstoy, E. Bibcode: 2007astro.ph..1729B Altcode: We present preliminary results concerning the search for short-period variable stars in Tucana and LGS3 based on very deep HST/ACS imaging. In the fraction of the observed field we studied in each galaxy, a total of 133 and 30 variables were found, respectively. For Tucana, we identified 76 of them as RR Lyrae (RRL) stars pulsating in the fundamental mode (RRab) and 32 in the first-overtone mode (RRc), as well as 2 anomalous Cepheids (AC). The mean period of the RRab and RRc is 0.59 and 0.35 days, respectively. In the case of LGS3, we found 24 RRab and 4 RRc, with mean periods 0.61and 0.39 days, respectively, plus two candidate ACs. These values place both galaxies in the Oosterhoff gap. Title: The hot stars in orbit around the M 31 central supermassive black hole: are they young or old? Authors: Demarque, P.; Virani, S. Bibcode: 2007A&A...461..651D Altcode: 2006astro.ph..3326D Aims:The cluster of hot stars observed in orbit around the central black hole of M 31 has been interpreted as a 200 Myr starburst. The formation of a population of young stars in close proximity to a massive black hole presents a difficult challenge to star formation theory. We point out that in a high stellar density environment, the course of stellar evolution is modified by frequent collisions and mergers.
Methods: Blue stragglers, which are the results of mergers in globular clusters, occupy the same position in the color-magnitude diagram as the observed hot stars in M 31. For confirmation, the integrated spectrum of P3 is shown to be compatible with the spectral energy distribution of a blue horizontal branch field star.
Results: We suggest an old stellar population of evolved blue horizontal-branch stars and of merger products cannot be ruled out on the basis of the available data. Observations are suggested that would help distinguish between a "young" and "old" stellar population interpretation of the observations. Title: Asteroseismic diagnostics of stellar convective cores Authors: Mazumdar, A.; Basu, S.; Collier, B. L.; Demarque, P. Bibcode: 2006ESASP.624E.109M Altcode: 2006soho...18E.109M No abstract at ADS Title: Asteroseismic diagnostics of stellar convective cores Authors: Mazumdar, Anwesh; Basu, Sarbani; Collier, Braxton L.; Demarque, Pierre Bibcode: 2006MNRAS.372..949M Altcode: 2006astro.ph..8331M; 2006MNRAS.tmp.1010M We present a detailed study of the small frequency separations as diagnostics of the mass of the convective core and evolutionary stage of solar-type stars. We demonstrate how the small separations can be combined to provide sensitive tests for the presence of convective overshoot at the edge of the core. These studies are focused on low-degree oscillation modes, the only modes expected to be detected in distant stars. Using simulated data with realistic errors, we find that the mass of the convective core can be estimated to within 5 per cent if the total stellar mass is known. Systematic errors arising due to uncertainty in the mass could be up to 20 per cent. The evolutionary stage of the star, determined in terms of the central hydrogen abundance using our proposed technique, however, is much less sensitive to the mass estimate. Title: NGC 6791: An Exotic Open Cluster or the Nucleus of a Tidally Disrupted Galaxy? Authors: Carraro, Giovanni; Villanova, Sandro; Demarque, Pierre; McSwain, M. Virginia; Piotto, Giampaolo; Bedin, Luigi R. Bibcode: 2006ApJ...643.1151C Altcode: 2005astro.ph.12650C We report on high-resolution echelle spectroscopy of 20 giant stars in the Galactic old open cluster NGC 6791, obtained with Hydra at the WIYN telescope. High-precision radial velocity allows us to isolate 15 bona fide cluster members. From 10 of them we derive a global [M/H]=+0.39+/-0.05. We therefore confirm that NGC 6791 is extremely metal-rich, exhibits a few marginally subsolar abundance ratios, and within the resolution of our spectra does not show evidence of spread in metal abundance. With these new data we rederive the fundamental cluster parameters, suggesting that it is about 8 Gyr old and 4.3 kpc from the Sun. The combination of its chemical properties, age, position, and Galactic orbit hardly makes NGC 6791 a genuine Population I open cluster. We discuss possible interpretations of the cluster peculiarities, suggesting that the cluster might be what remains of a much larger system whose initial potential well could have been sufficient to produce high-metallicity stars and which has been depopulated by the tidal field of the Galaxy. Alternatively, its current properties may be explained by the perturbation of the Galactic bar on an object that originated well inside the solar ring, where the metal enrichment was very fast. Title: Dynamical Characteristics Of Surface Convection In The Sun And A Population II Star Authors: Bach, Kiehunn; Jung, Y. K.; Kim, Y. C.; Robinson, F. J.; Demarque, P.; Chan, K. L. Bibcode: 2006AAS...208.0602B Altcode: 2006BAAS...38...84B We present the results of the 3 dimensional radiative hydrodynamic numerical large eddy simulations of the outer layers of stars. The goal of this study is to compare convection phenomenon of a Pop II star with the Sun. We have constructed a Pop II star of 0.7 solar mass and metallicity <i>z</i>=0.0004. The results are compared to a similar simulation performed for the Sun using the same detailed physics input and numerical code. The domains of simulations extend from the observable atmospheric layers in radiative equilibrium and the outer layers of the convection zones, down to a depth at which the temperature gradient exceeds the adiabatic value by only a very small amount. Each simulation has been made for 32 and 24 minutes for the Sun and a Pop II star, respectively, and the thermal and dynamical structures of each star including overshoot are explored. The vertical and horizontal sizes of granules, dominantly affected by the turbulence, are smaller in a Pop II star. These results indicate that a Pop II star has less turbulent structures than that of the Sun because of its higher density distribution and surface gravity. Title: Two-dimensional Stellar Evolution Code Including Arbitrary Magnetic Fields. I. Mathematical Techniques and Test Cases Authors: Li, L. H.; Ventura, P.; Basu, S.; Sofia, S.; Demarque, P. Bibcode: 2006ApJS..164..215L Altcode: 2005astro.ph.11238L A high-precision two-dimensional stellar evolution code has been developed for studying solar variability due to structural changes produced by varying internal magnetic fields of arbitrary configurations. Specifically, we are interested in modeling the effects of a dynamo-type field on the detailed internal structure and on the global parameters of the Sun. The high precision is required to model both very small solar changes (of the order of 10-4) and short timescales (of the order of 1 yr). It is accomplished by using the mass coordinate to replace the radial coordinate, by using fixed and adjustable time steps, a realistic stellar atmosphere, and element diffusion, and by adjusting the grid points. We have also built into the code the potential to subsequently include rotation and turbulence. The current code has been tested for several cases, including its ability to reproduce the one-dimensional results. Title: Turbulence in models of a star other than Sun: matching η Bootis observed p-modes Authors: Straka, C. W.; Demarque, P.; Guenther, D. B.; Li, L.; Robinson, F. J. Bibcode: 2006MmSAI..77..421S Altcode: Standard stellar models for η Boo fail to reproduce the newly observed low frequency p-modes from space and the high frequency p-modes observed from the ground, simultaneously. This discrepancy can be removed by including turbulence in the modeling of the outer layers of η Boo. We include turbulence by applying the effects of turbulent pressure and turbulent kinetic energy -- extracted from a hydrodynamical 3D convection simulation for the Sun -- at the correct depth in the 1D models of η Boo. Title: Space- and Ground-based Pulsation Data of η Bootis Explained with Stellar Models Including Turbulence Authors: Straka, Christian W.; Demarque, Pierre; Guenther, D. B.; Li, Linghuai; Robinson, Frank J. Bibcode: 2006ApJ...636.1078S Altcode: 2005astro.ph..9403S The space telescope MOST is now providing us with extremely accurate low-frequency p-mode oscillation data for the star η Boo. We demonstrate in this paper that these data, when combined with ground-based measurements of the high-frequency p-mode spectrum, can be reproduced with stellar models that include the effects of turbulence in their outer layers. Without turbulence, the l=0 modes of our models deviate from either the ground-based or the space data by about 1.5-4 μHz. This discrepancy can be completely removed by including turbulence in the models, and we can exactly match 12 out of 13 MOST frequencies that we identified as l=0 modes, in addition to 13 out of 21 ground-based frequencies within their observational 2 σ tolerances. The better agreement between model frequencies and observed frequencies depends for the most part on the turbulent kinetic energy that was taken from a three-dimensional convection simulation for the Sun. Title: Turbulence in η Bootis outer layers as derived from 3D convection simulations to explain its observed p-mode spectrum Authors: Straka, C. W.; Demarque, P.; Robinson, F. J.; Guenther, D. B.; Li, L. Bibcode: 2005AAS...207.6913S Altcode: 2005BAAS...37Q1272S Standard stellar models for η Boo fail to reproduce the observed low frequency p-modes from space and the high frequency p-modes observed from ground, simultaneously. This discrepancy can be mitigated by including turbulence in the modeling of the outer layers of η Boo. We present new results of our full 3D turbulent convection simulation of η Boo's outer layers. With these simulations we are able to answer the question whether the discrepancy between models and observations can be fully attributed to the effects of turbulence.

This research was supported by NASA grant NAG5-13299, and in part by the NASA EOS/IDS program. Title: Asteroseismic Diagnostics of Stellar Convective Cores Authors: Mazumdar, A.; Collier, B. L.; Basu, S.; Demarque, P. Bibcode: 2005AAS...207.6912M Altcode: 2005BAAS...37.1272M The extent of the central convective region is one of the crucial factors that govern the structure and evolution of massive stars. It has been suggested that observations of seismic waves that penetrate the deepest layers of a star might be used to estimate the size of such convective cores. This would allow a rigorous test of the current theories of convection and stellar evolution. We investigate whether suitable diagnostics can be constructed from the frequencies of low degree modes of oscillation which are sensitive to the size and evolution of the stellar core.

SB and BC are partially supported in this research by grant ATM-0348837 from NSF. PD is supported by NASA grant NAG5-13299. Title: The Nonhomologous Nature of Solar Diameter Variations Authors: Sofia, Sabatino; Basu, Sarbani; Demarque, Pierre; Li, Linghuai; Thuillier, Gerard Bibcode: 2005ApJ...632L.147S Altcode: 2005astro.ph..9816S We show in this Letter that the changes of the solar diameter in response to variations of large-scale magnetic fields and turbulence are not homologous. For the best current model, the variation at the photospheric level is over 1000 times larger than the variation at a depth of 5 Mm, which is about the level at which f-mode solar oscillations determine diameter variations. This model is supported by observations that indicate larger diameter changes for high-degree f-modes than for low-degree f-modes, since the energy of the former is concentrated at shallower layers than the latter. Title: Simulating the outer layers of Procyon A: a comparison with the Sun Authors: Robinson, F. J.; Demarque, P.; Guenther, D. B.; Kim, Y. -C.; Chan, K. L. Bibcode: 2005MNRAS.362.1031R Altcode: 2005astro.ph..7005R; 2005MNRAS.tmp..730R We compare a new 3D radiative hydrodynamical simulation of the surface layers of Procyon A to a similar 3D simulation of the surface layers of the Sun. Both simulations include realistic input physics and are performed using the same numerical techniques and computer codes.

Convection in the surface layers of Procyon A is very different from the Sun. Compared with the Sun, the atmospheric structure and convective flow in Procyon A exhibit the following characteristics. (i) The highly superadiabatic transition layer (SAL) is located at a much shallower optical depth; it is in a dynamically active region and its outer region is sometimes located in the optically thin atmosphere. (ii) The outer region of the SAL moves from an optically thin region to a thick region and back again over a time of 20-30 min. This motion, which is driven by the granulation, takes place in a time approximately half the turnover time of the largest granules. (iii) The peak rms velocity in the vertical direction is much larger in Procyon A. The main reason for the radically different radiative-convective behaviour in Procyon A compared with the Sun is the role played by turbulent eddies in determining the overall flow/thermal structure. The turbulent pressure and turbulent kinetic energy can exceed 50 per cent of the local gas pressure (compared with about 10-20 per cent in the Sun). In such regions, the mixing lengthy theory is a poor approximation.

The Procyon A simulation thus reveals two distinct time-scales: the autocorrelation time of the vertical velocity and the characteristic time-scale of the SAL, which is tied to granulation. Just below the surface, the autocorrelation decay time is about 5 min in Procyon A and the SAL motion time-scale is 20-30 min. In the simulations, the peak value of the superadiabaticity varies between 0.5 and 3. When the SAL penetrates the optically thin region, there are efficient radiative losses and the peak of the SAL is low. We speculate that these losses damp out the relative amplitudes in luminosity (temperature fluctuations) compared with velocity (Doppler). Although this will not affect the frequencies of the peaks in the power spectrum, it will probably lower the average amplitude of the peaks relative to the noise background. Title: Core Overshoot: An Improved Treatment and Constraints from Seismic Data Authors: Straka, Christian W.; Demarque, Pierre; Guenther, D. B. Bibcode: 2005ApJ...629.1075S Altcode: 2005astro.ph..5009S We present a comprehensive set of stellar evolution models for Procyon A in an effort to guide future measurements of both traditional stellar parameters and seismic frequencies toward constraining the amount of core overshoot in Procyon A and possibly other stars. Current observational measurements of Procyon A when combined with traditional stellar modeling only place a large upper limit on overshoot of αOV<1.1. By carrying out a detailed pulsation analysis, we further demonstrate how p- and g-mode averaged spacings can be used to gain better estimates of the core size. For both p- and g-modes, the frequency spacings for models without overshoot are clearly separated from the models with overshoot. In addition, measurements of the l=0 averaged small p-mode spacings could be used to establish Procyon A's evolutionary stage. For a fixed implementation of overshoot and under favorable circumstances, the g-mode spacings can be used to determine the overshoot extent to an accuracy of +/-0.05HP. However, we stress that considerable confusion is added due to the unknown treatment of the overshoot region. This ambiguity might be removed by analyzing many different stars. A simple nonlocal convection theory developed by Kuhfuss is implemented in our stellar evolution code and contrasted with the traditional approaches. We show that this theory supports a moderate increase of the amount of convective overshoot with stellar mass of ΔαOV~=+0.10 between 1.5 and 15 Msolar. This theory places an upper limit on Procyon A's core overshoot extent of ~0.4HP, which matches the limit imposed by Roxburgh's integral criterion. Title: Numerical Simulation of the Outer Layers of Procyon A: a Comparison with the Sun Authors: Demarque, P.; Robinson, F. J.; Guenther, D. B.; Kim, Y. -C.; Chan, K. L. Bibcode: 2005AAS...206.3501D Altcode: 2005BAAS...37..486D We present a 3D radiative-hydrodynamical simulation of the surface layers of Procyon A, and compare it to a similar simulation for the Sun. Procyon has a thin outer convection zone, and its atmosphere, like the solar atmosphere, is characterized by granulation. Procyon granules are much larger than in the Sun, with a mean diameter near 10,000 km, in comparison with 1,200 km for a solar granule. The dynamics of the Procyon atmosphere differ from the solar atmosphere dynamics in fundamental ways: (1) the ratio of turbulent to gas pressure is larger in the Procyon atmosphere (2) the highly superadiabatic transition layer (SAL) is located at much shallower optical depth (3) the location of the SAL maximum moves back and forth in the atmosphere, from an optically thin to optically thick region in a quasi-periodic manner. This motion is driven by the penetration of granules into the outermost atmospheric layers while they overturn. The timescale of this phenomenon is roughly half the full granule lifetime (which is typically in the range of 40 to 60 minutes); (3) the maximum temperature contrast at a given optical depth varies rapidly on a short timescale resulting in locally large radiative losses and strong intensity contrasts (hot spots). The simulation also shows that the autocorrelation timescale in Procyon's atmosphere is favorable to the stochastic excitation of p-modes by turbulence. However highly efficient radiative damping in Procyon's atmosphere is likely to result in much shorter p-mode coherence lifetimes than in the Sun.

This research was supported in part by NASA grant NAG5-13299 (PD), the NASA EOS/IDS Program (FJR), NSERC of Canada (DBG) and the Korea Research Foundation grant KRF-2003-015-C00249 (YCK). Title: Super-Helium-rich Populations and the Origin of Extreme Horizontal-Branch Stars in Globular Clusters Authors: Lee, Young-Wook; Joo, Seok-Joo; Han, Sang-Il; Chung, Chul; Ree, Chang H.; Sohn, Young-Jong; Kim, Yong-Cheol; Yoon, Suk-Jin; Yi, Sukyoung K.; Demarque, Pierre Bibcode: 2005ApJ...621L..57L Altcode: 2005astro.ph..1500L Recent observations for the color-magnitude diagrams (CMDs) of the massive globular cluster ω Centauri have shown that it has a striking double main sequence (MS), with a minority population of bluer and fainter MS stars well separated from a majority population of MS stars. Here we confirm, with the most up-to-date Y2 isochrones, that this special feature can only be reproduced by assuming a large variation (ΔY=0.15) of primordial helium abundance among several distinct populations in this cluster. We further show that the same helium enhancement required for this special feature on the MS can by itself reproduce the extreme horizontal-branch (HB) stars observed in ω Cen, which are hotter than normal HB stars. Similarly, the complex features on the HBs of other globular clusters, such as NGC 2808, are explained by large internal variations of helium abundance. Supporting evidence for the helium-rich population is also provided by the far-UV (FUV) observations of extreme HB stars in these clusters, where the enhancement of helium can naturally explain the observed fainter FUV luminosity for these stars. The presence of super-helium-rich populations in some globular clusters suggests that a third parameter, other than metallicity and age, also influences the CMD morphology of these clusters. Title: Y2 Isochrones with an Improved Core Overshoot Treatment Authors: Demarque, Pierre; Woo, Jong-Hak; Kim, Yong-Cheol; Yi, Sukyoung K. Bibcode: 2004ApJS..155..667D Altcode: Convective core overshoot affects stellar evolution rates and the dating of stellar populations. In this paper, we provide a patch to the Y2 isochrones with an improved treatment of convective core overshoot. The new tracks cover the transition mass range from no convective core to a fully developed convective core. We compare the improved isochrones to CMDs of a few well-observed open star clusters in the Galaxy and the Large Magellanic Cloud. Finally, we discuss future prospects for improving the treatment of core overshoot with the help of asteroseismology. Title: a 2-D Stellar Evolution Code Including Arbitrary Magnetic Fields Authors: Li, L.; Basu, S.; Sofia, S.; Demarque, P. Bibcode: 2004ESASP.559..536L Altcode: 2004soho...14..536L No abstract at ADS Title: Solar Luminosity Variations may Occur on Short Time Scales Authors: Li, L.; Basu, S.; Sofia, S.; Demarque, P. Bibcode: 2004ESASP.559..544L Altcode: 2004soho...14..544L No abstract at ADS Title: a Radiation-Hydrodynamical Simulation of the Envelope of Procyon a Authors: Robinson, F. J.; Demarque, P.; Kim, Y. -C. Bibcode: 2004ESASP.559..602R Altcode: 2004soho...14..602R No abstract at ADS Title: Asteroseismic Determination of Helium Abundance in Solar-Type Stars Authors: Basu, S.; Mazumdar, A.; Antia, H. M.; Demarque, P. Bibcode: 2004ESASP.559..313B Altcode: 2004soho...14..313B No abstract at ADS Title: The Y 2 Isochrones Authors: Yi, Sukyoung K.; Demarque, Pierre; Kim, Yong-Cheol Bibcode: 2004Ap&SS.291..261Y Altcode: 2004astro.ph..9024D We present a new set of isochrones in which the effect of the α-element enhancement is fully incorporated. The isochrone tables, together with interpolation routines in age and chemical composition, and in [α/Fe] are available on request as well as from the electronic version of our ApJ papers. Newly derived ages of Galactic globular clusters using these isochrones are 20 to 30% smaller than previous estimates. Title: Convection in the envelope of Procyon A Authors: Demarque, P.; Basu, S.; Guenther, D. B.; Li, L. H.; Robinson, F. J. Bibcode: 2004AAS...204.0706D Altcode: 2004BAAS...36R.785D Theoretical models are presented for the envelope of Procyon A. Procyon A is known to have a shallow convection zone. We investigate the depth of the convection zone, the efficiency of convection in the convection region, convective velocities and uncertainties in the efficiency of diffusion from the convection zone into the adjacent radiative layers. The effects of turbulent pressure and turbulent kinetic energy are included in the models, based on a parameterization of 3D simulations of radiative hydrodynamics. The conditions for the stochastic excitation of acoustic non-radial oscillations are investigated.

This work is supported in part by grants from NASA (PD, SB, FJR), NSF (SB, LHL) and NSERC (DBG). Title: Asteroseismic determination of helium abundance in stellar envelopes Authors: Basu, Sarbani; Mazumdar, Anwesh; Antia, H. M.; Demarque, Pierre Bibcode: 2004MNRAS.350..277B Altcode: 2004astro.ph..2360B Intermediate degree modes of the solar oscillations have previously been used to determine the solar helium abundance to a high degree of precision. However, we cannot expect to observe such modes in other stars. In this work we investigate whether low degree modes that should be available from space-based asteroseismology missions can be used to determine the helium abundance, Y, in stellar envelopes with sufficient precision. We find that the oscillatory signal in the frequencies caused by the depression in Γ1 in the second helium ionization zone can be used to determine the envelope helium abundance of low-mass main-sequence stars. For frequency errors of one part in 104, we expect errors σY in the estimated helium abundance to range from 0.03 for 0.8-Msolar stars to 0.01 for 1.2-Msolar stars. The task is more complicated in evolved stars, such as subgiants, but is still feasible if the relative errors in the frequencies are less than 10-4. Title: A Preliminary Seismic Analysis of 51 Pegasi: Large and Small Spacings from Standard Models Authors: Murphy, Eric J.; Demarque, Pierre; Guenther, D. B. Bibcode: 2004ApJ...605..472M Altcode: 2003astro.ph.12554M We present a preliminary theoretical seismic study of the astronomically famous star 51 Peg. This is done by first performing a detailed analysis within the Hertzsprung-Russell diagram (HRD). Using the Yale stellar evolution code (YREC), a grid of stellar evolutionary tracks has been constructed for the masses 1.00, 1.05, and 1.10 Msolar, in the metallicity range Z=0.024-0.044, and for values of the Galactic helium enrichment ratio (ΔY/ΔZ) in the range 0-2.5. Along these evolutionary tracks, we select 75 stellar model candidates that fall within the 51 Peg observational error box in the HRD (all turn out to have masses of 1.05 and 1.10Msolar). The corresponding allowable age range for these models, which depends sensitively on the parameters of the model, is relatively large, ~2.5-5.5 Gyr. For each of the 75 models, a nonradial pulsation analysis is carried out and the large- and small-frequency spacings are calculated. The results show that just measuring the large- and small-frequency spacings will greatly reduce the present uncertainties in the derived physical parameters and in the age of 51 Peg. Finally, we briefly discuss refinements in the physics of the models and in the method of analysis, which will have to be included in future models to make the best of the precise frequency determinations expected from space observations. Title: Three-dimensional simulations of the upper radiation-convection transition layer in subgiant stars Authors: Robinson, F. J.; Demarque, P.; Li, L. H.; Sofia, S.; Kim, Y. -C.; Chan, K. L.; Guenther, D. B. Bibcode: 2004MNRAS.347.1208R Altcode: This paper describes three-dimensional (3D) large eddy simulations of stellar surface convection using realistic model physics. The simulations include the present Sun, a subgiant of one solar mass and a lower-gravity subgiant, also of one solar mass. We examine the thermal structure (superadiabaticity) after modification by 3D turbulence, the overshoot of convective motions into the radiative atmosphere and the range of convection cell sizes. Differences between models based on the mixing length theory (MLT) and the simulations are found to increase significantly in the more evolved stages as the surface gravity decreases. We find that the full width at half maximum (FWHM) of the turbulent vertical velocity correlation provides a good objective measure of the vertical size of the convective cells. Just below the convection surface, the FWHM is close to the mean vertical size of the granules and 2 × FWHM is close to the mean horizontal diameter of the granules. For the Sun, 2 × FWHM = 1200 km, a value close to the observed mean granule size. For all the simulations, the mean horizontal diameter is close to 10 times the pressure scaleheight at the photospheric surface, in agreement with previous work. Title: Determining the helium abundance of stellar envelopes Authors: Basu, S.; Antia, H. M.; Demarque, P. Bibcode: 2004ESASP.538..273B Altcode: 2004sshp.conf..273B Intermediate-degree mode solar oscillation have been used to determine the solar helium abundance to a high degree of precision. However, we cannot expect to observe these modes in other stars. We investigate whether low degree modes that should be available from space-based asteroseismology missions can be used to determine the helium abundance with sufficient precision. We find that the oscillatory signal in the frequencies caused by the depression in Γ1 in the second helium ionisation zone can be used to determine the envelope helium abundance of low mass stars. Title: The Production of Elements in Low-Mass Stars Authors: Demarque, Pierre Bibcode: 2003APS..DNP4WB002D Altcode: A major source of many of the light elements, including carbon and nitrogen, as well as many of the s-process elements, is low-mass stars. In this context, not only are nuclear rates important, but so to is the astrophysics of convective dredge-up and rotation. In this talk, I will review the major processes that result in the ejection from low-mass stars of an important complement of the elements of the universe. Title: Global Parameter and Helioseismic Tests of Solar Variability Models Authors: Li, L. H.; Basu, S.; Sofia, S.; Robinson, F. J.; Demarque, P.; Guenther, D. B. Bibcode: 2003ApJ...591.1267L Altcode: We construct models of the structure and evolution of the Sun which include variable magnetic fields and turbulence. The magnetic effects are (1) magnetic pressure, (2) magnetic energy, and (3) magnetic modulation to turbulence. The effects of turbulence are (1) turbulent pressure, (2) turbulent kinetic energy, and (3) turbulent inhibition of the radiative energy loss of a convective eddy, and (4) turbulent generation of magnetic fields. Using these ingredients we construct five types of solar variability models (including the standard solar model) with magnetic effects. These models are in part based on three-dimensional numerical simulations of the superadiabatic layers near the surface of the Sun. The models are tested with several sets of observational data, namely, the changes of (1) the total solar irradiance, (2) the photospheric temperature, (3) radius, (4) the position of the convection zone base, and (5) low- and medium-degree solar oscillation frequencies. We find that turbulence plays a major role in solar variability, and only a model that includes a magnetically modulated turbulent mechanism can agree with all the current available observational data. We find that because of the somewhat poor quality of all observations (other than the helioseismological ones), we need all data sets in order to restrict the range of models. Title: WOCS: Astrometry and Membership to V=21 in NGC 188 Authors: Platais, I.; Kozhurina-Platais, V.; Mathieu, R. D.; Demarque, P.; Girard, T. M.; van Altena, W. F. Bibcode: 2003AAS...202.2705P Altcode: 2003BAAS...35..734P The old open cluster NGC 188 is a cornerstone cluster for the WIYN Open Cluster Study (WOCS). In 2000 we presented first results of deep BV photometry and proper motions of NGC 188. In the meantime, the astrometric properties of the NOAO CCD Mosaic Imager have become better understood. Also, to the existing measurements of KPNO 4-m telescope plates we have added Mt. Wilson 60-in and Palomar 5-m telescope plates taken in the nineteen fifties. As a result, the precision of proper motions has improved by a factor of three, reaching 0.15 mas/yr for the best measured stars. These high-quality proper motions allow us to separate field stars from the cluster members with confidence. Some 1,100 bona fide cluster members are found over the one-degree area down to V=21. We discuss essential properties of NGC 188 in light of the new astrometry, photometry and radial velocities.

This study is supported by NSF grant AST-0196075 to The Johns Hopkins University and NSF grant AST-9731302 to the University of Wisconsin - Madison Title: Convection cells in a red giant atmosphere from 3D numerical simulations Authors: Demarque, P.; Robinson, F. J.; Sofia, S.; Kim, Y. -C.; Chan, K. L.; Guenther, D. B. Bibcode: 2003AAS...202.3208D Altcode: 2003BAAS...35..744D Results of a 3D radiative hydrodynamical simulation of atmospheric convection, applicable to a red giant star of one solar mass with log g = 2.6, are presented, with special emphasis on the scale of the convective cells. Since the work of Schwarzschild (1975), it has been expected, by analogy with the Sun, that convective cell size scales with the atmospheric scale height Hp and the width of the highly superadiabatic transition layer (SAL). Based on his estimate of the SAL, Schwarzschild (1975) predicted a few hundred cells on a giant and fewer (possibly even just a few) cells on a supergiant like Betelgeuse, in contrast with the two million cells observed on the solar surface. Freytag et al (1999)'s 10Hp scaling law, based on 2D numerical simulations, suggests a few hundred cells on a supergiant.

This work is an extension to a lower surface gravity of earlier 3D simulations for the Sun (Robinson et al. 2003a) and subgiant stars (Robinson et al. 2003b). Preliminary extrapolation from these simulations predicts that the surface of a giant with log g = 2.6 has approximately 40,000 cells. Betelgeuse, with log g = -0.5, would have approximately 300 cells. This number of cells is compatible with Gray's (2001) spectroscopic observations of Betelgeuse, although other observations of its surface brightness have been interpreted as due to fewer giant cells.

The convection scale size from the giant simulation is compared to previous extrapolations and to observation.

This work was supported in part by research grants from NASA (NAG5-8406), Yonsei University and NSERC of Canada. Title: Three-dimensional convection simulations of the outer layers of the Sun using realistic physics Authors: Robinson, F. J.; Demarque, P.; Li, L. H.; Sofia, S.; Kim, Y. -C.; Chan, K. L.; Guenther, D. B. Bibcode: 2003MNRAS.340..923R Altcode: 2002astro.ph.12296R This paper describes a series of three-dimensional simulations of shallow inefficient convection in the outer layers of the Sun. The computational domain is a closed box containing the convection-radiation transition layer, located at the top of the solar convection zone. The most salient features of the simulations are that: (i) the position of the lower boundary can have a major effect on the characteristics of solar surface convection (thermal structure, kinetic energy and turbulent pressure); (ii) the width of the box has only a minor effect on the thermal structure, but a more significant effect on the dynamics (rms velocities); (iii) between the surface and a depth of 1 Mm, even though the density and pressure increase by an order of magnitude, the vertical correlation length of vertical velocity is always close to 600 km; (iv) in this region the vertical velocity cannot be scaled by the pressure or the density scaleheight; this casts doubt on the applicability of the mixing length theory, not only in the superadiabatic layer, but also in the adjacent underlying layers; (v) the final statistically steady state is not strictly dependent on the initial atmospheric stratification. Title: Helioseismic tests of solar variability models Authors: Li, L. H.; Basu, S.; Sofia, S.; Robinson, F. J.; Demarque, P.; Guenther, D. B. Bibcode: 2003ESASP.517..329L Altcode: 2003soho...12..329L We construct models of the structure and evolution of the Sun which includes variable magnetic fields and turbulence. The magnetic effects included are: (1) magnetic pressure, (2) magnetic energy, and (3) magnetic modulation of turbulence. The effects of turbulence are: (1) turbulent pressure, (2) turbulent kinetic energy, (3) inhibition of radiative losses from a convective eddy, and (4) generation of magnetic fields. Using these ingredients we construct various types of solar models and test them against helioseismic data. We find that only a solar variability model which includes a magnetically-modulated mechanism to generate turbulence can agree with all the current available observational data. Title: The Y2 Stellar Evolutionary Tracks Authors: Yi, Sukyoung K.; Kim, Yong-Cheol; Demarque, Pierre Bibcode: 2003ApJS..144..259Y Altcode: 2002astro.ph.10201Y We present a database of the latest stellar models of the Y2 (Yonsei-Yale) collaboration. This database contains the stellar evolutionary tracks from the pre-main-sequence birthline to the helium core flash that were used to construct the Y2 isochrones. We also provide a simple interpolation routine that generates stellar tracks for given sets of parameters (metallicity, mass, and α-enhancement). Title: Testing Intermediate-Age Stellar Evolution Models with VLT Photometry of Large Magellanic Cloud Clusters. I. The Data Authors: Gallart, Carme; Zoccali, Manuela; Bertelli, Gianpaolo; Chiosi, Cesare; Demarque, Pierre; Girardi, Leo; Nasi, Emma; Woo, Jong-Hak; Yi, Sukyoung Bibcode: 2003AJ....125..742G Altcode: 2002astro.ph..8188G This is the first of a series of three papers devoted to the calibration of a few parameters of crucial importance in the modeling of the evolution of intermediate-mass stars, giving special attention to the amount of convective core overshoot. To this end we acquired deep V and R photometry for three globular clusters of the Large Magellanic Cloud, namely, NGC 2173, SL 556, and NGC 2155, in the age interval 1-3 Gyr. In this first paper, we describe the aim of the project and Very Large Telescope observations and data reduction and we make preliminary comparisons of the color-magnitude diagrams with both the Padova and Yonsei-Yale isochrones. Two following papers in this series present the results of a detailed analysis of these data, independently carried out by members of the Yale and Padova stellar evolution groups. This allows us to compare both sets of models and discuss their main differences, as well as the systematic effects that they would have on the determination of the ages and metallicities of intermediate-age single-stellar populations.

