Author name code: shu ADS astronomy entries on 2022-09-14 author:Shu, Frank H. ------------------------------------------------------------------------ Title: The impact of new estimates of models of stellar motion from VLBI on the alignment of the optically bright Gaia frame to ICRF3 Authors: Lunz, S.; Anderson, J.; Xu, M.; Heinkelmann, R.; Titov, O.; Lestrade, J. F.; Johnson, M. C.; Shu, F.; Chen, W.; Melnikov, A.; McCallum, J.; Lopez, Y.; Mikhailov, A.; Abad, P. de Vicente; Schuh, H. Bibcode: 2022evlb.confE..32L Altcode: 2022PoS...399E..32L No abstract at ADS Title: A superconductor filter installed in the broadband feed of Ishioka VLBI station Authors: Takagi, Y.; Ueshiba, H.; Nakakuki, T.; Matsumoto, S.; Hayashi, K.; Yutsudo, T.; Mori, K.; Kobayashi, T.; Sekido, M.; McCallum, J.; Shu, F. Bibcode: 2021evga.conf...24T Altcode: No abstract at ADS Title: Improving the S/X Celestial Reference Frame in the South: A Status Update Authors: de Witt, A.; Basu, S.; Charlot, P.; Gordon, D.; Jacobs, C.; Johnson, M.; Krásná, H.; Le Bail, K.; Shu, F.; Titov, O.; Schartner, M. Bibcode: 2021evga.conf...85D Altcode: No abstract at ADS Title: The K Band Geodesy with the East Asian VLBI Network Authors: Xu, S.; Jike, T.; Jung, T.; Shu, F.; Cui, L.; Melnikov, A.; McCallum, J.; Yi, S.; Zhang, B.; Sakai, N.; He, X.; Imai, H.; Kawaguchi, N.; Sakai, D.; Oh, C.; Jiang, P.; Xu, M.; Wang, G. Bibcode: 2021evga.conf...71X Altcode: No abstract at ADS Title: Observations of radio stars in geodetic VLBI experiments Authors: Titov, O.; Shu, F.; Chen, W. Bibcode: 2020jsrs.conf..173T Altcode: Proper motion and parallaxes of radio stars will enable us to align the latest International Celestial Reference Frame (ICRF3) based on VLBI observations with the optical astrometric catalogue obtained by the Gaia mission.

The Gaia mission has observed approximately 500,000 extragalactic objects in optical with a competitive precision, and produced an independent astrometric catalogue of 2820 objects common to the ICRF2 (Gaia Collaboration et al. 2018)

and contains astrometric solutions of parallaxes and proper motions for significant amount of stars. This provides a rare opportunity for independent verification of the VLBI astrometric catalogues. However, due to an average parallax

zero-point of -29 µas of the Gaia catalogue (Lindegren et al. 2018), independent assessment of the Gaia parallaxes is required. Unfortunately, very few optically bright radio stars were observed with VLBI,

therefore it is important to identify more Galactic stars that could be observed by both VLBI and optical mission for the parallax verification.

Here we show the astrometric results for five radio stars (HR1099, UX Ari, HR132742, HR5907 and LSI+61 303) in several VLBI experiments between 2015 and 2019. Title: Observations of Radio Sources Near the Sun Authors: Titov, O.; Lambert, S.; Soja, B.; Shu, F.; Melnikov, A.; McCallum, J.; McCallum, L.; Schartner, M.; de Witt, A.; Ivanov, D.; Mikhailov, A.; Yi, S. O.; Chen, W.; Xia, B.; Ishigaki, M.; Gulyaev, S.; Natusch, T.; Weston, S. Bibcode: 2019evga.conf..138T Altcode: 2019arXiv190800973T Geodetic Very Long Baseline Interferometry (VLBI) data are capable of measuring the light deflection caused by the gravitational field of the Sun and large planets with high accuracy. The parameter $\gamma$ of the parametrized Post-Newtonian (PPN) formalism estimated using observations of reference radio sources near the Sun should be equal to unity in the general relativity. We have run several VLBI experiments tracking reference radio sources from 1 to 3 degrees from the Sun. The best formal accuracy of the parameter $\gamma$ achieved in the single-session mode is less than 0.01 percent, or better than the formal accuracy obtained with a global solution included all available observations at arbitrary elongation from the Sun. We are planning more experiments starting from 2020 using better observing conditions near the minimum of the Solar activity cycle. Title: Mixed-mode VLBI Experiment with Chinese Stations in APSG40 Authors: He, X.; Shu, F.; Jiang, W.; Ma, L.; Yang, X.; Chen, L. Bibcode: 2019evga.conf..148H Altcode: No abstract at ADS Title: Efforts and Attempts to Develop VGOS-like Stations in China Authors: Li, J.; Liu, C.; Zhang, H.; Zhao, D.; Fan, Q.; Shu, F.; Zhao, R.; Wang, J.; Zhang, X.; Wang, G.; Yang, X.; Yu, L.; Jiang, Y.; Guo, S.; Wang, L. Bibcode: 2019evga.conf...12L Altcode: No abstract at ADS Title: Comparison of Results Between CVN and K5 Software Correlators Authors: Kondo, T.; Zheng, W.; Liu, L.; Zhang, J.; Shu, F.; Tong, L.; Tong, F. Bibcode: 2019evga.conf...62K Altcode: No abstract at ADS Title: Preliminary Work on Promoting Radar Astronomical Study Authors: Sun, J.; Ping, J.; Han, S.; Shu, F.; Ma, L.; Chen, W.; Cui, L. Bibcode: 2019evga.conf..134S Altcode: No abstract at ADS Title: Research and Application of VLBI Differential Phase Delays in Lunar Exploration Authors: Zheng, X.; Liu, Q.; Shu, F.; Huang, Y.; Li, P.; Xu, Z.; Chu, Z. Bibcode: 2019evga.conf...57Z Altcode: No abstract at ADS Title: VLBI monitoring of two distant quasars as a showcase for 'EVN Lite' Authors: Frey, S.; Titov, O.; Melnikov, A.; de Vicente, P.; Shu, F. Bibcode: 2018evn..confE..97F Altcode: 2018PoS...344E..97F No abstract at ADS Title: High-resolution radio imaging of two luminous quasars beyond redshift 4.5 Authors: Frey, S.; Titov, O.; Melnikov, A. E.; de Vicente, P.; Shu, F. Bibcode: 2018A&A...618A..68F Altcode: 2018arXiv180706837F Context. Radio-loud active galactic nuclei in the early Universe are rare. The quasars J0906+6930 at redshift z = 5.47 and J2102+6015 at z = 4.57 stand out from the known sample with their compact emission on milliarcsecond (mas) angular scale with high (0.1 Jy level) flux densities measured at GHz radio frequencies. This makes them ideal targets for very long baseline interferometry (VLBI) observations.
Aims: By means of VLBI imaging we can reveal the inner radio structure of quasars and model their brightness distribution to better understand the geometry of the jet and the physics of the sources.
Methods: We present sensitive high-resolution VLBI images of J0906+6930 and J2102+6015 at two observing frequencies, 2.3 and 8.6 GHz. The data were taken in an astrometric observing programme involving a global five-element radio telescope array. We combined the data from five different epochs from 2017 February to August.
Results: For one of the highest redshift blazars known, J0906+6930, we present the first-ever VLBI image obtained at a frequency below 8 GHz. Based on our images at 2.3 and 8.6 GHz, we confirm that this source has a sharply bent helical inner jet structure within ∼3 mas from the core. The quasar J2102+6015 shows an elongated radio structure in the east-west direction within the innermost ∼2 mas that can be described with a symmetric three-component brightness distribution model at 8.6 GHz. Because of their non-pointlike mas-scale structure, these sources are not ideal as astrometric reference objects. Our results demonstrate that VLBI observing programmes conducted primarily with astrometric or geodetic goals can be utilized for astrophysical purposes as well. Title: Testing general relativity with geodetic VLBI. What a single, specially designed experiment can teach us Authors: Titov, O.; Girdiuk, A.; Lambert, S. B.; Lovell, J.; McCallum, J.; Shabala, S.; McCallum, L.; Mayer, D.; Schartner, M.; de Witt, A.; Shu, F.; Melnikov, A.; Ivanov, D.; Mikhailov, A.; Yi, S.; Soja, B.; Xia, B.; Jiang, T. Bibcode: 2018A&A...618A...8T Altcode: 2018arXiv180611299T Context. We highlight the capabilities of geodetic VLBI technique to test general relativity in the classical astrometric style, i.e. measuring the deflection of light in the vicinity of the Sun.
Aims: In previous studies, the parameter γ was estimated by global analyses of thousands of geodetic VLBI sessions. Here we estimate γ from a single session where the Sun has approached two strong reference radio sources, 0229+131 and 0235+164, at an elongation angle of 1-3°.
Methods: The AUA020 VLBI session of 1 May 2017 was designed to obtain more than 1000 group delays from the two radio sources. The solar corona effect was effectively calibrated with the dual-frequency observations even at small elongation.
Results: We obtained γ with a greater precision (0.9 × 10-4) than has been obtained through global analyses of thousands of standard geodetic sessions over decades. Current results demonstrate that the modern VLBI technology is capable of establishing new limits on observational tests of general relativity. Title: The Progress of VLBI Terminal and Software Correlator in SHAO Authors: Zheng, W.; Renjie, Z.; Zhang, J.; Tong, L.; Liu, L.; Li, J.; Guo, S.; Tong, F.; Shu, F. Bibcode: 2017evga.conf...56Z Altcode: No abstract at ADS Title: VizieR Online Data Catalog: VLBI Ecliptic Plane Survey: VEPS-1 (Shu+, 2017) Authors: Shu, F.; Petrov, L.; Jiang, W.; Xia, B.; Jiang, T.; Cui, Y.; Takefuji, K.; McCallum, J.; Lovell, J.; Yi, S. -O.; Hao, L.; Yang, W.; Zhang, H.; Chen, Z.; Li, J. Bibcode: 2017yCat..22300013S Altcode: We began observations in the search mode in 2015 February. The participating stations included the three core stations of the Chinese VLBI Network (CVN): seshan25, kunming, and urumqi. Depending on the participating stations, the longest baseline length in each session can be varied from 3200km to 9800km. Our observations were performed at a 2048Mbps data rate, with 16 Intermediate Frequency (IF) channels and 2-bit sampling. The first eight IFs of 32MHz bandwidth were distributed in the range of [8.188, 8.444]GHz, and the remaining eight IFs of 32MHz bandwidth were in the range of [8.700, 8.956]GHz.

Table 1: Summary of the VLBI Ecliptic Plane Survey (VEPS) observations in search mode: --------------------------------------------------- Date Dur. Code Stations Number of (Y/M/D) (h) Targets --------------------------------------------------- 2015 Feb 13 24 VEPS01 ShKmUr 293 2015 Feb 14 24 VEPS02 ShKmUr 338 2015 Apr 23 24 VEPS03 UrKv 300 2015 Apr 24 24 VEPS04 ShKmUrKv 400 2015 Aug 10 25 VEPS05 ShKmKvHo 252 2015 Aug 19 25 VEPS06 ShKmKvHo 277 2016 Mar 02 24 VEPS07 ShKmUrKb 333 2016 Mar 11 24 VEPS08 ShKmUrKb 477 2016 May 13 24 VEPS09 ShUrHo 291 2016 May 14 22 VEPS10 ShUrKv 322 2016 Jul 06 24 VEPS11 ShUrKb 307 2016 Sep 02 23 VEPS12 ShUr 424 2016 Sep 03 23 VEPS13 ShKmUr 344 --------------------------------------------------- Sh=Seshan25; Km=Kunming; Ur=Urumqi; Kv=Sejong; Kb=Kashim34; Ho=Hobart26. ---------------------------------------------------

We ran two absolute astrometry dual-band VLBA programs that targeted ecliptic plane compact radio sources: the dedicated survey of weak ecliptic plane calibrators with the VLBA BS250 program in 2016 March-May, and the VLBA Calibrator Survey 9 (VCS-9) in 2015 August-2016 September.

The International VLBI Service for Geodesy and Astrometry (IVS) runs a number of VLBI observing programs. We made an attempt to improve the coordinates of some VEPS sources detected in the search mode and provide additional measurements of telescope position with the same experiments in two such 24hr sessions, AOV010 in July and AUA012 in 2016 August.

(2 data files). Title: Results from the VLBI Analysis Software Comparison Campaign 2015 Authors: Klopotek, G.; Artz, T.; Bellanger, A.; Bourda, G.; Gerstl, M.; Gordon, D.; Haas, R.; Halsig, S.; Hjelle, G. A.; Hobiger, T.; Hugentobler, U.; Iddink, A.; Kirkvik, A. S.; Lambert, S.; Plank, L.; Schmid, R.; Shu, F.; Titov, O.; Tong, F.; Wang, G.; Xu, M.; Zheng, W. Bibcode: 2016ivs..conf..203K Altcode: The aim of the VLBI Analysis Software Comparison Campaign 2015 (VASCC2015) was to compare different VLBI analysis software packages on the basis of computed theoretical delays. Eleven research groups and institutes participated in this project, which allowed us to compare software packages that are used in operational VLBI analyses or that are still under development. We present the first results, and we show how well the individual software packages agree at this stage. Title: Six Decades of Spiral Density Wave Theory Authors: Shu, Frank H. Bibcode: 2016ARA&A..54..667S Altcode: The theory of spiral density waves had its origin approximately six decades ago in an attempt to reconcile the winding dilemma of material spiral arms in flattened disk galaxies. We begin with the earliest calculations of linear and nonlinear spiral density waves in disk galaxies, in which the hypothesis of quasi-stationary spiral structure (QSSS) plays a central role. The earliest success was the prediction of the nonlinear compression of the interstellar medium and its embedded magnetic field; the earliest failure, seemingly, was not detecting color gradients associated with the migration of OB stars whose formation is triggered downstream from the spiral shock front. We give the reasons for this apparent failure with an update on the current status of the problem of OB star formation, including its relationship to the feathering substructure of galactic spiral arms. Infrared images can show two-armed, grand design spirals, even when the optical and UV images show flocculent structures. We suggest how the nonlinear response of the interstellar gas, coupled with overlapping subharmonic resonances, might introduce chaotic behavior in the dynamics of the interstellar medium and Population I objects, even though the underlying forces to which they are subject are regular. We then move to a discussion of resonantly forced spiral density waves in a planetary ring and their relationship to the ideas of disk truncation, and the shepherding of narrow rings by satellites orbiting nearby. The back reaction of the rings on the satellites led to the prediction of planet migration in protoplanetary disks, which has had widespread application in the exploding data sets concerning hot Jupiters and extrasolar planetary systems. We then return to the issue of global normal modes in the stellar disk of spiral galaxies and its relationship to the QSSS hypothesis, where the central theoretical concepts involve waves with negative and positive surface densities of energy and angular momentum in the regions interior and exterior, respectively, to the corotation circle; the consequent transmission and overreflection of propagating spiral density waves incident on the corotation circle; and the role of feedback from the central regions. Title: The CVN Geodetic Observation and its Result Authors: Wang, G.; Xu, M.; Zhang, Z.; Xu, S.; Li, L.; Shu, F. Bibcode: 2015evga.conf..228W Altcode: No abstract at ADS Title: The Asia-Oceania VLBI Group for Geodesy and Astrometry Authors: Lovell, J.; Kawabata, R.; Kurihara, S.; Shu, F.; Cho, J. Bibcode: 2015evga.conf..117L Altcode: No abstract at ADS Title: On the Role of Tianma Radio Telescope for Improving Celestial Reference Frames Authors: Shu, F.; Wang, J.; Jiang, W.; Wang, G.; Shen, Z. Bibcode: 2015evga.conf...71S Altcode: No abstract at ADS Title: VLBI Phase-referencing Experiments for Deep Space Probes Authors: Zheng, W.; Tong, F.; Zhang, J.; Shu, F.; Liu, L. Bibcode: 2015evga.conf..268Z Altcode: No abstract at ADS Title: Erratum: "Gas Dynamics of Semidetached Binaries" (1975, ApJ, 198, 383) Authors: Lubow, Stephen H.; Shu, Frank H. Bibcode: 2014ApJ...788...95L Altcode: No abstract at ADS Title: Erratum: "Gas Dynamics of Semidetached Binaries. II - The Vertical Structure of the Stream" (1976, ApJ, 207, L53) Authors: Lubow, Stephen H.; Shu, Frank H. Bibcode: 2014ApJ...787L..38L Altcode: No abstract at ADS Title: First 3-Way Lunar Radio Phase Ranging and Doppler Experiment in Chang'E-3 Lander Mission Authors: PING, J.; Meng, Q.; Tang, G.; Jian, N.; Wang, Z.; Li, W.; Chen, C.; Wang, M.; Wang, M.; Lu, Y.; Yu, Q.; Mao, Y.; Miao, C.; Lei, Y.; Shu, F.; Cao, J. Bibcode: 2014EPSC....9..226P Altcode: Radio science experiments have been involved in all of the Chinese lunar missions with different research objectives. In Chang'E-3 landing mission, a 3-way open loop lunar radio phase ranging and Doppler technique was suggested and tested. This technique is modified and updated from early multi-channel oneway Doppler deep space tracking technique developed for Chinese Mars mission Yinghuo-1. In the 1st preliminary experiments, we obtained 1sps continuous phase ranging data before and after the successful landing period, with a resolution of 0.5 millimeter or better. This method, called Lunar Radio Phase Ranging (LRPR) can be a new space geodetic technique to measure the station position, earth tide and rotation, lunar orbit, tide and liberation, by means of independent observation, or to work together with Lunar Laser Ranging. Also, it can be used in future Mars mission. Title: Spiral Arm Substructure and Massive Star Formation Authors: Lee, Wing-Kit; Shu, F. H. Bibcode: 2013AAS...22122502L Altcode: A theoretical framework is developed to understand the substructures of spiral arms in galaxies and its relation to the massive star formation. In particular, quasi-periodic gas density enhancements along the spiral arms, which are called feathers, can be formed by the instability of spiral arms. Such feathering instability may trigger the formation of Giant Molecular Clouds (GMCs) by the collapse of high density region. The analytic framework has advantages for the large parameter space in the problem. Parameter study of the problem using the framework are done to understand which factor is most important among self-gravity, magnetic field or local shear in the inter-arm region. Observational applications (on nearby spiral galaxies) are also being developed to understand the physics of large-scale star formation quantitatively. Ultimately, we hope to apply what we know about star formation in the nearby galaxies to the high-redshift universe. Title: Feathering Instability of Spiral Arms. I. Formulation of the Problem Authors: Lee, Wing-Kit; Shu, Frank H. Bibcode: 2012ApJ...756...45L Altcode: 2012arXiv1207.0875L In this paper, we study the feathering substructures along spiral arms by considering the perturbational gas response to a spiral shock. Feathers are density fluctuations that jut out from the spiral arm to the interarm region at pitch angles given by the quantum numbers of the doubly periodic structure. In a localized asymptotic approximation, related to the shearing sheet except that the inhomogeneities occur in space rather than in time, we derive the linearized perturbation equations for a razor-thin disk with turbulent interstellar gas, frozen-in magnetic field, and gaseous self-gravity. Apart from the modal quantum numbers, the individual normal modes of the system depend on seven dimensionless quantities that characterize the underlying time-independent axisymmetric state plus its steady, nonlinear, two-armed spiral-shock response to a hypothesized background density wave supported by the disk stars of the galaxy. We show that some of these normal modes have positive growth rates. Their overdensity contours in the post-shock region are very reminiscent of observed feathering substructures in full magnetohydrodynamic simulations. The feathering substructures are parasitic instabilities intrinsic to the system; thus, their study not only provides potential diagnostics for important parameters that characterize the interstellar medium of external galaxies, but also yields a deeper understanding of the basic mechanism that drives the formation of the giant molecular clouds and the OB stars that outline observed grand-design spirals. Title: ERRATUM: "Gravitational Collapse of Magnetized Clouds. I. Ideal Magnetohydrodynamic Accretion Flow" (2006, ApJ, 647, 374) Authors: Galli, Daniele; Lizano, Susana; Shu, Frank H.; Allen, Anthony Bibcode: 2012ApJ...754...78G Altcode: No abstract at ADS Title: Feathering Instability of Spiral Arms and OB Star Formation Authors: Lee, Wing-Kit; Shu, F. H. Bibcode: 2012AAS...21944109L Altcode: Quasi-regular substructures of the spiral arm are commonly found in spiral galaxies. These substructures are known as feathers or spurs, and they jut out perpendicularly into the inter-arm region. They also associate with the Giant Molecular Clouds where massive star formation occurs. The formation of these density fluctuation can be studied from the perspective of perturbation of galactic spiral shock. We investigate the gas response under the influcence of the shock perturbation, and formulate the MHD equations in a local two-dimensional quasi-rectangular region between tightly-winding spiral arms. Our theoretical model includes the effect of magnetic field and self-gravity of the gas, we are able to reproduce feather-like structures in the post-shock region. In this semi-analytical framework, the periodic density fluctuations depends on the various background parameters such as pattern speed, strength of spiral arm, surface density of the gas and strength of magnetic field. Potentially this study can help understand the inter-arm environment that will be observed in the nearby galaxies using submillimeter telescope such ALMA in the coming years. Title: Magnetic Interactions in Pre-main-sequence Binaries Authors: Adams, Fred C.; Cai, Michael J.; Galli, Daniele; Lizano, Susana; Shu, Frank H. Bibcode: 2011ApJ...743..175A Altcode: 