Based on observations collected at the European Southern Observatory, Paranal, Chile (ESO 64.L-0385). Title: Testing Intermediate-Age Stellar Evolution Models with VLT Photometry of Large Magellanic Cloud Clusters. II. Analysis with the Yale Models Authors: Woo, Jong-Hak; Gallart, Carme; Demarque, Pierre; Yi, Sukyoung; Zoccali, Manuela Bibcode: 2003AJ....125..754W Altcode: 2002astro.ph..8142W Using Yale stellar evolution models, we present an analysis of the color-magnitude diagrams (CMDs) of three intermediate-age LMC clusters, namely, NGC 2173, SL 556, and NGC 2155, obtained with the Very Large Telescope. The main goal of our project is to investigate the amount of convective core overshoot necessary to reproduce the CMDs of relatively metal-poor, intermediate-age stellar populations, to check whether the extrapolation that is usually made from solar metallicity is valid. In the process, we obtained values for the binary fraction of each cluster, together with refined age estimates. Our method involved the comparison of the observed CMDs with synthetic CMDs computed using various values of the overshoot parameter and binary fraction. We conclude that a moderate amount of overshoot and some fraction of binary stars are essential for reproducing the observed shapes around the turnoff in the CMDs of all three clusters: unresolved binary stars fill in the expected core contraction gap and make a unique sequence near the gap, which cannot be reproduced by single stars alone, even with a larger amount of overshoot. We utilize ratios of the number of stars in different areas around the core contraction gap to constrain the binary fraction, which is around 10%-20% (for primary-to-secondary mass ratio >=0.7) in all three clusters. Even if binary stars contaminate the core contraction gap, it is shown that the overshoot parameter can be inferred from the color dispersion of the stars around the contraction gap, regardless of the assumed binary fraction. From our overall analysis such as shape of isochrones, star counts, color distribution, and synthetic CMD comparisons, we conclude that overshoot ~20% of the local pressure scale height best reproduces the CMD properties of all three clusters. The best age estimates are 1.5, 2.1, and 2.9 Gyr for NGC 2173, SL 556, and NGC 2155, respectively.

Based on observations collected at the European Southern Observatory, Paranal, Chile (ESO 64.L-0385). Title: The Y2 Isochrones Authors: Yi, S.; Demarque, P.; Kim, Y. C. Bibcode: 2003ASPC..296..240Y Altcode: 2003nhgc.conf..240Y No abstract at ADS Title: The Y 2 Isochrones and Stellar Evolution Tracks Authors: Yi, Sukyoung K.; Demarque, Pierre; Kim, Yong-Cheol Bibcode: 2003egcs.conf..255Y Altcode: We present a new set of isochrones in which the effect of the α-element enhancement is fully incorporated. The isochrone tables, together with interpolation routines in age and chemical composition, and in [α/Fe] are available on request as well as from the electronic version of our ApJ papers. Newly-derived ages of Galactic globular clusters using these isochrones are 20 to 30% smaller than previous estimates. This is a summary of our upcoming papers (Kim et al. 2002; Yi et al. 2003). Title: Radiative Hydrodynamical Simulations of Granulation in the Sun at Different Stages Along the Evolutionary Track Authors: Robinson, F. J.; Demarque, P.; Li, L. H.; Sofia, S.; Kim, Y. -C.; Guenther, D. B. Bibcode: 2003ASPC..293..221R Altcode: 2003tdse.conf..221R Realistic solar surface simulations in 3 dimensions have been primarily done for the Sun. This is a logical first step as there is much more accurate observational data available for the Sun than for any other star. After reproducing observed characteristics of solar surface convection, we apply our 3D model to the Sun at later evolutionary stages. We find that while the superadiabatic layer (SAL) in the subgiant is similar to the Sun, the red giant has a much broader and lower SAL. The maximum turbulent pressure and maximum turbulent kinetic energy in the giant are almost double that of the other models. The asymmetry between upflow and downflow seems to decrease as the Sun evolves. This will reduce the size of the kinetic energy flux. We find a linear relationship between the depth of the granules and the surface gravity of the model. Extrapolating to log g = -0.5 (estimated surface gravity of Betelgeuse) suggests about 1000 granules would be required to cover the entire surface of Betelgeuse. Title: Three-Dimensional Radiative Hydrodynamical Simulations of the Outer Layers of Sun-Like Stars Authors: Demarque, P.; Li, L. H.; Robinson, F. J.; Sofia, S.; Guenther, D. B. Bibcode: 2003aahd.conf..509D Altcode: No abstract at ADS Title: Stellar Convection Authors: Demarque, Pierre; Robinson, Francis J. Bibcode: 2003Ap&SS.284..193D Altcode: Seismology provides powerful tests of convection deep in stellar interiors. First, the role of convective overshoot and low efficiency convection, two areas of uncertainty with important astrophysical implications, are reviewed briefly. In the rest of the talk, a critical introduction to numerical simulations of radiative hydrodynamics will be given. The basic underlying assumptions and challenges are explained, and some recent results are presented. Title: Effects of Domain Size on Surface Simulations of the Sun Authors: Robinson, F. J.; Li, L. H.; Demarque, P.; Sofia, S.; Kim, Y. -C.; Guenther, D. B. Bibcode: 2003IAUS..210P..C3R Altcode: No abstract at ADS Title: The Scale of Granulation in Red Giant Atmospheres Authors: Demarque, P.; Robinson, F. J.; Chan, K. L.; Kim, Y. -C.; Guenther, D. B.; Sofia, S. Bibcode: 2002AAS...20111506D Altcode: 2002BAAS...34Q1291D The scale of photospheric convection in red giants is generally associated, by analogy with the Sun, with the atmospheric pressure scale height and the thickness of the superadiabatic transition layer. As pointed out by Schwarzschild (1975), both of these quantities are relatively much larger in terms of the stellar radius in a red giant than in the Sun. On these grounds, Schwarzschild concluded that only a few cells must be present on a red giant surface, in contrast with two millions cells observed on the solar surface. Observations of brightness variations in the TiO band on Betelgeuse (Gaustad 1986) and direct imaging of Betelgeuse (Gilliland & Dupree 1996) with HST have yielded results that are compatible with giant cells, although other interpretations (e.g. pulsation) are possible. 2D numerical simulations (Freytag et al. 2001) also claim compatibility with the giant cell interpretation. On the other hand, Gray (2001) has pointed out that his extensive spectroscopic observations of Betelgeuse are more easily interpreted in terms of many convection cells per hemisphere. We present preliminary results of 3D radiative hydrodynamical simulations which corroborate Gray's interpretation. This work is supported in part by NASA grant NAG5-8406 Title: The Y2 Isochrones for α-Element Enhanced Mixtures Authors: Kim, Yong-Cheol; Demarque, Pierre; Yi, Sukyoung K.; Alexander, David R. Bibcode: 2002ApJS..143..499K Altcode: 2002astro.ph..8175.; 2002astro.ph..8175C; 2002astro.ph..8175K We present a new set of isochrones in which the effect of the α-element enhancement is fully incorporated. These isochrones are an extension of the already published set of Y2 Isochrones (our Paper I), constructed for the scaled-solar mixture. As in Paper I, helium diffusion and convective core overshoot have been taken into account. The range of chemical compositions covered is 0.00001<=Z<=0.08. The models were evolved from the pre-main-sequence stellar birthline to the onset of helium burning in the core. The age range of the full isochrone set is 0.1-20 Gyr, while younger isochrones of age 1-80 Myr are also presented up to the main-sequence turn-off. Combining this set with that of Paper I for scaled-solar mixture isochrones, we provide a consistent set of isochrones that can be used to investigate populations of any value of α-enhancement. We confirm the earlier results of Paper I that inclusion of α-enhancement effects further reduces the age estimates of globular clusters by approximately 8% if [α/Fe]=+0.3. It is important to note the metallicity dependence of the change in age estimates (larger age reductions in lower metallicities). This reduces the age gap between the oldest metal-rich and metal-poor Galactic stellar populations and between the halo and the disk populations. We also investigate whether the effects of α-enhancement can be mimicked by increasing the total metal abundance in the manner proposed by Salaris and collaborators. We find such simple scaling formulae are valid at low metallicities but not at all at high metallicities near and above solar. Thus, it is essential to use the isochrones rigorously computed for α-enhancement when modeling metal-rich populations, such as bright galaxies. The isochrone tables, together with interpolation routines have been made available via internet. Title: Seismic Test of Solar Models, Solar Neutrinos, and Implications for Metal-rich Accretion Authors: Winnick, R. A.; Demarque, Pierre; Basu, Sarbani; Guenther, D. B. Bibcode: 2002ApJ...576.1075W Altcode: 2001astro.ph.11096W The Sun is believed to have been the recipient of a substantial amount of metal-rich material over the course of its evolution, particularly in the early stages of the solar system. With a long diffusion timescale, the majority of this accreted matter should still exist in the solar convection zone, enhancing its observed surface abundance, and implying a lower abundance core. While helioseismology rules out solar models with near-zero metallicity cores, some solar models with enhanced metallicity in the convection zone might be viable, as small perturbations to the standard model. Because of the reduced interior opacity and core temperature, the neutrino flux predicted for such models is lower than that predicted by the standard solar model. This paper examines how compatible inhomogeneous solar models of this kind are with the observed low and intermediate degree p-mode oscillation data, and with the solar neutrino data from the Sudbury Neutrino Observatory Collaboration. We set an upper limit on how much metal-rich accretion took place during the early evolution of the Sun at ~2 M⊕ of iron (or ~40 M⊕ of meteoric material). Title: High-Amplitude δ Scuti and SX Phoenicis Stars: The Effects of Chemical Composition on Pulsations and the Period-Luminosity Relation Authors: Templeton, Matthew; Basu, Sarbani; Demarque, Pierre Bibcode: 2002ApJ...576..963T Altcode: We present a theoretical study of the radially pulsating δ Scuti and SX Phoenicis variables, concentrating on the blue straggler SX Phoenicis variables found in globular clusters. We have evolved a grid of stellar models with the metal abundance of the globular cluster M55, including models with alpha-enhanced metal abundances, and tested these models for radial pulsations observed in the high-amplitude δ Scuti and SX Phoenicis stars. Our grid includes models with globally enriched helium content to simulate the effects of stellar collisions and global mixing possible in blue stragglers. We find that global enrichment of helium strongly affects the temperature and luminosity of a given star, but the location of the instability strip blue edge and the slope of the period-luminosity (PL) relation are unchanged. This suggests that the PL relation is not affected by blue straggler formation if blue stragglers are fully mixed stellar mergers. Our blue edges and PL relations are in agreement with other theoretical determinations and also with the observational PL relation of M55, but they are not in agreement with the PL relation previously derived for high-amplitude δ Scuti stars in the field. Analysis of the double-mode variable, V41, suggests either that the star may not be pulsating in the first and second overtones as claimed or that normal stellar models may not be accurate models of blue straggler stars. Title: Helioseismic tests of a new category of solar models Authors: Li, L. H.; Basu, Sarbani; Sofia, S.; Demarque, Pierre Bibcode: 2002ESASP.508...83L Altcode: 2002soho...11...83L We have constructed a series of solar models that unlike others which treat the effects of magnetic fields as a perturbation, incorporate the effect of magnetic fields into the stellar structure equations. Such models can reproduce the observed irradiance and effective temperature variations of the Sun. We study the helioseismic properties of these models representing different epochs of the solar cycle. In particular we examine whether the frequency differences between the models reproduce the observed solar-cycle related change of solar oscillation frequencies. Title: Inclusion of Turbulence in Solar Modeling Authors: Li, L. H.; Robinson, F. J.; Demarque, P.; Sofia, S.; Guenther, D. B. Bibcode: 2002ApJ...567.1192L Altcode: 2001astro.ph..9078L The general consensus is that in order to reproduce the observed solar p-mode oscillation frequencies, turbulence should be included in solar models. However, until now there has not been any well-tested efficient method to incorporate turbulence into solar modeling. We present here two methods to include turbulence in solar modeling within the framework of the mixing length theory, using the turbulent velocity obtained from numerical simulations of the highly superadiabatic layer (SAL) of the Sun at three stages of its evolution. The first approach is to include the turbulent pressure alone, and the second is to include both the turbulent pressure and the turbulent kinetic energy. The latter is achieved by introducing two variables: the turbulent kinetic energy per unit mass and the effective ratio of specific heats owing to the turbulent perturbation. These are treated as additions to the standard thermodynamic coordinates (e.g., pressure and temperature). We investigate the effects of both treatments of turbulence on the structure variables, the adiabatic sound speed, the structure of the highly superadiabatic layer, and the p-mode frequencies. We find that the second method reproduces the SAL structure obtained in three-dimensional simulations and produces a p-mode frequency correction an order of magnitude better than the first method. Title: Testing Overshoot with Intermediate-age Magellanic Cloud Clusters Authors: Woo, J. -H.; Gallart, C.; Demarque, P.; Yi, S.; Zoccali, M. Bibcode: 2002ASPC..274..377W Altcode: 2002ohds.conf..377W No abstract at ADS Title: Modification of the Core Overshoot Treatment Authors: Woo, Jong-Hak; Demarque, Pierre; Yi, Sukyoung Bibcode: 2002IAUS..207..736W Altcode: Following Roxburgh's integral constraint, we implemented an upper limit of overshoot in the conventional method of α parameterization in order to remove an overly large overshoot effect for low-mass stars. The erroneously large effect of overshoot due to the failure of α parameterization can be effectively corrected by limiting the amount of overshoot to 15 % of the core radius. Title: The Y2 Isochrones Authors: Yi, S. K.; Demarque, P.; Kim, Y. -C.; Lee, Y. -W.; Ree, C. H.; Lejeune, T. Bibcode: 2002ASPC..274...45Y Altcode: 2002ohds.conf...45Y No abstract at ADS Title: Testing Intermediate-age Stellar Evolution with Magellanic Cloud Clusters Authors: Gallart, Carme; Zoccali, Manuela; Bertelli, Giampaolo; Chiosi, Cesare; Demarque, Pierre; Girardi, Leo; Yi, Sukyoung Bibcode: 2002IAUS..207..713G Altcode: We present new deep VLT color-magnitude diagrams of three intermediate-age populous star clusters in the LMC. These data will be used independently by the Padova and Yale stellar evolution Groups to address issues relevant to the evolution of intermediate-mass stars: mainly the extent of the core convective overshoot, as well as other poorly determined parameters, such as mass loss during stellar evolution, internal rotation, and He content. Title: VizieR Online Data Catalog: NGC 2451A proper motions and membership (Platais+, 2001) Authors: Platais, I.; Kozhurina-Platais, V.; Barnes, S.; Girard, T. M.; Demarque, P.; van Altena, W. F.; Deliyannis, C. P.; Horch, E. Bibcode: 2002yCat..51221486P Altcode: We provide new evidence that NGC 2451A is undoubtedly a young open cluster, although sparsely populated. New cluster membership has been derived from relative proper motions of 5868 stars. In total, 136 stars down to V~15 5 have membership probability Pμ>=2%. New CCD BV photometry indicates that about 70 stars are indeed main-sequence stars of NGC 2451A. This is also supported by our measurements of radial velocities. A total of 34 very likely cluster members yield a mean heliocentric radial velocity for NGC 2451A equal to +22.9km/s. The high quality of our BV photometry, a confirmation of cluster membership from proper motions and radial velocities, and a recently obtained metallicity estimate for several cluster stars allow us to perform a precise isochrone fit. The Yale isochrones, updated by the latest available input physics, have been fitted to the cluster's color-magnitude diagram, yielding a distance modulus V0-MV in the range 6.35 to 6.38, which is in excellent agreement with the distance modulus (m-M)0=6.38 derived from the Hipparcos data recently by van Leeuwen and Robichon et al. (1998, Cat. <J/A+AS/130/157>). We have used the high-resolution spectra acquired using the Hydra multiobject spectrograph at the WIYN telescope (Kitt Peak, Arizona) in 1997 November and 1998 October.