2011arXiv1110.4562A Young stars typically have strong magnetic fields, so that the magnetospheres of newly formed close binaries can interact, dissipate energy, and produce synchrotron radiation. The V773 Tau A binary system, a pair of T Tauri stars with a 51 day orbit, displays such a signature, with peak emission taking place near periastron. This paper proposes that the observed emission arises from the change in energy stored in the composite magnetic field of the system. We model the fields using the leading order (dipole) components and show that this picture is consistent with current observations. In this model, the observed radiation accounts for a fraction of the available energy of interaction between the magnetic fields from the two stars. Assuming antisymmetry, we compute the interaction energy E int as a function of the stellar radii, the stellar magnetic field strengths, the binary semimajor axis, and orbital eccentricity, all of which can be measured independently of the synchrotron radiation. The variability in time and energetics of the synchrotron radiation depend on the details of the annihilation of magnetic fields through reconnection events, which generate electric fields that accelerate charged particles, and how those charged particles, especially fast electrons, are removed from the interaction region. However, the major qualitative features are well described by the background changes in the global magnetic configuration driven by the orbital motion. The theory can be tested by observing a collection of pre-main-sequence binary systems. Title: From Magnetized Cores to Protoplanetary Disks Authors: Lizano, Susana; Shu, Frank H. Bibcode: 2010HiA....15..440L Altcode: We highlight several recent theoretical results that show how magnetic fields, with the magnitudes currently observed in molecular clouds, affect the structure and evolution of dense cores and protoplanetary disks to form stars and planets. Title: The Role of Magnetic Fields in the Protostellar Accretion Phase Authors: Galli, Daniele; Shu, Frank H.; Lizano, Susana; Cai, Michael J. Bibcode: 2010AIPC.1242..231G Altcode: We summarize recent work addressing the role of magnetic fields in the process of star formation and disk accretion. After a short review of the basic observational results, we concentrate on the efficiency of magnetic braking during cloud collapse and its consequences on the formation of centrifugally supported disks around young stars. Then, we relate this issue to the well-known magnetic flux problem of star formation, and we show that the introduction of non-ideal MHD effects is a necessary step toward the development of self-consistent models for the collapse of molecular clouds and the formation of disks. Finally, we discuss the structure and evolution of magnetized accretion disks around young stars that have dragged their magnetic field in the phase of gravitational collapse and we focus on two main diffusive processes: viscous stresses that redistribute mass and angular momentum, and the resistive diffusion of mass across magnetic field lines. Title: The SMA and Galactic Star Formation Authors: Shu, Frank H. Bibcode: 2010AAS...21523202S Altcode: 2010BAAS...42R.595S We review observations of the SMA telescope that have had a large impact in the field of Galactic star formation. We divide the subject into the origin of low-mass and high-mass stars, and we focus on the subjects of protostellar collapse and outflow. We conclude that the SMA indicates that (a) magnetic fields have a stronger influence in controlling star formation than interstellar turbulence, (b) until the appearance of H II regions, high-mass star formation is just a scaled-up version of low-mass star formation, and (c) the basic drivers for bipolar outflows originate near the inner edges of the accretion disks that surround young stellar objects. Title: Current status of Chinese VLBI network software correlator Authors: Zheng, W.; Shu, F.; Luo, W.; Yu, Y.; Wang, W. Bibcode: 2009evga.conf...84Z Altcode: No abstract at ADS Title: Shanghai correlation system upgrade for geodetic application Authors: Shu, F.; Zheng, W.; Zhang, X.; Xu, Z.; Wang, W.; Chen, Z. Bibcode: 2009evga.conf...87S Altcode: No abstract at ADS Title: Magnetized Disks around Young Stars Authors: Lizano, S.; Shu, F. H.; Galli, D.; Glassgold, A. Bibcode: 2009RMxAC..36..149L Altcode: We discuss the structure and evolution of a magnetized accretion disks around young stars that have dragged their magnetic field in the process of gravitational collapse. The disk evolves due to two diffusive processes: viscous stresses that redistribute mass and angular momentum, and the resistive diffusion of mass across magnetic field lines due to imperfect conduction. In steady-state there is an analytic model of the structure of these magnetized disks. We discuss the application of this model to disks around low and high mass young stars and recent results of time dependent models. Title: Magnetic Braking and Field Dissipation in the Protostellar Accretion Phase Authors: Galli, D.; Cai, M.; Lizano, S.; Shu, F. H. Bibcode: 2009RMxAC..36..143G Altcode: We summarize recent theoretical work addressing the role of magnetic fields in the process of star formation. First, we concentrate on the efficiency of magnetic braking during cloud collapse and its consequences on the formation of centrifugally supported disks around young stars. Then, we relate this issue to the well-known magnetic flux problem of star formation, and we show that the introduction of non-ideal MHD effects is a necessary step toward the development of self-consistent models for the collapse of molecular clouds and the formation and evolution of accretion disks around young stars. Title: Generalized Multipole X-Wind Model Authors: Mohanty, Subhanjoy; Shu, Frank H. Bibcode: 2009ASSP...13...51M Altcode: 2009pjc..book...51M The X-wind model for magnetospheric accretion and outflow in classical T Tauri stars (CTTS) has gained credence in recent years for a variety of theoretical and observational reasons. However, both this model as well as other theoretical scenarios for explaining magnetospheric disk accretion assume that the stellar field, were it not perturbed by an electrically conducting accretion disk, would have a dipolar geometry (e.g., [5]; OS95 hereafter). Observations of accretion hot spot sizes and net field polarization on the surface of CTTS, however, clearly indicate that the stellar field has a complex multipolar structure. To overcome this discrepancy between theory and data, we reformulate X-wind theory without the dipole constraint. This contribution represents a brief summary of the paper by Mohanty and Shu [6]. In Sect. 1 we present the fundamental physical ideas of the generalized theory, and the associated equations; in Sect. 2 we compare the resulting theoretical prediction to recent observations, and provide some illustrative numerical simulations with multipole stellar fields. Title: Magnetocentrifugally Driven Flows from Young Stars and Disks. VI. Accretion with a Multipole Stellar Field Authors: Mohanty, Subhanjoy; Shu, Frank H. Bibcode: 2008ApJ...687.1323M Altcode: 2008arXiv0806.4769M Previous analyses of magnetospheric accretion and outflow in classical T Tauri stars (CTTSs), within the context of both the X-wind model and other theoretical scenarios, have assumed a dipolar geometry for the stellar magnetic field if it were not perturbed by the presence of an accreting, electrically conducting disk. However, CTTS surveys reveal that accretion hot spots cover a small fraction of the stellar surface and that the net field polarization on the stellar surface is small. Both facts imply that the magnetic field generated by the star has a complex nondipolar structure. To address this discrepancy between theory and observations, we reexamine X-wind theory without the dipole constraint. Using simple physical arguments based on the concept of trapped flux, we show that a dipole configuration is in fact not essential. Independent of the precise geometry of the stellar magnetosphere, the requirement for a certain level of trapped flux predicts a definite relationship among various CTTS observables. Moreover, superposition of multipole stellar fields naturally yield small observed hot spot covering fractions and small net surface polarizations. The generalized X-wind picture remains viable under these conditions, with the outflow from a small annulus near the inner disk edge little affected by the modified geometry, but with inflow highly dependent on the details of how the emergent stellar flux is linked and trapped by the inner disk regions. Our model is consistent with data, including recent spectropolarimetric measurements of the hot spot sizes and field strengths in V2129 Oph and BP Tau. Title: The Challenge of Sub-Keplerian Rotation for Disk Winds Authors: Shu, Frank H.; Lizano, Susana; Galli, Daniele; Cai, Mike J.; Mohanty, Subhanjoy Bibcode: 2008ApJ...682L.121S Altcode: 2008arXiv0806.2137S Strong magnetization makes the disks surrounding young stellar objects rotate at rates that are too sub-Keplerian to enable the thermal launching of disk winds from their surfaces unless the rate of gas diffusion across field lines is dynamically fast. This underappreciated implication of disk magnetization poses a considerable challenge for disk-wind theory. Title: Astrophysics of X-ray Irradiated Protoplanetary Disks Authors: Shu, Frank H. Bibcode: 2008HEAD...10.1703S Altcode: No abstract at ADS Title: X-Winds in Action Authors: Cai, Mike J.; Shang, Hsien; Lin, Hsiao-Hsuan; Shu, Frank H. Bibcode: 2008ApJ...672..489C Altcode: 2007arXiv0709.3087C The interaction of accretion disks with the magnetospheres of young stars can produce X-winds and funnel flows. With the assumption of axial symmetry and steady state flow, the problem can be formulated in terms of quantities that are conserved along streamlines, such as the Bernoulli integral (BI), plus a partial differential equation (PDE), called the Grad-Shafranov equation (GSE), that governs the distribution of streamlines in the meridional plane. The GSE plus BI yields a PDE of mixed type, elliptic before critical surfaces where the flow speed equals certain characteristic wave speeds are crossed and hyperbolic afterward. The computational difficulties are exacerbated by the locations of the critical surfaces not being known in advance. To overcome these obstacles, we consider a variational principle by which the GSE can be attacked by extremizing an action integral, with all other conserved quantities of the problem explicitly included as part of the overall formulation. To simplify actual applications we adopt the cold limit of a negligibly small ratio of the sound speed to the speed of Keplerian rotation in the disk where the X-wind is launched. We also ignore the obstructing effects of any magnetic fields that might thread a disk approximated to be infinitesimally thin. We then introduce trial functions with adjustable coefficients to minimize the variations that give the GSE. We tabulate the resulting coefficients so that other workers can have analytic forms to reconstruct X-wind solutions for various astronomical, cosmochemical, and meteoritical applications. Title: Ambipolar Diffusion In Molecular Cloud Cores and the Gravomagneto Catastrophe Authors: Adams, Fred C.; Shu, Frank H. Bibcode: 2007ApJ...671..497A Altcode: 2007arXiv0708.4238A This paper reexamines the problem of ambipolar diffusion as a mechanism for the production and runaway evolution of centrally condensed molecular cloud cores, a process that has been termed the gravomagneto catastrophe. Our calculation applies in the geometric limit of a highly flattened core and allows for a semianalytic treatment of the full problem, although physical fixes are required to resolve a poor representation of the central region. A noteworthy feature of the overall formulation is that the solutions for the ambipolar diffusion portion of the evolution for negative times (t<0) match smoothly onto the collapse solutions for positive times (t>0). The treatment shows that the resulting cores display nonzero, but submagnetosonic, inward velocities at the end of the diffusion epoch, in agreement with current observations. Another important result is the derivation of an analytic relationship between the dimensionless mass-to-flux ratio λ0≡f-10 of the central regions produced by runaway core condensation and the dimensionless measure of the rate of ambipolar diffusion ɛ. In conjunction with previous work showing that ambipolar diffusion takes place more quickly in the presence of turbulent fluctuations, i.e., that the effective value of ɛ can be enhanced by turbulence, the resultant theory provides a viable working hypothesis for the formation of isolated molecular cloud cores and their subsequent collapse to form stars and planetary systems. Title: Mean Field Magnetohydrodynamics of Accretion Disks Authors: Shu, Frank H.; Galli, Daniele; Lizano, Susana; Glassgold, Alfred E.; Diamond, Patrick H. Bibcode: 2007ApJ...665..535S Altcode: 2007arXiv0705.0421S We consider the accretion process in a disk with magnetic fields that are dragged in from the interstellar medium by gravitational collapse. Two diffusive processes are at work in the system: (1) ``viscous'' torques exerted by turbulent and magnetic stresses, and (2) ``resistive'' redistribution of mass with respect to the magnetic flux arising from the imperfect conduction of current. In steady state, self-consistency between the two rates of drift requires that a relationship exists between the coefficients of turbulent viscosity and turbulent resistivity. Ignoring any interactions with a stellar magnetosphere, we solve the steady-state equations for a magnetized disk under the gravitational attraction of a mass point and threaded by an amount of magnetic flux consistent with calculations of magnetized gravitational collapse in star formation. Our model mean field equations have an exact analytical solution that corresponds to magnetically diluted Keplerian rotation about the central mass point. The solution yields the strength of the magnetic field and the surface density as functions of radial position in the disk and their connection with the departure from pure Keplerian rotation in representative cases. We compare the predictions of the theory with the available observations concerning T Tauri stars, FU Orionis stars, and low- and high-mass protostars. Finally, we speculate on the physical causes for high and low states of the accretion disks that surround young stellar objects. One of the more important results of this study is the physical derivation of analytic expressions for the turbulent viscosity and turbulent resistivity. Title: Review of Theory Authors: Shu, Frank Bibcode: 2007sftn.confE..32S Altcode: No abstract at ADS Title: Magnetohydrodynamics of Star Formation Authors: Shu, Frank H. Bibcode: 2007DDA....38.0101S Altcode: We present self-consistent models of disk accretion driven by the magneto-rotational instability associated with magnetic fields dragged in by a process of gravitational collapse from rotating, magnetized, molecular cloud cores. We compare such star-formation models with constraints from astronomical observations, meteoritic investigations, and comet sample returns. We show that previous theoretical studies have missed two crucial effects: (1) the fact that diffusion is occurring not only via a viscous redistribution of angular momentum but also by a non-ideal drift of inwardly moving matter across magnetic field lines that thread vertically through the disk, and (2) that realistic circumstances may result in magnetically pinched disks which rotate at substantially sub-Keplerian speeds. We also argue that the complete data set cannot be understood for sunlike stars without incorporating the interaction of the inner edge of the accretion disk with the magnetosphere of the central star that results in X-winds and funnel flows, with important, incompletely examined, consequences for the processes of planet formation. Title: Formation of OB Associations in Galaxies Authors: Shu, Frank H.; Allen, Ronald J.; Lizano, Susana; Galli, Daniele Bibcode: 2007ApJ...662L..75S Altcode: 2007arXiv0705.0634S We consider the formation of OB associations from two perspectives: (1) the fractional gas consumption in star formation, ɛ, per dynamical timescale tdyn in a galaxy, and (2) the origin of the so-called Kennicutt-Schmidt law, that the rate of star formation per unit area is proportional to a power, α, of the surface density in H I and H2 gas when certain thresholds are crossed. The empirical findings that ɛ~10-2 and α~1.4 or 1.5 have simple explanations if the rate of star formation is magnetically regulated. An empirical test of the ideas resides in an analysis of why giant OB associations are ``strung out like pearls along the arms'' of spiral galaxies. Title: Magnetization, accretion, and outflows in young stellar objects Authors: Shu, Frank H.; Galli, Daniele; Lizano, Susana; Cai, Mike J. Bibcode: 2007IAUS..243..249S Altcode: We review the theory of the formation and gravitational collapse of magnetized molecular cloud cores, leading to the birth of T Tauri stars surrounded by quasi-Keplerian disks whose accretion is driven by the magnetorotational instability (MRI). Some loss of magnetic flux during the collapse results typically in a dimensionless mass-to-flux ratio for the star plus disk of λ0 ≈ 4. Most of the mass ends up in the star, while almost all of the flux and the angular momentum ends up in the disk; therefore, a known mass for the central star implies a computable flux in the surrounding disk. A self-contained theory of the MRI that drives the viscous/resistive spreading in such circumstances then yields the disk radius needed to contain the flux trapped in the disk as a function of the age t. This theory yields analytic predictions of the distributions with distance from the central star of the surface density Σ(), the vertical magnetic field Bz(), and the (sub-Keplerian) angular rotation rate Ω (). We discuss the implications of this picture for disk-winds, X-winds, and funnel flows, and we summarize the global situation by giving the energy and angular-momentum budget for the overall problem. Title: Star formation and radio astronomy Authors: Shu, F. Bibcode: 2007mru..confE..60S Altcode: 2007PoS....52E..60S No abstract at ADS Title: Gravitational Collapse of Magnetized Clouds. II. The Role of Ohmic Dissipation Authors: Shu, Frank H.; Galli, Daniele; Lizano, Susana; Cai, Mike Bibcode: 2006ApJ...647..382S Altcode: 2006astro.ph..4574S We formulate the problem of magnetic field dissipation during the accretion phase of low-mass star formation, and we carry out the first step of an iterative solution procedure by assuming that the gas is in free fall along radial field lines. This so-called ``kinematic approximation'' ignores the back reaction of the Lorentz force on the accretion flow. In quasi-steady state and assuming the resistivity coefficient to be spatially uniform, the problem is analytically soluble in terms of Legendre's polynomials and hypergeometric confluent functions. The dissipation of the magnetic field occurs inside a region of radius inversely proportional to the mass of the central star (the ``Ohm radius''), where the magnetic field becomes asymptotically straight and uniform. In our solution the magnetic flux problem of star formation is avoided because the magnetic flux dragged in the accreting protostar is always zero. Our results imply that the effective resistivity of the infalling gas must be higher by at least 1 order of magnitude than the microscopic electric resistivity, to avoid conflict with measurements of paleomagnetism in meteorites and with the observed luminosity of regions of low-mass star formation. Title: Gravitational Collapse of Magnetized Clouds. I. Ideal Magnetohydrodynamic Accretion Flow Authors: Galli, Daniele; Lizano, Susana; Shu, Frank H.; Allen, Anthony Bibcode: 2006ApJ...647..374G Altcode: 2006astro.ph..4573G We study the self-similar collapse of an isothermal magnetized rotating cloud in the ideal magnetohydrodynamic (MHD) regime. In the limit of small distance from the accreting protostar, we find an analytic solution that corresponds to free fall onto a central mass point. The density distribution is not spherically symmetric but depends on the mass loading of magnetic field lines, which can be obtained by matching our inner solution to an outer collapse solution previously computed by Allen et al. The concentration of magnetic field trapped by the central mass point under field freezing, independent on the details of the starting state, creates a split-monopole configuration in which the magnetic field strength increases as the inverse square of the distance from the center. Under such conditions, the inflow eventually becomes sub-Alfvènic and the outward transfer of angular momentum by magnetic braking very efficient, thus preventing the formation of a centrifugally supported disk. Instead, the azimuthal velocity of the infalling gas decreases to zero at the center, and the gas spirals into the star. Therefore, the dissipation of dynamically important levels of magnetic field is a fundamental requisite for the formation of protoplanetary disks around young stars. Title: The Irradiation Origin of Beryllium Radioisotopes and Other Short-lived Radionuclides Authors: Gounelle, Matthieu; Shu, Frank H.; Shang, Hsien; Glassgold, A. E.; Rehm, K. E.; Lee, Typhoon Bibcode: 2006ApJ...640.1163G Altcode: 2005astro.ph.12517G Two explanations exist for the short-lived radionuclides (T1/2<=5 Myr) present in the solar system when the calcium-aluminum-rich inclusions (CAIs) first formed. They originated either from the ejecta of a supernova or by the in situ irradiation of nebular dust by energetic particles. With a half-life of only 53 days, 7Be is then the key discriminant, since it can be made only by irradiation. Using the same irradiation model developed earlier by our group, we calculate the yield of 7Be. Within model uncertainties associated mainly with nuclear cross sections, we obtain agreement with the experimental value. Moreover, if 7Be and 10Be have the same origin, the irradiation time must be short (a few to tens of years), and the proton flux must be of order F~2×1010 cm-2 s-1. The X-wind model provides a natural astrophysical setting that gives the requisite conditions. In the same irradiation environment, 26Al, 36Cl, and 53Mn are also generated at the measured levels within model uncertainties, provided that irradiation occurs under conditions reminiscent of solar impulsive events (steep energy spectra and high 3He abundance). The decoupling of the 26Al and 10Be observed in some rare CAIs receives a quantitative explanation when rare gradual events (shallow energy spectra and low 3He abundance) are considered. The yields of 41Ca are compatible with an initial solar system value inferred from the measured initial 41Ca/40Ca ratio and an estimate of the thermal metamorphism time (from Young et al.), alleviating the need for two-layer proto-CAIs. Finally, we show that the presence of supernova-produced 60Fe in the solar accretion disk does not necessarily mean that other short-lived radionuclides have a stellar origin. Title: Indeterminate-origin nozzles to control jet structure and evolution Authors: Shu, F.; Plesniak, M. W.; Sojka, P. E. Bibcode: 2005JTurb...6...26S Altcode: 2005JTurb...6N..26S An indeterminate-origin (IO) nozzle consisting of a four-point tapered crown geometry is used to control jet structure and evolution. The near-field structure and flow field of round water jets were studied with particle image velocimetry (PIV) and laser-induced fluorescence (LIF) techniques. Typical jet Reynolds numbers (based on the nozzle diameter) are 5 000 10 000. The jet structures for IO nozzles are compared with those produced by conventional round nozzles. The IO nozzles introduce strong streamwise vortex pairs, which influence the near-nozzle structure and spreading by deforming the Kelvin Helmholtz vortex rings. A pair of counter-rotating vortices forms at each valley plane, within the interior of the jet. Adjacent vortex pairs reorganize themselves and form another set of counter-rotating vortex pairs that propagate radially outwards from the jet. The evolution of these effects with streamwise distance and their implications are discussed. Title: A Theory of the IMF Authors: Shu, Frank H.; Li, Zhi-Yun; Allen, Anthony Bibcode: 2005ASSL..327..401S Altcode: 2005imf..conf..401S No abstract at ADS Title: Collapse of Singular Isothermal Spheres to Black Holes Authors: Cai, Mike J.; Shu, Frank H. Bibcode: 2005ApJ...618..438C Altcode: We study the gravitational collapse of a relativistic singular isothermal sphere that is initially in unstable equilibrium. In the subsequent collapse, the dynamic spacetime is self-similar. The infall proceeds in an inside-out fashion, mimicking its Newtonian counterpart in star formation. A spherical expansion wave propagates outward at the speed of sound, initiating an inward collapse relative to local static observers. Outside of the expansion wave front, matter remains in local equilibrium. Inside, fluid elements are accelerated from rest toward the expanding black hole event horizon. When the singular isothermal sphere is initially threaded by a uniform but weak magnetic field, the frozen-in field lines accumulate above the horizon according to a distant observer, while assuming a split-monopole configuration on a larger scale. When the magnetized system also possesses rotation, such a configuration may naturally develop a vigorous outflow in the simultaneous presence of an accretion inflow. We speculate that such a process underlies the well-known relationship between mass and bulge velocity dispersion of supermassive black holes in the nuclei of galaxies. Title: Making Radio Jets from the X-wind Model Authors: Shang, H.; Lizano, S.; Glassgold, A.; Shu, F. Bibcode: 2004ASPC..323..299S Altcode: We calculate free-free emission at centimeter wavelengths from jets from Young Stellar Objects (YSOs). We use the x-wind model which is heated and ionized by several physical mechanisms, being of primary importance ionization by x-rays and heating by dissipation of mechanical energy (Shang et al. 2002). With stellar parameters, mass-loss rates, and x-ray luminosities characteristic of Class 0 or Class I YSOs, we recover realistic radio spectral indices and radio maps. In particular, the 3.6 cm flux in models with mass loss rates ∼ 10-6 M⊙/yr are comparable to existing observations of radio jets at 0''.1 resolution. Lower mass-loss rates characteristic of Class II YSOs with revealed optical jets generally produce radio jets that are too weak to be detected at current sensitivity levels. We discuss the implications of these comparisons for theoretical models of the YSO outflow phenomenon. Title: The Stellar Initial Mass Function Authors: Shu, F. H.; Li, Z. -Y.; Allen, A. Bibcode: 2004ASPC..323...37S Altcode: If magnetic fields are frozen in gravitational collapse, the resulting magnetic tension can prevent the outer part of the subcritical envelope of a molecular cloud from falling in with the supercritical core. However, the implied surface magnetic fields much exceed measured values for young stars. Moreover, it is virtually impossible for Keplerian disks to form in these circumstances. Magnetic reconnection can eliminate the long lever arms of the split monopole formed by the gravitational collapse that contibutes to catastrophic magnetic braking. The natural appearance then of a Keplerian disk adjoining a rotating star with an outer convective envelope will lead to an X-wind driven magnetocentrifugally from the inner edge of the disk. This wind can cut off the continued infall from the envelope and build-up of the central stellar mass. We use these ideas and results to calculate the initial mass function and star formation efficiency for the distributed and clustered modes of star formation. Title: The Origin of Short-lived Radionuclides and Early Solar System Irradiation Authors: Gounelle, M.; Shu, F. H.; Shang, H.; Glassgold, A. E.; Rehm, K. E.; Lee, T. Bibcode: 2004cpd..work.9051G Altcode: No abstract at ADS Title: Singular isothermal sphere and black hole formation Authors: Cai, Mike J.; Shu, Frank H. Bibcode: 2004IAUS..222..551C Altcode: We present here the self-similar collapse of a relativistic singular isothermal sphere (SIS) that leads to monolithic black hole formation. The dynamic evolution is triggered by the central portion of the SIS collapsing to for an infinitesimal black hole. This perturbation removes the pressure support for the layer immediately above, which causes it to collapse, and so on. The influence of gravitational collapse propagates out as an expansion wave moving at the speed of sound. Outside of the expansion wave, matter remains in local equilibrium. Title: Jet Flows: Formation and Thermal Processing of Solids in Protoplanetary Disks Authors: Shu, F. H.; Shang, H.; Lee, T. Bibcode: 2004cpd..work.9008S Altcode: No abstract at ADS Title: Free-free Radio Emission from Young Stellar Objects Authors: Shang, Hsien; Lizano, Susana; Glassgold, Al; Shu, Frank Bibcode: 2004ApJ...612L..69S Altcode: We calculate the centimeter wavelength free-free emission of the jets of young stellar objects (YSOs) with the X-wind model enhanced by a variety of physical processes. Using parameters characteristic of a Class I YSO with a mass-loss rate of ~10-6 Msolar yr-1, we obtain a 3.6 cm map and a spectral index that compare well with high spatial resolution observations of L1551 IRS 5. Models with lower mass-loss rates, appropriate for Class II YSOs with revealed optical jets, produce radio jets that are too weak to be detected at current sensitivity levels. In addition to demonstrating the consistency of the density distribution of the X-wind model with observations, we are able to obtain information on the processes that heat and ionize the inner jet, i.e., X-ray ionization and shock heating and ionization. Title: Nonaxisymmetric Neutral Modes in Relativistic Disks Authors: Cai, Mike J.; Shu, Frank H. Bibcode: 2004ApJ...611.1054C Altcode: 2004astro.ph..5247C We perform a linear stability analysis of the axisymmetric, relativistic, self-similar, isothermal disk against nonaxisymmetric perturbations. Two sets of neutral modes are discovered. The first set corresponds to marginally unstable perturbations driven by gravitational radiation, and the other signals the onset of bifurcation to nonaxisymmetric equilibrium solutions to the Einstein equations. Title: The Origin of Short-lived Radionuclides and Early Solar System Irradiation Authors: Gounelle, M.; Shu, F. H.; Shang, H.; Glassgold, A. E.; Rehm, K. E.; Lee, T. Bibcode: 2004LPI....35.1829G Altcode: Using the irradiation model developed by Gounelle et al. (2001), we can reproduce the abundance of ^7Be measured by Chaussidon,et al. (2004, this conference). We also provide a tentative explanation for the hibonite grains that show a decoupling betwen ^26Al and ^10Be (Marhas et al. 2002). Title: Does Magnetic Levitation or Suspension Define the Masses of Forming Stars? Authors: Shu, Frank H.; Li, Zhi-Yun; Allen, Anthony Bibcode: 2004ApJ...601..930S Altcode: 2003astro.ph.11426S We investigate whether magnetic tension can define the masses of forming stars by holding up the subcritical envelope of a molecular cloud that suffers gravitational collapse of its supercritical core. We perform an equilibrium analysis of the initial and final states assuming perfect field freezing, no rotation, isothermality, and a completely flattened configuration. The sheet geometry allows us to separate the magnetic tension into a levitation associated with the split monopole formed by the trapped flux of the central star and a suspension associated with curved field lines that thread the static pseudodisk and envelope of material external to the star. We find solutions where the eigenvalue for the stellar mass is a fixed multiple of the initial core mass of the cloud. We verify the analytically derived result by an explicit numerical simulation of a closely related three-dimensional axisymmetric system. However, with field freezing, the implied surface magnetic fields much exceed measured values for young stars. If the pinch by the central split monopole were to be eliminated by magnetic reconnection, then magnetic suspension alone cannot keep the subcritical envelope (i.e., the entire model cloud) from falling onto the star. We argue that this answer has general validity, even if the initial state lacked any kind of symmetry, possessed rotation, and had a substantial level of turbulence. These findings strongly support a picture for the halt of infall that invokes dynamic levitation by YSO winds and jets, but the breakdown of ideal magnetohydrodynamics is required to allow the appearance in the problem of a rapidly rotating, centrifugally supported disk. We use these results to calculate the initial mass function and star formation efficiency for the distributed and clustered modes of star formation. Title: Planetesimal Formation by Gravitational Instability - The Goldreich-Ward Hypothesis Revisited Authors: Youdin, Andrew N.; Shu, Frank H. Bibcode: 2004IAUS..202..250Y Altcode: No abstract at ADS Title: Chaos in Spiral Galaxies Authors: Shu, F. H.; Chakrabarti, S.; Laughlin, G. Bibcode: 2004ASSL..319..581S Altcode: 2004pbmc.conf..581S No abstract at ADS Title: Chaos in Spiral Galaxies Authors: Shu, Frank H.; Chakrabarti, Sukanya; Laughlin, Gregory Bibcode: 2004csg..book.....S Altcode: We review spiral density wave theory and its relationship to the disordered appearance of population I objects in some disk galaxies. We discuss mechanisms proposed for the formation of (a) feathers by gravitational instability behind galactic shocks, (b) branches by the action of ultraharmonic resonances, (c) spurs by reflection of leading waves off sharp features induced by nonlinear dredgin, and (d) flocculence by the chaos produced from overlapping resonances. We conclude that disorder arises is spiral galaxies not so much from disorderly causes as from too sensitive a response of the interstellar medium to an orderly but nonlinear spiral gravitational field. Title: Non-Axisymmetric Modes in Relativistic Singular Isothermal Disks Authors: Cai, M. J.; Shu, F. H. Bibcode: 2004PThPS.155..317C Altcode: The relativistic singular isothermal disk equilibriumsolutions are self-similar and form a two-parameter family described by the sound speed and the linear rotational velocity. Using the equilibrium model as the base state, we study its stability against self-similar but non-axisymmetric perturbations. We discovered that instability can be manifested through radiation driven neutral modes and bifurcation points to non-axisymmetric equilibria. Title: Collapse of Magnetized Singular Isothermal Toroids. II. Rotation and Magnetic Braking Authors: Allen, Anthony; Li, Zhi-Yun; Shu, Frank H. Bibcode: 2003ApJ...599..363A Altcode: 2003astro.ph.11377A We study numerically the collapse of rotating magnetized molecular cloud cores, focusing on rotation and magnetic braking during the main accretion phase of isolated star formation. Motivated by previous numerical work and analytic considerations, we idealize the precollapse core as a magnetized singular isothermal toroid, with a constant rotational speed everywhere. The collapse starts from the center and propagates outward in an inside-out fashion, satisfying exact self-similarity in space and time. For rotation rates and field strengths typical of dense low-mass cores, the main feature remains the flattening of the mass distribution along field lines-the formation of a pseudodisk, as in the nonrotating cases. The density distribution of the pseudodisk is little affected by rotation. On the other hand, the rotation rate is strongly modified by pseudodisk formation. Most of the centrally accreted material reaches the vicinity of the protostar through the pseudodisk. The specific angular momentum can be greatly reduced on the way, by an order of magnitude or more, even when the precollapse field strength is substantially below the critical value for dominant cloud support. The efficient magnetic braking is due to the pinched geometry of the magnetic field in the pseudodisk, which strengthens the magnetic field and lengthens the level arm for braking. Both effects enhance the magnetic transport of angular momentum from inside to outside. The excess angular momentum is carried away in a low-speed outflow that has, despite claims made by other workers, little in common with observed bipolar molecular outflows. We discuss the implications of our calculations for the formation of true disks that are supported against gravity by rotation. Title: Collapse of Magnetized Singular Isothermal Toroids. I. The Nonrotating Case Authors: Allen, Anthony; Shu, Frank H.; Li, Zhi-Yun Bibcode: 2003ApJ...599..351A Altcode: 2003astro.ph.11376A We study numerically the collapse of nonrotating self-gravitating magnetized singular isothermal toroids characterized by sound speed, a, and level of magnetic to thermal support, H0. In qualitative agreement with treatments by Galli & Shu and other workers, we find that the infalling material is deflected by the field lines toward the equatorial plane, creating a high-density flattened structure, a pseudodisk. The pseudodisk contracts dynamically in the radial direction, dragging the field lines and threading them into a highly pinched configuration that resembles a split monopole. The oppositely directed field lines across the midplane and the large implied stresses may play a role in how magnetic flux is lost in the actual situation in the presence of finite resistivity or ambipolar diffusion. The infall rate into the central regions is given to 5% uncertainty by the formula M=(1+H0)a3/G, where G is the universal gravitational constant, anticipated by semianalytical studies of the self-similar gravitational collapses of the singular isothermal sphere and isopedically magnetized disks. The introduction of finite initial rotation results in a complex interplay between pseudodisk and true (Keplerian) disk formation that is examined in a companion paper. Title: Radio Free-Free Emission from Young Stellar Objects Authors: Shang, H.; Lizano, S.; Glassgold, A.; Shu, F. Bibcode: 2003AAS...203.3305S Altcode: 2003BAAS...35R1257S We use x-winds heated and ionized by the physical mechanisms discussed in earlier communications by our group to model the free-free emission at centimeter wavelengths of the jets from young stellar objects (YSOs). With stellar parameters, mass-loss rates, and x-ray luminosities characteristic of Class 0 or Class I YSOs, we recover realistic spectral indices and radio maps. In particular, the 3.6 cm flux in models with mass loss rates ∼ 10-6 M⊙/yr or higher are comparable to existing observations of radio jets at 0".1 resolution. Lower mass-loss rates characteristic of Class II YSOs with revealed optical jets generally produce radio jets that are too weak to be detected at current sensitivity levels. We discuss the implications of these comparisons for theoretical models of the YSO outflow phenomenon. Title: Branch, Spur, and Feather Formation in Spiral Galaxies Authors: Chakrabarti, S.; Laughlin, G.; Shu, F. H. Bibcode: 2003ApJ...596..220C Altcode: 2003astro.ph..6472C We use hydrodynamic simulations to investigate the response of geometrically thin, self-gravitating, singular isothermal disks of gas to imposed rigidly rotating spiral potentials. By minimizing reflection-induced feedback from boundaries, and by restricting our attention to models where the swing parameter X~10, we minimize the swing amplification of global normal modes even in models where Toomre's Qg~1-2 in the gas disk. We perform two classes of simulations: short-term ones over a few galactic revolutions where the background spiral forcing is large, and long-term ones over many galactic revolutions where the spiral forcing is considerably smaller. In both classes of simulations, the initial response of the gas disk is smooth and mimics the driving spiral field. At late times, many of the models evince substructure akin to the so-called branches, spurs, and feathers observed in real spiral galaxies. We comment on the parts played respectively by ultraharmonic resonances, reflection off internal features produced by nonlinear dredging, and local, transient, gravitational instabilities within spiral arms in the generation of such features. Our simulations reinforce the idea that spiral structure in the gaseous component becomes increasingly flocculent and disordered with the passage of time, even when the background population of old disk stars is a grand-design spiral. We speculate that truly chaotic behavior arises when many overlapping ultraharmonic resonances develop in reaction to an imposed spiral forcing that has itself a nonlinear, yet smooth, wave profile. Title: Chondritic meteorites and X-wind Authors: Shu, Frank H. Bibcode: 2003GeCAS..67R.435S Altcode: No abstract at ADS Title: The Epoch of Planetesimal Formation Authors: Youdin, A. N.; Shu, F. H. Bibcode: 2003AAS...202.2405Y Altcode: 2003BAAS...35..729Y The Epoch of Planetesimal Formation

Our research addresses the hypothesis that kilometer sized planetesimals form directly by the gravitational fragmentation of a layer of small (⪉ cm) midplane solids. The advantage of this mechanism is that one need not rely on uncertain particle sticking efficiencies. It is particularly advantageous to bypass the difficulties of pairwise collisional growth in the meter size range. However small solids are tightly coupled to the gas motions. Any midplane turbulence must therefore be characterized by very low α values to allow efficient settling to the midplane. Thus, for the gravitational instability mechanism, planetesimals should form relatively late in the lifetime of a YSO, e.g. near the boundary between Class II (classical) and Class III (weak-lined) T-Tauri stars. Additionally our model argues for a gradual buildup of the solid/gas ratio of protoplanetary disks which greatly aids particle settling to the midplane. Several mechanisms (bipolar outflows, radial migration, photoionization, and layered accretion) can contribute to this enrichment on relevant 105 to 106 year timescales.