(4 data files). Title: Towards Better Age Estimates for Stellar Populations: The Y2 Isochrones Authors: Demarque, Pierre; Yi, Sukyoung; Kim, Yong-Cheol Bibcode: 2002IAUS..207..711D Altcode: New theoretical isochrones are presented and compared to the observational data. Title: Asteroseismology of Delta Scuti Stars - A Parameter Study Authors: Templeton, M.; Basu, S.; Demarque, P. Bibcode: 2002ASPC..259..600T Altcode: 2002rnpp.conf..600T; 2002IAUCo.185..600T No abstract at ADS Title: Update on globular cluster ages Authors: Demarque, Pierre; Kim, Young-Cheo; Yi, Sukyoung Bibcode: 2002ASSL..274..111D Altcode: We update globular cluster age estimates and discuss their implications in the context of present knowledge in cosmology. On the basis of the best information now available, the ages of the oldest globular clusters are estimated at 13.0±1.5 Gyr. If this result, whose main uncertainty is primarily in the distance scale, continues to hold, the long standing discrepancy between the stellar evolution timescale and the universal expansion timescale will have been resolved. Title: Asteroseismology of δ Scuti Stars: A Parameter Study and Application to Seismology of FG Virginis Authors: Templeton, Matthew; Basu, Sarbani; Demarque, Pierre Bibcode: 2001ApJ...563..999T Altcode: 2001astro.ph..8458T We assess the potential of asteroseismology for determining the fundamental properties of individual δ Scuti stars. We computed a grid of evolution and adiabatic pulsation models using the Yale Rotating Evolution Code to study the systematic changes in low-order (l=0, 1, 2, and 3) modes as functions of fundamental stellar properties. Changes to the stellar mass, chemical composition, and convective core overshooting length change the observable pulsation spectrum significantly. In general, mass has the strongest effect on evolution and on pulsation followed by the metal abundance. Changes to the helium content have very little effect on the frequencies until near the end of the main sequence. Changes to each of the four parameters change the p-mode frequencies more, in both absolute and relative terms, than they do the g- and mixed-mode frequencies, suggesting that these parameters have a greater effect on the outer layers of the star. We also present evolution and pulsation models of the well-studied star FG Virginis, outlining a possible method of locating favorable models in the stellar parameter space based on a definitive identification of only two modes. Specifically, we plot evolution models on the (period-period ratio) and (temperature-period ratio) planes to select candidate models and modify the core overshooting parameter to fit the observed star. For these tests, we adjusted only the mass, helium and metal abundances, and core overshooting parameter, but this method can be extended to include the effects of first-order rotational splitting and second-order rotational distortion of pulsation spectra. Title: Toward Better Age Estimates for Stellar Populations: The Y2 Isochrones for Solar Mixture Authors: Yi, Sukyoung; Demarque, Pierre; Kim, Yong-Cheol; Lee, Young-Wook; Ree, Chang H.; Lejeune, Thibault; Barnes, Sydney Bibcode: 2001ApJS..136..417Y Altcode: 2001astro.ph..4292Y We have constructed a new set of isochrones, called the Y2 Isochrones, that represent an update of the Revised Yale Isochrones (RYI), using improved opacities and equations of state. Helium diffusion and convective core overshoot have also been taken into consideration. This first set of isochrones is for the scaled solar mixture. A subsequent paper will consider the effects of α-element enhancement, believed to be relevant in many stellar systems. Two additionally significant features of these isochrones are that (1) the stellar models start their evolution from the pre-main-sequence birthline instead of from the zero-age main sequence and (2) the color transformation has been performed using both the latest table of Lejeune et al., and the older, but now modified, Green et al. table. The isochrones have performed well under the tests conducted thus far. The reduction in the age of the Galactic globular clusters caused by this update in stellar models alone is approximately 15% relative to RYI-based studies. When the suggested modification for the α-element enhancement is made as well, the total age reduction becomes approximately 20%. When post-RGB evolutionary stages are included, we find that the ages of globular clusters derived from integrated colors are consistent with the isochrone fitting ages. Title: Empirical Constraints on Convective Core Overshoot Authors: Woo, Jong-Hak; Demarque, Pierre Bibcode: 2001AJ....122.1602W Altcode: 2001astro.ph..6061W In stellar evolution calculations, the local pressure scale height is often used to empirically constrain the amount of convective core overshoot. However, this method brings unsatisfactory results for low-mass stars (<=1.1-1.2 Msolar for Z=Zsolar), which have very small cores or no convective core at all. Following Roxburgh's integral constraint, we implemented an upper limit of overshoot within the conventional method of α parameterization to remove an overly large overshoot effect on low-mass stars. The erroneously large effect of core overshoot due to the failure of α parameterization can be effectively corrected by limiting the amount of overshoot to <=15% of the core radius; 15% of the core radius would be a proper limit of overshoot, which can be implemented in a stellar evolution code for intermediate- to low-mass stars. The temperature structure of the overshoot region does not play a crucial role in stellar evolution since this transition region is very thin. Title: WIYN Open Cluster Study. VII. NGC 2451A and the Hipparcos Distance Scale Authors: Platais, Imants; Kozhurina-Platais, Vera; Barnes, Sydney; Girard, Terrence M.; Demarque, Pierre; van Altena, William F.; Deliyannis, Constantine P.; Horch, Elliott Bibcode: 2001AJ....122.1486P Altcode: We provide new evidence that NGC 2451A is undoubtedly a young open cluster, although sparsely populated. New cluster membership has been derived from relative proper motions of 5868 stars. In total, 136 stars down to V~15 have membership probability Pμ>=2%. New CCD BV photometry indicates that about 70 stars are indeed main-sequence stars of NGC 2451A. This is also supported by our measurements of radial velocities. A total of 34 very likely cluster members yield a mean heliocentric radial velocity for NGC 2451A equal to +22.9 km s-1. The high quality of our BV photometry, a confirmation of cluster membership from proper motions and radial velocities, and a recently obtained metallicity estimate for several cluster stars allow us to perform a precise isochrone fit. The Yale isochrones, updated by the latest available input physics, have been fitted to the cluster's color-magnitude diagram, yielding a distance modulus V0-MV in the range 6.35 to 6.38, which is in excellent agreement with the distance modulus (m-M)0=6.38 derived from the Hipparcos data recently by van Leeuwen and Robichon et al. For NGC 2451A the isochrone fit yields an age of 60+/-20 Myr; hence, the cluster appears to be somewhat younger than the Pleiades. We also present alternative evidence suggesting that the cluster could be slightly older than the Pleiades. The most important result of this study is an excellent match between the main-sequence fitting and Hipparcos distances to NGC 2451A. If we consider noticeable similarities between NGC 2451A and Pleiades, then the Hipparcos distance anomaly for Pleiades may not require an explanation of astrophysical nature. Title: Yonsei-Yale Isochrones: the status report Authors: Kim, Y. -C.; Yi, S. K.; Demarque, P. Bibcode: 2001AAS...198.4301K Altcode: 2001BAAS...33..844K This poster is the first major progress report of the Yonsei-Yale Isochrone Project. The use of isochrones has grown steadily. The isochrone fitting technique has been a corner stone in the study of formation and evolution of the Milky Way. Thanks to the recent improvement in the input physics (opacities and EOS, etc.), we now boast of knowing the age of the Milky Way within 20% uncertainty. Recently, various population synthesis studies have shown that isochrones can be of great power in understanding the evolution of external galaxies as well, by matching their colors and spectra. Numerous studies demonstrated that isochrones can even help us to select a more likely cosmology model, not just the age of the universe. Those also showed only carefully constructed isochrones can provide the required accuracy. Therefore, there have been demands for an updated full-set of isochrones. The Yonsei-Yale Isochrone Project has been designed and carried out to fulfill such needs. This new isochrones would create particularly positive impacts to the astronomical community in general. In this poster, we report the availability of the Yonsei-Yale Isochrone standard set, and the current status of the project will be reported. Title: Radiative hydrodynamics in the highly super adiabatic layer of stellar evolution models Authors: Robinson, F. J.; Demarque, P.; Sofia, S.; Chan, K. L.; Kim, Y. -C.; Guenther, D. B. Bibcode: 2001ESASP.464..443R Altcode: 2000astro.ph.11065R; 2001soho...10..443R We present results of three dimensional simulations of the uppermost part of the sun, at 3 stages of its evolution. Each model includes physically realistic radiative-hydrodynamics (the Eddington approximation is used in the optically thin region), varying opacities and a realistic equation of state (full treatment of the ionization of H and He). In each evolution model, we investigate a domain, which starts at the top of the photosphere and ends just inside the convection zone (about 2400 km in the sun model). This includes all of the super-adiabatic layer (SAL). Due to the different positions of the three models in the log(g) vs logTeff plane, the more evolved models haved lower density atmospheres. The reduction in density causes the extent of overshoot into the radiation layer, to be greater in the more evolved models. Title: The effect of turbulent pressure on the p-mode frequencies in stellar models Authors: Demarque, P.; Li, L. H.; Robinson, F. J.; Sofia, S.; Kim, Y. -C.; Chan, K. L.; Guenther, D. B. Bibcode: 2001ESASP.464..423D Altcode: 2000astro.ph.11064D; 2001soho...10..423D We have constructed models for the sun at three stages of its evolution: a zero-age main sequence model, the present sun, and a subgiant model. For each model, the turbulent pressure and turbulent kinetic energy were calculated from 3-d radiative hydrodynamical simulations (described in the poster by Robinson et al.), and inserted into the 1-d stellar models. We note that in these simulations, the turbulent pressure is not a free parameter, but can be computed from the resulting velocity field. We show the calculated p-mode frequencies for the model of the present sun, with and without turbulent pressure, and compare them to the observed solar frequencies. When the turbulent pressure is included in the models, the calculated frequencies are brought closer to the observed frequencies in the sun by up to two μHz, strictly from structural effects. The effect of including turbulent pressure on p-mode frequencies is also shown for the zero-age main sequence model. Our models also suggest that the importance of turbulent pressure increases as the star evolves into the subgiant region. We discuss the importance of also including realistic turbulence as well as radiation in the non-adiabatic calculation of oscillation frequencies. Title: Convection, Angular Momentum Transfer and Mixing Mechanisms in Late-Type Stars (CD-ROM Directory: contribs/demarque) Authors: Demarque, P. Bibcode: 2001ASPC..223..179D Altcode: 2001csss...11..179D No abstract at ADS Title: Hubble Space Telescope Photometry of the Metal-rich Globular Clusters NGC 6624 and NGC 6637 Authors: Heasley, J. N.; Janes, K. A.; Zinn, Robert; Demarque, Pierre; Da Costa, Gary S.; Christian, Carol A. Bibcode: 2000AJ....120..879H Altcode: We have observed the metal-rich globular clusters NGC 6624 and NGC 6637 (M69) using the planetary camera of the WFPC2 on the Hubble Space Telescope (HST). Observations of the Ca II triplet lines in giant stars in these clusters show that NGC 6624 and NGC 6637 have metallicities on the Zinn and West scale of [Fe/H]=-0.63+/-0.09 and -0.65+/-0.09, only slightly more metal rich than 47 Tuc [Fe/H]=-0.71+/-0.07. For clusters of identical (or nearly so) metallicity, one can make a direct comparison of the color-magnitude diagrams to derive the relative ages of the clusters. From the color-magnitude diagrams derived from the HST photometry, we find that NGC 6624 and NGC 6637 differ in age by less than 0.5 Gyr. Their color-magnitude diagrams are also compared with those of 47 Tuc and NGC 6352, and while these latter diagrams are of somewhat lower quality, they are consistent with all of these clusters having the same ages. Adopting an apparent distance modulus of 13.40 and reddening E(B-V)=0.04 for 47 Tuc, the new Yale isochrones yield an age for the clusters of 14 Gyr. The positions of NGC 6624 and NGC 6637 in the Galaxy suggest that they belong to the bulge population of globular clusters. The only other bulge clusters that have been dated so far are the more metal rich clusters NGC 6528 and NGC 6553, which also appear to be very old. Consequently, the age-metallicity relation of the bulge may be very steep. The close similarity of the ages and metallicities of NGC 6624 and NGC 6637 to the thick-disk globular clusters 47 Tuc and NGC 6352 indicates that the age-metallicity relations of these populations intersect. We briefly discuss the possibility that these populations had a common origin. Based on observations made with the Anglo-Australian Telescope, Canada-France-Hawaii Telescope, and the NASA/ESA Hubble Space Telescope. The observations from the Hubble Space Telescope were obtained from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Title: Mass Loss on the Horizontal Branch: An Application to NGC 6791 Authors: Yong, Hwanjong; Demarque, Pierre; Yi, Sukyoung Bibcode: 2000ApJ...539..928Y Altcode: 2000astro.ph..3425Y The presence of a substantial number of hot stars in the extremely metal-rich open cluster NGC 6791 has been a mystery. If these hot stars are in their core helium-burning phase, they are significantly bluer (hotter) than predicted by canonical stellar evolution theory. No obvious explanation is available yet. We consider the effects of mass loss during the evolution of horizontal-branch (HB) stars as their possible origin. We find that the addition of mass loss causes HB stars to evolve to be hotter and fainter. Mass loss has a more pronounced effect for less massive stars and thus naturally widens the temperature (and color) distribution on the HB. If mass-loss rates are higher for more metal-rich stars, this phenomenon would be even more pronounced in the metal-rich populations, such as NGC 6791. We find that mass loss on the HB may be a viable method of forming subdwarf B (sdB) stars both in the field and in clusters, especially when the metallicity is high. WIYN Open Cluster Study. IV. Title: WOCS: Pushing the Astrometric Envelope of the Open Cluster NGC 188 Authors: Platais, I.; Kozhurina-Platais, V.; Demarque, P.; Girard, T. M.; van Altena, W. F.; Klemola, A. R.; WOCS Collaboration Bibcode: 2000AAS...196.4203P Altcode: 2000BAAS...32..741P The well-known 7-Gyr-old open cluster NGC 188 has been a subject of numerous studies. It is also a cornerstone cluster for the WIYN Open Cluster Study (WOCS) project which aims to produce a comprehensive photometric, astrometric, and spectroscopic database for selected fundamental open clusters. The astrometric aspects of WOCS benefit from deep KPNO and CTIO 4m telescope old material in combination with the NOAO CCD Mosaic Imager frames. The latter presents a certain challenge to obtain astrometric grade positions (see accompanying poster on the CCD Mosaic's geometry). As an example of WOCS potential we present the first results of BV photometry and new proper motions of NGC 188 down to V=21 mag. We show that there are some cluster members, mostly faint ones, beyond the adopted tidal radius and that the Present Day Mass Function (PDMF) strongly differs from the traditionally accepted shape of the IMF. Hopefully, the WOCS results will trigger more work on N-body simulations from which the IMF can be reconstructed using the observed PDMF. This study is supported in part by NSF grant AST9819777 to Yale University and NSF grant AST9530632 to UCO/Lick Observatory. Title: α Centauri AB Authors: Guenther, D. B.; Demarque, P. Bibcode: 2000ApJ...531..503G Altcode: Detailed models of α Centauri A and B based on the Hipparcos, Yale, and Söderhjelm parallaxes are compared. The consequences of the uncertainty in mass, luminosity, surface temperature, and composition on the structure and the p-mode pulsation spectrum of the models are presented. All of the models were constructed using the most current stellar structure physics available to us, including helium and heavy-element diffusion, OPAL (Lawrence Livermore Opacity Library) equation of state, and OPAL and Alexander opacities. Self-consistent models of α Cen A and B that satisfy the observational constraints have an initial helium mass fraction YZAMS=~0.28. The age of the system depends critically on whether or not α Cen A has a convective core. If it does (our best model), then α Cen AB is ~7.6 Gyr old, and if it does not, then the binary system is ~6.8 Gyr old. Both ages and YZAMS are accurate to +/-~10% owing to observational uncertainties. The Galactic enrichment parameter (ΔY/ΔZ) for our best model pair is less than 1. Pulsation analyses of our best models yield an average large and small spacing of 101+/-3 μHz and 4.6+/-0.4 μHz, respectively, for α Cen A, and 173+/-6 μHz and 15+/-1 μHz for α Cen B. Some methodologies that use p-mode frequency observations to constrain the system further are outlined. We include a simple test to determine whether or not α Cen A has a convective core and introduce a method to use the small frequency spacing to determine the age of system, overcoming the limitation that it is also sensitive to composition. Title: The Metallicity Dependence of RR Lyrae Absolute Magnitudes from Synthetic Horizontal-Branch Models Authors: Demarque, Pierre; Zinn, Robert; Lee, Young-Wook; Yi, Sukyoung Bibcode: 2000AJ....119.1398D Altcode: 1999astro.ph.12226D A grid of synthetic horizontal-branch (SHB) models based on horizontal-branch (HB) evolutionary tracks with improved physics has been constructed to reconsider the theoretical calibration of the dependence of Mv(RR) on metallicity in globular clusters and the slope of the mean <MV(RR)>-[Fe/H] relation. The SHB models confirm Lee's earlier finding that the slope of the <MV(RR)>-[Fe/H] relation is itself a function of the metallicity range considered and that, in addition, for a given [Fe/H] RR Lyrae luminosities depend on HB morphology. This is due to the fact that HB stars pass through the RR Lyrae instability strip at different evolutionary stages, depending on their original position on the HB. At [Fe/H]=-1.9, and for HB type 0, the models yield MV(RR)=0.47+/-0.10. The mean slope for the zero-age HB models is 0.204. Since there is no simple universal relation between Mv(RR) and metallicity that is applicable to all globular clusters, the HB morphology of each individual cluster must be taken into account, in addition to [Fe/H], in deriving the appropriate Mv(RR). Taking HB morphology into account, we find that the slope of the mean <MV(RR)>-[Fe/H] relation varies between 0.36 for the clusters with Galactocentric distances Rgc less than 6 kpc and 0.22 for clusters with 6 kpc<Rgc<=20 kpc. Implications for interpreting observations of field RR Lyrae variables and for absolute globular cluster ages and Galactic chronology are briefly discussed. Title: Evolutionary Model and Oscillation Frequencies for α Ursae Majoris: A Comparison with Observations Authors: Guenther, D. B.; Demarque, P.; Buzasi, D.; Catanzarite, J.; Laher, R.; Conrow, T.; Kreidl, T. Bibcode: 2000ApJ...530L..45G Altcode: Inspired by the observations of low-amplitude oscillations of α Ursae Majoris A by Buzasi et al. using the WIRE satellite, a grid of stellar evolutionary tracks has been constructed to derive physically consistent interior models for the nearby red giant. The pulsation properties of these models were then calculated and compared with the observations. It is found that, by adopting the correct metallicity and for a normal helium abundance, only models in the mass range of 4.0-4.5 Msolar fall within the observational error box for α UMa A. This mass range is compatible, within the uncertainties, with the mass derived from the astrometric mass function. Analysis of the pulsation spectra of the models indicates that the observed α UMa oscillations can be most simply interpreted as radial (i.e., l=0) p-mode oscillations of low radial order n. The lowest frequencies observed by Buzasi et al. are compatible, within the observational errors, with model frequencies of radial orders n=0, 1, and 2 for models in the mass range of 4.0-4.5 Msolar. The higher frequencies observed can also be tentatively interpreted as higher n-valued radial p-modes, if we allow that some n-values are not presently observed. The theoretical l=1, 2, and 3 modes in the observed frequency range are g-modes with a mixed mode character, that is, with p-mode-like characteristics near the surface and g-mode-like characteristics in the interior. The calculated radial p-mode frequencies are nearly equally spaced, separated by 2-3 μHz. The nonradial modes are very densely packed throughout the observed frequency range and, even if excited to significant amplitudes at the surface, are unlikely to be resolved by the present observations. Title: Precision Photometry of Key Open Clusters Authors: Sarajedini, Ata; Deliyannis, Constantine; Bailyn, Charles; Mathieu, Robert; Demarque, Pierre Bibcode: 2000noao.prop..312S Altcode: We propose to carry out wide-field imaging observations of 8 open clusters which are an important part of the WIYN Open Cluster Study. These data will allow us to construct high precision color-magnitude diagrams for the clusters, which will then be used to investigate a number of astrophysical questions. In addition to publication in refereed journals, the detailed data will be made available to the community as part of the WIYN Open Cluster Study database. The number of filters, spatial coverage, and precision of the data we propose to obtain constitutes a substantial improvement over previous studies of these, and most other, open clusters. Title: The WIYN open Cluster Study: A test of convective overshoot and the age of NGC 752 Authors: Kozhurina-Platais, V.; Platais, I.; Demarque, P.; Yong, H.; Lee, J.; Sarajedini, A. Bibcode: 2000ASPC..198...79K Altcode: 2000scac.conf...79K No abstract at ADS Title: The WIYN Open Cluster Study and a Distance Scale Authors: Platais, I.; Girard, T.; van Altena, W.; Koshurina-Platais, V.; Demarque, P. Bibcode: 2000IAUJD..13E..17P Altcode: The WIYN consortium (Wisconsin, Indiana, Yale and NOAO) astronomers have started the WIYN Open Cluster Study (WOCS) with the goals of 1) producing comprehensive photometric, astrometric and spectroscopic data for selected fundamental open clusters, and 2) addressing key problems in astrophysics, stellar dynamics and star formation. These fundamental clusters were selected to cover a wide range of ages, metallicity and location in the Galaxy and hence be representative of the whole system of open clusters. The Pleiades perhaps is the most publicised case of disagreement between the space-based and classical distance determination techniques. The isochrone fitting technique applied to the WOCS data is expected to provide more insights on some other clusters showing a similar discrepancy. The open cluster NGC 2451 is of particular interest because it is a nearby young cluster, is well-sampled by Hipparcos and also appears to have the Pleiades-like paradigm. Title: The WIYN open cluster study: A new color-magnitude diagram for M35 - a twin of the Pleiades? Authors: von Hippel, T.; Kozhurina-Platais, V.; Platais, I.; Demarque, P.; Sarajedini, A. Bibcode: 2000ASPC..198...75V Altcode: 2000scac.conf...75V No abstract at ADS Title: WIYN Open Cluster Study. II. UBVRI CCD Photometry of the Open Cluster NGC 188 Authors: Sarajedini, Ata; von Hippel, Ted; Kozhurina-Platais, Vera; Demarque, Pierre Bibcode: 1999AJ....118.2894S Altcode: We present high-precision UBVRI CCD photometry of the old open cluster NGC 188. Our color-magnitude diagram extends from near the red giant branch tip to as faint as ~5 mag below the main-sequence turnoff. From an analysis of these data along with published photometry for M67, we draw the following conclusions: (1) From the UBV two-color diagram, we find a reddening of E(B-V)=0.04+/-0.02 for M67 and E(B-V)=0.09+/-0.02 for NGC 188. (2) Based on main-sequence fitting to solar abundance isochrones, the distance moduli turn out to be (m-M)v=9.69+/-0.11 for M67 and (m-M)v=11.44+/-0.08 for NGC 188. (3) The comparison of the CMDs to theoretical isochrones indicates that an amount of core convective overshoot equivalent to 0.10 of a pressure scale height is appropriate for M67, while no overshoot is required to fit the CMD of NGC 188. These isochrones suggest that NGC 188 is 3.0+/-0.7 Gyr older than M67. (4) There is a clear indication of mass segregation in both M67 and NGC 188, with the most massive stars (M/Msolar>1.1) being more centrally concentrated than those that are the least massive (0.8>=M/Msolar>0.65). Title: The Pulsation Properties of Procyon A Authors: Chaboyer, Brian; Demarque, P.; Guenther, D. B. Bibcode: 1999ApJ...525L..41C Altcode: 1999astro.ph..9015C A grid of stellar evolution models for Procyon A has been calculated. These models include the best physics available to us (including the latest opacities and equation of state) and are based on the revised astrometric mass of Girard et al. Models were calculated with helium diffusion and with the combined effects of helium and heavy-element diffusion. Oscillation frequencies for l=0, 1, 2, and 3 p-modes and the characteristic period spacing for the g-modes were calculated for these models. We find that g-modes are sensitive to model parameters that effect the structure of the core, such as convective core overshoot, the heavy-element abundance, and the evolutionary state (main sequence or shell hydrogen burning) of Procyon A. The p-modes are relatively insensitive to the details of the physics used to model Procyon A and only depend on the evolutionary state of Procyon A. Hence, observations of p-mode frequencies on Procyon A will serve as a robust test of stellar evolution models. Title: Toward a Pop II Mass-Luminosity Relation Authors: Rubenstein, Eric P.; Orosz, Jerome A.; Deliyannis, Constantine; Demarque, Pierre Bibcode: 1999noao.prop..116R Altcode: The observing runs contained within this proposal are part of a long term goal of constraining the variation of the Mass-Luminosity Relation as a function of composition. Using a sequence of Mosaic II images (U,V,R,I & H-alpha) we propose to make a detailed study of binaries and the general population of 2 globular clusters and three open clusters. The specific clusters were selected to have a wide range of chemical composition, from -2.2 <=[Fe/H]<=0.2. I will make a sensitive search for main-sequence binary stars, blue straggler stars, SX Phe pulsators and cataclysmic variables and construct a precise CMD to search for evidence of a binary sequence. The luminosity function of M30 will be compared with that derived from my team's HST data of that cluster's core (HST-GO-7379). Comparison of these two data sets, combined with Yale stellar evolutionary models (with co-I Demarque) will allow me to study the degree of mass segregation in this dynamically evolved cluster. As part of my ongoing project to constrain the Pop II Mass-Luminosity Relation (NSF-9902667) I will follow-up the detection of any detached main-sequence binaries with spectroscopic observations (with co-Is Orosz & Deliyannis) to determine the masses of the components. We also propose to obtain spectroscopic observations of an uncrowded binary candidate detected during previous work on NGC 6752. If this object and other binaries we hope to detect are double- lined spectroscopic binaries, our observations of radial velocity variations will allow the precise determination of masses. Single-lined spectroscopic binaries will allow us to derive the mass-ratio of the systems and begin to constrain the unknown mass-ratio distribution of binaries in clusters. I will be spending this coming year at CTIO on an NSF fellowship specifically to pursue this project. Title: Helioseismology: Probing the Interior of a Star Authors: Demarque, P.; Guenther, D. B. Bibcode: 1999PNAS...96.5356D Altcode: Helioseismology offers, for the first time, an opportunity to probe in detail the deep interior of a star (our Sun). The results will have a profound impact on our understanding not only of the solar interior, but also neutrino physics, stellar evolution theory, and stellar population studies in astrophysics. Title: The Run of Superadiabaticity in Stellar Convection Zones. II. Effect of Photospheric Convection on Solar p-Mode Frequencies Authors: Demarque, Pierre; Guenther, D. B.; Kim, Yong-Cheol Bibcode: 1999ApJ...517..510D Altcode: Kim and Chan have completed a three-dimensional numerical simulation of the interaction between turbulent convection and radiation in and above the highly superadiabatic layer (SAL) in the Sun. They have shown that the dynamics of the domain dictates a SAL structure different from that of a traditional hydrostatic solar model. The top boundary of the convection zone is moved outward by about 0.3 pressure scale height, and in addition, convective overshoot extends into the radiative atmosphere layers by 0.45 pressure scale height. Using our one-dimensional stellar evolution code, we have studied the sensitivity of the calculated p-mode frequencies to a modification of the SAL structure similar to that predicted by Kim and Chan. We find that it is possible to remove in this way the main discrepancy between observed and calculated p-mode frequencies. This experiment is a promising first step in the use of physically realistic three-dimensional radiative-hydrodynamic numerical simulations to derive reliable surface boundary conditions for one-dimensional stellar models. Title: The Ultraviolet Upturn in Elliptical Galaxies as an Age Indicator Authors: Yi, Sukyoung; Lee, Young-Wook; Woo, Jong-Hak; Park, Jang-Hyun; Demarque, Pierre; Oemler, Augustus, Jr. Bibcode: 1999ApJ...513..128Y Altcode: 1998astro.ph.10022Y The UV upturn phenomenon in elliptical galaxies, although challenging because of its complexity, is attractive for its potential value as an age indicator of old stellar systems. This work represents the combined efforts of two population synthesis groups with substantially different views to work together to minimize uncertainties in modeling and analysis. Unfortunately, this study, using the currently available data, cannot determine the metallicity of the dominant UV sources, one of the most outstanding problems related to the UV upturn phenomenon, as some input parameters need to be constrained better. We have found, however, that it is feasible to select a more likely model empirically because different models predict substantially different UV-to-V flux ratios as functions of redshift: metal-rich solutions predict a much steeper decline in the UV-to-V flux ratio than metal-poor solutions. We show that such differences in model predictions are quite independent of cosmology and are detectable using current and upcoming space UV facilities. The various alternatives suggest significantly different ages for the present-epoch giant ellipticals: the metal-rich solutions suggest 30%-50% smaller ages than the metal-poor solutions. Thus, an empirical fitting would not only reveal the origin of the UV upturn but yield independent age estimations for ellipticals. We show that this may effectively constrain some of the cosmological parameters that predict a unique age for the present-epoch galaxies. If we use the most recent estimations of H0 and Ω0, the younger, metal-rich models would have no conflict with a cosmology of a negligibly small Λ0, whereas the older, metal-poor models unavoidably suggest a substantially large value of Λ0 (i.e., Λ0>~0.63 for zfor=∞) in the context of an inflationary universe. Title: Precision Photometry of Key Open Clusters Authors: Sarajedini, Ata; Deliyannis, Constantine; Demarque, Pierre; Bailyn, Charles; von Hippel, Ted; Mathieu, Robert; van Altena, William Bibcode: 1999noao.prop...98S Altcode: We propose to carry out wide-field imaging observations of 5 open clusters which are an integral part of the WIYN Open Cluster Study. These data will allow us to construct high precision color-magnitude diagrams for the clusters, which will then be used to investigate a number of astrophysical questions. In addition to publication in refereed journals, the detailed data will be made available to the community as part of the WIYN Open Cluster Study database. The number of filters, spatial coverage, and precision of the data we propose to obtain constitutes a substantial improvement over previous studies of these, and most other, open clusters Title: Which Are the Most Realistic Models to be Used? Authors: Demarque, P. Bibcode: 1999ASPC..173..373D Altcode: 1999sstt.conf..373D No abstract at ADS Title: Globular Cluster Ages: Overview Authors: Demarque, P. Bibcode: 1999ASPC..192..177D Altcode: 1999sdsg.conf..177D No abstract at ADS Title: The Search for Pulsations in the sdB Stars of NGC 6791 Authors: Yong, H.; Bailyn, C. D.; Demarque, P. Bibcode: 1998AAS...193.6403Y Altcode: Pulsations in sdB stars were predicted by Charpinet et al. (1996) and discovered observationally by Kilkenny et al. (1997). The theoretical models of Charpinet et al. suggest that the pulsation mechanism is enhanced with metallicity. Thus, the sdB stars in the metal rich open cluster NGC 6791 are good candidates for such pulsations. We present the results of CCD photometry taken at the WIYN Observatory of NGC 6791. Pulsations in the sdB star B5 are found with a period of about 101 sec. and an amplitude of 0.04 mag. in B. Both pulsation period and amplitude are consistent with previous observations of field sdB stars. This is the first detection of pulsations in a sdB star in a cluster environment. The detection of pulsations in sdB stars in a cluster gives us a unique opportunity to calculate physical properties of sdB stars (such as the core mass) through comparison with theoretical models. This may also give us the opportunity to independently determine the metallicity and initial helium content of NGC 6791. Title: The search for pulsations in the sdB stars of NGC 6791. Authors: Yong, H.; Bailyn, C. D.; Demarque, P. Bibcode: 1998BAAS...30.1338Y Altcode: No abstract at ADS Title: Testing the Constancy of the Gravitational Constant Using Helioseismology Authors: Guenther, D. B.; Krauss, L. M.; Demarque, P. Bibcode: 1998ApJ...498..871G Altcode: We compare the p-mode oscillation spectra of solar models, constructed under the assumption that the universal gravitation constant G varied monotonically over the course of the solar lifetime, to the most recent solar p-mode frequency observations from the Global Oscillation Network Group (GONG) instrument and Birmingham Solar Oscillation Network (BiSON). The GONG instrument consists of six telescopes spread over different longitude around the Earth, each recording, once every minute, Doppler shift images of the Sun's surface. BiSON also consists of a network of six telescopes specifically designed to observe low-l p-modes. We find that only those solar models constructed with (1/G)dG/dt <= ~1.6 × 10-12 yr-1 are consistent with the observations. This constraint not only probes G over cosmic time, it is stronger by almost one order of magnitude than constraints on the current maximum time variation coming from radar ranging and binary pulsar timing measurements. Title: The Age of Globular Clusters in Light of Hipparcos: Resolving the Age Problem? Authors: Chaboyer, Brian; Demarque, P.; Kernan, Peter J.; Krauss, Lawrence M. Bibcode: 1998ApJ...494...96C Altcode: 1997astro.ph..6128C We review five independent techniques that are used to set the distance scale to globular clusters, including subdwarf main-sequence fitting utilizing the recent Hipparcos parallax catalog. These data together all indicate that globular clusters are farther away than previously believed, implying a reduction in age estimates. We now adopt a best-fit value Mv (RR Lyrae stars) = 0.39 +/- 0.08 (statistical) at [Fe/H] = -1.9 with an additional uniform systematic uncertainty of +0.13-0.18. This new distance scale estimate is combined with a detailed numerical Monte Carlo study (previously reported by Chaboyer et al.) designed to assess the uncertainty associated with the theoretical age-turnoff luminosity relationship in order to estimate both the absolute age and uncertainty in age of the oldest globular clusters.

Our best estimate for the mean age of the oldest globular clusters is now 11.5 +/- 1.3 Gyr, with a one-sided 95% confidence level lower limit of 9.5 Gyr. This represents a systematic shift of over 2 σ compared to our earlier estimate, owing completely to the new distance scale--a shift which we emphasize results not only from the Hipparcos data. This now provides a lower limit on the age of the universe that is consistent with either an open universe or with a flat matter-dominated universe (the latter requiring H0 <= 67 km s-1 Mpc-1). Our new study also explicitly quantifies how remaining uncertainties in the distance scale and stellar evolution models translate into uncertainties in the derived globular cluster ages. Simple formulae are provided that can be used to update our age estimate as improved determinations for various quantities become available. Formulae are also provided that can be used to derive the age and its uncertainty for a globular cluster, given the absolute magnitude of the turnoff or the point on the subgiant branch 0.05 mag redder than the turnoff. Title: The internal structure and age of the sun from seismology Authors: Demarque, P.; Guenther, D. B.; Kim, Y. -C. Bibcode: 1998ASPC..138...79D Altcode: 1998stas.conf...79D No abstract at ADS Title: The Evolution of Procyon A Authors: Chaboyer, B.; Demarque, P.; Guenther, D. B. Bibcode: 1998ASPC..135...67C Altcode: 1998hcsp.conf...67C; 1997astro.ph.10034C A grid of stellar evolution models for Procyon A have been calculated. These models include the best physics available to us (including the latest opacities and equation of state) and are based on the revised astrometric mass of Girrard et al. (1996, BAAS, 28, 919). The long standing descrepency between the evolutionary mass and the astometric mass is now resolved, a result of the newly determined astrometric mass. Models were calculated with helium diffusion and with the combined effects of helium and heavy element diffusion. Oscillation frequencies for l=0,1,2 and 3 p-modes (and g-modes) were calculated for these models, and we discuss how future obsevations of p-modes in Procyon may be used to constrain the stellar evolution models and the evolutionary stage of Procyon A. Title: On the Origin of the Ultraviolet Upturn in Elliptical Galaxies. II. Test of the Horizontal Branch Hypothesis Authors: Yi, Sukyoung; Demarque, Pierre; Oemler, Augustus, Jr. Bibcode: 1998ApJ...492..480Y Altcode: 1997astro.ph..8185Y Whether or not metal-rich HB stars are the dominant UV source in giant elliptical galaxies (GEs) is an important question in current astronomical research. We follow up our previous evolutionary population synthesis study with quantitative tests to answer this question affirmatively under the following three conditions: (1) Reimers's empirical mass-loss formula is proper, (2) the mass-loss efficiency parameter η in metal-rich stars is somewhat larger than the value estimated from the metal-poor star studies, and (3) the true value of the helium-enrichment parameter (ΔY/ΔZ) is positive. All three important empirical characteristics of the UV upturn (i.e., the fact that strong UV upturns are restricted to GEs, the positive UV upturn-metallicity correlation, and the narrow range of the Teff of the UV sources) are closely reproduced for reasonable ranges of input parameters. We discuss the major sources of uncertainties in the models, such as the production and role of hot horizontal-branch stars in GEs, and the importance of galactic nucleosynthesis. Title: Report from Working Group Session 2: Seismology of Sun-like Stars: Techniques and Strategies Authors: Demarque, P. Bibcode: 1998ESASP.418..413D Altcode: 1998soho....6..413D No abstract at ADS Title: Testing the Constancy of Newton's Gravitational Constant using Helioseismology Authors: Guenther, D. B.; Demarque, P.; Krauss, L. M. Bibcode: 1998ESASP.418..469G Altcode: 1998soho....6..469G Using low-l p-mode oscillation data from GONG and BiSON, and solar models constructed with a varying gravitational constant G, we are able to set limits on the maximum allowable monotonic variation in G during the current epoch. The helioseismic limit is almost an order of magnitude tighter than previously established limits using radar ranging and binary pulsar timing (Guenther et al. 1998) This research was supported in part by NSERC, to DBG, and DOE, to LMK. Title: Testing Models of the Solar Highly Superadiabatic Layer with Seismology Authors: Demarque, P.; Guenther, D. B.; Kim, Y. -C. Bibcode: 1998ESASP.418..699D Altcode: 1998soho....6..699D The discrepancy between the computed and observed solar p-mode frequencies can be used to determine which parts of the solar model needs improvement. The increase of the discrepancy toward higher frequencies reveals that the problem lies near the surface (Guenther 1994). Solar models constructed in the standard way with radiative atmospheres and using the mixing length approximation do not represent faithfully the structure of the photospheric and highly superadiabatic subphotospheric layers of the Sun. Recent 3D numerical simulations of radiative hydrodynamics in the solar outer convection zone and atmosphere by Kim & Chan (1998) show that there is convective overshoot into the atmosphere and that the convection zone top boundary is moved outward. Guided by these results we study the sensitivity of the p-mode frequencies to outward shifts in the position of the convection zone top boundary. This research was supported in part by grants from NASA (PD), NSERC (DBG) and the Korean Scientific Research Agency (YCK). Title: Mass Loss on the Horizontal Branch Authors: Yong, H.; Demarque, P.; Yi, S. Bibcode: 1997AAS...191.1209Y Altcode: 1997BAAS...29.1229Y Mass loss is an important mechanism in various stages of stellar evolution. Despite this, mass loss remains one of the more poorly understood aspects of stellar evolution. We present preliminary results of a systematic study of mass loss effects on the evolution of horizontal branch stars. Since mass loss is poorly understood, the simplest approach was taken. Several constant mass loss rates of {10(-9) , 3 x 10(-9}) , and {10(-10}) {Msun yr(-1}) were assumed in the evolution of horizontal branch stars and compared to those with no mass loss. Horizontal branch models were constructed with envelope masses ranging from {0.005 Msun} to {0.46 Msun}. While it is unclear whether mass loss occurs at all on the horizontal branch, we examine its effects, first, to attempt to explain the origin of some extreme blue horizontal branch stars and second, to constrain mass loss rates on the horizontal branch. We also study the effect it has in stars of different metallicities. Preliminary results suggest that mass loss tends to evolve stars blueward in the HR diagram in comparison to their normal course of evolution. This effect is somewhat enhanced with larger metallicities. Title: The Relative Ages of Galactic Globular Clusters Authors: Sarajedini, A.; Chaboyer, B.; Demarque, P. Bibcode: 1997PASP..109.1321S Altcode: 1997astro.ph.10245S We present a review of the present state of knowledge regarding the relative ages of Galactic globular clusters. First, we discuss the relevant galaxy formation models and describe the detailed predictions they make with respect to the formation timescale and chemical evolution of the globular clusters. Next, the techniques used to estimate globular cluster ages are described and evaluated with particular emphasis on the advantages and disadvantages of each method. With these techniques as a foundation, we present arguments in favor of the following assertions: 1) The age of a globular cluster is the likeliest candidate to be the global second parameter, which along with metal abundance, controls the morphology of the horizonal branch. 2) A total age range of as much as ~ 5 Gyr exists among the bulk of the Galactic globulars. 3) There is a significant relation between age and metallicity among the Galactic globular clusters if the slope of the \mvrr-\feh\ relation is less than ~ 0.23. These conclusions along with other supporting evidence favor a formation scenario in which the inner regions of the Galactic halo collapsed in a monotonic fashion over a short time period much less than 1 Gyr. In contrast, the outer regions of the halo fragmented and collapsed in a chaotic manner over several Gyrs. (SECTION: Invited Review Articles) Title: Evolution of Stellar Collision Products in Globular Clusters. I. Head-on Collisions Authors: Sills, Alison; Lombardi, James C., Jr.; Bailyn, Charles D.; Demarque, Pierre; Rasio, Frederic A.; Shapiro, Stuart L. Bibcode: 1997ApJ...487..290S Altcode: 1997astro.ph..5019S We explore the evolution of collisionally merged stars in the blue straggler region of the H-R diagram. The starting models for our stellar evolution calculations are the results of the smoothed particle hydrodynamics (SPH) simulations of parabolic collisions between main-sequence stars performed by Lombardi, Rasio, & Shapiro. Since SPH and stellar evolution codes employ different and often contradictory approximations, it is necessary to treat the evolution of these products carefully. The mixture and disparity of the relevant timescales (hydrodynamic, thermal relaxation, and nuclear burning) and of the important physical assumptions between the codes makes the combined analysis of the problem challenging, especially during the initial thermal relaxation of the star. In particular, the treatment of convection is important and semiconvection must be modeled in some detail.

The products of seven head-on collisions are evolved through their initial thermal relaxation and then through the main-sequence phase to the base of the giant branch. Their evolutionary tracks are presented. In contrast to what was assumed in previous work, these collision products do not develop substantial convective regions during their thermal relaxation and therefore are not mixed significantly after the collision. Title: On the Origin of the UV Upturn in Elliptical Galaxies. I. Sensitivity of UV Population Synthesis to Various Input Parameters Authors: Yi, Sukyoung; Demarque, Pierre; Oemler, Augustus, Jr. Bibcode: 1997ApJ...486..201Y Altcode: 1997astro.ph..5173Y We present models of the late stages of stellar evolution intended to explain the ``UV upturn'' phenomenon in elliptical galaxies. Such models are sensitive to values of a number of poorly constrained physical parameters, including metallicity, age, stellar mass loss, helium enrichment, and the distribution of stars on the zero-age horizontal branch (HB). We explore the sensitivity of the results to values of these parameters and reach the following conclusions.

Old, metal-rich galaxies, such as giant ellipticals, naturally develop a UV upturn within a reasonable timescale--less than a Hubble time--without the presence of young stars. The most likely stars to dominate the UV flux of such populations are low-mass, core helium-burning (HB and evolved HB) stars. Metal-poor populations produce a higher ratio of UV-to-V flux, owing to opacity effects, but only metal-rich stars develop a UV upturn, in which the flux increases toward shorter UV wavelengths. Title: Seismic Tests of the Sun's Interior Structure, Composition, and Age, and Implications for Solar Neutrinos Authors: Guenther, D. B.; Demarque, P. Bibcode: 1997ApJ...484..937G Altcode: The structure of the deep interior of a variety of standard and nonstandard solar models constrained by the low-l p-mode oscillation data from GONG are presented. For standard models, we show that the effects of both helium and heavy-element diffusion must be included in order to find simultaneous agreement with both the p-mode spectrum and the observed value of (Z/X)⊙. Related to this conclusion, we find that the average interior heavy-element abundance is greater than the observed surface abundance in models whose small spacings, which are derived from the p-mode oscillation spectra, best match the observations. The high-quality data from the Global Oscillation Network Group (GONG) now permit a precise determination of the seismic age of the Sun. The best agreement with the calculated oscillation spectra is achieved for 4.5 +/- 0.1 Gyr, an age closely consistent with the age of the Sun inferred from meteorites, i.e., 4.53 +/- 0.04 Gyr. This result lends strong support to the standard assumption of the theory of stellar evolution. With regard to the nonstandard solar models, we set limits on the extent to which the nonstandard assumptions can be applied to the model while still being consistent with the observed p-modes. The nonstandard assumptions investigated here are: forced mixing in the core, forced mixing in a shell surrounding the core, and near-zero heavy-element abundance in the core. These assumptions were selected because at one time or another they have all been proposed to reduce the neutrino flux of the solar model, thereby bringing the flux of the model more in line with the observed flux. All nonstandard models include helium and heavy element diffusion. We confirm, now using the latest solar model physics, that these nonstandard assumptions, when capable of reducing significantly the solar neutrino flux, perturb the interior structure too much to be consistent with p-mode observations. In addition, we set strict limits on the extent to which these nonstandard assumptions are tolerated by the current p-mode observations. For example, we show that the p-mode small spacings are incompatible with a low-Z core larger than 0.06 M⊙ in the Sun. And we show that if the Sun's core is chemically mixed, the extent of the mixed core cannot exceed 0.02 M⊙. The seismic data are also incompatible with extensive rapid mixing of 4He in the solar envelope. This, we believe, also argues against the possibility of slow mixing of 3He occurring in a shell, as was recently proposed by Cumming and Haxton to lower the 7Be/8B neutrino flux ratio. But we note that the occurrence of some mixing of 3He and other trace elements and isotopes in the region of the solar interior where the initial 4He abundance is nearly uniform (which could not at this point be detected by seismology) might modify the calculated neutrino fluxes. Title: On the Ultraviolet-bright Phase of Metal-rich Horizontal-Branch Stars Authors: Yi, Sukyoung; Demarque, Pierre; Kim, Yong-Cheol Bibcode: 1997ApJ...482..677Y Altcode: 1997astro.ph..5111Y We consider the origin of the UV-bright phase of metal-rich helium-burning stars, the slow blue phase (SBP) that was predicted by various earlier works. Based on improved physics including OPAL opacities, which is the same physics that was used in the construction of the new Yale Isochrones, we confirm the existence of the SBP. In addition to our grid of evolutionary tracks, we provide an analytical understanding of the main characteristics of the SBP phenomenon.

The SBP is slow because it is a slow-evolving, helium-shell-burning phase that is analogous to the early asymptotic giant branch phase. The SBP of a more metal-rich star is slower than a metal-poor counterpart if they have the same Teff because a more metal-rich helium-burning star has a smaller mass than a metal-poor one and because lifetime increases as mass decreases.