The late formation of planetesimals has implications for the mass of planetary systems and for the metallicity enrichment of gas giant planets and their host stars. The talk will also present recent results on the non-linear development of the two phase (gas and solid) gravitational instability. The focus will be on the timescale of this process and the implications for subsequent phases of planet formation. Title: Early Solar System Irradiation and Beryllium-7 Synthesis Authors: Gounelle, M.; Shang, S.; Glassgold, A. E.; Shu, F. H.; Rehm, E. K.; Lee, T. Bibcode: 2003LPI....34.1833G Altcode: In the framework of the x-wind model, we have calculated the yields of the extinct short-lived 7Be (T=53 days) produced via irradiation by protoSolar Energetic Particles. 7Be -and other radionuclides- yields are compatible with initial abundances inferred from meteorites. Title: Relativistic Singular Isothermal Toroids Authors: Cai, Mike J.; Shu, Frank H. Bibcode: 2003ApJ...583..391C Altcode: 2003astro.ph..2474C We construct self-similar, axisymmetric, time-independent solutions to Einstein's field equations for an isothermal gas with a flat rotation curve in the equatorial plane. The metric scales as ds2-->α2ds2 under the transformation r-->αr and t-->α1-nt, where n is a dimensionless measure of the strength of the gravitational field. The solution space forms a two-parameter family characterized by the ratios of the isothermal sound speed and the equatorial rotation speed to the speed of light. The isodensity surfaces are toroids, empty of matter along the rotation axis. Unlike the Newtonian case, the velocity field is not constant on a cylindrical radius because of frame dragging. As the configuration rotates faster, an ergoregion develops in the form of the exterior of a cone centered about the rotation axis. The sequence of solutions terminates when frame dragging becomes infinite and the ergocone closes onto the axis. The fluid velocity of the last solution has a modest value in the midplane but reaches the speed of light on the axis. Title: Collapse of Magnetized, Rotating, Toroidal Clouds Authors: Allen, Anthony; Shu, Frank; Li, Zhi-Yun; Choi, Minho; Chuang, Hui-Wen Bibcode: 2003IAUS..221P..85A Altcode: Models of molecular cloud cores have been proposed by Frank Shu and Zhi-Yun Li. These models have been used as initial states for magnetohydrodynamic collapse calculations to study the dynamics of the early stages of star formation. Implications about accretion rates outflows and angular momentum transport will be summarized. Synthetic maps and spectra generated from simulation data will be shown. Title: Champagne Flows and Winds in H II Regions Authors: Lizano, S.; Galli, D.; Shu, F.; Cantó, J. Bibcode: 2003RMxAC..15..166L Altcode: We discuss the expansion of an initially self-gravitating, static, singular cloud core characterized by a power-law density distribution, rho ~ r^-n, with 3/2 < n < 3. This core is heated out of mechanical balance by the formation of a massive star at its center. If the initial ionization and heating is approximated to occur instantaneously at t = 0, the subsequent flow (for r >> 100 AU) caused by the resulting imbalance between self-gravity and thermal pressure is self-similar. Because of the steep density profile, pressure gradients produce a shock front that travels into the cloud, accelerating the gas to supersonic velocities in what has been called the ``champagne phase''. The expansion of the inner region at t > 0 is connected to the outer envelope of the now ionized cloud core through this shock whose strength is an increasing function of the exponent n. We also discuss the evolution of the strong stellar winds of massive stars inside these champagne flows. Title: CVN Correlator and Its Future Authors: Zhang, X.; Zheng, W.; Shu, F.; Han, Z.; Xiang, Y.; Chen, Z.; Zhu, R. Bibcode: 2003ASPC..306..287Z Altcode: 2003ntvl.conf..287Z No abstract at ADS Title: Self-similar Champagne Flows in H II Regions Authors: Shu, Frank H.; Lizano, Susana; Galli, Daniele; Cantó, Jorge; Laughlin, Gregory Bibcode: 2002ApJ...580..969S Altcode: 2002astro.ph..9036S We consider the idealized expansion of an initially self-gravitating, static, singular, isothermal cloud core. For t>=0, the gas is ionized and heated to a higher uniform temperature by the formation of a luminous but massless star in its center. The approximation that the mass and gravity of the central star are negligible for the subsequent motion of the H II region holds for distances r much greater than ~100 AU and for the massive cloud cores that give rise to high-mass stars. If the initial ionization and heating are approximated to occur instantaneously at t=0, then the subsequent flow (for r>>100 AU) caused by the resulting imbalance between self-gravity and thermal pressure is self-similar. Because of the steep density profile (ρ~r-2), pressure gradients produce a shock front that travels into the cloud, accelerating the gas to supersonic velocities in what has been called the ``champagne phase.'' The expansion of the inner region at t>0 is connected to the outer envelope of the now ionized cloud core through this shock, whose strength depends on the temperature of the H II gas. In particular, we find a modified Larson-Penston (L-P) type of solution as part of the linear sequence of self-similar champagne outflows. The modification involves the proper insertion of a shock and produces the right behavior at infinity (v-->0) for an outflow of finite duration, reconciling the long-standing conflict on the correct (inflow or outflow) interpretation for the original L-P solution. For realistic heating due to a massive young central star that ionizes and heats the gas to ~104 K, we show that even the self-gravity of the ionized gas of the massive molecular cloud core can be neglected. We then study the self-similar solutions of the expansion of H II regions embedded in molecular clouds characterized by more general power-law density distributions: ρ~r-n with 3/2<n<3. In these cases, the shock velocity is an increasing function of the exponent n and diverges as n-->3. We show that this happens because the model includes an origin where the pressure driving the shock diverges because the enclosed heated mass is infinite. Our results imply that the continued photoevaporation of massive reservoirs of neutral gas (e.g., surrounding disks and/or globules) near the embedded ionizing source is required in order to maintain over a significant timescale the emission measure observed in champagne flows. Title: Planetesimal Formation by Gravitational Instability Authors: Youdin, Andrew N.; Shu, Frank H. Bibcode: 2002ApJ...580..494Y Altcode: 2002astro.ph..7536Y We investigate the formation of planetesimals via the gravitational instability of solids that have settled to the midplane of a circumstellar disk. Vertical shear between the gas and a subdisk of solids induces turbulent mixing that inhibits gravitational instability. Working in the limit of small, well-coupled particles, we find that the mixing becomes ineffective when the surface density ratio of solids to gas exceeds a critical value. Solids in excess of this precipitation limit can undergo midplane gravitational instability and form planetesimals. However, this saturation effect typically requires increasing the local ratio of solid to gaseous surface density by factors of 2-10 times cosmic abundances, depending on the exact properties of the gas disk. We discuss existing astrophysical mechanisms for augmenting the ratio of solids to gas in protoplanetary disks by such factors and investigate a particular process that depends on the radial variations of orbital drift speeds induced by gas drag. This mechanism can concentrate millimeter-sized chondrules to the supercritical surface density in <=few×106 yr, a suggestive timescale for the disappearance of dusty disks around T Tauri stars. We discuss the relevance of our results to some outstanding puzzles in planet formation theory-the size of the observed solar system and the rapid type I migration of Earth-mass bodies. Title: Activity in Very Cool Stars: Magnetic Dissipation in Late M and L Dwarf Atmospheres Authors: Mohanty, Subhanjoy; Basri, Gibor; Shu, Frank; Allard, France; Chabrier, Gilles Bibcode: 2002ApJ...571..469M Altcode: 2002astro.ph..1518M Recent observations show that chromospheric Hα activity in late M and L dwarfs is much lower than in the earlier M types. This is particularly surprising given that the late M and L dwarfs are comparatively very rapid rotators: in the early M dwarfs, rapid rotation is associated with high activity levels. One possibility is that the drop-off in activity in the late M and L dwarfs is a result of very high electrical resistivities in their dense, cool, and predominantly neutral atmospheres.We calculate the magnetic field diffusivity in the atmospheres of objects with Teff in the range 3000-1500 K (mid M to late L) using the atmospheric structure models of Allard and Hauschildt. We find that the combination of very low ionization fraction and high density in these atmospheres results in very large resistivities and thus efficient field diffusion. While both ambipolar diffusion and Ohmic decay of currents due to ion-electron collisions occur, the primary diffusion effects are due to current decay through collisions of charged particles with neutrals. Moreover, the latter resistivity is a strong function of both effective temperature and optical depth, increasing rapidly as either Teff or optical depth decreases. This has two implications: (1) Any magnetic field present is increasingly decoupled from atmospheric fluid motions as one moves from mid M to L. In the late M and L dwarfs, atmospheric motions cannot lead to equilibrium field configurations very different from potential ones. That is, the magnitude of magnetic stresses generated by atmospheric motions is very small in these objects. We quantify this effect by a simple Reynolds number calculation. (2) Even if magnetic stresses are easily produced by fluid motions in the hot interior (where the coupling between field and matter is good), their propagation up through the atmosphere will be increasingly hampered by the growing atmospheric resistivity as one moves from mid M to late L. Thus both the generation and propagation of magnetic stresses are increasingly damped with decreasing Teff in these cool dwarfs. As a result, the magnetic free energy available for the support of a chromosphere, and activity becomes smaller and smaller with later type. This can account for the observed drop in Hα activity from mid M to L, assuming that activity in these dwarfs is magnetically driven. To check the latter assumption, we estimate the emergent acoustic fluxes in these objects through a Lighthill-Proudman calculation. While the acoustic fluxes also decrease with decreasing Teff, they appear inadequate to explain the observed Hα fluxes in mid M to L dwarfs. In the absence of acoustic heating, magnetic heating indeed seems the most viable way of generating activity. We emphasize that our calculations are equilibrium ones and do not address time-dependent phenomena. We also do not examine the highest atmospheric layers, where correction is not expected, but which we show are rarefied enough to permit charged particles to remain coupled to the field. Finally, while our calculations do not address flares in late M and L dwarfs, we speculate that the latter could be created by buoyant flux tubes that are generated in the interior and rise rapidly through the atmosphere, dissipating their associated currents in the upper atmospheric layers. Title: Star-Disk Coupling Mechanisms Authors: Shu, F. H. Bibcode: 2002AAS...200.5406S Altcode: 2002BAAS...34..731S We attempt to clarify the confusion concerning angular-momentum coupling mechanisms when closed and open magnetic fields originating from a young star thread through a surrounding disk. We argue that the traditional Ghosh & Lamb description represents only a transient behavior that does not account for important longer-term effects that arise because of accretion and if the disk is highly, but imperfectly, electrically conducting. In the latter case, we argue that the steady-state response of the system is to form a funnel-flow/x-wind geometry. We describe approximate, self-consistent, calculations of the gas flow for the case when the unperturbed magnetic-field configuration of the star would have been a pure dipole in the absence of the disk. We show that the disk-star interactions considerably modifies the actual magnetospheric structure of the system. We also show calculations where we drop the assumption that the unperturbed magnetosphere is a pure dipole. As long as the radius of the inner edge of the disk is a few or more times the radius of the star, we find that the properties of the x-wind are little changed by the relaxation of the dipole assumption. However, the size and geometry of the hot spots where the funnel flow impacts the star can be greatly affected by the exact mixture of multipoles chosen to model the magnetic fields on the stellar surface. The crucial invariant in our theory is the amount of trapped flux required to truncate a disk of a certain accretion rate before the flow reaches the equator of a star of given mass. We present empirical evidence that trapped flux is indeed the relevant concept for the explanation of the hot-spot properties of T Tauri stars. We close with a qualitative discussion of the limits of the validity of the concept of disk locking. This research is supported in part by grants from NASA and the National Science Foundation. Title: Relativistic Self-similar Equilibria and Non-axisymmetric Neutral Modes Authors: Cai, Mike J.; Shu, F. H. Bibcode: 2002AAS...200.8004C Altcode: 2002BAAS...34..781C We have constructed semi-analytic axisymmetric scale free solutions to Einstein field equations with perfect fluid matter source. These spacetimes are self-similar under the simultaneous transformation r'= ar and t'=a1-nt. We explored the two dimensional solution space parameterized by the rescaling index n and the isothermal sound speed γ 1/2. The isopycnic surfaces are in general toroids. As the equilibrium configuration rotates faster, an ergo region develops in the form of the exterior of a cone centered about the symmetry axis. The sequence of solution terminates when frame dragging becomes infinite and the ergo cone closes onto the axis. In the extreme flattening limit, we have also searched for non-axisymmetric neutral modes in a self-similar disk. Two separate sets of tracks are discovered in the solution space. One corresponds to the bifurcation points to non-axisymmetric equilibria, which is confined in the non-ergo solutions. The other track signals the onset of instability driven by gravitational radiation. These solutions are formally infinite in extent, and thus can not represent realistic astrophysical systems. However, if these properties do not alter qualitatively when the self-similar configurations are truncated, then these solutions may serve as initial data for dynamic collapse in super massive black hole formation. Title: Relativistic Self-similar Disks Authors: Cai, Mike J.; Shu, Frank H. Bibcode: 2002ApJ...567..477C Altcode: 2001astro.ph.11344C We formulate and solve by semianalytic means the axisymmetric equilibria of relativistic self-similar disks of infinitesimal vertical thickness. These disks are supported in the horizontal directions against their self-gravity by a combination of isothermal (two-dimensional) pressure and a flat rotation curve. The dragging of inertial frames restricts possible solutions to rotation speeds that are always less than 0.438 times the speed of light, a result first obtained by Lynden-Bell & Pineault in 1978 for a cold disk. We show that prograde circular orbits of massive test particles exist and are stable for all of our model disks but that retrograde circular orbits cannot be maintained with particle velocities less than the speed of light once the disk develops an ergoregion. We also compute photon trajectories, planar and nonplanar, in the resulting spacetime for disks with and without ergoregions. We find that all photon orbits, except for a set of measure zero, tend to be focused by the gravity of the flattened mass-energy distribution toward the plane of the disk. This result suggests that strongly relativistic, rapidly rotating, compact objects may have difficulty ejecting collimated beams of matter or light along the rotation axes until the flows get well beyond the flattened parts of the relativistic mass distribution (which cannot happen in the self-similar models considered in this paper). Title: Heating and Ionization of X-Winds Authors: Shang, Hsien; Glassgold, Alfred E.; Shu, Frank H.; Lizano, Susana Bibcode: 2002ApJ...564..853S Altcode: 2001astro.ph.10539S In order to compare the X-wind with observations, one needs to be able to calculate its thermal and ionization properties. We formulate the physical basis for the streamline-by-streamline integration of the ionization and heat equations of the steady X-wind. In addition to the well-known processes associated with the interaction of stellar and accretion funnel hot spot radiation with the wind, we include X-ray heating and ionization, mechanical heating, and a revised calculation of ambipolar diffusion heating. The mechanical heating arises from fluctuations produced by star-disk interactions of the time-dependent X-wind that are carried by the wind to large distances where they are dissipated in shocks, MHD waves, and turbulent cascades. We model the time-averaged heating by the scale-free volumetric heating rate, Γmech=αρv3s-1, where ρ and v are the local mass density and wind speed, respectively, s is the distance from the origin, and α is a phenomenological constant. When we consider a partially revealed but active young stellar object, we find that choosing α~10-3 in our numerical calculations produces temperatures and electron fractions that are high enough for the X-wind jet to radiate in the optical forbidden lines at the level and on the spatial scales that are observed. We also discuss a variety of applications of our thermal-chemical calculations that can lead to further observational checks of X-wind theory. Title: Collapse of Magnetised, Singular Isothermal Toroids Authors: Allen, Anthony; Shu, Frank; Li, Zhi-Yun Bibcode: 2001JKAS...34..325A Altcode: This poster summarizes numerical collapse calculations of non-rotating and rotating singular, isothermal toroids that employed the zeus2d (Norman and Stone 1992) magnetohydrodynamics package. In the non-rotating collapse calculations, it is seen that infall proceeds at a constant rate and magnetically supported, high density pseudo-disks form in the equatorial plane. With rotating clouds, however, toroidal magnetic fields grow as infall proceeds, teaming with angular momentum to slow the inflow to the center and generate outflow. Title: Heating and Ionization of X-Winds Authors: Shang, H.; Glassgold, A. E.; Shu, F. H.; Lizano, S. Bibcode: 2001AAS...198.2801S Altcode: 2001BAAS...33..824S We formulate the physical basis for a streamline-by-streamline integration of the ionization and heat equations of the steady x-wind. In addition to th well-known processes associated with the interaction of stellar and accretion-funnel hot-spot radiation with the wind, we include X-ray heating and ionization, mechanical heating, and a new calculation of ambipolar diffusion heating. The mechanical heating arises from the fluctuations produced by the time dependent x-wind star-disk interaction, which generates magnetic fluctuations at the source that are carried by the wind to large distances where they are dissipated in shocks, MHD waves, and turbulent cascades. We model the time-averaged heating by the scale-free volumetric heating rate Γ mech = α ρ v3 s-1, where ρ and v are the local mass density and wind speed, respectively, s is the distance from the origin, and α is a phenomenological constant. When we consider a revealed but active young stellar object, we find that choosing α ~ 10-3 in our numerical calculations produces temperatures and electron fractions that are high enough for the x-wind jet to radiate in the optical forbidden lines at the level and on the spatial and kinematic scales that are observed. We also discuss a variety of applications of our thermal-chemical calculations that lead to further observational checks of the theory. This research has been supported in part by the NSF. Title: The X-Wind Theory for the Origin of Chondritic Meteorites Authors: Shu, F. H. Bibcode: 2001eag..conf.3700S Altcode: No abstract at ADS Title: Ambipolar Diffusion and X-ray Heating of the X-wind Jet Authors: Glassgold, A. E.; Shang, H.; Shu, F. H.; Lizano, S. Bibcode: 2001AAS...198.2802G Altcode: 2001BAAS...33..824G Ambipolar diffusion and X-ray interactions have the potential to heat the inner part (or jet) of the X-wind. We have recalculated the ambipolar diffusion coupling coefficient for atomic regions (where H+ is the dominant ion) using recently calculated cross sections by Krstíc and Schultz and by others. The coefficient for hot regions (T ≈ 104 K) is an order of magnitude larger than the values used in the past because the short-range part of the H-H+ interaction (including exchange effects) is larger than previously assumed. Consequently ambipolar diffusion plays only a minor role in heating the inner part of the X-wind and very likely in YSO jets in general. We also find that X-ray heating is enhanced in atomic regions through the significant probability for populating excited levels of the H atom in collisions with X-ray induced secondary electrons (e.g., Dalgarno, Yan, and Liu 1999). The standard Lyman-alpha cooling must then be reduced by an amount that can be greater than direct collisional X-ray heating. The excess population in the n = 2 level leads to heating by collisional de-excitation in the presence of trapped Lyman-alpha line radiation. However, even with this enhancement, X-ray heating is usually less important than the main heating mechanism for the inner X-wind, which involves the dissipation of shock and turbulent energy. This research has been supported in part by the NSF. Title: Self-similar Relativisitic Disks revisted Authors: Cai, M. J.; Shu, F. H. Bibcode: 2001AAS...198.3706C Altcode: 2001BAAS...33..836C We revisit the rotating self-similar disk first studied by Lynden-Bell and Pineault and extend it to include pressure. A two-parameter family of solutions is constructed numerically. These disks are parameterized by the constant linear rotation velocity v, and the isothermal sound speed γ 1/2. For sufficiently high velocities, an ergo region develops in the form of the exterior of a cone. For each value of γ , there is a maximum velocity vc above which there is no equilibrium solutions. For this solution the frame dragging is infinite and the ergo cone closes on the rotation axis. The null geodesic equations are also integrated numerically. Due to the infinite extend and mass of the system, all photon trajectories are focused towards the disk. The behavior of equatorial photons orbits is qualitatively the same as that of cold disks. Title: Singular Isothermal Disks. II. Nonaxisymmetric Bifurcations and Equilibria Authors: Galli, Daniele; Shu, Frank H.; Laughlin, Gregory; Lizano, Susana Bibcode: 2001ApJ...551..367G Altcode: 2000astro.ph.12242G We review the difficulties of the classical fission and fragmentation hypotheses for the formation of binary and multiple stars. A crucial missing ingredient in previous theoretical studies is the inclusion of dynamically important levels of magnetic fields. As a minimal model for a candidate precursor to the formation of binary and multiple stars, we therefore formulate and solve the problem of the equilibria of isopedically magnetized, singular isothermal disks, without the assumption of axial symmetry. Considerable analytical progress can be made if we restrict our attention to models that are scale-free, i.e., that have surface densities that vary inversely with distance ϖ from the rotation axis of the system. In agreement with earlier analysis by Syer & Tremaine, we find that lopsided (M=1) configurations exist at any dimensionless rotation rate, including zero. Multiple-lobed (M=2, 3, 4,...) configurations bifurcate from an underlying axisymmetric sequence at progressively higher dimensionless rates of rotation, but such nonaxisymmetric sequences always terminate in shock waves before they have a chance to fission into M=2, 3, 4,... separate bodies. On the basis of our experience in this paper and the preceding Paper I, we advance the hypothesis that binary and multiple star formation from smooth (i.e., not highly turbulent) starting states that are supercritical but in unstable mechanical balance requires the rapid (i.e., dynamical) loss of magnetic flux at some stage of the ensuing gravitational collapse. Title: Refractory Phases of Micrometeorites and the ``Primitivity'' of Cometary Nuclei Authors: Gounelle, M.; Maurette, M.; Engrand, C.; Kurat, G.; Shu, F. Bibcode: 2001LPI....32.1626G Altcode: We used common characteristics of refractory phases in modern and early micrometeorites to both further constrain the scenarios proposed for the origin of the solar system and confirm that cometary nuclei are not as primitive as previously thought. Title: The Origin of Chondrules and Refractory Inclusions in Chondritic Meteorites Authors: Shu, Frank H.; Shang, Hsien; Gounelle, Matthieu; Glassgold, Alfred E.; Lee, Typhoon Bibcode: 2001ApJ...548.1029S Altcode: Examples of calcium-aluminum-rich inclusions (CAIs) surrounded by thick chondrule mantles have been found in chondritic meteorites and cast doubt on the conventional belief that CAIs and chondrules possessed different spacetime origins in the primitive solar nebula. We study specific processes by which such objects, and the more common ordinary CAIs and chondrules, might have formed by flare heating of primitive rocks interior to the inner edge of a gaseous accretion disk that has been truncated by magnetized funnel flow onto the central proto-Sun. Motivated by the appearance of the chains of Herbig-Haro knots that define collimated optical jets from many young stellar objects (YSOs), we adopt the model of a fluctuating X-wind, where the inner edge of the solar nebula undergoes periodic radial excursions on a timescale of ~30 yr, perhaps in response to protosolar magnetic cycles. Flares induced by the stressing of magnetic fields threading both the star and the inner edge of the fluctuating disk melt or partially melt solids in the transition zone between the base of the funnel flow and the reconnection ring, and in the reconnection ring itself. The rock melts stick when they collide at low velocities. Surface tension pulls the melt aggregate into a quasi-spherical core/mantle structure, where the core consists mainly of refractories and the mantle mainly of moderate volatiles. Orbital drift of rocks past the inner edge of the disk or infall of large objects from the funnel flow replaces the steady loss of material by the plasma drag of the coronal gas that corotates with the stellar magnetosphere. In quasi-steady state, agglomeration of molten or heat-softened rocks leads to a differential size-distribution in radius R proportional to R-3e-Lt/tLR, where tL~20 yr is the drift time of an object of fiducial radius L≡1 cm and t is the time since the last inward excursion of the base of the funnel flow and X-wind. Thus, during the ~30 yr interval between successive flushing of the reconnection ring, flash-heated and irradiated rocks have a chance to grow to millimeter and centimeter sizes. The evaporation of the moderately volatile mantles above large refractory cores, or the dissolving of small refractory cores inside thick ferromagnesian mantles before launch, plus extended heating in the X-wind produce the CAIs or chondrules that end up at planetary distances in the parent bodies of chondritic meteorites. Title: Extinct Radioactivities and Protosolar Cosmic Rays: Self-Shielding and Light Elements Authors: Gounelle, Matthieu; Shu, Frank H.; Shang, Hsien; Glassgold, A. E.; Rehm, K. E.; Lee, Typhoon Bibcode: 2001ApJ...548.1051G Altcode: We study the effects of self-shielding in the X-wind model of protosolar cosmic-ray irradiation of early solar-system rocks. We adopt a two-component picture of protoCAIs consisting of cores with the elemental abundances of type B1 CAIs (calcium-aluminum-rich inclusions) and mantles of less refractory material. The cores have a power-law distribution of sizes between Rmin and Rmax. The mantles have a uniform thickness, whose value is chosen to bring the total inventory of elements at least as refractory as sulfur to cosmic abundances for the entire population of protoCAIs. Each object is irradiated with a fluence consistent with the product of their residence time in the reconnection ring and the flux of solar cosmic rays obtained by a scaling of impulsive flares from the hard X-rays observed from low-mass protostars. For Rmin in the 50 μm regime and Rmax in the few centimeter regime, which corresponds to the range of sizes of observed CAIs in micrometeorites and chondrites, we recover approximately the canonical values quoted for the ratios 26Al/27Al, 53Mn/55Mn, and 41Ca/40Ca in CV3 meteorites. Moreover, the excess 138La (denoted as 138La*) produced by proton bombardment of 138Ba lies within the CAI range obtained in the experiments of Shen et al. When we include fragmentation reactions that produce 10Be from the impact of protons, alphas, and 3He on the 16O that is bound up in rocks, we further obtain a level of 10Be/9Be that agrees approximately with the report of McKeegan et al. for a CAI from the Allende meteorite. Similar calculations for the expected anomalies in the stable isotopes of lithium show rough consistency with the measured values and further support our interpretation. The value for 10Be/9Be is particularly difficult to produce by any other astrophysical mechanism. Thus, the 10Be discovery greatly strengthens the case for an origin in early solar-system irradiation, rather than external stellar seeding, for the shortest-lived radionuclides inferred from CAIs in chondritic meteorites. Title: Singular Isothermal Disks and the Formation of Multiple Stars Authors: Galli, D.; Shu, F. H.; Laughlin, G.; Lizano, S. Bibcode: 2001ASPC..243...93G Altcode: 2000astro.ph.12019G; 2001fdtl.conf...93G A crucial missing ingredient in previous theoretical studies of fragmentation is the inclusion of dynamically important levels of magnetic fields. As a minimal model for a candidate presursor to the formation of binary and multiple stars, we therefore consider the equilibrium configuration of isopedically magnetized, scale-free, singular isothermal disks, without the assumption of axial symmetry. We find that lopsided (M=1) configurations exist at any dimensionless rotation rate, including zero. Multiple-lobed (M = 2, 3, 4, ...) configurations bifurcate from an underlying axisymmetric sequence at progressively higher dimensionless rates of rotation, but such nonaxisymmetric sequences always terminate in shockwaves before they have a chance to fission into separate bodies. We advance the hypothesis that binary and multiple star-formation from smooth (i.e., not highly turbulent) starting states that are supercritical but in unstable mechanical balance requires the rapid (i.e., dynamical) loss of magnetic flux at some stage of the ensuing gravitational collapse. Title: Time Scale of Disk and Planet Formation Authors: Shu, Frank H. Bibcode: 2001ASPC..245...89S Altcode: 2001aats.conf...89S No abstract at ADS Title: Excitation Conditions and Line Ratios from X-wind Jets Authors: Shang, H.; Glassgold, A. E.; Shu, F. H.; Lizano, S. Bibcode: 2000AAS...19713209S Altcode: 2001BAAS...33R.718S Thermal structures of the X-wind proves to be a necessary yet very important foundation for realistic modeling of the jet phenomena. X-rays and magnetic disturbances emanating from the central source, are identified as the self-sustaining energy generators in the x-wind star-disk interacting system. Following the demonstration of density structure and kinematic information in X-winds in Shang et al. (1998) with the assumption of uniform ionization conditions to calculate the emissions in the [S II]λ 6716/6731 and [O I]λ 6300/6364 lines, we investigate the self-consistent excitation conditions powered by the energy sources in the X-wind system. We perform streamline-by-streamline integrations of the rate and heat equations on the steady-state x-wind. Results show that X-rays are capable of providing typical ionization fractions inferred from the optical jets. We model the external heating mechanism as he averaging of mechanical energies deposited by shocks and/or MHD turbulence cascades. Shockwaves or turbulence cascades of magnetic origins are needed to provide the sufficiently high temperatures in enough emitting areas of the jets observed in the forbidden lines at thousands of AU from the stars to maintain the excitation. A good demonstration of such diagnostics would be line ratios from the optical forbidden lines. We will report in this presentation that the electron densities and temperatures achieved in the overall self-consistent model yield line ratios similar to those observed in a large set of YSO jet sources. Title: What Do Meteorites Tell Us about the Formation of the Solar System? Authors: Shu, F. H. Bibcode: 2000AAS...197.0201S Altcode: 2000AAS...197..201S; 2000BAAS...32.1385S The existence of chondritic meteorites has posed deep puzzles for our understanding of the origin of the solar system for two hundred years. Chondrites are believed to be primitive material that recorded the physical conditions under which solids in the solar nebula aggregated to form the interiors of the terrestrial planets and the cores of the giant planets. Chondritic meteorites contain a grainy matrix that has obviously never been heated to more than a few hundred K, intimately mixed with mm- to cm-sized balls of rock that have been brought briefly (hours for chondrules to days for refractory inclusions) to the melting temperatures of iron-magnesium silicates or calcium-aluminum silicates and oxides. Moreover, the calcium-aluminum-rich inclusions (CAIs) contain evidence for short-lived radioactivities that should have been long extinct at the birth of the sun and its planets, and yet were clearly alive at the origin of the meteoritical parent bodies. According to modern astronomical observations and theory, the solar nebula was not nearly hot enough to melt rocks at the distance of the asteroid belt where the parent bodies of meteorites reside, nor are time scales as short as hours to days natural for such regions. I will review a new theory developed by our group which suggests that chondrules and CAIs originated much closer to the protosun, and that they were irradiated by protosolar flares and cosmic-ray particles in the fierce interaction region between a strongly magnetized protosun and its surrounding protoplanetary disk, before being thrown to the distances of their modern host bodies by a powerful bipolar jet that is another byproduct of this interaction. I will present evidence unearthed since the initial development of the theory that supports its assumptions and conclusions, and I will conclude with some brief remarks about the changes brought to the question of our astronomical origins by the new perspective. This work was funded by NASA's program on Origins of Solar Systems. Title: Making Calcium-Aluminum-rich Inclusions and Chondrules near the Young Sun by Flares Authors: Shu, F. H.; Shang, H.; Gounelle, M.; Glassgold, A. E.; Lee, T. Bibcode: 2000M&PSA..35Q.145S Altcode: No abstract at ADS Title: Extinct Radioactivities in Meteorites and Protosolar Cosmic Rays: The Effect of Self-Shielding Authors: Gounelle, M.; Shu, F. H.; Shang, S.; Glassgold, A. E.; Rehm, K. E.; Lee, T. Bibcode: 2000M&PSA..35R..62G Altcode: No abstract at ADS Title: A Toy Model of Giant Molecular Clouds Authors: Allen, Anthony; Shu, Frank H. Bibcode: 2000ApJ...536..368A Altcode: We idealize giant molecular clouds as flattened sheets, incorporating the concepts of strong magnetization, star formation from dense cores, and efficient bipolar outflows. This toy model reproduces the observed tendency of molecular clouds to form filaments without the need to invoke large-scale overall gravitational collapse that would yield a rate of star formation far in excess of empirical Galactic values. It yields line width-size relationships that are in rough accord with observations, although better simulations are needed that remove the imposition of periodic boundary conditions and allow for a more systematic treatment of subgrid turbulence. The model lends credence to earlier ideas concerning the self-regulation of star formation by turbulence and photoionization. Title: X-Winds Theory and Observations Authors: Shu, F. H.; Najita, J. R.; Shang, H.; Li, Z. -Y. Bibcode: 2000prpl.conf..789S Altcode: We review the theory of x-winds in young stellar objects (YSOs), and we compare its predictions with a variety of astronomical observations. Such flows arise magnetocentrifugally from accretion disks when their inner edges interact with strongly magnetized central stars. X-winds collimate logarithmically slowly into jets, and their interactions with the surrounding molecular cloud cores of YSOs yield bipolar molecular outflows. Title: Singular Isothermal Disks. I. Linear Stability Analysis Authors: Shu, Frank H.; Laughlin, Gregory; Lizano, Susana; Galli, Daniele Bibcode: 2000ApJ...535..190S Altcode: As part of a larger effort to understand how binary and single stars form from the collapse of magnetized molecular cloud cores, we perform a global stability analysis of isopedically magnetized, singular isothermal disks (SIDs). The work described here has precedents in earlier studies of disturbances in power-law disks by Zang in 1976, Toomre in 1977, Lynden-Bell & Lemos in 1993, Syer & Tremaine in 1996, and Goodman & Evans in 1999. We find the analytic criteria for the bifurcation of axisymmetric disks into nonaxisymmetric forms with azimuthal periodicities m=1 (eccentric displacements), 2 (oval distortions), 3 (triangular distortions), etc. These bifurcations, which occur at zero frequency, are the compressible and differentially rotating analogs of how the classical sequence of incompressible and uniformly rotating Maclaurin spheroids bifurcate (secularly, under dissipative forces) to become Dedekind ellipsoids with figure axes that remain fixed in space. Like Syer & Tremaine and Lynden-Bell & Lemos, we also find that zero-frequency logarithmic spirals are possible scale-free disturbances, but our interpretation of the existence of such steadily propagating wavetrains is different. We give a dynamical instability interpretation based on the onset of swing amplification by overreflection at the corotation circle of prograde spiral density waves the pattern speeds of which have nonzero and positive values. Our analysis yields identical instability criteria as the global normal-modes treatment of Goodman & Evans, and we tentatively also identify dynamical barred-spiral instabilities as the ``breathing mode'' limit of two-armed ordinary-spiral instabilities. We prove a general ``reciprocity theorem,'' which states that the overreflection factors are identical for spiral density waves launched from cavities interior or