Metal-rich helium-burning stars easily become hot because the luminosity from the hydrogen-burning shell is extremely sensitive to the mean molecular weight μ, whereas the luminosity from the helium-burning core is not. Under the assumption of a positive ΔY/ΔZ, helium abundance plays the most important role in governing μ, and thus Dorman et al. found that the SBP occurs only when Y >~ 0.4 when ΔY/ΔZ >~ 0. We suggest that the SBP phenomenon is a major cause of the UV upturn phenomenon in giant elliptical galaxies, as will be shown in subsequent papers. The new HB tracks can be retrieved from S. Y.'s web site http://shemesh.gsfc.nasa.gov/astronomy.html. Title: The Age of NGC 3680 and a Test of Convective Overshoot Authors: Kozhurina-Platais, Vera; Demarque, Pierre; Platais, Imants; Orosz, Jerome A.; Barnes, Sydney Bibcode: 1997AJ....113.1045K Altcode: A new CCD BV color-magnitude diagram has been derived for the intermediate age open cluster NGC 3680. The quality of photometry coupled with the best-to-date knowledge of cluster memberships allows for a detailed isochrone fit to the CMD. The theoretical isochrones have been constructed using the Yale Rotating Evolution Code (YREC) in its non-rotating mode and the OPAL opacities (circa 1991). Four sets of isochrones have been calculated: one for the standard stellar model calibrated to the Sun and three others for models with different amounts of convective overshoot at the edge of the convective core, namely, 0.15, 0.20 and 0.25Hp, where Hp is the pressure scale height at the core edge. All four sets of theoretical isochrones were adjusted to an adopted distance modulus of Vdeg-MV=10.20 and reddening E(B-V)=0.075 which leads to ages of 1.3+/-0.15, 1.5+/-0.15, 1.6+/-0.15 and 1.7+/-0.15 Gyr, respectively. The uncertainties in age mainly reflect a subjective decision in differentiating a good fit from a poorer one. The model with a convective overshoot of 0.20Hp seems to yield the best fit to the sharply curved upper main-sequence. Hence, the estimated age of NGC 3680 is 1.6+/-0.15 Gyr. The adopted overshoot parameter ostensibly is uncertain by +/-0.05H_p. For comparison, the same stellar models with overshoot of 0.25Hp were fit to the color-magnitude diagram of the cluster NGC 752, which is similar in age and composition to NGC 3680. The age estimate for NGC 752 is 1.6+/-0.2 Gyr, assuming Vdeg-MV=8.20 and E(B-V)=0.030 for the cluster. Title: The Run of Superadiabaticity in Stellar Convection Zones. I. The Sun Authors: Demarque, P.; Guenther, D. B.; Kim, Y. -C. Bibcode: 1997ApJ...474..790D Altcode: The effect of adjustments to the superadiabatic layer (SAL) in a model of the Sun on the p-mode oscillation frequencies has been studied. Numerical simulations of solar convection by Kim and coworkers have shown that the usual mixing length approximation (MLA) overestimates the convective efficiency in the SAL. To correct for the overestimated convective efficiency in the calculation of the temperature gradient and the mean structure of the SAL, we have adopted a variable mixing length parameter, which decreases as the surface is approached, based on a simple parameterization suggested by the simulations. We find that these changes to the structure of the SAL reduce the discrepancies between observed and calculated oscillation frequencies in the low to intermediate l range. Title: The Ages of Globular Star Clusters Authors: Demarque, Pierre Bibcode: 1997eds..proc...30D Altcode: A summary of the chronology of the Galaxy is presented. The best determined age is that of the sun, near 4.5 Gyr, which is derived by combining the radioactive age of the solar system with our knowledge of the premain sequence evolutionary timescales. It is argued that some open clusters, and a fraction of the disk field population may be as old as 10 Gyr. Title: Evolution of Blue Stragglers Authors: Sills, A.; Lombardi, J.; Bailyn, C.; Demarque, P.; Rasio, F.; Shapiro, S. Bibcode: 1996AAS...189.7207S Altcode: 1996BAAS...28.1366S Blue stragglers, stars in clusters which are bluer and brighter than the main sequence turnoff, were discovered over forty years ago. However, a definitive theory to explain their formation and subsequent evolution still does not exist. One currently favoured idea is that some blue stragglers are the products of stellar collisions. We have been investigating this hypothesis by combining the results of Smoothed Particle Hydrodynamics (SPH) simulations of the stellar collision (Lombardi, Rasio and Shapiro 1996) with stellar evolution calculations. We convert the results of the SPH simulations into a starting model for our evolution calculations, and then evolve the resulting star through the initial thermal relaxation phase to the main sequence, up the giant branch, and onto the horizontal branch. In this way we can determine lifetimes and distributions of blue stragglers in globular clusters without resorting to previously-employed ad hoc assumptions (eg. Bailyn and Pinsonneault 1995, Ouellette and Prichet 1996, Sandquist, Bolte and Hernquist 1996). The difficulties of converting the results of a hydrodynamic calculation into a starting model for a stellar evolution code which assumes hydrostatic equilibrium will be discussed, and preliminary results will be presented. Bailyn, C. D. and Pinsonneault, M. H. 1995, ApJ, 439, 705. Lombardi, J. C., Rasio, F. A. and Shapiro, S.L. 1996 ApJ, 468, 797. Ouellette, J. and Prichet, C. 1996, in The Origins, Evolution and Destinies of Binary Stars in Globular Clusters, ed. E. F. Milone, p 356. Sandquist, Bolte and Hernquist, 1996, ApJ in press. Title: An accurate relative age estimator for globular clusters. Authors: Chaboyer, B.; Demarque, P.; Kernan, P. J.; Krauss, L. M.; Sarajedini, A. Bibcode: 1996MNRAS.283..683C Altcode: Globular cluster age estimates based on the absolute magnitude of the main-sequence turn-off [Mv(TO)] are generally considered to be the most reliable from a theoretical viewpoint. However, the difficulty in determining Mv(TO) in observed colour-magnitude diagrams leads to a large error in the derived age. In this paper, the authors advocate the use of the absolute magnitude of the point which is brighter than the turn-off and 0.05 mag redder [Mv(BTO)] as a precision age indicator. It is easy to measure this point on observed colour-magnitude diagrams, leading to small observational error bars. Furthermore, an extensive Monte Carlo calculation indicates that the theoretical uncertainty in Mv(BTO) is similar to Mv(TO). As a result, ages derived using Mv(BTO) are at least a factor of 2 more precise than those derived using Mv(TO). This technique is applied to the globular cluster M68 and an age of 12.8±0.3 Gyr is derived, indicating that M68 is a "young" globular cluster. A homogeneous set of globular cluster age estimates with this precision would provide unprecedented insight into the formation of the Galactic halo. Title: Testing the Structure of the Solar Superadiabatic Layer with Seismology Authors: Demarque, P.; Guenther, D. B.; Kim, Y. -C. Bibcode: 1996AAS...189.1801D Altcode: 1996BAAS...28.1297D Kim et al.(1995 ApJ,442,422;1996 ApJ,461,499) have recently carried out 3D numerical simulations of shallow convection in the Sun. These calculations, which apply to the transition from the inner part of the superadiabatic peak into the deeper adiabatic layers of the convection zone, treat the coupling of radiative and convective transport and include realistic physics in the solar model. Radiative transfer is treated in the diffusion approximation. The simulations show that the conventional mixing length formalism underestimates the ratio of radiative to convective transport in the outer layers of the solar convection zone. This effect has been simulated in the solar models by calculating the temperature gradient within the mixing length framework, adopting a variable mixing length parameter alpha . We have constructed standard solar models precisely tuned to the Sun, using a simple linear dependence of alpha on depth in the outer part of the solar convection zone. We show that increasing alpha with depth in this way modifies the structure of the superadiabatic layer in the direction of improving agreement between the calculated and observed solar p-mode frequencies. Title: VizieR Online Data Catalog: Revised Yale Isochrones and Luminosity Functions (Green+ 1987) Authors: Green, E. M.; Demarque, P. Bibcode: 1996yCat.6040....0G Altcode: The revised isochrones and luminosity functions (LFs) are based on the stellar evolution calculations of Mengel et al. (1979) and Sweigart and Gross (1978). The revised tables comprise the most complete (as of 1987) isochrone grid available in both age and composition. The improvement in the revised catalog is the inclusion of UBVRI data as well as the theoretical quantities, which are empirically matched to a wide range of observational UBVRI data (Green et al. 1987, unpublished). These tables enable users to interpolate between isochrones, to plot isochrones in order to fit color-magnitude diagrams, luminosity functions to compare with observational data, and to construct stellar population models. To facilitate the use of isochrones, we (at the Astronomical Data Center, in 1987) had made available several FORTRAN programs, ISOTRP, LF, and subroutines. They incorporate our experience of the best methods of interpolation, minimization of precision problems, and how to deal with compositions and ages that are not always complete. ISOTRP allows the user to interpolate for any helium abundance or metallicity in the range of the Revised Yale Isochrone tables. LF constructs customized differential luminosity functions for Mbol or a UBVRI magnitude, for any bin size, and for any initial mass functions. In addition to the 24 isochrone files and the FORTRAN programs, we (at ADC) have included the color table that was used to convert from [Fe/H], log Teff, and log g to BC, U-B, B-V, V-R, R-I, in the isochrone data files. The data cover a complete grid with [Fe/H] from 1.00 to -3.5 in steps of 0.50, Teff from 2800 to 20000 K, and log g from 0.00 to 6.00. (26 data files). Title: Photometry of Star Clusters Near the Galactic Center Authors: Heasley, J.; Janes, K.; Demarque, P.; Zinn, R. Bibcode: 1996AAS...188.0303H Altcode: 1996BAAS...28U.821H Observations from the WFPC2 on the Hubble Space Telescope have been combined with ground-based CCD calibration frames to derive new color-magnitude diagrams for the globular clusters NGC 6624, NGC 6293, and NGC 6333. All three clusters lie in crowded fields in the direction of the galactic center. NGC 6624 is a metal-rich cluster ([Fe/H] = -0.4) thought to be part of the thick disk population, while NGC 6293 and 6333 are metal-poor clusters ([Fe/H] = -1.9 and -1.8 respectively) located within 3 kpc of the galactic center. The new color-magnitude diagrams are compared with theoretical isochrones and color-magnitude diagrams of clusters with similar metallicity located further from the galactic center. Title: The Current State of Solar Modeling Authors: Christensen-Dalsgaard, J.; Dappen, W.; Ajukov, S. V.; Anderson, E. R.; Antia, H. M.; Basu, S.; Baturin, V. A.; Berthomieu, G.; Chaboyer, B.; Chitre, S. M.; Cox, A. N.; Demarque, P.; Donatowicz, J.; Dziembowski, W. A.; Gabriel, M.; Gough, D. O.; Guenther, D. B.; Guzik, J. A.; Harvey, J. W.; Hill, F.; Houdek, G.; Iglesias, C. A.; Kosovichev, A. G.; Leibacher, J. W.; Morel, P.; Proffitt, C. R.; Provost, J.; Reiter, J.; Rhodes, E. J., Jr.; Rogers, F. J.; Roxburgh, I. W.; Thompson, M. J.; Ulrich, R. K. Bibcode: 1996Sci...272.1286C Altcode: Data from the Global Oscillation Network Group (GONG) project and other helioseismic experiments provide a test for models of stellar interiors and for the thermodynamic and radiative properties, on which the models depend, of matter under the extreme conditions found in the sun. Current models are in agreement with the helioseismic inferences, which suggests, for example, that the disagreement between the predicted and observed fluxes of neutrinos from the sun is not caused by errors in the models. However, the GONG data reveal subtle errors in the models, such as an excess in sound speed just beneath the convection zone. These discrepancies indicate effects that have so far not been correctly accounted for; for example, it is plausible that the sound-speed differences reflect weak mixing in stellar interiors, of potential importance to the overall evolution of stars and ultimately to estimates of the age of the galaxy based on stellar evolution calculations. Title: The Prospects for Asteroseismology from Ground-based Sites Authors: Heasley, J. N.; Janes, Kenneth; Labonte, Barry; Guenther, David; Mickey, Donald; Demarque, Pierre Bibcode: 1996PASP..108..385H Altcode: We reexamine the possibility of detecting p-mode oscillations in Sun-like stars with ground-based telescopes. Previous attempts to make such observations with photometric techniques have been limited to subgiant stars in M67 and have illustrated the great difficulties involved in performing ground-based asteroseismology. Substantial gains in observing efficiency can be realized from new diagnostic techniques and improvements in instrumentation, especially with newer CCD camera systems. We show that for appropriately selected field stars observed with a network of telescopes or at a high duty cycle site, it will be possible to detect p-mode oscillations from the ground. An alternative to a network of telescopes for asteroseismology would be to develop a dedicated observatory for this purpose at a high duty cycle site, i.e., the South Pole. We estimate the scintillation, the main noise source in asteroseismology, at the pole by modeling the index of refraction structure parameter from meterological data. The model results show that at the Pole the variance of the relative intensity fluctuations--i.e., the scintillation--should be a factor of 5 smaller than at at Mauna Kea. Taking into account the improvements possible with target selection and instrumentation, the South Pole would be an excellent site for asteroseismological work on Sun-like stars. (SECTION: Stars) Title: Seismology of the Standard Solar Model: Tests of Diffusion and the OPAL and MHD Equations of State Authors: Guenther, D. B.; Kim, Y. -C.; Demarque, P. Bibcode: 1996ApJ...463..382G Altcode: The OPAL equation-of-state tables and the MHD equation-of-state tables are utilized in the calculation of new standard solar models that also include helium diffusion. The structure of the models and their l = 0-100 p-mode oscillation spectra are compared. The p-mode frequencies of the standard solar model are dramatically and nearly identically affected by the inclusion of these detailed equation-of-state formulations. The sensitivities of the p-mode frequencies to other physical uncertainties, in particular the surface conditions, are also discussed. Title: Testing Solar Models with the GONG Data Authors: Christensen-Dalsgaard, J.; Dappen, W.; Demarque, P.; GONG Models Team Bibcode: 1996AAS...188.5302C Altcode: 1996BAAS...28..903C The wealth of very precise data from the GONG network and other helioseismic experiments provide a unique opportunity for testing the modeling of stellar interiors. Stellar modeling forms the basis for much of astrophysics. Furthermore, the application and calibration of solar models to the helioseismic data provides a test of the physics on which the models depend; thus we can investigate the thermodynamic and radiative properties of matter under the extreme conditions found in the Sun. Current models are in remarkable agreement with the helioseismic inferences within the errors in the input data; this suggests, for example, that the disagreement between the predicted and observed fluxes of neutrinos from the Sun is unlikely to be caused by errors in the standard solar model. However, careful analysis of the GONG data reveals subtle discrepancies that we attribute to uncertainties in some of the model input physics and the neglect of other subtle physical processes. The role of mixing inside the Sun, due for example to rotationally induced instabilities during its past evolution, the precise form of the hydrogen depletion profile near the center, the efficiency of helium and heavier element diffusion below the convection zone, and the structure of the highly superadiabatic layer just below the photosphere, are among the significant details in solar models, which the high-quality GONG data will greatly help to resolve. Comparisons between theoretical predictions of solar p-mode frequencies, previous p-mode observations and the first GONG data are used to illustrate these points. Title: A Precision Age Determination Technique for Globular Clusters Authors: Chaboyer, Brian; Demarque, Pierre; Kernan, Peter; Krauss, Lawrence M.; Sarajedini, Ata Bibcode: 1996astro.ph..4122C Altcode: Globular cluster age estimates based on the absolute magnitude of the main sequence turn-off (\mvto) are generally considered to be the most reliable from a theoretical viewpoint. However, the difficulty in determining \mvto\ in observed colour-magnitude diagrams leads to a large error in the derived age. In this paper, we advocate the use of the absolute magnitude of the point which is brighter than the turn-off and 0.05 mag redder (\mvb) as a precision age indicator. It is easy to measure this point on observed colour-magnitude diagrams, leading to small observational error bars. Furthermore, an extensive Monte Carlo calculation indicates that the theoretical uncertainty in \mvb\ is similar to \mvto. As a result, ages derived using \mvb\ are at least a factor of 2 more precise than those derived using \mvto. This technique is applied to the globular cluster M68 and an age of $12.8\pm 0.3\,$Gyr is derived (assuming $\mvrr = 0.20\,\feh + 0.98$), indicating that M68 is a `young' globular cluster. A homogeneous set of globular cluster age estimates with this precision would provide unprecedented insight into the formation of the Galactic halo. Title: Modeling Convection in the Outer Layers of the Sun: A Comparison with Predictions of the Mixing-Length Approximation Authors: Kim, Yong-Cheol; Fox, Peter A.; Demarque, Pierre; Sofia, Sabatino Bibcode: 1996ApJ...461..499K Altcode: The mixing-length theory (MLT) approximation (Vitense 1953) is used in most stellar evolution codes to describe the structure of the outer, highly superadiabatic, layers of the Sun. This procedure is known to be incorrect because of the MLT's inadequacies in describing convection and because of the need to include the strong coupling between radiation and convection in modeling this region. However, it is not known to what extent and precisely in what ways the MLT approximation distorts the structure of the highly superadiabatic peak in the outer convection zone. The purpose of this paper is to compare the statistical results of a more realistic three-dimensional numerical simulation of shallow convection to the predictions of the MLT. The simulations differ from the previous simulations of Chan & Sofia (1989) in that they include a treatment of radiative transfer (in the diffusion approximation). The layers are super- adiabatic and exhibit a sharp peak in the temperature gradient. The results we derive from this simulation provide much more information than conventional one-dimensional theories of convective energy transport. We attempt to analyze or condense the information from the simulation to be compared with a traditional "theory" in an effort to establish how much a large eddy simulation can teach us about mean convective transport theories. In this paper we chose to use the mixing-length approximation for comparison.