exterior to Q-barriers that straddle the corotation circle. This globally valid result supports a unifying interpretation, advocated for many years by C. C. Lin and his colleagues (see, e.g., work by Bertin & Lin): the coexistence of spiral structure in galaxies arising from the instability of internal normal modes in the combined star/gas disk or from driving by external tidal influences associated with the chance passages of companion bodies. Title: Protostellar Winds and Chondritic Meteorites Authors: Shang, Hsien; Shu, Frank H.; Lee, Typhoon; Glassgold, Alfred E. Bibcode: 2000SSRv...92..153S Altcode: We discuss the interaction between the magnetosphere of a young star and its surrounding accretion disk. We consider how an X-wind can be driven magnetocentrifugally from the inner edge of the disk where accreting gas is diverted onto stellar field lines either to flow onto the Sun or to be flung outwards with the wind. The X-wind satisfies many observational tests concerning optical jets, Herbig-Haro objects, and molecular outflows. Connections may exist between primitive solar system materials and X-winds. Chondrules and calcium-aluminum-rich inclusions (CAIs) experienced short melting events uncharacteristic of the asteroid belt where meteorites originate. The inner edge of the solar nebula has the shortest orbital timescale available to the system, a few days. Protosolar flares introduce another timescale, tens of minutes to hours. CAIs may form when solids are lifted from shaded portions of the disk close to the Sun and are exposed to its intense light for a day or so before they are flung by the X-wind to much larger distances. Chondrules were melted, perhaps many times, by flares at larger distances from the Sun before being launched and annealed, but not remelted, in the X-wind. Aerodynamic sorting explains the narrow range of sizes with which CAIs and chondrules are found in chondritic meteorites. Flare-generated cosmic-rays may induce spallation reactions that produce some of the short-lived radioactivities associated with primitive solar system rocks. Title: Protostellar Winds and Chondritic Meteorites Authors: Shang, H.; Shu, F. H.; Lee, T.; Glassgold, A. E. Bibcode: 2000fdtp.conf..153S Altcode: No abstract at ADS Title: Singular Isothermal Disks Authors: Galli, Danielle; Shu, Frank H.; Laughlin, Gregory; Lizano, Susana Bibcode: 2000STIN...0107251G Altcode: We review the difficulties of the classical fission and fragmentation hypotheses for the formation of binary and multiple stars. A crucial missing ingredient in previous theoretical studies is the inclusion of dynamically important levels of magnetic fields. As a minimal model for a candidate presursor to the formation of binary and multiple stars, we therefore formulate and solve the problem of the equilibria of isopedically magnetized, singular isothermal disks, without the assumption of axial symmetry. Considerable analytical progress can be made if we restrict our attention to models that are scale-free, i.e., that have surface densities that vary inversely with distance omega from the rotation axis of the system. In agreement with earlier analysis by Syer and Tremaine, we find that lopsided (M = 1) configurations exist at any dimensionless rotation rate, including zero. Multiple-lobed (M = 2, 3, 4, ...) configurations bifurcate from an underlying axisymmetric sequence at progressively higher dimensionless rates of rotation, but such nonaxisymmetric sequences always terminate in shockwaves before they have a chance to fission into M = 2, 3, 4, ... separate bodies. On the basis of our experience in this paper, we advance the hypothesis that binary and multiple star-formation from smooth (i.e., not highly turbulent) starting states that are supercritical but in unstable mechanical balance requires the rapid (i.e., dynamical) loss of magnetic flux at some stage of the ensuing gravitational collapse. Title: Chandrasekhar's Study of Dynamical Friction Authors: Shu, Frank H. Bibcode: 1999ApJ...525C.347S Altcode: 1999ApJC..525..347S No abstract at ADS Title: Scale-free Equilibria of Isopedic Polytropic Clouds Authors: Galli, D.; Lizano, S.; Li, Z. Y.; Adams, F. C.; Shu, F. H. Bibcode: 1999ApJ...521..630G Altcode: 1999astro.ph..4028G We investigate the equilibrium properties of self-gravitating magnetized clouds with polytropic equations of state with negative index n. In particular, we consider scale-free isopedic configurations that have constant dimensionless spherical mass-to-flux ratio λr and that may constitute ``pivotal'' states for subsequent dynamical collapse to form groups or clusters of stars. For given Γ=1+1/n, equilibria with smaller values of λr are more flattened, ranging from spherical configurations with λr=∞ to completely flattened states for λr=1. For a given amount of support provided by the magnetic field as measured by the dimensionless parameter H0, equilibria with smaller values of Γ are more flattened. However, logatropic (defined by Γ=0) disks do not exist. The only possible scale-free isopedic equilibria with logatropic equation of state are spherical uniformly magnetized clouds. Title: Annual review of astronomy and astrophysics. Vol. 37. Authors: Burbidge, G.; Sandage, A.; Shu, F. H. Bibcode: 1999ARA&A..37.....B Altcode: No abstract at ADS Title: Our Changing Society Authors: Shu, Frank H. Bibcode: 1999aasf.book..307S Altcode: No abstract at ADS Title: Low-Mass Star Formation: Theory Authors: Shu, Frank H.; Allen, Anthony; Shang, Hsien; Ostriker, Eve C.; Li, Zhi-Yun Bibcode: 1999ASIC..540..193S Altcode: 1999osps.conf..193S Magnetic Support of Clouds Fragmentation Empirical Measurements of the Mass-to-Flux Ration Turbulent Support of Molecular Clouds Filamentary Structure in Molecular Clouds Sustenance of Cloud Turbulence Through YSO Outflows A Toy Model of GMC Structure and Evolution Formation and Evolution of Molecular Cloud Cores The Pivotal State Self-Similar Collapse of Singular Isothermal Toroids Mass Loss from Stars and Disks Generalized X-wind Model Summary Title: Protostellar Cosmic Rays and Extinct Radioactivities in Meteorites Authors: Lee, Typhoon; Shu, Frank H.; Shang, Hsien; Glassgold, Alfred E.; Rehm, K. E. Bibcode: 1998ApJ...506..898L Altcode: Calcium-aluminum-rich inclusions (CAIs) and chondrules of chondritic meteorites may originate with the melting of dustballs launched by a magnetically driven bipolar outflow from the inner edge of the primitive solar nebula. Bombardment by protostellar cosmic rays may make the rock precursors of CAIs and chondrules radioactive, producing radionuclides found in meteorites that are difficult to obtain with other mechanisms. Reasonable scalings from the observed hard X-rays for the cosmic-ray protons released by flares in young stellar objects yield the correct amounts of 41Ca, 53Mn, and 138La inferred for meteorites, but proton- and α-induced transformations underproduce 26Al by a factor of about 20. The missing 26Al may be synthesized by 3He nuclei accelerated in impulsive flares reacting primarily with 24Mg, an abundant isotope in the target precursor rocks. The mechanism allows a simple explanation for the very different ratios of 26Al/27Al inferred for normal CAIs, CAIs with fractionated and unidentified nuclear (FUN) anomalies, and chondrules. The overproduction of 41Ca by analogous 3He reactions and the case of 60Fe inferred for eucritic meteorites require special interpretations in this picture. Title: Star Formation, Near and Far Authors: Shu, Frank H. Bibcode: 1998AAS...192.3107S Altcode: 1998BAAS...30..855S We review what has been learned during the past two decades from observations of nearby star-forming regions, and the theory developed to describe the simplest case of the birth of low-mass stars from isolated molecular cloud cores. We then outline the principal difficulties in understanding the formation of high-mass stars, especially when they occur under crowded and turbulent conditions, as they usually do. Finally, we speculate on possible differences made by much lower metallicities and magnetic field strengths in the case of the formation of the first generation of stars. Throughout our discussion, we emphasize the importance of having measurements at the highest spatial and spectral resolutions if we are to make breakthroughs on the most obstinate problems in the field. Large-aperture telescopes and interferometric arrays at infrared, submillimeter, and millimeter wavelengths, operating from high mountaintops, or borne on airplanes or into space, are the crucial components for the observational attack on these very challenging problems. Title: Synthetic Images and Long-Slit Spectra of Protostellar Jets Authors: Shang, Hsien; Shu, Frank H.; Glassgold, Alfred E. Bibcode: 1998ApJ...493L..91S Altcode: We construct synthetic images and long-slit spectra of protostellar jets modeled as steady state X-winds. Assuming uniform ionization fractions and electron temperatures, we calculate non-LTE level populations for five-level atoms. Synthetic images in the [S II] λ6716/λ6731 and [O I] λ6300/λ6364 lines have roughly the same surface brightness as observed jets if we ignore their knotty structure. Long-slit spectra taken with the slit placed along the central axis of the jet, or slightly displaced laterally from it, provide strong evidence in support of the X-wind theory. In both the models and the actual objects, wide line profiles, containing both large positive and negative velocities, are often seen at the base of the flow, indicative of a wide-angle wind. As one progresses up the length of the slit, the line profiles narrow to straddle the projected velocity of a highly collimated jet. Title: Annual review of astronomy and astrophysics. Vol. 36. Authors: Burbidge, G.; Sandage, A.; Shu, F. H. Bibcode: 1998ARA&A..36.....B Altcode: No abstract at ADS Title: Synthetic Images and Long-Slit Spectra of Protostellar Jets Authors: Shu, F. H.; Shang, Hsien; Glassgold, A. E. Bibcode: 1997AAS...19112107S Altcode: 1997BAAS...29.1404S We construct synthetic images and long-slit spectra of protostellar jets modeled as steady-state x-winds. Assuming uniform ionization fractions and electron temperatures, we calculate non-LTE level populations for five-level atoms. Synthetic images in the [S II]lambda 6716/6731 and [O I]lambda 6300/6364 lines have approximately the same surface brightness distribution as observed jets. Thus, observed jets may be optical illusions at their base, since x-winds become highly focused only at large distances from the source. Long-slit spectra taken with the slit placed along the central axis of the jet, or slightly diplaced from it, provide strong evidence in support of the x-wind theory. In both the models and the actual objects, wide line-profiles, containing both large positive and negative velocities, are often seen at the base of the flow, indicative of a wide-angle wind. As one progresses uo the length of the slit, the line profiles narrow to straddle the projected velocity of a well-collimated jet. Title: The Origin of Chondrites and Extinct Radioactivities in the Solar System Authors: Shu, F. H.; Shang, H.; Lee, T.; Glassgold, A. E. Bibcode: 1997AAS...190.4905S Altcode: 1997BAAS...29..845S We will review current ideas concerning the formation of sunlike stars and planetary systems. In particular, we discuss the idea that the chondrules and CAIs found in chondritic meteorites might have been formed as solids entrained and melted in the bipolar wind that results from the interaction of the accreting protosolar nebula and the magnetosphere of the young protosun. Aerodynamic sorting and a mechanical selection for molten droplets that rain back onto the disk at planetary distances explain the size distributions and patterns of element segregation that we observe in carbonaceous and ordinary chondrties. Cosmic ray ions generated in the flares that accompany the general magnetic activity of the inner region may irradiate the precursor rocks before they are launched in the bipolar wind. Under reasonable scaling assumptions for the efficiency of the process in protostars, cosmic-ray bombardment suffices to generate the short-lived radionuclides (26) Al, (41) Ca, and (53) Mn at their inferred meteoritic levels. Title: Magnetic Forces in an Isopedic Disk Authors: Shu, Frank H.; Li, Zhi-Yun Bibcode: 1997ApJ...475..251S Altcode: We consider the magnetic forces in electrically conducting thin disks threaded by magnetic fields originating in the external (interstellar) medium. We focus on disks that have dimensionless ratios λ of the mass to flux that are spatially constant, a condition that we term isopedic. For arbitrary distributions of the surface density Σ (which can be nonaxisymmetric and time dependent), we show that the magnetic tension exerts a force in the plane of the disk equal to -1/λ2 times the self-gravitational force. In addition, if the disk maintains magnetostatic equilibrium in the vertical direction, the magnetic pressure, integrated over the z-height of the disk, may be approximated as (1 + η2)/(λ2 + η2) times the gas pressure integrated over z, where η ≡ f||/2πGΣ and f|| is the component of the local gravitational field parallel to the plane of the disk. We apply these results to the problem of the stability of magnetized isothermal disks to gravitational fragmentation into subcondensations of a size comparable to the vertical scale height of the disk. Contrary to common belief, such dynamical fragmentation probably does not occur. In particular, the case of the magnetized singular isothermal disk undergoes not dynamical fragmentation into many subcondensations, but inside-out collapse into a single compact object, a self similar problem that is studied in a companion paper (Li & Shu 1997). Title: Self-Similar Collapse of an Isopedic Isothermal Disk Authors: Li, Zhi-Yun; Shu, Frank H. Bibcode: 1997ApJ...475..237L Altcode: We study the gravitational collapse of an isothermal disk which is isopedically magnetized (i.e., with a mass-to-flux ratio that is spatially constant). The two theorems concerning magnetic forces in such a disk proven in a companion paper (Shu & Li 1997), plus the self-similar nature of the overall problem, allow a semianalytical treatment. The inflow occurs in an inside-out manner similar to that which applies in the collapse of the unmagnetized singular isothermal sphere (Shu 1977). These two cases (singular sphere and disk) bracket the range of possible collapse behaviors expected for the family of isopedic singular isothermal toroids described by Li & Shu (1996b). Although the strong magnetic fields dilute the effects of self-gravity in the isopedic isothermal disk, they do not prevent its outer parts (envelope) from falling onto the central condensed object (protostar) at a fixed infall rate Ṁ, even when the field is perfectly frozen to the matter. Indeed, the higher densities supported by the fields in the equilibrium state increase Ṁ during collapse in comparison with the unmagnetized case. The larger effective speed of sound due to magnetization produces a smaller effect. The flattened geometry enforced by the strong magnetic fields introduces a complication: the appearance of an outwardly propagating shock wave that runs ahead of the region of infall (also studied by Tsai & Hsu 1995 in a different context). We discuss the implications of our results for the magnetic-flux problem and for the formation of centrifugally supported disks in the presence of rotation. Title: Protostellar cosmic rays and extinct radioactivities Authors: Shang, Hsien; Shu, Frank; Lee, Typhoon; Glassgold, Alfred E. Bibcode: 1997IAUS..182P.312S Altcode: A magnetocentrifugally driven X-wind from the inner edge of the solar nebula around the young sun may process dustballs into the CAIs and chondrules found in chondritic meteorites (\cite{Shu-Shang-Lee). Time variability of the magnetic configuration could trigger flares, accelerating cosmic rays that spallogenically produce the short-lived radionuclides inferred for CAIs and chondrules. Title: Protostellar X-rays, Jets, and Bipolar Outflows Authors: Shu, Frank H.; Shang, Hsien Bibcode: 1997IAUS..182..225S Altcode: We review the theory of x-winds in young stellar objects (YSOs). In particular, we consider how a model where the central star does not corotate with the inner edge of the accretion disk may help to explain the enhanced emission of X-rays from embedded protostars. We argue, however, that the departure from corotation is not large, so a mathematical formulation that treats the long-term average state as steady and axisymmetric represents a useful approximation. Magnetocentrifugally driven x-winds of this description collimate into jets, and their interactions with the surrounding molecular cloud cores of YSOs yield bipolar molecular outflows. Title: X-rays and fluctuating X-winds from protostars. Authors: Shu, F. H.; Shang, H.; Glassgold, A. E.; Lee, T. Bibcode: 1997Sci...277.1475S Altcode: Protostars emit more X-rays, hard and soft, than young Sun-like stars in more advanced stages of formation. The X-ray emission becomes harder and stronger during flares. The excess X-rays may arise as a result of the time-dependent interaction of an accretion disk with the magnetosphere of the central star. Flares produced by such fluctuations have important implications for the X-wind model of protostellar jets, for the flash-heating of the chondrules found in chondritic meteorites, and for the production of short-lived radioactivities through the bombardment of primitive rocks by solar cosmic rays. Title: C. C. Lin's Influence on Modern Ideas in Star Formation Authors: Shu, F. H. Bibcode: 1997tcca.conf...21S Altcode: No abstract at ADS Title: Annual review of astronomy and astrophysics. Vol. 35. Authors: Burbidge, G.; Sandage, A.; Shu, F. H. Bibcode: 1997ARA&A..35.....B Altcode: The following topics are discussed: Eta Carinae, solar variable radiation, luminous supersoft X-ray sources, extragalactic distance scale, low mass stars and brown dwarfs, dense photodissociation regions, high-velocity clouds, low surface brightness galaxies, supernova spectra, compact groups of galaxies, faint blue galaxies, AGN variability, abundance ratios and Galactic chemical evolution, mixing in stars, parsec-scale jets, and galactic bulges. Title: Molecules in star formation. Authors: Shu, F. H. Bibcode: 1997IAUS..178...19S Altcode: 1996IAUS..178...19S The author reviews current ideas and models in the problem of star formation from molecular cloud cores that are relatively isolated from the influences of other forming stars. He discusses the time scales, flow dynamics, and density and temperature structures applicable to each of the four stages of the entire process: (1) formation of a magnetized cloud core by ambipolar diffusion and evolution to a pivotal state of gravomagneto catastrophe; (2) self-similar collapse of the pivotal configuration and the formation of protostars, disks, and pseudo-disks; (3) onset of a magnetocentrifugally driven, lightly ionized wind from the interaction of an accretion disk and the magnetosphere of the central star, and the driving of bipolar molecular outflows; (4) evolution of pre-main-sequence stars surrounded by dusty accretion disks. For each of these stages and processes, he considers the characteristics of the molecular diagnostics needed to investigate the crucial aspects of the observational problem. Title: A Quantitative Model for Bipolar Molecular Outflows Authors: Li, Z.; Shu, F. H. Bibcode: 1997ASPC..121..757L Altcode: 1997IAUCo.163..757L; 1997apro.conf..757L No abstract at ADS Title: Magnetized Singular Isothermal Toroids Authors: Li, Zhi-Yun; Shu, Frank H. Bibcode: 1996ApJ...472..211L Altcode: We consider the end states for a quasi-magnetostatic phase of evolution of molecular cloud cores