The standard mixing-length approximation predicts a few linear relationships between local thermodynamic and dynamic quantities, the coefficients of which are functions of the mixing length. In these MLT relations, the ratio of mixing length to the local pressure scale is assumed to be constant over the entire convection zone, including the region of high superadiabaticity where convective energy transfer becomes less efficient. Title: The Age of NGC 5927: Formation Timescales of the Halo and Thick Disk Authors: Fullton, L. K.; Carney, B. W.; Olszewski, E. W.; Zinn, R.; Demarque, P.; Da Costa, G. S.; Janes, K. A.; Heasley, J. N. Bibcode: 1996ASPC...92..269F Altcode: 1996fogh.conf..269F We present the HST WFPC2 VIC color-magnitude diagram (CMD) of the metal-rich (thick) disk globular cluster NGC 5927. Our CMD indicates that NGC 5927 is somewhat younger than other disk globular clusters with known ages. Observations of the relative numbers of stars on the RGB and RHB indicate a large helium abundance, Y = +0.29+/-0.04. Comparison of the seven known thick disk globular clusters that have estimated ages with ages of globulars that belong to the halo reveals a significant overlap in age between the two cluster systems. As a group, the disk clusters appear somewhat younger than their halo counterparts, indicating that the Galactic halo began to form some 1--2 Gyr before the thick disk, in agreement with studies by Carney et al. (1990) and Marquez & Schuster (1994). Title: Globular Cluster Ages and the Formation of the Galactic Halo Authors: Chaboyer, Brian; Demarque, P.; Sarajedini, Ata Bibcode: 1996ApJ...459..558C Altcode: 1995astro.ph..9063C Main-sequence turnoff magnitudes from the recent set of Yale isochrones published by Chaboyer et al. in 1995 have been combined with a variety of relations for the absolute magnitude of RR Lyrae stars [Mv(RR)] to calibrate age as a function of the difference in magnitude between the main-sequence turnoff and the horizontal branch (A VHToB). A best estimate for the calibration of Mv(RR) is derived from a survey of the current literature: Mv(RR) = 0.20[Fe/H] + 0.98. This estimate, together with other calibrations (with slopes ranging from 0.15 to 0.30), has been used to derive A VTHoB ages for 43 Galactic globular clusters. Independent of the choice of Mv(RR), there is no strong evidence for an age- Galactocentric distance relationship among the 43 globular clusters. However, an age-metallicity relation exists, with the metal-poor clusters being the oldest. A study of the age distribution reveals that an age range of 5 Gyr exists among the bulk of the globular clusters. In addition, about 10% of the sample are substantially younger, and including them in the analysis increases the age range to 9 Gyr. Once again, these statements are independent of the Mv(RR) relation. Evidence for age being the second parameter governing horizontal-branch morphology is found by comparing the average A VHToB age of the second parameter clusters to the normal clusters. The second parameter clusters are found to be on average 2-3 Gyr younger than the other clusters, which is consistent with age being the second parameter. These results suggest that globular clusters were formed over an extended period of time, with progressively more metal-rich globular clusters ([Fe/H] - 1.7) being formed at later times. Title: A Lower Limit on the Age of the Universe Authors: Chaboyer, Brian; Demarque, Pierre; Kernan, Peter J.; Krauss, Lawrence M. Bibcode: 1996Sci...271..957C Altcode: 1995astro.ph..9115C A detailed numerical study was designed and conducted to estimate the absolute age and the uncertainty in age (with confidence limits) of the oldest globular clusters in our galaxy, and hence to put a robust lower bound on the age of the universe. Estimates of the uncertainty range and distribution in the input parameters of stellar evolution codes were used to produce 1000 Monte Carlo realizations of stellar isochrones, which were then used to derive ages for the 17 oldest globular clusters. A probability distribution for the mean age of these systems was derived by incorporating the observational uncertainties in the measured color-magnitude diagrams for these systems and the predicted isochrones. The dominant contribution to the width of the distribution (approximately ±5 percent) resulted from the observational uncertainty in RR-Lyrae variable absolute magnitudes. Subdominant contributions came from the choice of the color table used to translate theoretical luminosities and temperatures to observed magnitudes and colors, as well as from theoretical uncertainties in heavy element abundances and mixing length. The one-sided 95 percent confidence limit lower bound for this distribution occurs at an age of 12.07 x 10^9 years, and the median age for the distribution is 14.56 x 10^9 years. These age limits, when compared with the Hubble age estimate, put powerful constraints on cosmology. Title: Characteristic Temperature of UV Upturn of Elliptical Galaxies Authors: Demarque, P.; Yi, S.; Oemler, A., Jr. Bibcode: 1996ASPC...98..111D Altcode: 1996fstg.conf..111D No abstract at ADS Title: Are Blue Stragglers Mixed During Collisions? Authors: Procter, A.; Bailyn, C. D.; Demarque, P. Bibcode: 1996ASPC...90..380P Altcode: 1996oedb.conf..380P No abstract at ADS Title: Photometry of star clusters near the galactic center. Authors: Heasley, J.; Janes, K.; Demarque, P.; Zinn, R. Bibcode: 1996BAAS...28R.821H Altcode: No abstract at ADS Title: The Theoretical Calculation of the Rossby Number and the ``Nonlocal'' Convective Overturn Time for Pre-Main-Sequence and Early Post-Main-Sequence Stars Authors: Kim, Yong-Cheol; Demarque, Pierre Bibcode: 1996ApJ...457..340K Altcode: This paper provides estimates of convective turnover timescales for Sun-like stars in the pre-main-sequence and early post-main-sequence phases of evolution, based on up-to-date physical input for the stellar models. In this first study, all models have solar abundances, which is typical of the stars in the Galactic disk, where most of the available data have been collected. A new feature of these models is the inclusion of rotation in the evolutionary sequences, thus making it possible to derive theoretically the Rossby number for each star along its evolutionary track, based on its calculated rotation rate and its local convective turnover time near the base of the convection zone. Global turnover times are also calculated for the complete convection zone. This information should make possible a new class of observational tests of stellar theory that were previously impossible with semiempirical models, particularly in the study of stellar activity and in research related to angular momentum transfer in stellar interiors during the course of stellar evolution. Title: Seismology of η Bootis Authors: Guenther, D. B.; Demarque, P. Bibcode: 1996ApJ...456..798G Altcode: The p-mode frequencies recently observed by Kjeldsen and coworkers, along with other observables, are used to determine the mass, age, and helium abundance of η Bootis. We show, by direct application, how the p-model frequencies and stellar seismological tools aid in constraining the physical parameters of η Boo. We confirm the existence of mode bumping and discuss how it may be used to refine the estimate of η Boo's age. We describe the effect of the new OPAL equation-of-state tables (Rogers, Swenson, and Iglesias) on the p-mode frequencies. Title: Progress in helio- and astero-seismology Authors: Demarque, P.; Guenther, D. B. Bibcode: 1996ASPC..109...87D Altcode: 1996csss....9...87D No abstract at ADS Title: An answer to the UV upturn phenomenon of the elliptical galaxies? Authors: Yi, Sukyoung; Demarque, P.; Oemler, A., Jr. Bibcode: 1996ASPC...98..113Y Altcode: 1996fstg.conf..113Y Recent discoveries about the UV upturn of the elliptical galaxies including (1) presence of the UV upturn in the old, metal-rich systems, (2) a positive correlation between the magnitude of the UV upturn and metallicity (Burstein et al. 1988), and (3) the characteristic temperature of the UV flux which is about 20000-23000 K (Brown et al. 1995), are consistent with the hypothesis that the hot HB stars are the main sources of the UV upturn. The UV upturn is governed mainly by age and metallicity which are key elements in understanding the formation and evolution of galaxies. Thus, exploiting the UV upturn phenomenon not only tells us about the hot stellar content in the galaxies but also provides important clues to cosmology. However, the UV flux is a complex function of age and metallicity. There are multiple solutions of age and metallicity that produce a similar amount of UV flux, but, whose implications for galaxy evolution are drastically different. Title: Are Blue Stragglers Mixed during Collisions? Authors: Procter Sills, Alison; Bailyn, Charles D.; Demarque, Pierre Bibcode: 1995ApJ...455L.163P Altcode: 1995ApJ...455L.163S The smoothed particle hydrodynamics (SPH) simulations of Benz & Hills led to the assumption that blue stragglers created by collisions are initially fully mixed. The theoretical models of stars created under this assumption fit the observed blue straggler sequences well. Recently, however, Lombardi, Rasio, & Shapiro performed SPH calculations of stellar collisions using a more realistic polytropic index for the structure of stars near the main-sequence turnoff. Their results show that the collision products are not mixed and, in fact, have composition profiles similar to those of the original stars. We have calculated evolutionary tracks for both the "unmixed" products and fully mixed products and have compared the results to the very bright blue stragglers in the center of NGC 6397. The unmixed stars are not as blue and not as bright as the observed stars, and they only live on the main sequence for a very short time. To account for the observed blue stragglers in NGC 6397, the collision products must either be more than twice the turnoff mass or mixed by some process subsequent to the initial collision and merger. Title: Uncertainties in the Absolute Age of the Globular Clusters Authors: Chaboyer, Brian; Kernan, Peter J.; Krauss, Lawrence M.; Demarque, Pierre Bibcode: 1995AAS...187.0702C Altcode: 1995BAAS...27.1292C As the longstanding discrepancy between the expansion age of the universe and the age of the oldest globular clusters continues, it becomes critically important to provide a realistic estimate of the error associated with the globular cluster age estimates. We report the results of a detailed numerical study designed to answer this question. Utilizing estimates of the uncertainty range (and distribution) in the input parameters of stellar evolution codes we produced 1000 Monte Carlo realizations of stellar isochrones, with which we could determine the ages of the 18 oldest globular clusters using the difference in magnitude between the main-sequence turnoff and the horizontal branch. Incorporating the observational uncertainties in the measured color-magnitude diagrams for these systems and the predicted isochrones, we derived a probability distribution for the mean age of these systems. Our best estimate is 14.6+/- 1.7 Gyr, with the one-sided 95% C.L. lower bound of 12.1 Gyr. The total error budget is dominated by the uncertainty in the absolute magnitude of the horizontal branch. Simply varying this quantity over its full 2sigma range, keeping all other parameters fixed, would produce a +/- 16% change in globular cluster ages estimates. Other significant input parameter uncertainties in this same sense are [alpha /Fe] (+/- 7 % effect), mixing length (+/- 5% effect), and diffusion, (14) Np reaction rate, the choice of colour table, and the primordial Helium abundance, each of which would affect age estimates at the +/-3% level if allowed to vary over its entire range, keeping all other parameters fixed. Simple formulae are provided which can be used to update our age estimate and its error as improved determinations for the above quantities become available. Title: Horizontal Branch Morphology and the UV Upturn Phenomenon in the Elliptical Galaxies Authors: Yi, S.; Demarque, P.; Oemler, A. A., Jr. Bibcode: 1995AAS...187.8305Y Altcode: 1995BAAS...27.1407Y Recent HUT observation of the FUV flux of several elliptical galaxies has been interpreted as due to the very low-mass, modestly metal-rich (about 1-2 solar), helium burning (horizontal branch - HB) stars, with a narrow range of temperature (Brown, Ferguson, & Davidsen 1995, ApJ, 454, L15). It is interesting to see if population synthesis models based purely on standard stellar evolution theory can explain these observational suggestions. The discovery of several analogous hot stars in the old, metal-rich open cluster NGC 6791 also suggests that higher mass loss rates should be considered for more metal-rich stars (Liebert, Saffer, & Green, 1994, AJ, 107, 1408). We have constructed new helium burning phase evolutionary tracks using improved physics. Based on the tracks, synthetic HB models were built under the assumption of Delta Y/Delta Z=2 and 3 and a gaussian mass distribution on the HB with various mass loss efficiency parameters eta in the Reimers' formula. The mass distribution on the HB strongly affects the resulting composite UV spectra. We found that there is more than one solution which reproduces a similar magnitude of UV upturn. However, we prefer the modestly metal-rich populations with high mass loss rates as a solution because they can explain not only the magnitude of the UV flux but also strength of the absorption lines suggested by the HUT data, in a reasonable time scale (15-20 Gyr). Meanwhile, the physics which causes the unusually high mass loss for the metal-rich stars remains to be understood. Title: Observational Tests for Ultraviolet Sources in Elliptical Galaxies Authors: Yi, Sukyoung; Afshari, Ehsan; Demarque, Pierre; Oemler, Augustus, Jr. Bibcode: 1995ApJ...453L..69Y Altcode: An understanding of the empirical correlation between the strength of the UV upturn and the metallicity of luminous elliptical galaxies (Burstein and coworkers) is vital in the study of the evolution of elliptical galaxies. We support a modestly old (15--20 Gyr), metal-rich hypothesis ([Fe/H] >~ 0) for the UV sources in UV upturn galaxies as an alternative to the old (>~20 Gyr), metal-poor ([Fe/H] <~ -1.0) hypothesis of Park & Lee. This preliminary result of our population synthesis is based on a Gaussian mass distribution on the horizontal branch and a complete library of stellar evolution tracks, both of which are crucial in producing UV light in the metal-rich systems. The metal-rich hypothesis has an advantage over the metal-poor one not only because it explains the UV upturn--metallicity relation but also because it requires neither an uncomfortably large age nor a large age span for elliptical galaxies. We suggest several observational tests to constrain the metallicity of the UV sources. Title: Stellar Physics Using High-Resolution Spectroscopy with Large Telescopes Authors: Demarque, Pierre Bibcode: 1995PASP..107..994D Altcode: The traditional high resolution domain of stellar physics will be profoundly affected by the deployment of 8-10 meter class telescopes. We discuss here some of the advances anticipated in the fields of stellar convection, seismology and magentic structures. Also discussed are the tests of stellar evolution and star formation processes that one may expect based on high resolution observations of radial velocity, Doppler imaging, and chemical abundances. (SECTION: Workshop on High-Resolution Spectroscopy) Title: A VIC Color-Magnitude Diagram of the Globular Cluster NGC 6352 From Hubble Space Telescope Planetary Camera Observations Authors: Fullton, L. K.; Carney, B. W.; Olszewski, E. W.; Zinn, R.; Demarque, P.; Janes, K. A.; Da Costa, G. S.; Seitzer, P. Bibcode: 1995AJ....110..652F Altcode: The globular cluster NGC 6352, which on the basis of its Galactic position, radial velocity, and [Fe/H] is a member of the (thick) disk system of globular clusters, has been observed to below the main-sequence turnoff (MSTO) using the Hubble Space Telescope. These observations, which were obtained before the repair mission, were analyzed using the flux-conserving iterative/recursive deconvolution algorithm developed at the University of North Carolina. This algorithm can produce more precise photometry than standard PSF-fitting methods with the aberrated images. The V, (V-IC) color-magnitude diagram constructed from these observations places the cluster turnoff at V 18.80±0.10. Observations with the CTIO 0.9 m telescope have been used to photometer the brighter stars in the cluster, providing a calibration of the HST data and an estimate of the cluster's reddening, E(B-V) =0.21±0.03. From spectroscopic observations of the strengths of the Ca II triplet lines in red giants in the cluster and from previous measurements in the literature, we find that NGC 6352 is only slightly more metal rich (Δ[Fe/H]=0.08±0.05) than the prototypical disk globular cluster, 47 Tuc. From the difference in V magnitude between the horizontal branch and the MSTO, we find that NGC 6352 is essentially the same age as 47 Tuc (formally, older by 0.7±2.2 Gyr). Comparisons with the latest Yale isochrones support this result and yield 14.5±2 Gyr for the age of 47 Tuc from the photometry of Hesser et al. [PASP, 99, 739 (1987)], which is consistent with other determinations. The old age obtained for NGC 6352 provides additional evidence that the disk system of globular clusters is very old and is in fact older or comparable in age to several globular clusters populating the Galactic halo. Title: The theoretical calculation of the Rossby number and the `non-local' convective overturn time for pre-main sequence and early post-main sequence stars Authors: -Cheol Kim, Yong; Demarque, Pierre Bibcode: 1995astro.ph..7096. Altcode: 1995astro.ph..7096K This paper provides estimates of convective turnover time scales for Sun-like stars in the pre-main sequence and early post-main sequence phases of evolution, based on up-to-date physical input for the stellar models. In this first study, all models have solar abundances, which is typical of the stars in the Galactic disk where most of the available data have been collected. A new feature of these models is the inclusion of rotation in the evolutionary sequences, thus making it possible to derive theoretically the Rossby number for each star along its evolutionary track, based on its calculated rotation rate and its local convective turnover time near the base of the convection zone. Global turnover times are also calculated for the complete convection zone. This information should make possible a new class of observational tests of stellar theory which were previously impossible with semi-empirical models, particularly in the study of stellar activity and in research related to angular momentum transfer in stellar interiors during the course of stellar evolution. Title: Rotation, Diffusion, and Overshoot in the Sun: Effects on the Oscillation Frequencies and the Neutrino Flux Authors: Chaboyer, Brian; Demarque, P.; Guenther, D. B.; Pinsonneault, M. H. Bibcode: 1995ApJ...446..435C Altcode: 1995astro.ph..1058C We have studied the importance of the combined effects of rotation, diffusion, and convective overshoot on the p-mode oscillation spectrum and the neutrino flux of the standard solar model. To isolate the various physical affects included in the new rotation plus diffusion models we also constructed solar models to test the significance of diffusion and of overshoot by themselves. In previous studies, models that include helium diffusion during solar evolution were found to improve the predicted p-mode frequencies for some modes and worsen the agreement for others (Guenther et al. 1993). Here we verify this result for both the Bahcall & Loeb (1990) formulation of diffusion and the Proffitt & Michaud (1991) formulation of diffusion. We find that the effects of rotation on the Sun's structure in the outer layers perturbs the p-mode frequencies only slightly when compared to the more substantial effects due to diffusion. In the thin overshoot layer (taken here to be 0.1 Hp), we have compared the effect of over-mixing in a radiative layer versus convective (adiabatic) penetration. Neither radiative over-mixing nor adiabatic penetration has any significant effect on the p-modes, probably in part because the overshoot layer is constrained to be thin. The predicted neutrino flux in our diffusion plus rotation model is 7.12 SNU for Cl detectors, 127 SNU for Ga detectors, and 5.00 × 106 ergs cm-2 for the 8B neutrinos; this is approximately half-way between the standard solar model without diffusion, and the standard solar model with diffusion alone. Title: The Seismology of η Bootes Authors: Demarque, P.; Guenther, D. B. Bibcode: 1995ESASP.376b.555D Altcode: 1995help.confP.555D; 1995soho....2..555D No abstract at ADS Title: The Ages of the Disk Clusters NGC 188, M67, and NGC 752, Using Improved Opacities and Cluster Membership Data Authors: Dinescu, Dana I.; Demarque, Pierre; Guenther, D. B.; Pinsonneault, M. H. Bibcode: 1995AJ....109.2090D Altcode: Theoretical isochrones have been constructed using the OPAL opacities to derive the ages of the open clusters NGC 188, M67, and NGC 752 as well as to estimate the amount of convective overshoot at the convective core edge. We find that-under the assumptions made in the models-no overshoot, or a very small amount (up to 0.1 Hp) is allowed for the best fits. Very good agreement is achieved between models and the main sequence, turnoff, subgiant, and giant branch for the clusters NGC 188 and M67. For NGC 188 and M67 we derive the ages: 6.0+1.0-0.5 Gyr and 4.0±0.5 Gyr, respectively, where the main uncertainty is due to the estimate of the interstellar reddening. The age of NGC 752 is 2.0+0.5-0.3 Gyr if the adopted metallicity is [Fe/H] = - 0.27. Title: Sensitivity of Solar g-Modes to Varying G Cosmologies Authors: Guenther, D. B.; Sills, Ken; Demarque, Pierre; Krauss, Lawrence M. Bibcode: 1995ApJ...445..148G Altcode: The sensitivity of the solar g-mode oscillation spectrum to variability in the universal gravitational constant G is described. Solar models in varying G cosmologies were constructed by evolving a zero-age main-sequence stellar model to the Sun's current age, while allowing the value of G to change according to the power law G(t) proportional to t-beta, where Beta approximately equals delta G/GH and H is the Hubble constant. All solar models were constrained to the observed luminosity and radius at the current age of the Sun by adjusting the helium abundance and the mixing-length parameter of the models in the usual way for standard stellar models. Low-l g-mode oscillation periods were calculated for each of the models and compared to the claimed observation of the solar g-mode oscillation spectrum by Hill & Gu (1990). If one accepts Hill & Gu's claims, then within the uncertainties of the physics of the solar model calculation, our models rule out all but (delta G/GH) less than approximately 0.05. In other words, we conclude that G could not have varied by more than 2% over the past 4.5 Gyr, the lifetime of the present-day Sun. This result lends independent support to the validity of the standard solar model. Title: Stellar Models with Microscopic Diffusion and Rotational Mixing. II. Application to Open Clusters Authors: Chaboyer, B.; Demarque, P.; Pinsonneault, M. H. Bibcode: 1995ApJ...441..876C Altcode: 1994astro.ph..8059C Stellar models with masses ranging from 0.5 to $1.3~M_\odot$ were constructed in order to compare to young cluster observations of Li and of rotation velocities. The amount of Li depletion in cool stars is sensitive to the amount of overshoot at the base of the surface convection zone, and the exact metallicity of the models. Even when this uncertainty is taken into account, the Li observations are a severe constraint for the models and rule out standard models, and pure diffusion models. Stellar models which include diffusion and rotational mixing in the radiative regions of stars are able to simultaneously match the Li abundances observed in the Pleiades, UMaG, Hyades, Praesepe, NGC 752 and M67. They also match the observed rotation periods in the Hyades. However, these models are unable to simultaneously explain the presence of the rapidly rotating late G and K stars in the Pleiades and the absence of rapidly rotating late F and early G stars. Title: Modeling of Shallow and Inefficient Convection in the Outer Layers of the Sun Using Realistic Physics Authors: Kim, Yong-Cheol; Fox, Peter A.; Sofia, Sabatino; Demarque, Pierre Bibcode: 1995ApJ...442..422K Altcode: In an attempt to understand the properties of convective energy transport in the solar convective zone, a numerical model has been constructed for turbulent flows in a compressible, radiation-coupled, nonmagnetic, gravitationally stratified medium using a realistic equation of state and realistic opacities. The time-dependent, three-dimensional hydrodynamic equations are solved with minimal simplifications. The statistical information obtained from the present simulation provides an improved understanding of solar photospheric convection. The characteristics of solar convection in shallow regions is parameterized and compared with the results of Chan & Sofia's (1989) simulations of deep and efficient convection. We assess the importance of the zones of partial ionization in the simulation and confirm that the radiative energy transfer is negligible throughout the region except in the uppermost scale heights of the convection zone, a region of very high superadiabaticity. When the effects of partial ionization are included, the dynamics of flows are altered significantly. However, we confirm the Chan & Sofia result that kinetic energy flux is nonnegligible and can have a negative value in the convection zone. Title: Theoretical Color Calibration for HST FOC Ultraviolet Filters Authors: Yi, Sukyoung; Demarque, Pierre; Oemler, Augustus, Jr. Bibcode: 1995PASP..107..273Y Altcode: A theoretical color calibration is carried out for three (F140W, F220W, and F342W) of the most heavily used ultraviolet filters, and one visual band filter (F480LP) in the Faint Object Camera on the HST, using the new Kurucz model spectra which became available recently. It is known that the UV filters suffer from the visible leak problem, and it is apparent in this study that the visible leak problem can cause serious trouble to analyses of observed data. We show here that some of the colors defined from these filters do not have a monotonic relation with effective temperature; and thus, the colors fail to indicate a unique effective temperature in certain temperature ranges. This study shows that the problem can be solved by using one of the visual band filters, preferably F480LP, together with the UV filters. The expected model color-magnitude diagrams for a typical globular cluster with the HST ultraviolet colors are also provided. (SECTION: Astronomical Instrumentation) Title: Stellar Models with Microscopic Diffusion and Rotational Mixing. I. Application to the Sun Authors: Chaboyer, Brian; Demarque, P.; Pinsonneault, M. H. Bibcode: 1995ApJ...441..865C Altcode: 1994astro.ph..8058C The Yale stellar evolution code has been modified to include the combined effects of diffusion and rotational mixing on $^1$H, $^4$He and the trace elements $^3$He, $^6$Li, $^7$Li, and $^9$Be. The interaction between rotational mixing and diffusion is studied by calculating a number of calibrated solar models. The rotational mixing inhibits the diffusion in the outer parts of the models, leading to a decrease in the envelope diffusion by 25 -- 50\%. Conversely, diffusion leads to gradients in mean molecular weight which can inhibit the rotational mixing. The degree to which gradients in mean molecular weight inhibits the rotational mixing is somewhat uncertain. A comparison to the observed solar oblateness suggests that gradients in the mean molecular weight play a smaller role in inhibiting the rotational mixing previously believed. This is reinforced by the fact that the model with the standard value for the inhibiting effect of mean molecular weight on the rotational mixing depletes no Li on the main sequence. This is in clear contrast to the observations. A reduction in the inhibiting effect of mean molecular weight gradients by a factor of ten leads to noticeable main sequence Li depletion. Title: Main Sequence and H-Shell Burning: Convection and Seismology Authors: Demarque, P.; Guenther, D. B.; Kim, Y. -C. Bibcode: 1995LIACo..32..279D Altcode: 1995sews.book..279D No abstract at ADS Title: Modeling Convection in the Outer Layers of the Sun : Constancy of the Mixing Length Ratio near the Top of Convection Zone Authors: Kim, Y. -C.; Fox, P. A.; Demarque, P.; Sofia, S. Bibcode: 1995LIACo..32..195K Altcode: 1995sews.book..195K The purpose of this poster is to utilize the results of a numerical simulation of solar convection based on a physically realistic description of the shallow layers, and to compare the results with one of the basic assumption of the standard mixing length approximation, namely, a constant mixing length ratio throughout a convection zone. Title: Ages of galactic globular clusters from the new Yale isochrones. Authors: Chaboyer, B.; Demarque, P.; Guenther, D. B.; Pinsonneault, M. H.; Pinsonneault, L. L. Bibcode: 1995fmw..conf..289C Altcode: 1994astro.ph.11024C A new grid of theoretical isochrones based on the Yale stellar evolution code using the OPAL and Kurucz opacities has been constructed. The grid of isochrones spans a wide range of metallicities, helium abundances and masses. The construction of the isochrones is described and the isochrones are compared to galactic globular cluster observations. A solar calibrated mixing length (α = 1.7) yields a good fit to globular cluster colour-magnitude diagrams. Ages for 40 globular clusters are determined using the ΔV(TO-HB) method and the formation of the halo is discussed. Title: Concluding Remarks Authors: Demarque, P. Bibcode: 1995LIACo..32..489D Altcode: 1995sews.book..489D No abstract at ADS Title: Stellar evolution: what should be done Authors: Noels, A.; Fraipont-Caro, D.; Gabriel, M.; Grevesse, N.; Demarque, P. Bibcode: 1995sews.book.....N Altcode: No abstract at ADS Title: The Combined Effects of Rotation and Diffusion on the Standard Solar Model Authors: Demarque, P.; et al. Bibcode: 1995somo.conf..151D Altcode: No abstract at ADS Title: Helioseismology Authors: Demarque, P.; Chaboyer, B.; Guenther, D. B.; Pinsonneault, M. H. Bibcode: 1995ASPC...78..417D Altcode: 1995aapn.conf..417D No abstract at ADS Title: Helioseismology, solar evolution and other physics Authors: Demarque, P.; Chaboyer, B.; Guenther, D. B.; Pinsonneault, M. H. Bibcode: 1995HiA....10..332D Altcode: No abstract at ADS Title: The Sun as a Probe of Varying G Authors: Demarque, P.; Krauss, Lawrence M.; Guenther, D. B.; Nydam, David Bibcode: 1994ApJ...437..870D Altcode: In order to explore the ability of helioseismology and features of solar models to test the constancy of the gravitational constant G during the last 4.5 Gyr of solar evolution, we have constructed a grid of evolutionary sequences for solar models under the assumptions that G varies with time, and have explored the sensitivity of their nonradial acoustic mode oscillation spectra to G variability. All final models satisfy the standard constraints for the present Sun and, except for the variation in G, were constructed under the assumptions of a standard solar model. When compared with the observed solar p-mode spectrum, our models definitely rule out beta greater than 0.4 and beta less than -0.4, where G(t) proportional to t-beta over the last 4.5 Gyr. These limits can be tightened to rule out absolute value of beta greater than 0 (0.1) by the use of other solar observables. For nonmonotonic variation in G this suggests that long-timescale variations greater than 0 (5%) in G are ruled out over the last 4.5 Gyr. Future prospects for improving the sensitivity of helioseismic tests of a varying G are also discussed. Finally, we explore the sensitivity of the predicted solar neutrino flux to varying G. Title: Theoretical Color Calibration for HST FOC Ultraviolet Filters Authors: Yi, Sukyoung; Demarque, Pierre; Oemler, Augustus, Jr. Bibcode: 1994AAS...185.0910Y Altcode: 1994BAAS...26.1320Y Theoretical color calibration is carried out for three (F140W, F220W, and F342W) of the most heavily used ultraviolet filters and one visual band filter (F480LP) in the Faint Object Camera on the HST, using the new Kurucz model spectra which became available recently. It is known that the UV filters suffer from the visible leak problem, and it is apparent in this study that the visible leak problem can cause serious trouble to analyses of observed data. We show here that some of the colors defined from these filters do not have a monotonic relation with effective temperature; and thus, the colors fail to indicate a single effective temperature in certain temperature ranges. This study shows that the problem can be solved by using one of the visual band filters, preferably F480LP, together with the UV filters. The expected model color-magnitude diagrams for a typical globular cluster with the HST ultraviolet colors are also provided. Title: 7Li Abundances in Halo Stars: Testing Stellar Evolution Models and the Primordial 7Li Abundance Authors: Chaboyer, Brian; Demarque, P. Bibcode: 1994ApJ...433..510C Altcode: 1994astro.ph..3043C A large number of stellar evolution models with [Fe/H] = -2.3 and -3.3 have been calculated in order to determine the primordial $^7$Li abundance and to test current stellar evolution models by a comparison to the extensive database of Li abundances in extremely metal poor halo stars observed by Thorburn (1994). Standard models do a good job of fitting the observed Li abundances in stars hotter than 5600 K. They predict a primordial $^7$Li abundance of Log N(Li) = 2.24\pm 0.03$. Models which include microscopic diffusion predict a downward curvature in the $^7$Li destruction isochrones at hot temperatures which is not present in the observations. Thus, the observations clearly rule out models which include uninhibited microscopic diffusion of $^7$Li from the surface of the star. The [Fe/H] = -2.3 stellar models which include both diffusion and rotational mixing provide an excellent match to the observations. Both the plateau stars and the heavily depleted cool stars are well fit by these models. The rotational mixing leads to considerable $^7$Li depletion in these models and the primordial $^7$Li abundance inferred from these models is Log N(Li) = $3.08\pm 0.1$. Title: The Gap in the Color-Magnitude Diagram of NGC 2420: A Test of Convective Overshoot and Cluster Age Authors: Demarque, Pierre; Sarajedini, Ata; Guo, X. -J. Bibcode: 1994ApJ...426..165D Altcode: Theoretical isochrones have been constructed using the OPAL opacities specifically to study the color-magnitude diagram of the open star cluster NGC 2420. This cluster provides a rare test of core convection in intermediate-mass stars. At the same time, its age is of interest because of its low metallicity and relatively high Galactic latitude for an open cluster. The excellent color-magnitude diagram constructed by Anthony-Twarog et al. (1990) allows a detailed fit of the isochrones to the photometric data. We discuss the importance of convective overshoot at the convective core edge in determining the morphology of the gap located near the main-sequence turnoff. We find that given the assumptions made in the models, a modest amount of overshoot (0.23 Hp) is required for the best fit. Good agreement is achieved with all features of the turnoff gap for a cluster age of 2.4 +/- 0.2 Gyr. We note that a photometrically complete luminosity function near the main-sequence turnoff and subgiant branch would also provide an important test of the overshoot models. Title: Modelling of Shallow and Inefficient Convection in the Outer Layers of the Sun Using Realistic Physics Authors: -Cheol Kim, Yong; Fox, Peter A.; Sofia, Sabatino; Demarque, Pierre Bibcode: 1994astro.ph..3046. Altcode: 1994astro.ph..3046K In an attempt to understand the properties of convective energy transport in the solar convection zone, a numerical model has been constructed for turbulent flows in a compressible, radiation-coupled, non-magnetic, gravitationally stratified medium using a realistic equation of state and realistic opacities. The time-dependent, three-dimensional hydrodynamic equations are solved with minimal simplifications. The statistical information obtained from the present simulation provides an improved understanding of solar photospheric convection. The characteristics of solar convection in shallow regions is parameterized and compared with the results of Chan and Sofia's simulations of deep and efficient convection (Chan and Sofia 1989). We assess the importance of the zones of partial ionization in the simulation, and confirm that the radiative energy transfer is negligible throughout the region except in the uppermost scale heights of the convection zone, a region of very high super-adiabaticity. When the effects of partial ionization are included, the dynamics of flows are altered significantly. However, we confirm the Chan and Sofia result that kinetic energy flux is non-negligible and can have a negative value in the convection zone. Title: Mass Loss during the RR Lyrae Phase of the Horizontal Branch: Mass Dispersion on the Horizontal Branch and RR Lyrae Period Changes Authors: Koopmann, Rebecca A.; Lee, Young-Wook; Demarque, Pierre; Howard, Jamie M. Bibcode: 1994ApJ...423..380K Altcode: Mass loss on the horizontal branch has been invoked in the literature to explain such phenomena as the color (mass) dispersion of the horizontal branch and the observed distribution of period changes in RR Lyrae stars. To test these claims, the Yale stellar evolution code was used to evolve horizontal branch models of masses 0.64, 0.66, 0.68, 0.70, and 0.72 solar mass with Z of 0.001, core mass of 0.4893, main-sequence helium abundance of 0.23, and constant mass loss rates of 0, 10-10, 5 x 10-10, and 10-9 solar mass/yr. Mass loss was assumed to occur only in the instability strip, where a mechanism is most likely to exist. Synthetic horizontal branches, constructed from the models, show that mass loss on the horizontal branch cannot produce the observed color dispersion even for the highest mass-loss rate of 10-9 solar mass/yr. Mass loss is unlikely to occur at a higher rate without significant effects on the horizontal branch morphology, which would destroy the good agreement between standard synthetic models without mass loss and observed horizontal branches. Periods and period changes were calculated for all models. The period changes are not significantly larger for models with mass loss. The effect of mass loss in clusters of other metallicities is discussed. Title: The Horizontal-Branch Stars in Globular Clusters. II. The Second Parameter Phenomenon Authors: Lee, Young-Wook; Demarque, Pierre; Zinn, Robert Bibcode: 1994ApJ...423..248L Altcode: Using synthetic horizontal-branch models, we have investigated the origin of the systematic variation in horizontal-branch (HB) morphology with galactocentric distance (RG) among globular clusters. The variations in He abundance, CNO abundance, and core mass required separately to explain this effect are inconsistent with either the observed properties of the RR Lyrae variables or the observed main-sequence turnoffs in the clusters. There is also no clear evidence that the trend with RG is related to the central concentrations, central densities, or absolute magnitudes of the clusters. The variations in cluster age required to explain this effect are not in conflict with any observations. A detailed comparison of our synthetic HB calculations with pairs of clusters of very different HB morphology but similar (Fe/H) reveals reasonably good agreement between the age differences inferred from HB morphology and the main-sequence turnoff. The major source of uncertainty is the need for ad hoc hypotheses in the modeling of the HB morphologies of a few peculiar clusters (e.g., NGC 6752). Nonetheless, there is firm evidence for age variations of several gigayears (as much as approximately 5 Gyr) among the halo globular clusters. Our results support the hypothesis of Searle & Zinn that the inner halo is more uniform in age and is older in the mean than the outer halo, and we estimate this difference to be approximately 2 Gyr. Title: Report of IAU Commission 35: Stellar constitution (Constitution des étoiles). Authors: Demarque, P. Bibcode: 1994IAUTA..22..399D Altcode: No abstract at ADS Title: The Evolution of Rotating 15 M solar Stars Authors: Sofia, S.; Howard, J. M.; Demarque, P. Bibcode: 1994IAUS..162..131S Altcode: No abstract at ADS Title: The Extended Horizontal-Branch: A Challenge for Stellar Evolution Theory Authors: Demarque, Pierre Bibcode: 1994hsgh.conf..362D Altcode: No abstract at ADS Title: Diffusion and Rotational Mixing in Stellar Models Authors: Chaboyer, Brian; Demarque, P. Bibcode: 1994ASPC...64..273C Altcode: 1994csss....8..273C No abstract at ADS Title: The Effect of Livermore Opal Opacities on the Evolutionary Masses of RR Lyrae Stars Authors: Yi, Sukyoung; Lee, Young-Wook; Demarque, Pierre Bibcode: 1993ApJ...411L..25Y Altcode: We have investigated the effect of the new Livermore OPAL opacities on the evolution of horizontal-branch (HB) stars. This work was motivated by the recent stellar pulsation calculations using the new Livermore opacities, which suggest that the masses of double-mode RR Lyrae stars are 0.1-0.2 solar mass larger than those based on earlier opacities. Unlike the pulsation calculations, we find that the effect of opacity change on the evolution of HB stars is not significant. In particular, the effect of the mean masses of RR Lyrae stars is very small, showing a decrease of only 0.01-0.02 solar mass compared to the models based on old Cox-Stewart opacities. Consequently, with the new Livermore OPAL opacities, both the stellar pulsation and evolution models now predict approximately the same masses for the RR Lyrae stars. Our evolutionary models suggest that the mean masses of the RR Lyrae stars are about 0.76 and about 0.71 solar mass for M15 (Oosterhoff group II) and M3 (group I), respectively. If (alpha/Fe) = 0.4, these values are decreased by about 0.03 solar mass. Variations of the mean masses of RR Lyrae stars with HB morphology and metallicity are also presented. Title: Evolution and Seismology of Procyon Authors: Guenther, D. B.; Demarque, P. Bibcode: 1993ApJ...405..298G Altcode: We have calculated an array of stellar models for Procyon A that are based on the best physics available to us, including the latest opacities and nuclear cross sections. The array of models spans the error space centered on Procyon's mass, chemical composition, effective temperature, and luminosity. We find that with OPAL opacities no convective overshoot is needed at the edge of the convective core to match Procyon's position in the H-R diagram. We discuss the discrepancy between the astrophysical mass and the astrometric mass, which is reduced by the more up-to-date physics in the models, and describe how oscillation data can be used to distinguish among the possible models and help resolve the discrepancy. We have calculated oscillation frequencies for the l = 0, 1, 2, and 3 p-modes and oscillation periods for the l = 1, 2, and 3 g-modes for several of these models. Title: Seismology of Procyon Authors: Guenther, D. B.; Demarque, P. Bibcode: 1993ASPC...42..379G Altcode: 1993gong.conf..379G No abstract at ADS Title: New Physics, New Isochrones Authors: Demarque, P.; Guenther, D. B.; Pinsonneault, M. H.; Cheboyer, B.; Yi, S. Bibcode: 1993ASPC...48..191D Altcode: 1993gcgc.work..191D No abstract at ADS Title: Key issues - A round table discussion Authors: Brown, T. M.; Demarque, P.; Noyes, R.; Praderie, F.; Roxburgh, I. W.; Schatzman, E. Bibcode: 1993ASPC...40..776B Altcode: 1993ist..proc..776B; 1993IAUCo.137..776B An overview of a round table discussion on the internal dynamics of stars, some problems in stellar structure and evolution, a study of stellar activity mechanisms using PRISMA, the seismology of sunlike stars, and directions of future research is presented. It is concluded that models that take into account just one physical process generally do not agree with the observations. This provides evidence for the presence of other physical processes. In each phenomenon which is considered, a variety of physical processes are involved. All physical processes should be taken into account simultaneously. Stars need to be considered globally. It is recommended that attention be given to such unsolved problems as the helium content of the sun, the abundance of lithium in fast rotating stars, and the origin and evolution of stellar magnetic fields. Title: New physics, new evolutionary tracks, new isochrones Authors: Demarque, P.; Guenther, D. B.; Pinsonneault, B. Bibcode: 1993ASPC...40..448D Altcode: 1993IAUCo.137..448D; 1993ist..proc..448D No abstract at ADS Title: Effects of Microscopic Diffusion and Rotational Mixing on Stellar Models Authors: Chaboyer, Brian; Demarque, P.; Pinsonneault, M. H. Bibcode: 1992AAS...181.3609C Altcode: 1992BAAS...24.1176C Microscopic diffusion of the trace elements He3, Li7 and Be9 have been incorporated into the rotating stellar evolution code at Yale. The consequences of diffusion and rotational mixing are investigated for solar models and young cluster stars. The inhibition of diffusion due to rotational mixing (and vice versa) is explored. A comparison is made to abundance measurements of Li7 and rotational velocities in the Sun and young cluster stars. This work was partially supported by NASA grants NAG5--1486 and NAGW--2531. Title: The metal-rich globular cluster NGC 6553: clump morphology and age. Authors: Demarque, P.; Lee, Y. -W. Bibcode: 1992A&A...265...40D Altcode: Stellar evolutionary tracks for metal-rich HB stars have been used to construct synthetic population models for the peculiar HB clump in NGC 6553. Although the uncertainties are large, it is possible to derive an age for NGC 6553 in the range 8-13 Gyr, which supports the idea that the disk globular cluster system is younger than the oldest stars in the Galactic halo and bulge. Title: Stellar Photospheric Convection, and the Effect on Spectral Line Asymmetry Authors: Kim, Y. -C.; Demarque, P.; Fox, P. A. Bibcode: 1992AAS...181.3605K Altcode: 1992BAAS...24.1175K No abstract at ADS Title: Ages of Globular Clusters and Helium Diffusion Authors: Chaboyer, Brian; Sarajedini, Ata; Demarque, Pierre Bibcode: 1992ApJ...394..515C Altcode: Evolutionary tracks have been calculated with alpha-enhanced compositions which cover the entire globular cluster metallicity range and have constructed isochrones which include the effects of microscopic diffusion of helium. The turnoff magnitudes from the isochrones were combined with the theoretical RR Lyrae magnitudes from Lee to determine the ages of 32 Galactic globular clusters using the magnitude difference between the turnoff and horizontal branch. It is found that including the effects of helium diffusion has a negligible effect on the derived ages of globular clusters. Regardless of the inclusion of helium diffusion, a significant age spread of 5 Gyr among the globular clusters is found. The oldest globular clusters studied here are 17 +/- 2 Gyr old. Title: Evolutionary Models and the p-Mode Oscillation Spectrum of alpha Centauri A and B Authors: Edmonds, Peter; Cram, Lawrence; Demarque, Pierre; Guenther, D. B.; Pinsonneault, Marc H. Bibcode: 1992ApJ...394..313E Altcode: Spectroscopically observed abundances of Furenlid and Meylan (1990) and Los Alamos Opacity Library opacities specifically constructed for this mixture are used to construct the most detailed models to date for Alpha Cen A and B. Models including the effects of the diffusion of helium in Alpha Cen A's interior were constructed. Although the effect of helium diffusion is small in Alpha Cen A, it is slightly larger than in the sun because of the larger mass and therefore shallower convection zone of Alpha Cen A. Keeping the heavy-element mixture unchanged but varying Z within the uncertainties yields limits to the helium abundance of Y = 0.295 and 0.305, respectively. The derived age of Alpha Cen is 4.6 +/-0.4 Gyr, very similar to the solar age of 4.5 Gyr. Title: Standard Solar Model. II. g-Modes Authors: Guenther, D. B.; Demarque, P.; Pinsonneault, M. H.; Kim, Y. -C. Bibcode: 1992ApJ...392..328G Altcode: The paper presents the g-mode oscillation for a set of modern solar models. Each solar model is based on a single modification or improvement to the physics of a reference solar model. Improvements were made to the nuclear reaction rates, the equation of state, the opacities, and the treatment of the atmosphere. The error in the predicted g-mode periods associated with the uncertainties in the model physics is predicted and the specific sensitivities of the g-mode periods and their period spacings to the different model structures are described. In addition, these models are compared to a sample of published observations. A remarkably good agreement is found between the 'best' solar model and the observations of Hill and Gu (1990). Title: The Effect of Helium Diffusion on the Ages of Globular Clusters Authors: Chaboyer, Brian; Deliyannis, Constantine P.; Demarque, Pierre; Pinsonneault, M. H.; Sarajedini, Ata Bibcode: 1992ApJ...388..372C Altcode: Evolutionary tracks for halo stars were calculated, and isochrones which include the effects of microscopic diffusion of helium were constructed. The isochrones were fitted to a metal poor (M92) and a moderately metal rich (NGC 288) globular cluster using an updated version of the Revised Yale Isochrone color calibration. Ages of the two clusters were also determined using the difference between the turnoff magnitude and horizontal branch magnitude, and the difference in color between the main-sequence turnoff and lower giant branch. Considering all methods and constraints, diffusion is argued to reduce the derived ages of M92 and NGC 288 by 0.5-1 Gyr. The maximum age reduction that diffusion could cause is 3 Gyr. Age estimates including diffusion indicate that M92 is 16 +/- 2 Gyr old, and that M92 is about 3 Gyr older than NGC 288, assuming that the clusters have the same O/Fe of +0.4. Title: The Evolution of High-Metallicity Horizontal-Branch Stars and the Origin of the Ultraviolet Light in Elliptical Galaxies Authors: Horch, E.; Demarque, P.; Pinsonneault, M. Bibcode: 1992ApJ...388L..53H Altcode: Evolutionary calculations of high-metallicity horizontal-branch stars show that for the relevant masses and helium abundances, post-HB evolution in the HR diagram does not proceed toward and along the AGB, but rather toward a "slow blue phase" in the vicinity of the helium- burning main sequence, following the extinction of the hydrogen shell energy source. For solar and twice solar metallicity, the blue phase begins during the helium shell-burning phase (in agreement with the work of Brocato and Castellani & Tornambe); for 3 times solar metallicity, it begins earlier, during the helium core-burning phase. This behavior differs from what takes place at lower metallicities. The implications for high-metallicity old stellar populations in the Galactic bulge and for the integrated colors of elliptical galaxies are discussed. Title: Standard Solar Model Authors: Guenther, D. B.; Demarque, P.; Kim, Y. -C.; Pinsonneault, M. H. Bibcode: 1992ApJ...387..372G Altcode: A set of solar models have been constructed, each based on a single modification to the physics of a reference solar model. In addition, a model combining several of the improvements has been calculated to provide a best solar model. Improvements were made to the nuclear reaction rates, the equation of state, the opacities, and the treatment of the atmosphere. The impact on both the structure and the frequencies of the low-l p-modes of the model to these improvements are discussed. It is found that the combined solar model, which is based on the best physics available (and does not contain any ad hoc assumptions), reproduces the observed oscillation spectrum (for low-l) within the errors associated with the uncertainties in the model physics (primarily opacities). Title: Solar Calibration and the Age of the Old Disk Clusters M67, NGC 188, and NGC 6791 Authors: Demarque, Pierre; Green, E. M.; Guenther, D. B. Bibcode: 1992AJ....103..151D Altcode: The ages of the old disk star clusters M67, NGC 188, and NGC 6791 are derived on the basis of theoretical solar calibrated isochrones. The ages of M67 and NGC 188 were found to be about 4.0 +1.0 or -0.5 Gyr (close to the age of the sun) and 6.5 +1.5 or -0.5 Gyr, respectively. The age of NGC 6791 was found to be about 1.0 Gyr larger than that of NGC 188, assuming that the two clusters have the same metallicity. If, however, NGC 6791 is more metal rich, its age is less certain; it could be as low as 6.5 Gyr. Title: Evolutionary Models of Halo Stars with Rotation. II. Effects of Metallicity on Lithium Depletion, and Possible Implications for the Primordial Lithium Abundance Authors: Pinsonneault, M. H.; Deliyannis, Constantine P.; Demarque, P. Bibcode: 1992ApJS...78..179P Altcode: 1992ApJS...78...79P Models of metal-poor stars with rotation were computed and their lithium depletion was compared with observations of halo stars. The models that have turn-off ages compatible with the observations have a nearly flat Li-T(eff) relationship in the region of the Spite lithium 'plateau'. Depending on the initial angular momentum, the models have a depletion factor ranging between a factor of 5 and a factor of 10 at fixed T(eff), implying a maximum initial lithium abundance of 3.1. Both the dispersion and the overall depletion factor are much smaller for metal-poor models than for solar metallicity ones. The factors that determine lithium depletion in rotational models are discussed and the different depletion patterns in solar metallicity and metal-poor models are traced to differences in their structure and evolution. The dependence of the lithium depletion on age, mass, initial angular momentum, and metallicity is also discussed. The dispersion predicted from these models is not inconsistent with the observations. Title: The Effect of the Mihalas, Hummer, and Daeppen Equation of State and the Molecular Opacity on the Standard Solar Model Authors: Kim, Y. -C.; Demarque, P.; Guenther, D. B. Bibcode: 1991ApJ...378..407K Altcode: Improvements to the Yale Rotating Stellar Evolution Code (YREC) by incorporating the Mihalas-Hummer-Daeppen equation of state, an improved opacity interpolation routine, and the effects of molecular opacities, calculated at Los Alamos, have been made. the effect of each of the improvements on the standard solar model has been tested independently by computing the corresponding solar nonradial oscillation frequencies. According to these tests, the Mihalas-Hummer-Daeppen equation of state has very little effect on the model's low l p-mode oscillation spectrum compared to the model using the existing analytical equation of state implemented in YREC. On the other hand, the molecular opacity does improve the model's oscillation spectrum. The effect of molecular opacity on the computed solar oscillation frequencies is much larger than that of the Mihalas-Hummer-Daeppen equation of state. together, the two improvements to the physics reduce the discrepancy with observations by 10 microHz for the low l modes. Title: Lithium in Halo Stars: Constraining the Effects of Helium Diffusion on Globular Cluster Ages and Cosmology Authors: Deliyannis, Constantine P.; Demarque, Pierre Bibcode: 1991ApJ...379..216D Altcode: Stellar evolutionary models with diffusion are used to show that observations of lithium in extreme halo stars provide crucial constraints on the magnitude of the effects of helium diffusion. The flatness of the observed Li-T(eff) relation severely constrains diffusion Li isochrones, which tend to curve downward toward higher T(eff). It is argued that Li observations at the hot edge of the plateau are particularly important in constraining the effects of helium diffusion; yet, they are currently few in number. It is proposed that additional observations are required there, as well as below 5500 K, to define more securely the morphology of the halo Li abundances. Implications for the primordial Li abundance are considered. It is suggested that a conservative upper limit to the initial Li abundance, due to diffusive effects alone, is 2.35. Title: Lithium in the Most Extreme Halo Stars; Trends with Metallicity Authors: Deliyannis, Constantine P.; Demarque, Pierre Bibcode: 1991ApJ...370L..89D Altcode: A grid of extremely metal-poor stellar evolutionary tracks (Z = 0.00001) is constructed to compare the predicted lithium abundance depletion to observations of the lithium abundance in correspondingly metal-poor stars (Fe/H = -3.3), and, combined with an earlier work to investigate trends of lithium isochrones with metal abundance. Lithium abundances in the most extreme halo stars hint at the possibility of small downward slope toward lower T(eff) in the flat Li-T(eff) relation (the Spite lithium plateau) observed in slightly less extreme stars; the models also predict this. In the cool star region of the Li-T(eff) plane, higher metallicity lithium isochromes are cooler at fixed lithium abundance, consistent with the observations. Current data thus agree well with the lithium trends from standard model. Title: The Age Spread among Galactic Globular Clusters Authors: Demarque, Pierre Bibcode: 1991ppag.conf...45D Altcode: 1991ppag.proc...45D; 1991LDP....14...45D No abstract at ADS Title: Solar Calibration and the Ages of M67, NGC 188, and NGC 6791 Authors: Demarque, Pierre; Guenther, David B.; Green, Elizabeth M. Bibcode: 1991ASPC...13..259D Altcode: 1991fesc.book..259D No abstract at ADS Title: Mixing due to Angular Momentum Transfer in Evolving Sun-Like Stars Authors: Demarque, P. Bibcode: 1991IAUS..145...71D Altcode: No abstract at ADS Title: Post-main-sequence solar evolution. Authors: Demarque, Pierre; Guenther, David B. Bibcode: 1991sia..book.1186D Altcode: The authors' objective is to discuss some current research frontiers in solar-stellar physics. First, they describe and update the standard evolution of a 1-Msun star from the zero-age main sequence to the helium flash. Then, they focus on two areas of solar-stellar research which are presently in a state of rapid development. These two areas are: (1) the theory of solar and stellar nonradial acoustic (p-mode) oscillations, which can be used to test the pressure stratification in the stellar interior; and (2) the theory of the evolution of rotating stars, which predicts the evolution of surface rotational velocities and the mixing of chemical elements to the surface. The authors note in addition that oscillations and rotation are closely interrelated, because each individual p-mode is also split in frequency by internal rotation in the region through which it propagates. Observations of p-mode splittings thus can provide information on the state of rotation of the solar and stellar interiors. Title: Ages of Globular Clusters Authors: Demarque, P.; Deliyannis, C. P.; Sarajedini, A. Bibcode: 1991ASIC..348..111D Altcode: 1991otci.conf..111D No abstract at ADS Title: Evolutionary Models of Halo Stars with Rotation. I. Evidence for Differential Rotation with Depth in Stars Authors: Pinsonneault, Marc H.; Deliyannis, Constantine P.; Demarque, Pierre Bibcode: 1991ApJ...367..239P Altcode: Evolutionary models of metal-poor stars are computed including the effects of rotation, and their properties are compared with observations. The models rotate slowly at the surface, in agreement with the observed upper limits on the rotation velocity at main-sequence turnoff; they also have substantial differential rotation with depth. This differential rotation preserves a sufficient amount of internal angular momentum to explain the rapid rotation of evolved horizontal-branch stars. These results hold for a wide range of angular momentum loss and transport parameter values. Differences and similarities between the surface and internal rotation of solar metallicity and metal-poor models are discussed. Rigidly rotating models are found to be incompatible with the observations once giant branch mass loss is taken into account. Horizontal-branch rotation velocity measurements as a function of color are proposed as a test of the rotation law enforced in convection zones, and their dependence on cluster age and metallicity are discussed. Title: A New Age Diagnostic Applied to the Globular Clusters NGC 288 and NGC 362 Authors: Sarajedini, Ata; Demarque, Pierre Bibcode: 1990ApJ...365..219S Altcode: A new age diagnostic for globular clusters is described which uses the difference between the turnoff and the base of the giant branch as the age indicator. As a first application, it is shown that there is a difference in age of 3.1 + or - 0.9 Gyr between the classic 'second parameter' pair NGC 288 and NGC 362. The existence of this age difference is independent of metal abundance differences between the two clusters of up to 0.5 dex. This age difference is corrected for various combinations of relative oxygen enhancement, and it is concluded that the difference in age remains significant for all plausible scenarios. Title: Rotation of Low-Mass Stars: A New Probe of Stellar Evolution Authors: Pinsonneault, M. H.; Kawaler, Steven D.; Demarque, P. Bibcode: 1990ApJS...74..501P Altcode: Models of stars of various masses and rotational parameters were developed and compared with observations of stars in open clusters of various ages in order to analyze the evolution of rotating stars from the early premain sequence to an age of 1.7 x 10 to the 9th yrs. It is shown that, for stars older than 10 to the 8th yrs and less massive than 1.1 solar mass, the surface rotation rates depend most strongly on the properties of the angular momentum loss. The trends of the currently available observations suggest that the rotation periods are a good indicator of the field-star ages. Title: Effects of Helium Diffusion on Globular Cluster Ages Inferred from Isochrones Authors: Chaboyer, B.; Deliyannis, C. P.; Demarque, P.; Pinsonneault, M. H.; Sarajedini, A. Bibcode: 1990BAAS...22.1205C Altcode: No abstract at ADS Title: Primordial Lithium, Stellar Evolution, Big Bang Cosmology, and Dark Matter Authors: Deliyannis, C. P.; Demarque, P.; Pinsonneault, M. H. Bibcode: 1990BAAS...22.1214D Altcode: No abstract at ADS Title: The Effects of the MHD Equation of State and Molecular Opacities on the Oscillation Spectrum of the Standard Solar Model Authors: Kim, Y. -C.; Demarque, P.; Guenther, D. B. Bibcode: 1990BAAS...22Q1207K Altcode: No abstract at ADS Title: Effects of Main Sequence Rotational Mixing on Surface CNO Abundances in Red Giant Branch Stars Authors: Pinsonneault, M. H.; Demarque, P.; Sofia, S.; Deliyannis, C. P. Bibcode: 1990BAAS...22.1206P Altcode: No abstract at ADS Title: The Evolution of Horizontal-Branch Stars: Theoretical Sequences Authors: Lee, Young-Wook; Demarque, Pierre Bibcode: 1990ApJS...73..709L Altcode: A new grid of standard (i.e., solar CNO/Fe, no core rotation, and including semiconvection) horizontal-branch evolutionary sequences are presented which extend the grid of Sweigart both by including the final phase of core helium exhaustion and by using a finer grid of compositions. These sequences were constructed specifically for the studies of the Sandage period-shift effect and the second parameter phenomenon among the Galactic globular clusters. The numerical results for the observable characteristics of the sequences are tabulated which can be directly used to synthesize the observed horizontal-branch distributions. The present computations suggest that the observed difference in period change of RR Lyrae stars in the globular clusters M3 and M15 could be attributed, at least in the mean, to stellar evolution. Title: Lithium in Halo Stars from Standard Stellar Evolution Authors: Deliyannis, Constantine P.; Demarque, Pierre; Kawaler, Steven D. Bibcode: 1990ApJS...73...21D Altcode: A grid has been constructed of theoretical evolution sequences of models for low-metallicity stars from the premain-sequence to the giant branch phases. The grid is used to study the history of surface Li abundance during standard stellar evolution. The Li-7 observations of halo stars by Spite and Spite (1982) and subsequent observations are synthesized to separate the halo stars by age. The theory of surface Li abundance is illustrated by following the evolution of a reference halo star model from the contracting fully convective premain sequence to the giant branch phase. The theoretical models are compared with observed Li abundances. The results show that the halo star lithium abundances can be explained in the context of standard stellar evolution theory using completely standard assumptions and physics. Title: The Horizontal-Branch Stars in Globular Clusters. I. The Period-Shift Effect, the Luminosity of the Horizontal Branch, and the Age-Metallicity Relation Authors: Lee, Young-Wook; Demarque, Pierre; Zinn, Robert Bibcode: 1990ApJ...350..155L Altcode: Synthetic models of the horizontal branches in globular clusters are constructed from a grid of the standard horizontal branch evolutionary tracks. The models are used to study the period shifts at constant T(eff) between RR Lyrae variables in globular clusters of different metallicities and the variation in horizontal-branch luminosity with the Fe/H ratio. The results suggest that the observed differences in the mean periods of the ab variables and the fraction of c-type variables between the two Oosterhoff groups are caused by a difference in the mean luminosity of the ab variables of about 0.18 bolometric mass and by the uneven distribution of variables across the instability strip in the group II clusters. Title: The Effects of Internal Stellar Rotation on Age Estimates for Globular Cluster Stars Authors: Deliyannis, C. P.; Demarque, P.; Pinsonneault, M. H. Bibcode: 1990BAAS...22..746D Altcode: No abstract at ADS Title: On the Period-Shift Effect of the Field Rr-Lyrae Stars and the Ages of Galactic Globular Clusters Authors: Lee, Y. W.; Demarque, P.; Zinn, R. Bibcode: 1990aadm.conf..275L Altcode: No abstract at ADS Title: The Seismology of Sun-like Stars Authors: Demarque, P.; Guenther, D. B. Bibcode: 1990LNP...367..405D Altcode: 1990psss.conf..405D By observing the p-mode oscillation spectrum of a star and determining its first and second order frequency spacings, v and , it is possible to obtain constraints which are useful in modeling the star. We show, as an example, how the positions of the frequencies of the p-modes changes from the familiar picket fence or regular spacing to a more random irregular spacing as the sun evolves from the ZAMS to the base of the giant branch. For Cen, Eri, Procyon, Arcturus, and Cas we describe, in specific terms, how the frequency spacings can be used to improve our knowledge about each of these stars. We discuss the important advantages of observing the p-modes of stars in star clusters. We also discuss how rotational splittings could be used along with other stellar diagnostics to infer the rotational history of the star's interior. Title: Evolutionary Models Of Metal Poor Stars With Rotation Authors: Pinsonneault, M. H.; Deliyannis, C. P.; Demarque, P. Bibcode: 1990BAAS...22..746P Altcode: No abstract at ADS Title: The Ages of Globular Cluster Stars: Effects on Rotation of Pre--Main-Sequence, Main-Sequence, and Turnoff Evolution Authors: Deliyannis, Constantine P.; Demarque, Pierre; Pinsonneault, Marc H. Bibcode: 1989ApJ...347L..73D Altcode: Evolutionary sequences for low-metallicity stars (Z ranging from 0.001 to 0.0001) to study the effects of internal stellar rotation on the evolutionary time scales in the pre-main sequence, the main sequence (MS), and around the MS turnoff. Although a substantial amount of angular momentum remains in the interior, rotation is only a minor perturbation on the structure and ages of globular cluster stars. Even models with large initial angular momenta have MS lifetimes that are within 1 percent of those of standard models of the same mass and composition. Therefore, rotation does not affect age estimates of globular clusters from isochrone fitting. Furthermore, the models suggest that because rotation is not likely to affect horizontal-branch (HB) morphology, it does not affect significantly age estimates from the Delta-V method. Nevertheless, the internal angular momentum in the models is consistent with observations of surface rotational velocities on the HB, which require the preservation of a large reservoir of internal angular momentum. Title: The Standard Solar Model: Composition, Opacities, and Seismology Authors: Guenther, D. B.; Jaffe, A.; Demarque, P. Bibcode: 1989ApJ...345.1022G Altcode: High-precision standard solar models are used to investigate systematically the implications of current uncertainties in the interior radiative opacities for both the derivation of the helium content Y and the modeling of p-mode oscillation frequencies of the sun. The differences between the Cox-Stewart solar opacities and the Los Alamos Opacity Library solar opacities are found to be due principally to differences in the absorption coefficients themselves. The uncertainty of Ne in the solar chemical composition is shown to result in only a small uncertainty in Y. In the standard model framework, the uncertainty in Y is between 0.29 and 0.27. Only a small dependence of the asymptotic spacing of p-modes on chemical composition is detected. The effects of dependence on Y of the adiabatic exponent Gamma(1) and of the density of the structure of the outer layers is not monotonic because both these variables depend on the complex interplay of H and He ionization. The greatest sensitivity of sound speed to chemical composition is found below the surface convection zone. Title: A Map-based Determination of the Nature of Beta Delphini Authors: Gatewood, George; Castelaz, Michael; Persinger, Timothy; Stein, John; Demarque, Pierre; Sofia, Sabatino; Stephenson, Bruce Bibcode: 1989ApJ...342.1085G Altcode: The Beta Delphini binary system presents a stringent test of the theory of stellar evolution. Improved parallax and component masses are found for its giant (F5 III and F5 IV) stars. A study of the evolutionary status of the system indicates it to be 1.9 Gyr (1.9 billion years) old and to have a metallicity of approximately 1.5 times that of the Sun. The perturbation due to the 26.6 yr orbital motion is clearly shown in this 2.2 yr study and allows the most precise determination of the relative masses of the component stars to date. The next few months present an unusual opportunity for orbital study as the system passes through periastron. Two of the reference stars are found to have distances of less than 100 parsecs. Title: Primordial lithium and the standard model(s) Authors: Deliyannis, Constantine P.; Demarque, Pierre; Kawaler, Steven D.; Krauss, Lawrence M.; Romanelli, Paul Bibcode: 1989PhRvL..62.1583D Altcode: We present the results of new theoretical work on surface 7Li and 6Li evolution in the oldest halo stars along with a new and refined analysis of the predicted primordial lithium abundance resulting from big-bang nucleosynthesis. This allows us to determine the constraints which can be imposed upon cosmology by a consideration of primordial lithium using both standard big-bang and standard stellar-evolution models. Such considerations lead to a constraint on the baryon density today of 0.0044<Ωbh2<0.025 (where the Hubble constant is 100h Km sec-1 Mpc -1), and impose limitations on alternative nucleosynthesis scenarios. Title: Evolutionary Models of the Rotating Sun Authors: Pinsonneault, M. H.; Kawaler, Steven D.; Sofia, S.; Demarque, P. Bibcode: 1989ApJ...338..424P Altcode: A new rotating stellar evolution code is developed and applied to the sun. A hydrostatic fully convective premain-sequence model is evolved to the age of the sun. As the model evolves, it accounts for angular-momentum loss via a magnetic wind and angular-momentum redistribution by rotationally induced instabilities. The resulting models have an oblateness in agreement with observed upper limits. The rotation curves show two main features: the outer layers exhibit minimal radial differential rotation, and a rapidly rotating central core is preserved. These basic features persist through a wide range of model parameters. Title: Evidence for a spread in ages among galactic globular clusters: age as the second parameter in NGC 288 and NGC 362. Authors: Demarque, P.; Lee, Y. -W.; Zinn, R.; Green, E. M. Bibcode: 1989asgc.conf...97D Altcode: The differences in the C-M diagrams of NGC 288 and NGC 362 are discussed in the light of recent advances in the understanding of the horizontal-branch (HB), together with theoretical isochrones and CCD observations of their main sequence turnoffs. The authors show that consistency with current stellar evolution and pulsation theory can only be achieved if age (and not helium or oxygen abundance differences) is the second parameter of HB morphology. Title: Is the Sun really a rigid rotator? Authors: Demarque, P.; Guenther, D. B. Bibcode: 1988ESASP.286...99D Altcode: 1988ssls.rept...99D Attention is drawn to observations of surface rotational periods in subgiants which indicate that a deep-seated reservoir of angular momentum must exist in these stars. This interpretation is compatible with theoretical studies of the rotational history of the Sun and sun-like stars, but is in apparent conflict with observations of p-mode splittings in the Sun. In order to understand better the source of this apparent discrepancy, the authors explore the predicted rotational splittings of the several test rotation curves and compare them to the solar data. Finally, they discuss the implications of their findings for solar structure. Title: Primordial lithium and the standard model(s). Authors: Deliyannis, C. P.; Demarque, P.; Kawaler, S. D.; Krauss, L. M.; Romanelli, P. Bibcode: 1988plsm.book.....D Altcode: The authors present the results of new theoretical work on surface 7Li and 6Li evolution in the oldest halo stars along with a new and refined analysis of the predicted primordial lithium abundance resulting from big-bang nucleosynthesis. This allows to determine the constraints which can be imposed upon cosmology by a consideration of primordial lithium using both standard big-bang and standard stellar evolution models. Such considerations lead to a constraint on the baryon density today of 0.0044 < Ωbh2 < 0.025, and impose limitations on alternative nucleosynthesis scenarios. Title: Probing the Sun's Internal Rotation Authors: Demarque, P.; Guenther, D. B. Bibcode: 1988BAAS...20R1009D Altcode: No abstract at ADS Title: GONG Solar Model Workshop Yale Center for Solar and Space Research 1987MAY27-29 Authors: Demarque, P.; Christensen-Dalsgaard, J.; Guenther, D. B. Bibcode: 1988ComAp..12..157D Altcode: No abstract at ADS Title: Isochrone Comparisons Stellar Physics and Implications for Stellar Ages Authors: Demarque, P.; Guenther, D. B.; King, C. R.; Green, E. M. Bibcode: 1988csa..proc..101D Altcode: 1988LDP....10..101D In this paper, the authors address primarily problems of stellar interior physics and their implications for the calibration of stellar ages. Title: On the Bimodal Distrubutions of Horizontal Branches Authors: Lee, Young-Wook; Demarque, Pierre; Zinn, Robert Bibcode: 1988IAUS..126..505L Altcode: New synthetic horizontal branch (HB) models are presented for some globular clusters known to have bimodal HB distributions. Title: Sensitivity of Solar P-Modes to Solar Envelope Structure Authors: Demarque, P.; Guenther, D. B. Bibcode: 1988IAUS..123...91D Altcode: The authors present a study of the effects of diffusion of helium and heavy elements from the convection zone into the radiative layer immediately below the convection zone, on the structure of the solar envelope. A comparison is made of the p-mode oscillation spectrum and the run of the speed of sound in a model with diffusion, in a standard model, and with observation. Title: The Origin of Old Hot Stars in Super-Metal Stellar Systems Authors: Demarque, P.; Pinsonneault, M. Bibcode: 1988ASPC....1..371D Altcode: 1988posh.conf..371D Super-metal-rich (SMR) stars are observed in the bulges of elliptical galaxies and are believed to be a major component of the stellar population in elliptical galaxies. The authors present first results based on evolutionary calculations of SMR stars of low mass. They find that stellar evolution theory predicts the existence of an increased ratio of hot to cool stars in old SMR stellar populations, using standard estimates for mass loss and chemical enrichment. Title: Isochrone Fitting of Color Magnitude Diagrams of Old Star Clusters - Implications for Stellar Ages Authors: King, C. C.; Demarque, P.; Green, E. M. Bibcode: 1988csa..proc..211K Altcode: 1988LDP....10..211K No abstract at ADS Title: Evidence for a spread in ages among galactic globular clusters: age is the second parameter in NGC288 and NGC362 Authors: Demarque, P.; Lee, Y. W.; Zinn, R.; Green, E. M. Bibcode: 1988asgc.conf...97D Altcode: No abstract at ADS Title: Internal Constitution and Oscillation Spectrum of Procyon Authors: Demarque, P.; Guenther, D. B. Bibcode: 1988IAUS..123..287D Altcode: The aim of this paper is to explore theoretically the potential ability of the p-mode and g-mode non-radial oscillation spectrum to throw light on two fundamental problems raised by the authors' first study: Procyon A's evolutionary status, and its mass-luminosity relation. Title: Globular cluster luminosity functions Authors: Demarque, P. Bibcode: 1988IAUS..126..121D Altcode: The use of luminosity functions in determining the helium content, age, and initial mass function of globular clusters, and in testing the stellar structure theory is reviewed. The main sequence turnoff and subgiant phases are examined. The Thomas (1967) peak in the luminosity function and the gap near the base of the giant branch of globular clusters found in the CM are discussed as probes of stellar structure. Research in the area of faint star photometry, dealing with the lower main sequence luminosity function and the interplanetary magnetic field is presented. Title: Synthetic Models of the Horizontal Branch and the Calibration of Globular Clusters Ages Authors: Lee, Y. W.; Demarque, P.; Zinn, R. Bibcode: 1988csa..proc..149L Altcode: 1988LDP....10..149L In this paper, the authors summarize their progress in construction of synthetic models of the horizontal branch from stellar evolutionary tracks. Many of the details of their method are described in an earlier progress report (Lee, Demarque, and Zinn 1987). Title: ICCD Speckle Observations of Binary Stars. III. A Survey for Duplicity Among High Velocity Stars Authors: Lu, Phillip K.; Demarque, Pierre; van Altena, William; McAlister, Harold; Hartkopf, William Bibcode: 1987AJ.....94.1318L Altcode: A survey program to identify binary candidates among high-velocity dwarf stars using the GSU speckle camera has been carried out. The purposes of this study are: (1) to determine the binary frequency of the halo population to provide information on the star-formation processes in the galactic halo; and (2) to eventually derive the orbital elements of the newly discovered binaries. In this paper, the authors report speckle interferometry data that have been obtained and analyzed for a sample of 182 stars. Based on these data, ten stars are found to be binary. The authors find that their data are compatible with a total frequency for high-velocity long-period doubles as large as for low-velocity stars. Distances have been estimated for the ten binary stars using their spectroscopic parallaxes and visual magnitudes. Of these ten stars, all are within 100 pc of the Sun and eight have linear separations <20 AU. Using the mass-luminosity relation and assuming circular orbits, four stars are found to have periods less than 20 yr. These ten candidates will be monitored to determine their orbital elements. Title: The Evolution of Red Clump Stars: Theoretical Sequences Authors: Seidel, E.; Demarque, P.; Weinberg, D. Bibcode: 1987ApJS...63..917S Altcode: Two grids of horizontal-branch evolutionary sequences are presented which extend the grid of Sweigart and Gross (1976) to higher masses and higher metallicities. These two grids were constructed specifically for the study of red clump stars observed in intermediate-age star clusters in the Magellanic Clouds and in the Galaxy. An additional evolutionary track, with a thick hydrogen-burning shell which simulates the effect of a modest amount of mixing from the helium core into the shell at the time of the helium-core flash, is also presented. It is found that a small amount of mixing does not alter in a significant way the standard course of horizontal-branch evolution. Title: Intermediate-Age Core Helium-burning Stars and the Distance to the Magellanic Clouds Authors: Seidel, E.; Da Costa, G. S.; Demarque, Pierre Bibcode: 1987ApJ...313..192S Altcode: The luminosities of the core helium burning stars in six Magellanic Cloud intermediate-age star clusters, whose ages have recently been determined from photometry reaching below the main-sequence turnoff, are compared with the predictions of a new grid of theoretical models. It is found that for the so-called 'short' distance moduli consistent results can be achieved with moderate amounts of mass loss from the main sequence to the core helium burning stage for the older clusters, and with rather more substantial mass loss for the younger objects. For the 'long' moduli, however, the results of the comparison give inconsistent results in that the required masses for the clump stars generally exceed the corresponding turnoff masses, in some cases by several tenths of a solar mass. The results support the adoption of smaller distances to these galaxies. Title: The solar-stellar connection: Internal rotation in low-mass stars Authors: Demarque, Pierre Bibcode: 1987ASSL..137..121D Altcode: 1987isav.symp..121D The problem of the internal rotation of stars with masses and internal evolution similar to the sun is investigated, emphasizing the need to construct models of stellar convection zones which are consistent with full interior models. The affect of the distribution of angular momentum within a star on the formation process in a gas cloud and on the internal evolution throughout the premain-sequence phase and the early postmain-sequence phase is considered. The generation of magnetic fields associated with chromospheric activity via the interaction of rotation with convection is discussed. It is noted that rotationally induced mixing can alter the rate of evolution, thus affecting the luminosity function of star clusters. Title: The revised Yale isochrones and luminosity functions Authors: Green, Elizabeth M.; Demarque, Pierre; King, Christopher R. Bibcode: 1987ryil.book.....G Altcode: No abstract at ADS Title: Evolutionary Models of the Rotating Sun (Invited review) Authors: Sofia, Sabatino; Pinsonneault, Marc; Kawaler, Steven D.; Demarque, Pierre Bibcode: 1987LNP...291..205S Altcode: 1987csss....5..205S This paper reviews current work on the evolution of a differentially rotating solar model. Although we discuss global features of the evolution with rotation in general terms, the specific models described are those computed with the new Yale Rotating Evolution Code (YREC). YREC uses the Kippenhahn and Thomas (1970, KT) formalism as implemented by Endal and Sofia (1976), although the numerical formulation of our code is totally new. Particular calculations that we describe include the effects of different initial total angular momentum, the consequences of varying the properties and magnitude of angular momentum losses by wind torquing, and the consequences of specific composition and angular momentum redistribution mechanisms. This paper is a progress report which points out the complexity of the problem, and the need for a broad-based observational program to solve it. Because the final solution is not yet in hand, we outline the steps that, in our estimation, need to be undertaken in order to make progress. Title: Synthetic horizontal branch models for globular clusters: the luminosity of the horizontal branch and the Oosterhoff effect. Authors: Lee, Young-Wook; Demarque, Pierre; Zinn, Robert Bibcode: 1987fbs..conf..137L Altcode: 1987IAUCo..95..137L; 1987LDP.....8..137L The variation of horizontal-branch (HB) luminosities with metal abundances is analyzed on the basis of HB models synthesized from theoretical HB evolutionary tracks. The focus is on the Oosterhoff effect, as related to period shifts in globular-cluster RR Lyr variables. The construction of the models and the Oosterhoff period groups is explained in detail, and the implications for globular-cluster ages are considered. The ratio of Delta M(bol) (RR) to Delta Fe/H for the HB is calculated as 0.24, slightly steeper than that found by Sandage (1981 and 1982). Title: An Astrometry Explorer Authors: York, D.; Jones, B.; Demarque, P.; Faber, S.; Hughes, J.; Johnston, K.; Lin, D.; Silverglate, P.; van Altena, W. Bibcode: 1986BAAS...18.1012Y Altcode: No abstract at ADS Title: The Stellar Seismology of epsilon Eridani Authors: Guenther, D. B.; Demarque, P. Bibcode: 1986ApJ...301..207G Altcode: This calculated p-mode oscillation spectrum of several stellar models of ɛ Eridani are compared with the observational data of Noyes et al. Specifically, models with different masses, metallicities, and mixing length parameters are constructed, and for each model the low-degree p-mode oscillation spectrum is calculated and its characteristic frequency spacing is compared with the observed value of 172 μHz. Of all the models constructed, the model with a mass of 0.80 M_sun;, Z = 0.02, mixing length parameter α = 1.00, an age of 11.5 Gyr, and a radius of 0.80 R_sun; best fits the observed luminosity, effective temperature, and oscillation data. The characteristic frequency spacing of the oscillation spectrum of this model is 170 μHz. It is concluded that the structure of this model must be very close to the actual physical structure of ɛ Eri. Title: Interior Models for Procyon A: Evolutionary Status and Oscillation Properties Authors: Demarque, P.; Guenther, D. B. Bibcode: 1986LNP...254..187D Altcode: 1986csss....4..187D Evolutionary sequences have been constructed to determine the evolutionary status of Procyon A. We find that Procyon could be in any of the following three evolutionary phases: (a) core hydrogen burning; (b) core exhaustion; (c) hydrogen burning shell narrowing. However, both timescale and luminosity arguments strongly favor possibility (a). Title: The Case of alpha Centauri: Mass, Age, and p-Mode Oscillation Spectrum Authors: Demarque, P.; Guenther, D. B.; van Altena, W. F. Bibcode: 1986ApJ...300..773D Altcode: The authors have investigated the astrometric and physical properties of the α Centauri system, a nearby visual binary which shares a common space motion with the nearest star Proxima Cen. First, the parallax of the binary system is estimated to be 0arcsec.7506±0arcsec.0046 (s.e.). Using this absolute parallax, the masses of α Cen A and B are respectively 1.09±0.01 and 0.90±0.01 M_sun;. Second, the evolutionary status of α Cen A and B is determined with the help of theoretical stellar models. Evolutionary tracks yield the age of the α Cen system as a function of metallicity. The discussion points to a consistent model with an age in the vicinity of 4 - 4.5×109yr. Third, the nonradial oscillations of α Cen A are considered. The p-mode spectrum of the models of α Cen A is compared to that of the Sun and to the observations of Fossat et al. It is concluded that Fossat et al.'s observations are inconsistent with p-mode spectra constructed from standard theoretical models. Title: The luminosity function on the subgiant branch of 47 Tucanae : a comparison of observation and theory. Authors: King, C. R.; Da Costa, G. S.; Demarque, P. Bibcode: 1985ApJ...299..674K Altcode: Photographic photometry has been obtained for 1200 stars in the magnitude range of V = 13.5 to 17.5 in the field of the Galactic globular cluster 47 Tucanae and used to form both a color-magnitude diagram and a luminosity function. No gap at the base of the giant branch of size similar to those recently reported in other clusters was detected, but a prominent peak is present in the luminosity function at a level approximately 0.5 mag below the horizontal branch. This peak is identified with that which results from the passage of the hydrogen-burning shell through the composition discontinuity left by the deepest penetration of the convective envelope of the star. However, the observed peak is nearly 1 mag fainter than the prediction of standard stellar evolution theory for the age and composition of this cluster. The most plausible explanation for this discrepancy is that it results from overshoot at the base of the convective envelope of the stars. An overshoot of approximately one local pressure scale height is sufficient to produce agreement with the observations. Title: Binary Survey Among High Velocity Stars Using GSU Speckle Camera Authors: Lu, P. K.; Demarque, P.; van Altena, W.; Hartkopf, W.; McAlister, H. Bibcode: 1985BAAS...17Q.904L Altcode: No abstract at ADS Title: The Solar-Stellar Connection: II. - Stellar Studies Authors: Demarque, P. Bibcode: 1985BAAS...17..898D Altcode: No abstract at ADS Title: From solar dynamo to terrestrial climate Authors: Sofia, S.; Demarque, P.; Endal, A. Bibcode: 1985AmSci..73..326S Altcode: Recent progress made in measuring and characterizing variations in the global solar energy output are surveyed. The most reliable data on variations are gathered by spacecraft, most recently by Nimbus 7 and the SMM. The longest irradiance decrements are associated with the appearance of large sunspots, particularly at the center of the solar disk. Faculae, bright spots appearing with sunspots, redirect radiation to directions other than perpendicular. Global variations average less than 0.1 percent of the average annual irradiance, although some long-term drift may be occurring. Astrometric data on Mercury transit across the solar disk have, with 250 yr cumulative data, revealed no more than 0.2 arcsec variations in the solar diameter, while eclipse data indicate variations of up to 0.6 arcsec in recent epochs. Other data, however, have connected the diameter variations with magnetic field increases. A deeper understanding of the physics of the solar dynamo will be required to detect the significance of the observed changes relative to irradiance averages. Title: Current Problems in Horizontal Branch Theory - Implications Authors: Demarque, P. Bibcode: 1985cto..conf..268D Altcode: 1985IAUCo..82..268D No abstract at ADS Title: Convective heating of the inner core of red giants prior to the peak of the core helium flash Authors: Cole, P. W.; Demarque, P.; Deupree, R. G. Bibcode: 1985ApJ...291..291C Altcode: The effects of convective overshooting across the temperature inversion in red giant cores have been investigated theoretically, using a two-dimensional and three-dimensional hydrodynamic model of the core helium flash. The model is based on a simple heating algorithm which can be easily incorporated into a variety of stellar evolution codes. On the basis of the numerical results it is found that: (1) convective overshooting led to substantial heating of the material underneath the temperature inversion, producing a smoother temperature profile; (2) up until one week before the time of maximum temperature, interior heating is unimportant, but alters the standard relation of maximum temperature and density at the point of maximum temperature; and (3) overshooting redistributed the mass from the location of maximum temperature. The contribution of internal heating to the subsequent evolution of the core of a red giant is also discussed. Model estimates of the selected properties of core helium flash sequences at the time of maximum temperature are given in a table. Title: Mass Loss on the Blue Horizontal Branch and the Origin of the Subdwarf B Stars Authors: Demarque, Pierre; Eder, Jo-Ann Bibcode: 1985hbuv.proc...91D Altcode: 1985LDP.....6...91D The authors have investigated the effects of mass loss on the evolution of blue horizontal-branch stars. They find that mass loss rates in the range 2.5×10-10 < M < 1×10-9M_sun;/year are needed to explain the existence of the subdwarf B stars. Title: The resonant count diagram and solar g mode oscillations. Authors: Guenther, D. B.; Demarque, P. Bibcode: 1984sses.nasa..349G Altcode: 1984sss..conf..349G Evidence is provided to support the hypothesis that, because of the particular frequency separations of the solar g modes, resonant three-wave interactions stimulate only a selected few g modes. A resonant count diagram was obtained by plotting the total number of possible resonant three-wave interactions or a given beat frequency against the inverse of the beat frequency (the beat period), within a given frequency tolerance. The 1 = 1, 2, 3, 4 g modes calculated by Christensen-Dalsgaard, Gough and Morgan (1979) for a standard model of the Sun were used. The diagram has a significant peak at 160 minutes as well as other peaks at longer periods. The g modes that Delache and Scherrer (1983) tentatively identified from the Crimea-Stanford data were also plotted. These modes were found to correspond with the other peaks in the diagram. This coincidence between the observed g modes and the peaks in the resonant count diagram suggest that the observed g modes do owe their observability to resonant three-wave interactions. Title: An interpretation of the gap on the subgiant branch of several globular clusters Authors: Armandroff, T. E.; Demarque, P. Bibcode: 1984A&A...139..305A Altcode: Four globular clusters whose color-magnitude diagrams show a gap on the subgiant branch are discussed. Standard stellar evolution theory does not predict the existence of a rapid phase of evolution at this point during a star's lifetime. A mechanism is outlined where this rapid evolutionary phase is attributed to the hydrogen-burning shell passing through a discontinuity in chemical composition. This discontinuity is set up by an ad-hoc mixing process. Evolutionary tracks constructed result in a gap crossing time a factor of two smaller than the standard model and almost identical evolution outside the gap. Another possibility, that the gap is caused by a rapid change in atmopsheric structure, is investigated by varying the mixing length in the convection zone. Tracks constructed in this manner are not consistent with the observed gaps. Title: The Revised Yale Isochrones and Luminosity Functions Authors: Green, E. M.; Demarque, P.; King, C. R. Bibcode: 1984BAAS...16..997G Altcode: No abstract at ADS Title: Comparing α Cen with the Sun: Chemical Composition, Mixing-length, Age, and p-Mode Oscillation Spectrum Authors: Demarque, P.; Guenther, D. B. Bibcode: 1984BAAS...16.1001D Altcode: 1984BAAS...16.1000D No abstract at ADS Title: AM. 06 An Astrometry Satellite Authors: York, D. G.; Jones, B.; Faber, S.; Lin, D.; van Altena, W.; Demarque, P.; Hughes, J.; Johnston, K.; Bunner, A. Bibcode: 1984BAAS...16..775Y Altcode: No abstract at ADS Title: The Luminosity Function on the Subgiant Branch of 47 Tuc: A Comparison of Observation and Theory Authors: King, C. R.; Da Costa, G. S.; Demarque, P. Bibcode: 1984BAAS...16..529K Altcode: No abstract at ADS Title: Convective Heating of the Inner Core During the Core Helium Flash Authors: Cole, P. W.; Demarque, P.; Deupree, R. G. Bibcode: 1984BAAS...16..526C Altcode: No abstract at ADS Title: The Subgiant Branch Gap in Several Globular Clusters Authors: Demarque, P.; Armandroff, T. E. Bibcode: 1984BAAS...16..526D Altcode: No abstract at ADS Title: Resonant three-wave interactions of solar g-modes Authors: Guenther, D. B.; Demarque, P. Bibcode: 1984ApJ...277L..17G Altcode: The observed g-mode oscillation spectrum of the sun is compared with a plot of the number of possible resonant three-wave interactions of theoretical g-modes at a given period versus the frequency difference of the coupled modes. A very large peak is found near 160 minutes, corresponding to the well-known 160-minute solar oscillation. Peaks in the curve are also found to coincide with the recently identified g-mode data of Delache and Scherrer (1983). Title: Resonant Three-Wave Interactions and an Application to Solar G Mode Oscillations Authors: Guenther, D. B.; Demarque, P. Bibcode: 1984IAUS..105...67G Altcode: The authors present some initial results of a long-term investigation of resonant three-wave interactions in the solar interior, which indicate that these nonlinear interactions take place in the sun and are, in fact, responsible for the observed g mode spectrum. Title: Helium abundances in globular star clusters Authors: Cole, P. W.; Demarque, P.; Green, E. M. Bibcode: 1983prhe.work..235C Altcode: New results involving the determination of the abundances in galactic halo stars by examining the slopes of the subgiant luminosity functions in globular clusters and the R-method are reported. Theoretical luminosity functions for an age of 16 billion yr are applied to He abundances of 0.20-0.30, with error ranges included for 2000 stars in a 3 magnitude interval above the main sequence turn-off. The R method, comprising the ratio of theoretical horizontal branch (HB) to red giant (RG) lifetimes, is shown to be a function of the He abundance, since an He flash precedes the HB evolutionary phase and is associated with an He abundance of 0.16-0.23, which approaches Big Bang theory predictions of 0.24. Title: Evolution of a population III star of low mass Authors: Guenther, D. B.; Demarque, P. Bibcode: 1983A&A...118..262G Altcode: An evolutionary track for a metal-free star of low mass with parameters (M/M_sun;, X, Z)=(0.90, 0.80, 0.00) has been constructed from the main sequence to the base of the giant branch. It has been found that, although the evolutionary path of the star near the main sequence is similar to that of a star of very low metallicity, its internal structure is not. The zero-metallicity star failed to exhaust its central hydrogen. It maintained a trace amount of hydrogen in the core, through turnoff, up to the base of the giant branch where enough carbon was produced at the center by the triple-alpha reaction to start CN-burning. A convective core was formed which grew so rapidly that the calculations had to be terminated. Possible subsequent evolutionary outcomes are discussed, all of which suggest that compared to low-metallicity stars, the time spent in the giant phase will be sharply reduced. Title: The ages and compositions of old clusters. Authors: Janes, K.; Demarque, P. Bibcode: 1983ApJ...264..206J Altcode: From published color-magnitude diagrams of 23 globular and open clusters the authors have measured the mean color index or absolute magnitude at various points along the principal sequences. By relating these quantities to the analogous parameters derived from theoretical isochrones it is possible to solve simultaneously for age, heavy-element abundance, and helium abundance. It is concluded that there is no correlation of globular cluster age with composition and that the oldest open clusters are about one-half the age of the globular clusters. The evidence suggests that the open clusters have a higher helium abundance than the globular clusters, that the globulars have an age of about 16.6 ± 0.5 Gyr, and that the metal-rich clusters 47 Tuc and M71 have [Fe/H] ≡ -0.9. Title: Convective Heating of the Inner Core in the Core Helium Flash Authors: Cole, P. W.; Demarque, P.; Deupree, R. G. Bibcode: 1982BAAS...14..956C Altcode: No abstract at ADS Title: Effects of a Hydrodynamic Helium Flash on Horizontal-Branch Evolution Authors: Cole, P. W.; Demarque, P. Bibcode: 1982BAAS...14R.878C Altcode: No abstract at ADS Title: The globular cluster metallicity scale - Evidence from stellar models Authors: Demarque, P.; King, C. R.; Diaz, A. Bibcode: 1982ApJ...259..154D Altcode: Theoretical giant branches have been constructed to determine their relative positions for metallicities in the Fe/H ratio range of -2.3 to zero. The models included the following improvements over previous studies: (1) in the convection zone, use of the variable mixing length formalism of Deupree and Varner (1980) in conjunction with Deupree's (1979) horizontally averaged opacity; (2) for the surface boundary conditions, a fit to the grid of model atmospheres of Bell et al. (1976). A relation between the Fe/H ratio and (B-V)o,g based on these models is presented which yields good agreement over the observed range of metallicities for galactic globular clusters and old disk clusters. The metallicity of 47 Tuc and M71 given by this calibration is about -0.8 dex. Title: Nitrogen variations on the main sequence of 47 Tucanae - Implications from stellar structure theory. Authors: Da Costa, G. S.; Demarque, P. Bibcode: 1982ApJ...259..193D Altcode: We present evolutionary calculations of a 0.9 Msun metal-poor star in which the outer layers are artificially mixed throughout the main-sequence lifetime. For the case where the mixed region contains 85% of the mass of the star, evolution toward the subgiant branch is relatively unaffected but no significant enhancement of the surface nitrogen abundance occurs. Alternatively, for the case of deeper mixing, significant surface nitrogen enhancements are produced but the star evolves as a blue straggler and does not become a red giant in a normal way. Thus the observations of large nitrogen abundance variations among main-sequence stars andgiants in the globular cluster 47 Tuc are unlikely to be the result of deep envelope mixing on the main sequence. Mixing at an advanced phase of evolution and mass transfer in close binaries also seem unlikely. We conclude therefore that the most plausible explanation of the observed variations is that they are primordial in origin, that is, they reflect the presence of nitrogen abundance inhomogeneities in the protocluster gas cloud. Title: Metal abundances and ages for some Magellanic Cloud variable stars. Authors: Butler, D.; Demarque, P.; Smith, H. A. Bibcode: 1982ApJ...257..592B Altcode: The Delta-S metal abundance parameter, pulsation mass, and evolutionary age have been determined for 15 Magellanic Cloud field variable stars. It is found that the central region of the SMC is only now reaching the degree of chemical enrichment which the Galaxy attained 10-billion years ago. Both the LMC and SMC contain an old, metal-poor population of halo stars, just as the Galaxy does. In sharp contrast to the case for the Galaxy, however, much of the total SMC chemical enrichment has taken place within the past few billion years. The situation for the LMC is less clear, but the data are not inconsistent with a chemical evolution history similar to that for the SMC. Title: Stellar Physics. (Book Reviews: Physical Processes in Red Giants) Authors: Demarque, Pierre Bibcode: 1982Sci...216Q..46I Altcode: No abstract at ADS Title: Book-Review - Physical Processes in Red Giants - Proceedings - Erice Italy 1980SEP Authors: Iben, I., Jr.; Renzini, A.; Demarque, P. Bibcode: 1982Sci...216...46I Altcode: No abstract at ADS Title: Convection and the Thermonuclear Runaway in the Core Helium Flash Authors: Cole, P. W.; Deupree, R. G.; Demarque, P. Bibcode: 1982BAAS...14R.665C Altcode: No abstract at ADS Title: Models for Horizontal-Branch Stars with Cores Enriched in Carbon and Nitrogen Authors: Demarque, P. Bibcode: 1981apgc.conf..301D Altcode: 1981LDP.....2..301D; 1981IAUCo..68..301D No abstract at ADS Title: A Comparison of Stellar Models Constructed with Different Stellar Evolution Programs Authors: Demarque, P.; Laird, J. B.; Vandenberg, D. Bibcode: 1981apgc.conf..319D Altcode: 1981IAUCo..68..319D; 1981LDP.....2..319D No abstract at ADS Title: CH subgiants and the mixing hypothesis. Authors: Smith, J. A.; Demarque, P. Bibcode: 1980A&A....92..163S Altcode: Some consequences of internal mixing at the helium core flash have been investigated for low mass, Population II stars. On the basis of the luminosity criterion, it seems unlikely that the internal mixing hypothesis can explain the CH subgiants, though it cannot be ruled out for the CH giants or other chemically peculiar giants. A binary origin for the CH subgiants is proposed. Title: Implications of the new globular-cluster metal-abundance scale for the helium abundance in the Galactic halo Authors: Demarque, P.; McClure, R. D. Bibcode: 1980ApJ...242L...5D Altcode: In the adoption of the new abundance scale for globular clusters (Cohen, 1980; Pilachowski et al, 1980) a difficulty in fitting main sequences of 47 Tucanae and other well-studied globular clusters, like M 3, M 5, and M 13 is pointed out. In order to circumvent this problem, a substantial difference in helium abundance must be invoked. Such differences can also cause the very faint turnoff magnitudes found recently by Alcaino and Liller (1980) for NGC 6397 and by Cannon (1974) for Omega Centauri, without the need to assign excessive ages to these clusters. Other consequences of this helium-abundance variation are discussed. Title: Evolution of a 0.9M⊙ with Zero Heavy Element Content Authors: Demarque, P.; Guenther, D. B. Bibcode: 1980BAAS...12..833D Altcode: No abstract at ADS Title: Metal Abundance of Magellanic Cloud Variable Stars Authors: Butler, D.; Demarque, P.; Smith, H. A. Bibcode: 1980BAAS...12..538B Altcode: No abstract at ADS Title: Ages and abundances of globular clusters and the oldest open clusters. Authors: Demarque, P. Bibcode: 1980IAUS...85..281D Altcode: 1980stcl.conf..281D The ages and metal abundances of globular clusters and the oldest of the open clusters are reviewed. On the basis of a fitting of main-sequence turnoffs to standard theoretical isochrones, it is observed that the ages of well observed nearly globular clusters are between 10 and 15 billion years, with age negatively correlated with metallicity, while the ages of the oldest open clusters are near 6-7 billion years and their metallicities are almost as low as the most metal-rich globular clusters. Analysis of horizontal branch morphology compared with stellar models indicates that all globular clusters within 5 kpc of the galactic center have similar ages, consistent with a rapid collapse and chemical enrichment, and that globular cluster further than 12 kpc from the center lie on or below the standard age/abundance relation for nearby clusters. The need for reliable abundance determinations for faint stars and observations of main sequence turnoffs particularly for distance clusters is pointed out. Title: Stellar evolution from the zero-age main sequence. Authors: Mengel, J. G.; Sweigart, A. V.; Demarque, P.; Gross, P. G. Bibcode: 1979ApJS...40..733M Altcode: A consistent set of 247 evolutionary sequences extending from the ZAMS to the red-giant branch is presented for Y from 0.10 to 0.40, Z from 0.00001 to 0.10, and masses of 0.55 to 6.90 solar masses. Each sequence is started from a homogeneous ZAMS model, and almost all are evolved to the base of the red-giant branch. It is shown that: (1) the relative position of the main sequence can be determined as a function of composition; (2) theoretical luminosity functions can be derived from the relative evolutionary time scales; (3) a dip in luminosity sometimes occurs at the base of the red-giant branch and is most pronounced at larger Z values; (4) metal-poor stars evolve farther up along the main sequence before turning off toward the red-giant branch; and (5) the onset of helium burning halts the evolution across the Hertzsprung gap for the most massive and most metal-poor models, so that the star remains blue during its phase of core-helium burning. Title: Theoretical isochrones in the [log g - log Teff]-plane and in the [MV- (B-V)]-diagram. Authors: Ciardullo, R. B.; Demarque, P. Bibcode: 1979DudOR..14..317C Altcode: No abstract at ADS Title: The Evolution of the Integrated Colors of Globular Clusters and Elliptical Galaxies Authors: Ciardullo, R. B.; Demarque, P. Bibcode: 1978IAUS...80..345C Altcode: No abstract at ADS Title: Effects of CNO abundances on the Balmer jump of late-B horizontal-branch stars. Authors: Auer, L. H.; Demarque, P. Bibcode: 1977ApJ...216..791A Altcode: Model stellar atmospheres have been constructed to investigate the effects of the continuous opacity of C 1, N 1, and 0 1 on the Balmer jump of late-B horizontal-branch stars. The large opacity of C i and N 1 in the ultraviolet raises the Balmer continuum through back warming without changing the Paschen continuum. The models provide a natural explanation for the variations in the Balmer jump which have been observed from star to star by 0ke near 12,500 K on the horizontal branch of the globular cluster M92. We conclude that some stars on the horizontal branch of M92 have an atmospheric C and/or N content as much as 1000 times the cluster average. Although primordial chemical inhomogeneities within the cluster cannot be ruled out, the most plausible interpretation for these objects seems to be one in which surface enrichment occurs through mixing from a helium-burning region in the interior during the course of stellar evolution. The possible connection with Zinn's weak G-band stars on the asymptotic branch of M92 is discussed. The effects of C 1 and N 1 opacities may also explain Newell's gap 1 in the (U - B, B - V)diagram for field blue stars in the halo. Subject headings: opacities - stars: atmospheres - stars: horizontal branch Title: A comparison of observations with main-sequence evolutionary models. Authors: Philip, A. G. D.; Demarque, P.; Sweigart, A. V.; Ciardullo, R. B. Bibcode: 1977PASP...89..554P Altcode: A comparison of four-color observations, transformed to the (log g - log ) plane, with new theoretical isochrones for the composition (Y,Z = 0.2, 0.04) shows good agreement between theory and observation in the range from 0.5 to I x i0 years. Ages are derived for seven open clusters which are in accord with previous estimates. Key words: four-color photometry-open clusters- isochrones- cluster ages Title: The Distance Modulus of the Hyades from Stellar Models Authors: Anthony-Twarog, B. J.; Demarque, P. Bibcode: 1977A&A....57..471A Altcode: Summary. A comparison of stellar models to observational data f6r four Hyades binaries is made by demanding agreement of the empirical and theoretical main sequences and mass-luminosity relations. A Hyades distance modulus of 3.34 is obtained consistent with composition parameters (Y; Z) (0.30,0.03). A turnoff mass of 2.1 is deduced from the models, implying a cluster age of5 x 108 years. Key words: Hyades - distance chemical composition Title: The significance of the star clusters NGC 2420 and 47 Tucanae for galactic evolution. Authors: Demarque, P.; McClure, R. D. Bibcode: 1977ApJ...213..716D Altcode: Attention is drawn to the old disk "metal-poor" cluster NGC 2420 and the "metal-rich" globular cluster 47 Tuc, a member of the spheroidal component of the Galaxy, which have approximately the same metallicity index [8(U - B) = 0.11 mag for 47 Tuc and 8(U - B) = 0.10 mag for NGC 2420]. This similarity in metal abundance allows an instructive comparison to be made of the two components of the Galaxy. The color-magnitude (C-M) diagrams of the two clusters reveal differences which cannot be explained in terms of a difference in age alone. We conclude that the two clusters differ in the abundance of chemical elements other than those which contribute directly to the ultraviolet excess, i.e., either helium or the CNO elements. Comparison with theoretical isochrones yields agreement provided that either of two conditions is satisfied: (1) the helium abundance of 47 Tuc is greater than that of NGC 2420; or (2) the CNO content of 47 Tuc is less than that of NGC 2420. The implications of these alternatives in terms of models of galactic evolution are briefly discussed. The above relative fitting procedure yields a difference between the distance moduli of the two clusters of 1.2 mag. The difficulties in obtaining the absolute distance moduli are discussed. Best agreement is obtained for (m - M)47Tuc 13.0, corresponding to an age of about 13 x 10 years. Subject headings: clusters: globular - clusters: open - stars: abundances - stars: evolution Title: Mixing by Convective Overshoot in Population II Red Giants. Authors: Demarque, P.; Law, W. -Y. Bibcode: 1977BAAS....9..338D Altcode: No abstract at ADS Title: Tables of isochrones constructed from theoretical tracks of stellar evolution for ages between 200 and 25000 million years and chemical compositions in the ranges 0.1 ≤ Y ≤ 0.4 and 0.00001 ≤ Z ≤ 0.1. Authors: Ciardullo, Robin B.; Demarque, Pierre. Bibcode: 1977TOYal..33....1C Altcode: Star time-lines and stellar model masses calculated based from observations. Title: Tables of Bolometeric Luminosity Functions, Part II. Computed for ages between 200 and 25000 million years, and chemical compositions in the ranges of 0.1≤Y≤0.4 and 0.001≤Z≤0.1 Authors: Ciardullo, Robin B.; Demarque, Pierre. Bibcode: 1977TOYal..35....1C Altcode: Data tables from observation of stars containing chemical compositions and bolometric luminosity functions. Title: Sensitivity of internal structure to the surface boundary condition. Authors: Demarque, P. Bibcode: 1977saif.conf..137D Altcode: No abstract at ADS Title: Tables of bolometric luminosity functions. Part 1. Computed for ages between 200 and 25000 million years and chemical compositions in the ranges 0.1<=Y<=0.4 and 0.00001<=Z<=0.0004 Part 2. Computed for ages between 200 and 25000 million years and chemical compositions in the ranges 0.1<=Y<=0.4 and 0.001<=Z<=0.1 Authors: Ciardullo, R. B.; Demarque, P. Bibcode: 1977tblf.book.....C Altcode: No abstract at ADS Title: Tables of isochrones. Constructed from theoretical tracks of stellar evolution for ages between 200 and 25000 million years and chemical compositions in the ranges 0.1<=Y<=0.4 and 0.00001<=Z<=0.1 Authors: Ciardullo, Robin B.; Demarque, Pierre Bibcode: 1977tict.book.....C Altcode: No abstract at ADS Title: Stellar Populations in the Disk and Halo of the Galaxy (presented by R. D. McClure) Authors: Demarque, Pierre; McClure, Robert D. Bibcode: 1977egsp.conf..199D Altcode: No abstract at ADS Title: Tables of bolometric luminosity functions. Part I. Computed for ages between 200 and 25000 million years and chemical compositions in the ranges 0.1 ≤ Y ≤ 0.4 and 0.00001 ≤ Z ≤ 0.0004. Authors: Ciardullo, Robin B.; Demarque, Pierre. Bibcode: 1977TOYal..34....1C Altcode: Tables from data during star observation on bolometric luminosity and chemical compositions. Title: Possible Large Carbon and Nitrogen Abundance Variations on the Horizontal Branch of M92 Authors: Auer, L. H.; Demarque, P. Bibcode: 1977ASSL...67...25A Altcode: 1977cia..proc...25A Model stellar atmospheres have been constructed to investigate the effects of the continuous opacity of C I, N I, and O I on the Balmer jump of late-B horizontal-branch stars. The large opacity of C I and N I in the ultraviolet raises the Balmer continuum through back warming without changing the Paschen continuum, while O I has little effect. The models provide an explanation for the variations in the Balmer jump which have been observed from star to star by Oke (1975) near 12,500 K on the horizontal branch of the globular cluster M92. It is concluded that some stars in M92 appear to have an atmospheric C and/or N content between one hundred and one thousand times the cluster average. The same effect of the opacities of C I and N I may also explain Newell's (1973) gap N I in the (U-B)-(B-V) diagram for blue stars in the galactic halo. Title: Possible Effects of Meridian Circulation on the Evolution of Low-Mass Stars Authors: Stetson, P. B.; Demarque, P. Bibcode: 1976BAAS....8Q.319S Altcode: No abstract at ADS Title: The Importance of NGC 2420 and 47 Tuc. for Galactic Evolution Authors: Demarque, P.; McClure, R. D. Bibcode: 1976BAAS....8..333D Altcode: No abstract at ADS Title: On the nature of the bright variables in dwarf spheroidal galaxies. Authors: Demarque, P.; Hirshfeld, A. W. Bibcode: 1975ApJ...202..346D Altcode: Arguments are presented which suggest that the unusual 'bright variables' observed in Draco and other dwarf spheroidals are more massive than the RR Lyrae variables in the same systems. Models of very metal-poor stars on the horizontal branch with masses up to 2 solar masses confirm this interpretation in the sense that both the luminosities and the period-luminosity relation of the models are consistent with the observations of the bright variables. Some implications of the presence of stars about one billion years old in dwarf spheroidal galaxies are briefly discussed. Title: Theoretical isochrones and main sequences for old disk population stars. Authors: Demarque, P.; Gisler, G. R. Bibcode: 1975A&AS...20..237D Altcode: Theoretical isochrones have been constructed for fourteen mixtures in the ranges of chemical compositions (0.05 < Y< 0.45) and (0.01 <z <0.06) and for ages relevant to the old disk population stars. The effects of autoionization absorption as computed by Watson were added to the Cox-Stewart radiative opacities. Comparison with the colour-magnitude diagram of M67 and NGC 188 does not favour supermetallicity, but rather normal abundances. Helium abundance by mass Yin the range of 0.25-0.35 is found. The distance of the Hyades cluster is discussed in terms of main sequence models extended to higher masses. The distance modulus is consistent with the models for a shift of 0.27 mag. from that given by the convergent point method. This shift would be smaller for lower interior opacities (i.e. if we have overestimated the heavy element abundance Z, or alternately if Watson's autoionization opacities are too large). Key words: stellar evolution - disk population - Hyades Title: Models of the sun. Authors: Demarque, P.; Mengel, J. G.; Sweigart, A. V. Bibcode: 1975PhT....28b..71D Altcode: No abstract at ADS Title: Oblateness of solar models with rotating cores Authors: Demarque, Pierre; Mengel, John G.; Sweigart, Allen V. Bibcode: 1974Natur.252..368D Altcode: ROOD and Ulrich1 have discussed the oblateness of solar models with a rotating core. They find a larger oblateness for the same model than we did2. We have looked again into the oblateness calculation and now believe that the correct solution of the equations of Goldreich and Schubert3 gives an oblateness for the final model of our2 sequence C4 in agreement with the value derived by Rood and Ulrich. We also agree with Rood and Ulrich's explanation for the discrepancy. Title: Convective Overshoot Mixing in Old Open Clusters . Authors: Prather, M. J.; Demarque, P. Bibcode: 1974ApJ...193..109P Altcode: A simplified form of overshoot mixing of convective cores in intermediate-mass stars (0. .40 o) is investigated in order to explain the anomalous gap structure of M67. The amount of overshooting is assumed to be proportional to the pressure scale height at the convective core boundary, and the entire region is completely mixed. Evolutionary tracks are computed for both standard and overshoot models, and isochrones are constructed. in the presence of overshoot mixing, the isochrones show (1) a higher gap, (2) a lower stellar number density immediately above the gap, and (3) the persistence of the gap to greater ages. Subject headings: convection - interiors, stellar - open clusters - stellar evolution Title: Helium Inhomogeneities and the Solar Neutrino Problem Authors: Demarque, P.; Mengel, J. G.; Sweigart, A. V. Bibcode: 1974BAAS....6..271D Altcode: No abstract at ADS Title: Erratum: Rotating Solar Models with Low Neutrino Flux Authors: Demarque, Pierre; Mengel, John G.; Sweigart, Allen V. Bibcode: 1974ApJ...187..423D Altcode: No abstract at ADS Title: Yale University Observatory, New Haven, Connecticut. Observatory report. Authors: Demarque, P. Bibcode: 1974BAAS....6..199D Altcode: No abstract at ADS Title: Variable Stars and Evolution in Globular Clusters Authors: Demarque, P. Bibcode: 1974IAUS...59..105D Altcode: No abstract at ADS Title: On the Origin of the Blue Halo Stars Authors: Sweigart, A. V.; Mengel, J. G.; Demarque, P. Bibcode: 1974A&A....30...13S Altcode: Summary. The existence in the halo population of UV-bright stars lying above the horizontal branch in luminosity and blueward of the asymptotic branch is interpreted in terms of the final evolution of three stars with masses of 0.453, 0.51 and 0.60 M0. These stars, representing different possible evolutionary patterns, are consistent with observations of high-luminosity and subdwarf 0 stars. In addition the 0.51 M0 star provides a satisfactory explanation of the supra horizontal-branch stars. The stars of 0.51 and 0.60 M0 undergo a series of helium-shell flashes, leading to large and rapid excursions in the HR diagram. The present computations cannot account for the subdwarf B stars. Difficulties associated with several interpretations of the subdwarf B stars are discussed. Key words: shell flashes - halo stars - supra-horizontalbranch stars - subdwarf B stars - subdwarf 0 stars Title: Solar Rotation and the Neutrino Flux Authors: Demarque, P.; Mengel, J. G.; Sweigart, A. V. Bibcode: 1973NPhS..246...33D Altcode: 1973Natur.246...33D Some possible mechanisms for producing a low neutrino flux from solar models are discussed. Rapid internal rotation is