by ambipolar diffusion. The models yield good initial states for fully dynamical collapse to isolated protostellar systems. These pivotal transition states are self-gravitating, magnetized, isothermal masses of gas in which the density scales with spherical radius r as r-2 and the magnetic field as r-1. The dependences of these quantities with angle θ satisfy the constraints of magnetostatic equilibrium. Under these circumstances, we find a linear sequence of possible pivotal states, each member characterized by a separate value of the differential mass-to-flux ratio in dimensionless form = λ≡2πG½dM/dΦ≥1. In general, the pivotal states are toroids, with the density distribution on a circle of constant r in the meridional plane declining from a maximum value on the magnetic equator, θ = π/2, to zero over the magnetic poles, θ = 0 and θ = π. For λ≫ 1, the pivotal configurations approach the unmagnetized singular isothermal sphere, with volume density p = (a2/2πG)r-2. For λ close to 1, the pivotal configurations flatten to a thin disk, with surface density Σ = [(1 + H0)a2/πG]r-2, where H0 is a number ≫ 1. We comment on the implications of these results for observations and other theoretical investigations. Title: Toward an astrophysical theory of chondrites. Authors: Shang, H.; Shu, F. H.; Lee, T. Bibcode: 1996NASCP3343..191S Altcode: 1996ASPC..122..191S; 1996NASCP3339..191S The authors have proposed a theory where chondritic bodies form by compaction of the processed solids with the ambient nebular dust comprising the matrices after their reentry at great distances from the original launch radius. Title: Externally induced evaporation of young stellar disks in Orion. Authors: Johnstone, D.; Hollenbach, D.; Shu, F. Bibcode: 1996NASCP3343...55J Altcode: 1996NASCP3339...55J; 1996ASPC..122...55J The authors propose a model for the evaporation of disks around young low-mass stars by external sources of high energy photons. Two evaporation techniques are possible. Lyman continuum radiation can ionize hydrogen at the disk surface powering a steady thermal ionized disk-wind or FUV radiation can heat the disk through photo-electric grain processes powering a slower thermal neutral disk-wind. Applying these two models to the evaporating objects in the Trapezium produces a satisfactory solution to both the mass-loss rate and size of the ionized envelopes. Title: Interaction of Wide-Angle MHD Winds with Flared Disks Authors: Li, Zhi-Yun; Shu, Frank H. Bibcode: 1996ApJ...468..261L Altcode: A magnetocentrifugal mechanism probably underlies the narrow, high-speed, optical jets associated with young stellar objects. Recent theoretical calculations of jet formation in the X-wind model suggest the co-existence of a wide-angle component surrounding the well-collimated jet. We consider whether the predicted wide-angle component can reveal itself through interaction with an underlying flared accretion disk. We use the method of characteristics to calculate numerically the properties of the resulting shockwave. Because the frozen-in magnetic field reduces the effective Mach number of the flow, the shock is too weak at a distance of 10-100 AU to produce [O I] λ6300 or [S II] λ6717/6731 emission. Unless disks have obstructing features more extreme than conventional flaring (e.g., their own wind), the uncollimated component of an X-wind will remain optically hidden until it impacts material well off the disk plane at larger distances from the central star (e.g., an infalling envelope or the ambient molecular cloud). Title: Kinematic Diagnostics of Disks around Young Stars: CO Overtone Emission from WL 16 and 1548C27 Authors: Najita, Joan; Carr, John S.; Glassgold, Alfred E.; Shu, Frank H.; Tokunaga, Alan T. Bibcode: 1996ApJ...462..919N Altcode: 1995astro.ph.12109N We report high spectral resolution observations of the CO vibrational overtone emission from the young stellar object 1548C27; our observations include both the v = 2-0 and v = 5-3 band head regions. These data and similar observations of the young stellar object WL 16, reported in a previous contribution to this journal, provide some of the most compelling evidence to date for the existence of inner disks around young stars. We describe the simple procedure that we use to synthesize band head emission from disks including the effect of thermal dissociation of CO and non-LTE excitation of the vibrational levels. Using this spectral synthesis procedure to extract the kinematics and physical properties of the emitting gas from the overtone data, we show how these high signal-to-noise ratio data are also powerful probes of the stellar and inner disk properties of these systems. Our modeling is consistent with the identification of WL 16 and 1548C27 as Herbig AeBe stars with stellar masses of approximately 2 and 4 Msun, respectively. Thus, the kinematic signature of rotating disks in the overtone spectra of these sources provides strong support for the role of accretion disks in the formation of intermediate-mass stars. For both WL 16 and 1548C27, we interpret our modeling results as indicating that the overtone emission arises from a temperature inversion region in the inner disk atmosphere. We also find evidence for suprathermally broadened lines and are able to place useful constraints on the radial temperature and column density distributions of the CO line formation region of the disk atmosphere. Given these deduced properties, we discuss the constraints that our observations place on the physical processes responsible for the overtone emission in these sources. Title: Toward an Astrophysical Theory of Chondrites Authors: Shu, Frank H.; Shang, Hsien; Lee, Typhoon Bibcode: 1996Sci...271.1545S Altcode: The chondrules, calcium-aluminum-rich inclusions (CAIs), and rims in chondritic meteorites could be formed when solid bodies are lifted by the aerodynamic drag of a magnetocentrifugally driven wind out of the relative cool of a shaded disk close to the star into the heat of direct sunlight. For reasonable self-consistent parameters of the bipolar outflow, the base and peak temperatures reached by solid bodies resemble those needed to melt CAIs and chondrules. The process also yields a natural sorting mechanism that explains the size distribution of CAIs and chondrules, as well as their fine-grained and coarse-grained rims. After reentry at great distances from the original launch radius, the CAIs, chondrules, and their rims would be compacted with the ambient nebular dust comprising the matrices, forming the observed chondritic bodies. Title: Magnetocentrifugally Driven Flows from Young Stars and Disks. V. Collimation into Jets: Erratum Authors: Shu, Frank H.; Najita, Joan; Ostriker, Eve C.; Shang, Hsien Bibcode: 1996ApJ...459L..43S Altcode: No abstract at ADS Title: An X-Wind Scenario for Early Solar System Irradiation Authors: Lee, T.; Shu, F. H.; Glassgold, A.; Shang, H. Bibcode: 1996M&PSA..31R..79L Altcode: No abstract at ADS Title: Externally induced evaporation of young stellar disks in Orion. Authors: Johnstone, D.; Shu, F.; Hollenbach, D. Bibcode: 1996JRASC..90T.319J Altcode: No abstract at ADS Title: Magnetocentrifugally Driven Flows from Young Stars and Disks. V. Asymptotic Collimation into Jets Authors: Shu, Frank H.; Najita, Joan; Ostriker, Eve C.; Shang, Hsien Bibcode: 1995ApJ...455L.155S Altcode: We consider the asymptotic behavior of magnetocentrifugally driven X-winds, taking into proper account the pressure balance across the interface between the open stellar field lines of the dead zone devoid of matter and the open field lines of the wind. At large distances from the source the outflow collimates into jets along the rotation axis. Being almost cylindrically symmetric even close to the source of the outflow, the density distribution appears more jetlike than the collection of streamlines, which always contains a few members that skim over the surface of the disk. Title: Magnetocentrifugally Driven Flows from Young Stars and Disks. IV. The Accretion Funnel and Dead Zone Authors: Ostriker, Eve C.; Shu, Frank H. Bibcode: 1995ApJ...447..813O Altcode: We formulate the time-steady, axisymmetric problem of stellar magnetospheric inflow of gas from a surrounding accretion disk. The computational domain is bounded on the outside by a surface of given shape containing the open field lines associated with an induced disk wind. The mechanism for this wind has been investigated in previous publications in this journal. Our zeroth-order solution incorporates an acceptable accounting of the pressure balance between the magnetic field lines loaded with accreting gas (funnel flow) and those empty of matter (dead zone). In comparison with previous models, our funnel-flow/dead-zone solution has the following novel features: (1) Because of a natural tendency for the trapped stellar magnetic flux to pinch toward the corotation radius Rx (X-point of the effective potential), most of the interesting magnetohydrodynamics is initiated within a small neighborhood of Rx (X-region), where the Keplerian angular speed of rotation in the disk equals the spin rate of the star. (2) Unimpeded funnel flow from the inner portion of the X-region to the star can occur when the amount of trapped magnetic flux equals or exceeds 1.5 times the unperturbed dipole flux that would lie outside Rx in the absence of an accretion disk. (3). Near the equatorial plane, radial infall from the X-point is terminated at a "kink" point Rk = 0.74Rx that deflects the flow away from the midplane, mediating thereby between the field topology imposed by a magnetic fan of trapped flux at Rx and the geometry of a strong stellar dipole. (4) The excess angular momentum of accretion that would otherwise spin up the star rapidly is deposited by the magnetic torques of the funnel flow into the inner portion of the X-region of the disk. (5) An induced disk wind arises in the outer portion of the .X-region, where the stellar field lines have been blown open, and removes whatever excess angular momentum that viscous torques do not transport to the outer disk. (6) The interface between open field lines loaded with outflowing matter (connected to the disk) and those not loaded (connected to the star) forms a "helmet streamer," along which major mass-ejection and reconnection events may arise in response to changing boundary conditions (e.g., stellar magnetic cycles), much the way that such events occur in the active Sun. (7) Pressure balance across the dead-zone/wind interface will probably yield an asymptotically vertical (i.e., "jetlike") trajectory for the matter ejected along the helmet streamer, but mathematical demonstration of this fact is left for future studies. (8) In steady state the overall balance of angular momentum in the star/disk/ magnetosphere system fixes the fractions, f and 1 - f, of the disk mass accretion rate into the X-region carried away, respectively, by the wind and funnel flows. Title: The Birth of Sunlike Stars Authors: Shu, Frank H. Bibcode: 1995mcsf.conf...97S Altcode: No abstract at ADS Title: AAS Position on the Space Station Authors: Shu, Frank H. Bibcode: 1994Sci...265.1789S Altcode: No abstract at ADS Title: Magnetocentrifugally Driven Flows from Young Stars and Disks. I. A Generalized Model Authors: Shu, Frank; Najita, Joan; Ostriker, Eve; Wilkin, Frank; Ruden, Steven; Lizano, Susana Bibcode: 1994ApJ...429..781S Altcode: We propose a generalized model for stellar spin-down, disk accretion, and truncation, and the origin of winds, jets, and bipolar outflows from young stellar objects. We consider the steady state dynamics of accretion of matter from a viscous and imperfectly conducting disk onto a young star with a strong magnetic field. For an aligned stellar magnetosphere, shielding currents in the surface layers of the disk prevent stellar field lines from penetrating the disk everywhere except for a range of radii about pi = Rx, where the Keplerian angular speed of rotation Omegax equals the angular speed of the star Omega*. For the low disk accretion rates and high magnetic fields associated with typical T Tauri stars, Rx exceeds the radius of the star R* by a factor of a few, and the inner disk is effectively truncated at a radius Rt somewhat smaller than Rx. Where the closed field lines between Rt and Rx bow sufficiently inward, the accreting gas attaches itself to the field and is funneled dynamically down the effective potential (gravitational plus centrifugal) onto the star. Contrary to common belief, the accompanying magnetic torques associated with this accreting gas may transfer angular momentum mostly to the disk rather than to the star. Thus, the star can spin slowly as long as Rx remains significantly greater than R*. Exterior to Rx field lines threading the disk bow outward, which makes the gas off the mid-plane rotate at super-Keplerian velocities. This combination drives a magnetocentrifugal wind with a mass-loss rate Mw equal to a definite fraction f of the disk accretion rate MD. For high disk accretion rates, Rx is forced down to the stellar surface, the star is spun to breakup, and the wind is generated in a manner identical to that proposed by Shu, Lizano, Ruden, & Najita in a previous communication to this journal. In two companion papers (II and III), we develop a detailed but idealized theory of the magnetocentrifugal acceleration process. Title: Magnetocentrifugally Driven Flows from Young Stars and Disks. II. Formulation of the Dynamical Problem Authors: Shu, Frank H.; Najita, Joan; Ruden, Steven P.; Lizano, Susana Bibcode: 1994ApJ...429..797S Altcode: We formulate the dynamical problem of a cool wind centrifugally driven from the magnetic interface of a young star and an adjoining Keplerian disk. We examine the situation for mildly accreting T Tauri stars that rotate slowly as well as rapidly accreting protostars that rotate near break-up. In both cases a wind can be driven from a small X-region just outside the stellar magnetopause, where the field lines assume an open geometry and are rooted to material that rotates at an angular speed equal both to the local Keplerian value and to the stellar angular speed. Assuming axial symmetry for the ideal magnetohydrodynamic flow, which requires us to postpone asking how the (lightly ionized) gas is loaded onto field lines, we can formally integrate all the governing equations analytically except for a partial equation that describes how streamlines spread in the meridional plane. Apart from the difficulty of dealing with PDEs of mixed type, finding the functional forms of the conserved quantities along streamlines - the ratio beta of magnetic field to mass flux, the specific energy H of the fluid in the rotating frame, and the total specific angular momentum J carried in the matter and the field - constitutes a standard difficulty in this kind of (Grad-Shafranov) formalism. Fortunately, because the ratio of the thermal speed of the mass-loss regions to the Keplerian speed of rotation of the interface constitutes a small parameter epsilon, we can attack the overall problem by the method of matched asymptotic expansions. This procedure leads to a natural and systematic technique for obtaining the relevant functional dependences of beta, H, and J. Moreover, we are able to solve analytically for the properties of the flow emergent from the small transsonic region driven by gas pressure without having to specify the detailed form of any of the conserved functions, beta, H, and J. This analytical solution provides inner boundary conditions for the numerical computation in a companion paper by Najita & Shu of the larger region where the main acceleration to terminal speeds occurs. Title: Magnetocentrifugally Driven Flows from Young Stars and Disks. III. Numerical Solution of the Sub-Alfvenic Region Authors: Najita, Joan R.; Shu, Frank H. Bibcode: 1994ApJ...429..808N Altcode: We construct steady, axisymmetric, numerical models of the sub-Alfvenic regions of cool winds driven by the X-celerator mechanism. We find that smooth acceleration to speeds of the order of the escape speed accompanied by substantial collimation of the flow can be achieved within the Alfven surface, located characteristically at several stellar radii. We apply the nondimensional results to winds which emerge from the equator of magnetized YSOs rotating at breakup and from circumstellar disks interacting with the magnetospheres of slowly rotating T Tauri stars. Stellar magnetic fields of kilogauss strength can drive wind mass-loss rates of 10-6 solar mass/year from rapidly accreting YSOs and 10-8 solar mass/year from slowly accreting T Tauri stars. The X-celerator mechanism can accelerate winds from these systems to velocities of hundreds of km/sec within a few stellar radii in the case of embedded YSOs and approximately 10 stellar radii in the case of T Tauri stars. Title: Photoevaporation of Disks around Massive Stars and Application to Ultracompact H II Regions Authors: Hollenbach, David; Johnstone, Doug; Lizano, Susana; Shu, Frank Bibcode: 1994ApJ...428..654H Altcode: Young massive stars produce sufficient Lyman continuum photon luminosity Phii to significantly affect the structure and evolution of the accretion disks surrounding them. A nearly static, ionized, isothermal 104 K atmosphere forms above the neutral disk for disk radii r less than rg = 1015 M1 cm, where M* = 10 solar mass M1 is the stellar mass. For r approximately greater than rg the diffuse field created by hydrogen recombinations to the ground state in the photoionized gas above the disk produces a steady evaporation at the surface of the disk, and this H II gas flows freely out to the ISM (the 'disk wind'). The detailed structure depends on the mass-loss rate dot-Mw of the fast, approximately greater than 1000 km/sec, stellar wind from the massive star. A critical mass-loss rate dot-Mcr is defined such that the ram pressure of the stellar wind equals the thermal pressure of the H II atmosphere at rg. In the weak stellar wind solution, dot-Mw less than dot-Mcr, the diffuse photons from the atmosphere above rg produce a photoevaporative mass-loss rate from the disk at r approximately greater than rg of order 1 x 10-5(Phi49)1/2(M1)1/2 solar mass/year, where Phii = 1049 Phi49/sec. The resulting slow (10 to 50 km/sec) ionized outflow, which persists for approximately greater than 105 year for disk masses Md approximately 0.3 M*, may explain the observational characteri stics of unresolved, ultracompact H II regions. In the strong stellar wind solution, dot-Mw greater than dot-Mcr, the ram pressure of the stellar wind blows down the atmosphere for r less than rg and allows the stellar photons to penetrate to greater radii and smaller heights. A slow, ionized outflow produced mainly by diffuse photons is again created for r less than rg; however, it is now dominated by the flow at rw(greater than rg), the radius at which the stellar wind ram pressure equals the thermal pressure in the evaporating flow. The mass-loss rate from the disk is of order 6 x 10-5dot-Mw-6 vw8(Phi 49)-1/2 solar mass/year, where dot-Mw-6 = Mw/10-6 solar mass/year and vw8 = vw/1000 km/sec is the stellar wind velocity. The resulting outflow, which also persists for approximately greater than 105 year may explain many of the more extended (r approximately greater than 1016 cm) ultracompact H II regions. Both the weak-wind and the strong-wind models depend entirely on stellar parameters Phii, M*, dot-Mw) and are independent of disk parameters as long as an extended r much greater than (rg), neutral disk exists. We compare both weak-wind and strong-wind model results to the observed radio free-free spectra and luminosities of ultracompact H II regions and to the interesting source MWC 349. Title: Collapse of Magnetized Molecular Cloud Cores. I. Semianalytical Solution