shown to reduce the predicted neutrino flux to the upper limit observed by Davis. Title: Possible Effects of Internal Rotation on Population II Stars. Authors: Demarque, P.; Gross, P. G.; Mengel, J. G. Bibcode: 1973BAAS....5..406D Altcode: No abstract at ADS Title: The Effect of Overshoot Mixing in Old Galactic Clusters. Authors: Prather, M. J.; Demarque, P. Bibcode: 1973BAAS....5..405P Altcode: No abstract at ADS Title: Rotating Solar Models with Low Neutrino Flux Authors: Demarque, Pierre; Mengel, John G.; Sweigart, Allen V. Bibcode: 1973ApJ...183..997D Altcode: The effect of rotation on the structure of the Sun throughout its evolution has been investigated in an effort to explain the low upper limit to the solar neutrino flux measured by Davis. A class of models has been found which produce a predicted neutrino capture rate in the vicinity of 1 SNU. Such models are characterized by a rotating interior which contains initially 0.9 M0 and which is progressively spun down until a rotating remnant core of the order of 0.1 M0. remains in the present Sun. The dependence of the neutrino flux on the mass in the remnant rotating core, the degree of rotation, the assumed law of spindown, and the mixing produced by meridional circulation have been investigated. Finally the oblateness of each rotating model has been calculated. Subject headings: interiors, solar - neutrinos - rotation, solar Title: Rotating Solar Models with Low Neutrino Flux. Authors: Demarque, P.; Mengel, J. G.; Sweigart, A. V. Bibcode: 1973BAAS....5..313D Altcode: No abstract at ADS Title: Do helium inhomogeneities resolve the solar neutrino problem? Authors: Demarque, P.; Mengel, J. G.; Sweigart, A. V. Bibcode: 1973MNRAS.165P..19D Altcode: Detailed solar models do not support the conclusion of Prentice that the presence of a hydrogen-free core can greatly reduce the neutrino flux from the Sun. Title: Nitrogen enrichment due to meridional circulation in main sequence stars Authors: Demarque, Pierre; McClure, Robert D. Bibcode: 1973MNRAS.164P...5D Altcode: Paczyhski has presented arguments in support of the idea that rotating mainsequence stars in the mass range 2-10 M0 should show evidence for carbon depletion and nitrogen enrichment in their envelopes. An attempt to test this theory in terms of the relative CN-strength of K-giant stars tends to support Paczyn'ski's suggestion. Title: Semiconvection and the RR Lyrae Variables Authors: Sweigart, A. V.; Demarque, P. Bibcode: 1973ASSL...36..221S Altcode: 1973IAUCo..21..221S; 1973vsgc.coll..221S No abstract at ADS Title: Evolution from the Main Sequence to the Helium Flash for Population II Stars Authors: Demarque, P.; Mengel, J. G. Bibcode: 1973A&A....22..121D Altcode: Summary. Evolutionary turnoffs from the main sequence for stars of low mass with the composition parameters 0.50> Y> 0 and Z= t0- and t0- are presented and compared with previous results. The evolution of red giants to the helium flash is also investigated in an attempt to determine more reliably: (a) the effect of chemical composition on the position and slope of the giant branch; (b) the dependence on the original value of Y of the core mass and maximum luminosity at the helium flash; (c) the dependence of red-giant lifetimes on the total mass of the star. The position and extent of the luminosity dip discovered by Thomas (1967) is also briefly discussed. Key words: stellar evolution - red giants - globular clusters - population II stars Title: Variable Stars and Evolution in Globular Clusters (review Paper) Authors: Demarque, P. Bibcode: 1973ASSL...36..187D Altcode: 1973vsgc.coll..187D; 1973IAUCo..21..187D No abstract at ADS Title: Remarks on stellar abundances. Authors: Demarque, P. Bibcode: 1973stag.conf...21D Altcode: 1973IAUCo..17...21D No abstract at ADS Title: Yale University Observatory, New Haven, Connecticut. Observatory report. Authors: Demarque, P. Bibcode: 1973BAAS....5..258D Altcode: No abstract at ADS Title: The R-Method for Determining the Helium Abundance of Globular Clusters Authors: Demarque, P.; Sweigart, A. V.; Gross, P. G. Bibcode: 1972NPhS..239...85D Altcode: 1972Natur.239...85D ON the basis of calculations of evolution of Population II stars, Iben1 has shown that the ratio R of theoretical horizontal branch (HB) to red giant (RG) lifetimes (R = tHB/tRG) is a function of their helium abundance. This ratio can also be written where NHB and NRG represent the number of the two kinds of stars in an individual globular cluster. Number counts can then be used to estimate the helium abundance of the cluster. With the help of a theoretical calibration2, Iben et al.3 then determined the helium content of a number of globular clusters for which observations were available4, 5. They found an average value, R~=0.9, which yielded y~=0.27-0.31, a result in good agreement with other estimates of helium abundance in globular clusters. Title: Population II Stars and the Electron-Neutrino Weak Interaction Authors: Demarque, P.; Mengel, J. G. Bibcode: 1972NPhS..239...55D Altcode: 1972Natur.239...55D The theory of Feynman and Gell-Mann1 of weak interactions predicts an electron-neutrino interaction (\emacr ve) (\vmacr2 e) with the same coupling constant as that measured in β-decays. Owing to the lack of experimental verification of this prediction, a number of investigators have searched for possible astrophysical tests of the theory. The interaction cross-section of neutrinos with matter is very low and cosmic neutrinos are extremely difficult to detect directly. For this reason, however, they will escape unimpeded from stars and if produced in large quantities in stellar interiors, can have a marked effect on the total energy budget of some stars and consequently on their evolutionary time-scales. Ruderman2 and Chiu3 have discussed various aspects of this problem. Stothers4 has reviewed the relevant astrophysical data and concluded that so far all evidence confirms the existence of the interaction. He further concluded that g2=g2β × 10°+/-2, where gβ is the β-decay coupling constant. Title: Effects of Semiconvection on the Horizontal-Branch Authors: Sweigart, A. V.; Demarque, P. Bibcode: 1972A&A....20..445S Altcode: Summary. The effects of semiconvection on the horizontal-branch evolution of a 0.60 M0 Population II star with the envelope composition parameters X =0.732 and Z =0.001 are investigated. Following an initial phase of convective-core overshooting the models form a semiconvective zone which at its maximum extent results in the depletion of helium out to a mass value of 0.25 M0. A comparison between sequences computed with and without a treatment of semiconvection indicates that the neglect of semiconvection will (a) reduce the length of the horizontal-branch track by a factor of 1.8 and (b) decrease the horizontalbranch lifetime by a factor of 1.9. A number of additional horizontal-branch sequences including a treatment of semiconvection are presented over the mass range from 0.51-0.65 M0. These sequences suggest that a mass spread on the order of 0.05 M0 would be sufficient to produce a dispersion of 0.30 in log Teff over the log Teff interval from roughly 3.70-4.15. Key words: stellar evolution - semiconvection - honzontal-branch - Population II stars Title: Theoretical Isochrones for Disk Population Stars. Authors: Demarque, P.; Gisler, G. Bibcode: 1972BAAS....4..326D Altcode: No abstract at ADS Title: Some Uncertainties on Models for Horizontal-Branch Stars (These papers are represented by abstracts) Authors: Demarque, Pierre; Mengel, John G. Bibcode: 1972teps.proc...27D Altcode: No abstract at ADS Title: A Possible Explanation of the Gaps on the Giant Branch of M15 Authors: Demarque, Pierre; Mengel, John G.; Sweigart, Allen V. Bibcode: 1972ApJ...173L..27D Altcode: It is conlectured that fast rotating cores in the red giants of MIS might explain the possible existence of gaps on the giant branch of the cluster. In going outward through the transition layer between the rotating core and the nonrotating envelope the temperature gradient dT/dr will increase abruptly. As the hydrogen-burning shell traverses this transition layer, a sudden increase in the luminosity might therefore be expected. A numerical simulation of this phenomenon has confirmed this prediction. Some implications of this hypothesis on the evolution of Population II stars are briefly discussed. Title: Advanced Evolution of Population II Stars. III. Some Uncertainties in Horizontal-Branch Models Authors: Demarque, Pierre; Mengel, John G. Bibcode: 1972ApJ...171..583D Altcode: Evolution from the zero-age horizontal branch is investigated for stars with the chemical composition parameters (X, Z) = (0.75, 10- ) and masses 0.50, 0.56, and 0.60 M0. For the 0.60 M0 models, the effects of semiconvection and of the, choice of interpolation between opacity tables are studied. It is found that increasing the free-free opacities in the region enriched in carbon and oxygen affects the evolution markedly, yielding longer blue loops and a horizontal branch thicker in luminosity. Semiconvection, which has the effect of increasing the size of the convective core and of forming an extensive region of varying chemical composition, also leads to longer blueward loops and longer times spent on the horizontal branch as opposed to the asymptotic branch. Astronomical implications are briefly discussed. Title: Yale University Observatory, New Haven, Connecticut. Observatory report. Authors: Demarque, P. Bibcode: 1972BAAS....4..202D Altcode: No abstract at ADS Title: Concluding Remarks Authors: Demarque, P. Bibcode: 1972ade..coll...21D Altcode: 1972IAUCo..17...21D No abstract at ADS Title: Evolution Near the Main Sequence Authors: Demarque, P. Bibcode: 1972stev.conf..107D Altcode: No abstract at ADS Title: Effects of Semi-convection on the Horizontal Branch. Authors: Demarque, P.; Sweigart, A. Bibcode: 1971BAAS....3..479D Altcode: No abstract at ADS Title: Is There a Black Hole in ɛ Aurigae? Authors: Demarque, Pierre Bibcode: 1971Natur.230..516D Altcode: THE suggestion has been made1,2 that the secondary component of the eclipsing binary system ɛ Aurigae is a black hole, the result of a stellar implosion. Current interest in the physics of black holes3 and the research by Wilson4 on the dynamics of the supernova phenomenon, which indicates that collapsed objects may be quite common in the galaxy, make Cameron's1 suggestion particularly intriguing. Title: Advanced Evolution of Population II Stars. I. Red Giants and the Helium Flash Authors: Demarque, Pierre; Mengel, John G. Bibcode: 1971ApJ...164..317D Altcode: Models for red-giant stars of Population II have been constructed with emphasis on the determination of the critical mass of the helium core at the onset of the helium flash and on the subsequent development of the flash and core-cooling phase which precedes horixontal-branch evolution. Stars with masses 0.85, 0.60, and 0.502 and compositions (X, Z) = (0.75, 0.001) and (0.75, 0.0001) have been considered. The effects of including relativistic degeneracy have been investigated, together with the effects of the inclusion of neutrino losses due to weakAnteraction processes. No mixing between the helium core and the hydrogen-rich envelope is found in any of the models considered. The dependence of the mass of the helium core on total mass and on Z has been determined and found to be very weak in both cases. Finally, a tentative explanation for Barnard 29 is suggested. Title: Advanced Evolution of Population II Stars. II. The Horizontal Branch Authors: Demarque, Pierre; Mengel, John G. Bibcode: 1971ApJ...164..469D Altcode: Evolutionary tracks for post-helium-flash and horizontal-branch evolution are presented for masses 0.85,0.60,0.559, and 0.502 . The surface chemical composition is (X, Z) = (0.733,0.001) corresponding to (X, Z) = (0.75, 0.001) on the main sequence. Comparison with Newell's study of blue horizontalbranch stars yields agreement with his Group A for 0.60 . The very blue stars in Newell's Groups B and C, so far unexplained, are seen to have masses ranging from 0.50 to 0.56 . Mass loss and other less likely explanations of the low masses found are briefly discussed. Title: A Method for the Simultaneous Determination of Helium and Metal Abundances in Old Open Clusters Authors: Demarque, Pierre; Heasley, James N., Jr. Bibcode: 1971ApJ...163..547D Altcode: A method for the simultaneous determination of the helium content V, and metal content Z, for stars in old open clusters is described. The method is independent of the distance modulus of the cluster and of color corrections. It is found that the maximum luminosity during the phase of evolution in which hydrogen is burned in a thick shell is proportional to V for fixed Z. By using this effect and the analysis of the dependence of the gap width upon V for fixed Z determined by Aizenman, Demarque, and Miller it is possible in principle to determine V and Z for an old open cluster. In order to calibrate this new effect a series of theoretical evolutionary tracks was computed for stars evolving from the main sequence to the base of the giant branch for different chemical compositions. Problems of observational comparison are considered. Title: Yale University Observatory, New Haven, Connecticut. Observatory report. Authors: Demarque, P. Bibcode: 1971BAAS....3..432D Altcode: No abstract at ADS Title: The Early Evolution of Population II Stars. II Authors: Demarque, Pierre; Mengel, John G.; Aizenman, Morris L. Bibcode: 1971ApJ...163...37D Altcode: Evolutionary tracks for Population II stars of low mass are presented. The chemical composition lie: in the range 0.50 < X <0.999 and 10- <Z < 10- . Isochrones suitable for comparison with color magnitude diagrams of globular clusters have been constructed. The results are compared with earlie work, in particular with models by Simoda and Iben and Iben and Rood. Discrepancies are everywber less than 12 percent, except for the stellar radii which are subject to larger uncertainties due to th hydrogen convection zones. The strong dependence on Z of the turnoff luminosity for very low Z foun by Simoda and Iben is confirmed qualitatively. Title: Red giant evolution to the heliumflash of a super-metal-rich star Authors: Demarque, Pierre; Heasley, James N. Bibcode: 1971MNRAS.155...85D Altcode: An evolutionary sequence is presented for a star of 1 19 M , with composition parameters (X, Z) = ( , ). The evolution was followed from the main sequence to the onset of convection in the core during the helium flash. The resulting core mass at the helium flash, Me = M is smaller than those found for Population II stars with the same helium abundance. No mixing between hydrogen-rich envelope and the helium core can be expected due to the non-central development of the flash. The clumping of stars on the giant branch of old open clusters, discussed by Cannon, is considered. Title: A Method for the Simultaneous Determination of Helium and Metal Abundances in Old Galactic Clusters Authors: Demarque, Pierre; Heasley, James N., Jr. Bibcode: 1970BAAS....2Q.308D Altcode: No abstract at ADS Title: Early Evolution of Population II Stars Authors: Demarque, Pierre; Mengel, John G. Bibcode: 1970BAAS....2R.308D Altcode: No abstract at ADS Title: Stellar Evolution through the Helium Flash Authors: Demarque, Pierre; Mengel, John G. Bibcode: 1970BAAS....2S.308D Altcode: No abstract at ADS Title: Yale University Observatory, New Haven, Connecticut. Report 1968-1969. Authors: Demarque, P. Bibcode: 1970BAAS....2..166D Altcode: No abstract at ADS Title: ON the Color-Magnitude Diagrams of NGC 2360 and NGC 3680 Authors: Demarque, Pierre; Miller, Richard H. Bibcode: 1969ApJ...158.1037D Altcode: Eggen's recent observations of the color-magnitude diagrams of the two galactic clusters NGC 2360 and NGC 3680 are interpreted by the method previously applied to M67 and NGC 188 The width of the gap in NGC 2360 is the same as that in M67, implying similar chemical composition. The gap in NGC 3680 is significantly wider, suggesting a much higher helium abundance. NGC 3680 may be a `super-metal-rich" cluster Title: ON the Chemical Composition and Age of NGC 188 Authors: Demarque, Pierre; Schlesinger, Barry M. Bibcode: 1969ApJ...155..965D Altcode: Stellar models for a wide range of chemical compositions are constructed in an attempt to explain the presence of a gap above the main sequence in the C-M diagram of NGC 188. The procedure of fitting the theory to the observations is described. Agreement with observation is obtained for a high value of the metal content, Z ~ 0.07. The corresponding age for the cluster is of the order of 7 X 1O~ years Title: ON the Interpretation of the Color-Magnitude Diagrams of M67 and NGC 188 Authors: Aizenman, Morris L.; Demarque, Pierre; Miller, R. H. Bibcode: 1969ApJ...155..973A Altcode: A method for determining the helium content Y, for a given Z, of old galactic clusters is described. This method could be of considerable importance since it is independent of the distance modulus of the cluster and of color corrections such as those due to interstellar reddening and differential line blanketing. It is found that V is proportional to the width of the gap of the C-M diagram just above the main sequence In order to calibrate this effect, a number of theoretical evolutionary tracks was obtained from the main sequence to beyond the hydrogen-exhaustion phase for different chemical compositions. A sta- tistical analysis of the data of Eggen and Sandage was made which confirms the presence of such a gap in M67 and NGC 188 It shows that the gap width has a well-defined value equal to 0.167 ± 0.010 mag in M67. This last result yields V = 038 ± 0.02 at Z = 0 03 and V = 0.28 ± 0.02 at Z = 0.06. Higher V-values would be required if Z is as low as 001. The corresponding ages for M67 are about 3.5 X 1O~ years for V = 0.38, Z = 0.03, and 2 6 X 1O~ years for V = 0 28, Z = 0.06. In NGC 188, the gap, al- though present, does not appear to be fully developed, which leaves some uncertainty as to its chemical composition if this method is used Title: Some Uncertainties in Population II Stellar Models Near the Main Sequence Authors: Demarque, Pierre; Hartwick, F. D. A.; Naylor, M. D. T. Bibcode: 1968ApJ...154.1143D Altcode: Stellar models for Population II stars near the main sequence, computed in several different ways, are compared. It is found that, although there are considerable uncertainties in the radii of such models, the bolometric magnitude is nearly constant for a given age, mass, and chemical composition Title: Some Current Problems in the Theory of Late-Type Stars Authors: Demarque, Pierre Bibcode: 1968AJ.....73..669D Altcode: Present uncertainties in the position of the main sequence for Population I and Population II stars, and in the ages and chemical composition of old galactic clusters and globular clusters are discussed in terms of galactic evolution and current views in cosmology. Title: Interpretation of the C-M Diagrams of M67 and NGC 188 Authors: Aizenman, Morris L.; Demarque, Pierre; Miller, R. H. Bibcode: 1968AJS....73S.161A Altcode: A method for determining the helium content Y, for a given Z, of old galactic clusters is described. This method is independent of differential line blanketing and interstellar reddening. It is found that Y is proportional to the width of the gap of the C-M diagram just above the main sequence. In order to calibrate this effect, a number of theoretical evolutionary tracks was obtained from the main sequence to beyond the hydrogen exhaustion phase for different chemical compositions. A statistical analysis of the data of Eggen and Sandage (1964) was made which confirms the presence of such a gap in M67 and NGC 188 It shows that the gap width has a well-defined value equal to 0.167~0.010 mag in M67. This last result yields Y=0.38~0.02 atZ=0.03 and Y=0.28~0.02 at Z=0.06. Higher Y values would be required if Z is as low as 0.01. The corresponding ages for M67 are about 3.5X109 yr for Y=0.38, Z=0.03, and 2.6X109 yr for Y=0.28, Z=0.06. In NGC 188, the gap, although present does not appear to be fully developed, leaving some uncertainty as to its chemical composition if this method is used. Title: The Distance Modulus of Hyades: Distances in the Universe and Stellar Interiors Authors: Demarque, Pierre Bibcode: 1967ApJ...150..943D Altcode: It is shown that Hodge and Wallerstein's recent suggestion of a revision in the Hyades distance modulus does not remove the difficulties previously encountered by Iben and Bodenheimer in comparing the Hyades binaries with stellar interior models The problem seems to lie with the individual parallaxes attributed to each binary system It is also pointed out that a change in the Hyades distance modulus would not necessarily affect other distances in the Universe. Title: Dependence of the ^8{B} Solar Neutrino Flux on Heavy Element Composition Authors: Bahcall, John N.; Cooper, Martin; Demarque, Pierre Bibcode: 1967ApJ...150..723B Altcode: No abstract at ADS Title: Hyades Distance Modulus and Stellar Interiors. Authors: Demarque, Pierre Bibcode: 1967AJ.....72S.792D Altcode: It is shown that Hodge and Wallerstein's recent suggestion (Publ. Astron. Soc. Pacgic 78, 411,1966) of a revision in the Hyades distance modulus does not remove the difficulties previously encountered by Iben (Astrophys. J. 138, 452,1963) and Bodenheimer (Astrophys. J. 142, 451,1965) in comparing the Hyades binaries with stellar interior models. The problem seems to lie with the individual parallaxes attributed to each binary sytsem. It is also pointed out that a change in the Hyades distance modulus would not necessarily affect other distances in the universe. Title: The Early Evolution of Population II Stars Authors: Demarque, Pierre Bibcode: 1967ApJ...149..117D Altcode: Results of calculations of stellar evolution from the main sequence are presented for a range of masses and chemical compositions applicable to subdwarfs and globular-cluster members Twenty-two evolutionary tracks consisting of more than 770 models were obtained for the following compositions X = 0 999, 0 90, and 0 75 for Z = 0001 and X = 0 99 and 0.76 for Z = 0 01. These tracks were used to construct isochrones in the M i4og Teff diagram. It is emphasized that observed subdwarfs should not be compared with main-sequence models but rather with isochrones corresponding to their age and chemical composition. The slope of the observational data then favors a "normal" helium abundance Other evidence, however, suggests the possibility of variations in helium content from one Population II object to another. Uncertainties in the radii of the stellar models in this region of the H-R diagram are discussed, and it appears that the only fully reliable coordinate in the H-R diagram to be obtained from the theory is the value of at the turnoff point. No gap above the turnoff point similar to the one observed in M67 and confirmed by Population I models is predicted in globular-cluster color-magnitude arrays. Title: Hydrogen Convection Zones and Stellar Rotation Authors: Demarque, P.; Roeder, R. C. Bibcode: 1967ApJ...147.1188D Altcode: No abstract at ADS Title: The Hydrogen and Helium Content of Extreme Population i Objects Authors: Percy, John R.; Demarque, P. R. Bibcode: 1967ApJ...147.1200P Altcode: No abstract at ADS Title: The Early Evolution of a Star of 1.2 Solar Masses Authors: Hallgren, E. L.; Demarque, P. R. Bibcode: 1966ApJ...146..430H Altcode: Evolutionary models for a star of 1.2 Mo are constructed. The initial composition is X = 0.67 and Z = 0.03 with a mixing length equal to the pressure scale height. The star is evolved through the stages of hydrogen depletion, gravitational contraction, burning of the shell source, and a second gravitational contraction. The evolutionary track, in its main features, is found to be more similar to that of a 2.0 Mo star than to that of a 1.0 Mo star. Title: Solar Evolution and Brans-Dicke Cosmology Authors: Roeder, R. C.; Demarque, P. R. Bibcode: 1966ApJ...144.1016R Altcode: The evolution of the Sun from the zero-age main sequence has been studied using the assumption that the gravitational constant, G, is time-dependent and is given by the flat-space solution of the Brans-Dicke theory. The effects of the choice of the parameters co and H upon the evolutionary track are shown. It is concluded that a good fit to the present Sun in terms of Mbel and log Teff can be found for reasonable choices of the above parameters and chemical composition. The results obtained here also imply that the neutrino flux from the Sun could be appreciably different from that predicted by Ezer and Cameron without conflicting with the Brans-Dicke theory. Title: Evolution of Stars Near One Solar Mass Authors: Demarque, P. Bibcode: 1966stev.conf..231D Altcode: No abstract at ADS Title: The Upper Main Sequence Authors: Morris, Stephen C.; Demarque, Pierre Bibcode: 1966ZA.....64..238M Altcode: No abstract at ADS Title: On Fernie's Period-Radius Mass Relation for Variable Stars. Authors: Demarque, Pierre; Percy, Jojn R. Bibcode: 1965ApJ...142.1082D Altcode: A counterpart to Fernie's period-radius-mass relation for variable stars is obtained applying the classical theory of adiabatic pulsation to models for giant stars. The expression, which is to be regarded as approximate, is based on models with masses ranging from 1.2 to 20 solar masses. No attempt was made to study the physical processes responsible for the pulsation. Title: The Evolutionary Status of β Cephei Stars. Authors: Demarque, Pierre; Percy, John R. Bibcode: 1965AJ.....70S.136D Altcode: Use is made of the evolutionary tracks for stars of 10.1 and 15. SM presented at this Meeting by Demarque, Morris, and Percy. Following Schmalberger (Astrophys. J. 132, 591, 1960), the coincidence in the H-R diagram of the phase of hydrogen exhaustion in the core and the position of the p Cephei stars is emphasized. The effective ratio of specific heats in our models is found to be close to but somewhat below the value of 1.6 found by Chandrasekhar and Lebovitz (Astrophys. J. 136, 1105, 1962) to explain the nonradial pulsation models of p Canis Majoris. Changes in the period of adiabatic pulsation with time are also investigated and compared with observation. Title: Solar Evolution and Cosmology. Authors: Roeder, R. C.; Demarque, P. R. Bibcode: 1965AJ.....70S.689R Altcode: Investigations of solar evolution in the flat space model universe of Brans and Dicke [where the gravitational constant is a function of epoch given by G=G0(t0/t)n, n=2/(4+3co)] have been carried out for ~=4, 6, 8, and a Hubble constant of 75 and 100 km/sec/Mpc. Evolutionary tracks covering an interval of 4.5 x 1 0~ yr have been calculated using Larson and Demarque's version (Astrophys. J. 140, 524, 1964) of the Henyey method for various solar compositions having Z--0.03. The results show that the effect of increasing is to change the shape of the evolutionary track in the H-R diagram from a steep downward line-similar to that found by Pochoda and Schwarzschild (Astrophys. J. 139, 587, 1964) for co=2-tO a U-shaped curve and ultimately (as co~ co) to the conventional upward curve. It also appears that increasing the Hubble constant from 75 to 100 km/sec/Mpc makes relatively little difference in the composition needed to fit the present sun, but has a drastic effect on the shape of the evolutionary track. Title: Main Sequence and Early Evolutionary Models for Moderately Massive Stars. Authors: Morris, Stephen C.; Demarque, Pierre; Percy, John R. Bibcode: 1965AJ.....70Q.145M Altcode: Main sequence models have been constructed for stars of composition X=0.67, Z=0.03, with masses ranging from 1.4 to 15.9 solar masses. The method used is that described by Larson and Demarque (Larson, R. B., and Demarque, P. R., Astrophys. J. 140, 524, 1964), modified to allow for radiation pressure. The energy source is hydrogen burning, using the formulas given by Reeves (Reeves, H., Stellar Energy Sources, Institute for Space Studies Preprint). The opacity law used is a Kramers'type formula plus electron scattering, the latter being dominant for more massive stars. Radiative zero boundary conditions are assumed. The resulting models have been compared with the observed mass-luminosity relation and with the zero- age main sequence, after transforming M1101, log Te to Mv, (B - V). The agreement is entirely satisfactory, considering the uncertainties in the transformation. Models of 10.1 and 15.5 solar masses have been evolved to the stage of core exhaustion of hydrogen. Evolution proceeds in the direction of increasing Mv and (B- V) until core exhaustion is approached; then (B- V) decreases again, while Mv decreases slightly. This turning point is reached in 17.6 and 10.2 million years for stars of 10.1 and 15.5 solar masses, respectively. A comparison has been made with an evolutionary track calculated for a star of 15.6 solar masses, with X=.90, Z=.02 (Sakashita, S., Ono, Y., and Hayashi, C., Progr. Theoret. Phys. 21, 315,1959; Hayashi, C., and Cameron, R. C., Astrophys. J. 136, 166, 1962). The shape of this track compares well with that of our star of 15.5 solar masses, but the positions in the color-magnitude diagram and the evolutionary time scales are different, as is expected, owing to the different chemical compositions. Title: An Application of Henyey's Approach to the Integration of the Equations of Stellar Structure. Authors: Larson, R. B.; Demarque, P. R. Bibcode: 1964ApJ...140..524L Altcode: A method is described for the numerical solution of the equations of stellar structure by a technique especially designed for the solution of boundary-value problems. The numerical solution incorporates the boundary conditions from the start and eliminates the necessity for the trial-and-error calculations involved in many previous methods. Model-atmosphere calculations and mixing-length theory can readily be incorporated for accurate treatment of convective envelopes, and the method is capable of treating more or less exactly the energy generation and opacity laws and the equation of state for partial degeneracy. In addition, the method is well suited for the computation of evolutionary sequences of models. The method has been successfully used at the University of Toronto for a number of purposes, including the computation of evolutionary tracks from the main sequence for the purpose of estimating the ages of old star clusters. Title: A Series of Solar Models. Authors: Demarque, P. R.; Percy, J. R. Bibcode: 1964ApJ...140..541D Altcode: Models for the Sun are constructed, assuming a solar age between 4 5 and 5 X 10' years, and the hydrogen content corresponding to metal abundances between Z = 0 02 and Z = 0.04 are determined The mixing length yielding the present solar radius is also determined in terms of the pressure-scale height in the outer convection zone. Title: The Age of Galactic Cluster NGC 188. Authors: Demarque, P. R.; Larson, R. B. Bibcode: 1964ApJ...140..544D Altcode: The early evolution of lower main-sequence stars of Population I (composition parameters X = 0.67, Z = 0.03) is calculated on an IBM 7090 computer using the entirely automatic method of Larson and Demarque (1964). The agreement between theory and Sandage's color-maguitude diagram of NGC 188 is good and yields an age of 9- 10 X 10' years for that cluster. From another evolutionary track for X = 0 75 and Z = 0.01, the age of NGC 188 is estimated at 12 X 10' years, but it is found in this case that agreement with observation is not as satisfactory as for composition (X = 0 67, Z = 003). Title: Ages of Old Clusters. Authors: Demarque, P. R.; Larson, R. B. Bibcode: 1964AJ.....69R.136D Altcode: The ages of old star clusters are determined from theoretical evolutionary tracks for stars around one solar mass, using a method similar to that of Henyey et al. (Astrophys. J. 129, 628,1959). For a given mass and composition, the calculations, performed with the IBM 7090 of the University of Toronto, are entirely automatic, including the construction of the hydrogen convection zone on the outside. The age of the old Population I star cluster NGC 188 is found to be between 9 and lOX 10~ yr, under the assumption that X =0.67 and Z =0.03. Population II stars, for which Z = 0.001, have an uncertain hydrogen content and calculations were performed for X=0.999, X=0.90 and X=0.75, yielding corresponding ages for the globular clusters M3, MS, M13 between approximately 25X109 and 17X109 yr. Title: Models for Red Giant Stars. I. Authors: Demarque, Pierre; Geisler, John E. Bibcode: 1963ApJ...137.1102D Altcode: The evolution of population II stars into the red giant region of the H-R diagram is investigated, with a series of quasi-stationary models. The main features of the models are the following: a) The central region forms an isothermal partially degenerate core of helium. Both the pressure of the partially degenerate electrons and that of the non-degenerate nuclei are taken into account. The mass of the helium core increases as the star evolves b) The energy generation is principally due to the carbon-nitrogen cycle. Also included are the proton- proton chains through He3(He3, 2p)He4 and He3(a, )Be7( -, p)Li7(p, He4. Both C-N cycle and p-p chains are concentrated in a shell surrounding the helium core of the star. c) The opacity calculations of Keller and Meyerott are used. d) The outer boundary conditions are obtained from a gray model atmosphere. The structure of the convection zone assumes a mixing length equal to the local pressure scale height. In the result the slope of the giant branch is satisfactorily reproduced. The effect of the mass on the evolutionary track is investigated with stars of 1.0 and 1.2Mo. The effect of the chemical composition is also studied, metal content and helium content being varied independently. At constant Y (Y = 0), Z = 0, Z = 0.001, and Z = 0.01 evolutionary tracks are compared. At constant Z (Z = 0.001), Y = 0, and Y = 0 25 cases are similarly compared. Agreement with observation, for various metal contents is reasonable. There is no way to check the effect of helium content at the present time. Finally, uncertainties in our knowledge of bolometric corrections and (B - V) colors for a given effective temperature are pointed out. Title: Evolution of Population II Stars. Authors: Demarque, Pierre Bibcode: 1962AJ.....67Q.270D Altcode: The evolution of Population II stars into the red-giant region in the H-R diagram is investigated following a method recently used by Kippenhahn, Temesrary, and Biermann (Z. Astrophys. 46, 257, 1958) who emphasized the importance of the structure of the hydrogen convection zone to determine the radius of red giants, and consequently their position in the H-R diagram. (1) The central region forms an isothermal partially degenerate core of helium (Hoyle and Schwarzschild, Astrophys. J. Suppl. 2, 1, 1955). Both the pressure of the degenerate electron gas and the non- degenerate nuclei are taken into account. The mass of the core increases as the star evolves. (2) The energy generation is due to both proton- proton chain and carbon-nitrogen cycle. It is concentrated in a shell surrounding the helium core of the star. (3) The opacity is that given by Keller and Meyer- ott (Astrophys. J. 122, 32, 1955). (4) The outer boundary conditions are obtained from a model atmosphere. The structure of the hydrogen convection zone is calculated assuming the mixing length to be of the order of the local pressure scale height. The main purpose of this work is to investigate the differential effects of various chemical compositions on the evolutionary track. Calculations have been performed for stars of 1.2 and 1.0 solar masses. The results are compared to previous models and cluster color-magnitude diagrams. Title: Models for Lower Main-Sequence Population II Stars. Authors: Demarque, Pierre Bibcode: 1961ApJ...134....9D Altcode: Metal-deficient lower main-sequence stellar models by the author (Ap J, 132,366) are reconstructed by making a new assumption regarding the structure of the hydrogen convection zone. The mixing length 1 is taken to be 1 = 2H, instead of 1 = H as previously assumed The position of the resulting models in the N-R diagram is little changed, although, as expected, a shallow region of the hydrogen convection zone is considerably affected. Title: Interior models for subdwarf stars Authors: Demarque, Pierre Bibcode: 1960AJ.....65..396D Altcode: No abstract at ADS Title: The Structure of Population II Stars. Authors: Demarque, Pierre Bibcode: 1960ApJ...132..366D Altcode: Interior models for metal-deficient stars are constructed for values of Z = 0.001, Z = 0.005, and Z = 0.01, for masses ranging from 0.6 to 1.0 solar mass, and hydrogen content X varying from 0.75 to 0.99 by weight. The models have a structure similar to that of red dwarf stars, investigated by Osterbrock in 1953. They have a radiative core and a convective envelope in adiabatic equilibrium. The energy production is due to the proton-proton reaction, and Keller and Meyerott's tables (1955) are used for the opacity. In order to know the depth of the convective zone and therefore the position of the star in the Hertz- sprung-Russell dia,gram, the outer boundary conditions as given by the physical conditions in the subphotospheric layers must i?e determined. In the atmosphere of stars with effective temperatures in the vicinity of the sun's, the temperature gradient does not approach the adiabatic `gradient as soon as convection sets in; there is an intermediate zone where one bas to take into account the effect of the super- adiabatic gradient. Model atmospheres are constructed, making use of the theory of the mixing length as developed by Biermann. Both cores and atmospheres were integra'ted with the help of the University of Toronto IBM 650 computer. Integrations for adiabatic envelopes' have been given by Ost'erbrock (1953). The resulting models' enable `is to establish clearly the respective positions of main sequences for stars of various compositions in the theoretical Hertzsprung-Russeli diagram. It is found that the shifts in position between main sequences of different chemical contents do not exceed a few tenths of a magnitude, even in cases of extreme metal deficiency. This is in good agreement with recent observational results. The massluminosity law, nevertheless, is not unique for stars with different chemical abundances. Title: The Structure of Subdwarf Stars. Authors: Demarque, Pierre Raymond Bibcode: 1960PhDT.........2D Altcode: No abstract at ADS Title: The structure of subdwarf stars. Authors: Demarque, Pierre Bibcode: 1959AJ.....64..327D Altcode: It is generally accepted that the subdwarfs are characterized by a low metal abundance. The present investigation indicates that the helium content also plays a role in differentiating among subdwarfs. Interior models for subdwarf stars were constructed assuming a low heavy-element content, i.e., Z = 0.001, for masses ranging from 0.6 to 1.0 solar mass, and hydrogen content varying from 0.75 to 0.999 by weight. The models have a structure similar to that of red dwarf stars, investigated by Osterbrock: they have a radiative core and a convective envelope in adiabatic equilibrium. The energy production was assumed to be due to the proton-proton reaction, and Keller and Meyerott's tables were used for the opacity. In order to determine the depth of the convective zone, and therefore the position of the star in the H-R diagram, the outer boundary conditions as given by the physical conditions in the subphotospheric layers must be determined. In the atmospheres of stars with effective temperatures in the vicinity of the sun's, the temperature gradient does not approach the adiabatic gradient as soon as convection sets in. There is therefore an intermediate zone where one has to take into account the effect of the superadiabatic gradient. Such model atmospheres were constructed, making use of the theory of the mixing length as developed by Biermann. Both cores and atmospheres were integrated with the help of the University of Toronto IBM 650 computer. The resulting models are situated in the sub- dwarf region of the theoretical H-R diagram. One notices that for a given mass the helium content strongly affects the radius and therefore the effective temperature, in the sense that abundant helium makes for a smaller star with higher surface temperature. Unfortunately, comparison with observation is difficult because the actual position of observed subdwarfs in the L log - log Te diagram is still uncertain. The L0 results can be used to estimate the masses and helium content of stars in globular clusters. David Dunlap Observatory Richmond Hill, Ont., Canada