Authors: Galli, Daniele; Shu, Frank H. Bibcode: 1993ApJ...417..220G Altcode: In this paper we follow the evolution of an unstable magnetized cloud core modeled with the density distribution of a singular isothermal sphere and threaded by a uniform magnetic field. We include neutral-ion slip, and we solve the equations by an expansion about the known self-similar problem without magnetism. We find that the magnetic field does not significantly modify the standard rate of mass infall because of two offsetting effects: the Lorentz force that impedes gravitational collapse, and the increased characteristic speed that causes the initiation of infall to travel outward faster (as a fast magnetohydrodynamic wave rather than an acoustic wave). Strong magnetic pinching forces deflect infalling gas toward the equatorial plane to form a flattened disequilibrium structure ("pseudodisk') around the central proto star. The perturbative approach allows us to calculate analytically the dependence of the radius of the pseudodisk at small times on the physical parameters of the problem when a dimensionless coefficient of order unity is supplied by a separate numerical calculation for the nonlinear flow in the inner region (Paper II). Title: Collapse of Magnetized Molecular Cloud Cores. II. Numerical Results Authors: Galli, Daniele; Shu, Frank H. Bibcode: 1993ApJ...417..243G Altcode: In a previous paper (Paper I) we presented a perturbative analysis of the collapse of a molecular cloud core threaded by an ordered magnetic field, obtaining a semianalytical solution applicable over a moderate range of temporal and spatial scales. In the present paper we supplement this analysis with a numerical solution of the magnetohydrodynamic (MHD) equations that include the effects of ambipolar diffusion, valid in the region where magnetic effects dominate the dynamics of the collapse. We focus on the formation of a flattened disequilibrium structure ("pseudodisk") around the central protostar. The numerical solution gives dimensionless values for the radius of the pseudodisk as a function of time. Combined with the analytical scaling laws found in Paper I, these results provide in the small time limit a simple power-law expression for the dimensional radius of the pseudodisk as a function of the initial magnetic field B0 and effective sound speed a of the unstable molecular cloud core. We tabulate in nondimensional form the velocity, density, and magnetic fields as functions of the radius, polar angle, and time for two values (χ = 11.3 and ∞) of the ion-neutral coupling constant. We apply the results to the density and magnetic field structures on the astronomically interesting scale of a few hundred to a few thousand AU around protostars with mass in the range 0.57-2.0 Msun. The resultant magnetic field topology causes us to speculate on the importance of neutral-ion slip, ohmic dissipation, and reconnection in the overall problems of the loss of flux and the isolation of the magnetic fields in the pseudodisk (and smaller centrifugal disk) from their interstellar origins. We conclude by comparing our results with observations of flattened dense structures around young stellar objects in various stages of evolution. Title: Photoevaporation of the Solar Nebula and the Formation of the Giant Planets Authors: Shu, Frank H.; Johnstone, Doug; Hollenbach, David Bibcode: 1993Icar..106...92S Altcode: We review the prevailing theories for the formation of the jovian planets and comment that they do not provide a natural explanation for the oft-noted subdivision into two separate classes: the gas-rich giants, Jupiter and Saturn, and the gas-poor giants, Uranus and Neptune. To account for the observed differences in envelope mass relative to core mass, the conventional discriminants would seem to require special timing for the phases: assemblage of the protoplanetary cores, runaway accretion of gas, and the dispersal of nebular gases. We propose a discriminant that relies on photoevaporation by Lyman continuum photons of the outer parts of the disk as the primary hydrogen (and helium) loss mechanism. We show that Saturn's orbit constitutes the natural transitional radius between gas retainage and loss in this sort of picture and that the evaporative wind in the Uranus and Neptune regions would have been large enough to get rid of the hydrogen and helium gas before these planets assembled a critical core mass for runaway gas accretion, if the primitive Sun had an enhanced extreme ultraviolet luminosity for a duration comparable to those estimated for classical T Tauri stars. Title: Photoevaporation of disks around newly formed stars. Authors: Johnstone, D.; Hollenbach, D.; Shu, F. H. Bibcode: 1993JRASC..87..192J Altcode: No abstract at ADS Title: The Inner-Disk and Stellar Properties of the Young Stellar Object WL 16 Authors: Carr, John S.; Tokunaga, Alan T.; Najita, Joan; Shu, Frank H.; Glassgold, Alfred E. Bibcode: 1993ApJ...411L..37C Altcode: We present kinematic evidence for a rapidly rotating circumstellar disk around the young stellar object WL 16, based on new high-velocity-resolution data of the v = 2-0 CO bandhead emission. A Keplerian disk provides an excellent fit to the observed profile and requires a projected velocity for the CO-emitting region of roughly 250 km/s at the inner radius and 140 km/s at the outer radius, giving a ratio of the inner to the outer radius of about 0.3. We show that satisfying the constraints imposed by the gas kinematics, the observed CO flux, and the total source luminosity requires the mass of WL 16 to lie between 1.4 and 2.5 solar mass. The inner disk radius for the CO emission must be less than 8 solar radii. Title: Photoevaporation of Disks around Newly Formed Stars Authors: Johnstone, D.; Hollenbach, D.; Shu, F. H. Bibcode: 1993AAS...182.3704J Altcode: 1993BAAS...25..854J Newly formed stars produce sufficient Lyman continuum luminosity phi to significantly alter the structure and evolution of the accretion disk surrounding them. In the absence of a stellar wind, a nearly static, photoionized, 10(4) K, disk atmosphere, with a scale height that increases with r(3/2) , forms inside the gravitational radius r_g ~ 10(14) (M_* / M_⊙) cm where M_* is the mass of the central star. This ionized atmosphere is maintained by both the direct radiation from the central star and the diffuse field produced in the disk atmosphere by the significant fraction of hydrogen recombinations directly to the ground state. Beyond r_g the material evaporated from the disk is capable of escaping from the system and produces an ionized disk wind. The mass-loss due to this disk wind peaks at r_g. The inclusion of a stellar wind into the basic picture reduces the height of the inner disk atmosphere and introduces a new scale radius r_w where the thermal pressure of the material evaporated from the disk balances the ram pressure in the wind. In this case the mass-loss due to the disk wind peaks at r_w and is enhanced over the no-wind case. The photoevaporation of disks around newly formed stars has significance to both the UCHII problem and the dispersal of solar-type nebulae. High mass stars are intrinsically hot and thus yield sufficient Lyman luminosity to create disk mass-loss rates of order 2 times 10(-5) phi_ {49}(1/2) M_⊙ yr(-1) , where phi_ {49} = phi /(10(49) Lyman continuum photons s(-1) ) even without a stellar wind. This wind which will last for ~ 10(5) yrs if the disk mass is M_d ~ 0.3 M_*, yields sizes, emission measures and ages consistent with observations of UCHIIs. On the other end of the stellar scale, many newly formed low-mass stars are known to have enhanced extreme ultraviolet luminosity suggested to be due to boundary layer accretion. Assuming that the sun had such an enhanced Lyman luminosity phi ~ 10(41) s(-1) , for ~ 3times 10(7) yrs it is possible to remove all of the gas beyond the orbit of Saturn, r_g for the sun, associated with the minimum solar nebula. This process also has implications for the formation of the giant planets. Title: Book-Review - the Physics of Astrophysics - V.2 - Gas Dynamics Authors: Shu, F. H.; Pringle, J. E. Bibcode: 1993Obs...113...85S Altcode: No abstract at ADS Title: Parker Instability in a Realistic Gravitational Field Authors: Giz, Ahmet T.; Shu, Frank H. Bibcode: 1993ApJ...404..185G Altcode: The paper examines Parker's instability in a disk with a realistic vertical structure, but without including the effects of rotation and shear in the horizontal direction. In addition to the continuum modes found by previous authors for the idealized case when the vertical gravitational field is taken to be a simple step function, the possibility for discrete modes whose power is more spatially confined in z is discovered. When these discrete modes prove unstable, they favor condensations that are placed antisymmetrically with respect to the midplane, a feature found also in numerical simulations carried into the nonlinear regime by Matsumoto et al. (1988, 1990). For the continuum modes, the characteristic length scale for instability is typically half of the conventionally estimated value, yielding growth rates that are approximately double previous estimates. Title: The Collapse of Clouds and the Formation and Evolution of Stars and Disks Authors: Shu, Frank; Najita, Joan; Galli, Daniele; Ostriker, Eve; Lizano, Susana Bibcode: 1993prpl.conf....3S Altcode: We consider the interrelationships among the structure of molecular clouds; the collapse of rotating cloud cores; the formation of stars and disks; the origin of molecular outflows, protostellar winds, and highly collimated jets; the birth of planetary and binary systems; and the dynamics of star/disk/satellite interactions. Our discussion interweaves theory with the results of observations that span from millimeter wavelengths to X-rays. Title: The Inner Disk and Stellar Properties of the Young Stellar Object WL 16 Authors: Carr, J. S.; Tokunaga, A. T.; Najita, J.; Shu, F. H.; Glassgold, A. E. Bibcode: 1993AAS...18111605C Altcode: 1993BAAS...25Q.734C We present kinematic evidence for a rapidly rotating circumstellar disk around the young stellar object WL 16, based on new high velocity resolution data of the v = 2-0 CO bandhead emission. A Keplerian disk provides an excellent fit to the observed profile, with a projected velocity v sin i = 140 km s(-1) at the outer disk edge and about 250 km s(-1) at the inner edge. The disk must extend to within one stellar radius or less of the stellar surface. Constraints on the stellar mass of WL 16 and on other properties of the inner disk are discussed. Title: Photoevaporation of Disks Around Massive Stars and Ultracompact HH Regions Authors: Hollenbach, D.; Johnstone, D.; Shu, F. Bibcode: 1993ASPC...35...26H Altcode: 1993msli.conf...26H No abstract at ADS Title: Near-resonant Excitation and Propagation of Eccentric Density Waves by External Forcing Authors: Ostriker, Eve C.; Shu, Frank H.; Adams, Fred C. Bibcode: 1992ApJ...399..192O Altcode: An overview is presented of the astronomical evidence that relatively massive, distended, gaseous disks form as a natural by-product of the process of star formation, and also the numerical evidence that SLING-amplified eccentric modes in the outer parts of such disks can drive one-armed spiral density waves in the inner parts by near-resonant excitation and propagation. An ordinary differential equation (ODE) of the second order that approximately governs the nonlocalized forcing of waves in a disk satisfying Lindblad resonance almost everywhere is derived. When transformed and appended with an extra model term, this ODE implies, for free waves, the usual asymptotic results of the WKBJ dispersion relationship and the propagation Goldreich-Tremaine (1978) formula for the resonant torque exerted on a localized Lindblad resonance. An analytical solution is given for the rate of energy and angular momentum transfer by nonlocalized near-resonant forcing in the case when the disk has power-law dependences on the radius of the surface density and temperature. Title: Books-Received - the Physics of Astrophysics - V.2 - Gas Dynamics Authors: Shu, F. H. Bibcode: 1992JBAA..102..230S Altcode: No abstract at ADS Title: Book-Review - the Physics of Astrophysics - V.1&2 - Radiation and Gas Dynamics Authors: Shu, F. H.; Hughes, P. A. Bibcode: 1992Natur.357..122S Altcode: No abstract at ADS Title: Book-Review - Radiation - the Physics of Astrophysics - V.1 Authors: Shu, F. H.; Harper, G. Bibcode: 1992JBAA..102..108S Altcode: No abstract at ADS Title: Book-Review - the Physics of Astrophysics - V.2 - Gas Dynamics Authors: Shu, F. H. Bibcode: 1992Sci...256Q.253S Altcode: No abstract at ADS Title: Book-Review - the Physics of Astrophysics - V.1 - Radiation Authors: Shu, F. H. Bibcode: 1992JBAA..102Q..62S Altcode: No abstract at ADS Title: The physics of astrophysics. Volume II: Gas dynamics. Authors: Shu, F. H. Bibcode: 1992pavi.book.....S Altcode: Volume II is a self-contained textbook, and is not dependent on Volume I (see 53.003.096). It can be used as the text for a separate, one-semester course on its subject matter, which includes the interactions of matter and radiation, and electromagnetic fields of macroscopic scale in both the strongly collisional and collisionless regimes. It covers such fields as single-fluid theory, including radiative processes; waves, shocks, and fronts; magnetohydrodynamics and plasma physics; as well as their applications to such topics as self-gravitating spherical masses, accretion disks, spiral density waves, star formation, and dynamo theory. Title: Physics of Astrophysics, Vol. II Authors: Shu, Frank H. Bibcode: 1992phas.book.....S Altcode: Presented in two volumes, The Physics of Astrophysics is ideally suited for a year-long astrophysics course for university seniors and first-year graduate students. The first volume deals with the emission, absorption, and scattering of radiation by matter, as well as covering related topics such as radiative transfer, statistical physics, classical electrodynamics, and atomic and molecular structure. Title: Star Formation and the Nature of Bipolar Outflows Authors: Shu, Frank H.; Ruden, Steven P.; Lada, Charles J.; Lizano, Susana Bibcode: 1991ApJ...370L..31S Altcode: This paper presents a simple physical model for the bipolar molecular outflows that frequently accompany star formation. The model forges an intrinsic link between the bipolar flow phenomenon and the process of star formation, and it helps to explain many of the systematics known for existing sources. Title: Physics of Astrophysics, Vol. II: Gas Dynamics Authors: Shu, Frank Bibcode: 1991pagd.book.....S Altcode: This upper-level text deals with interactions of matter and radiation, electromagnetic fields, single-fluid theory, waves, fronts, magnetohydrodynamics, and plasma physics. Title: The Formation of Low Mass Stars: Theory Authors: Shu, Frank H. Bibcode: 1991ASIC..342..365S Altcode: 1991psfe.conf..365S Overview Bimodal Star Formation The Bipolar Outflow Phase: Observations Rotating, Magnetized, Molecular Cloud Cores Protostar Formation from Collapsing Cloud Cores Infrared Appearance of Rotating Protostellar Objects Protostar Formation By Disk Accretion Stellar Winds and Bipolar Flows: Theory Revealed T Tauri Stars The Disks Inferred for T Tauri Stars Binary Stars and Planetary Systems Title: The physics of astrophysics. Volume 1: Radiation. Authors: Shu, F. H. Bibcode: 1991pav..book.....S Altcode: This textbook deals with the emission, absorption, and scattering of radiation by matter, as well as radiative transfer, statistical physics, classical electrodynamics, and atomic and molecular structure. The volume include problem sets designed to bring the student to the level of competence in calculation currently expected in astronomy. Title: Physics of Astrophysics, Vol. I Authors: Shu, Frank H. Bibcode: 1991phas.book.....S Altcode: 1991QB461.S448..... Presented in two volumes, The Physics of Astrophysics is ideally suited for a year-long astrophysics course for university seniors and first-year graduate students. The first volume deals with the emission, absorption, and scattering of radiation by matter, as well as covering related topics such as radiative transfer, statistical physics, classical electrodynamics, and atomic and molecular structure. Title: Physics of Astrophysics: Volume I Radiation Authors: Shu, Frank Bibcode: 1991par..book.....S Altcode: This upper-level text deals with the emission, absorption, and scattering of radiation by mater, as well as radiative transfer, statistical physics, classical electrodynamics, and atomic and molecular structure. Title: Book-Review - the Physics of Astrophysics - V.1 - Radiation Authors: Shu, F. H. Bibcode: 1991AstQ....8..262S Altcode: No abstract at ADS Title: Ekman Pumping in a Rotating Protostar Authors: Galli, D.; Shu, F. H. Bibcode: 1991ASIC..340...33G Altcode: 1991amey.conf...33G No abstract at ADS Title: Star Formation - a Theoretician's View Authors: Shu, Frank H. Bibcode: 1991ASPC...20...23S Altcode: The present evaluation of the hypothesis that the birth of a sunlike star occurs in four stages, with a critical bipolar outflow phase, notes that observed molecular outflows cannot be initiated from circumstellar disks and must instead represent swept-up shells of gas that are driven by strong protostellar winds. The star can resolve its growing angular-momentum problem only by rejecting a fraction of the material that is fed through the disk in a powerful, magnetocentrifugally-driven wind. Empirical properties measured for well-collimated outflow sources can be accounted for by the interaction of this anisotropic wind with the ambient molecular cloud core. Title: Thermal Structure of Neutral Winds from Young Stellar Objects Authors: Ruden, Steven P.; Glassgold, Alfred E.; Shu, Frank H. Bibcode: 1990ApJ...361..546R Altcode: The physical processes that control the thermal structure of lightly ionized winds from cool protostars are discussed. Attention is concentrated on the hydrogen gas, and the heating, cooling, and chemical processes that affect the neutral and ionic species of atomic and molecular hydrogen are examined. Warm silicate dust may condense out of the cooling wind and may heat the gas through collisions. Singly ionized sodium atoms, which do not recombine for the mass-loss rates considered, set a lower limit to the ionization fraction in the wind. Magnetic fields, which are presumed to accelerate the wind, couple directly to the ionic component of the gas and transfer momentum and energy to the neutral component through collisions. This process of ambipolar diffusion is found to be the dominant source of heat input to the gas. Title: SLING Amplification and Eccentric Gravitational Instabilities in Gaseous Disks Authors: Shu, Frank H.; Tremaine, Scott; Adams, Fred C.; Ruden, Steven P. Bibcode: 1990ApJ...358..495S Altcode: An analytical description is presented of the modal mechanisms relevant to a recently discovered type of eccentric gravitational instability in nearly Keplerian disks. A quantum condition is derived which accurately predicts the pattern speeds for these modes. The growth rates for the modes are determined, and it is shown that the mode can grow when the disk is safely stable to axisymmetric disturbances. The case of marginal stability for the outside edge is discussed, and the implications of the results for the formation of binary companions and/or giant planets within disks associated with young stellar objects are considered. Title: Star formation and the lunar initiative Authors: Shu, Frank H. Bibcode: 1990AIPC..207..161S Altcode: 1990am...proc..161S This paper considers the opportunities offered by proposed lunar observatories for studies of the origin of stars and related objects. We summarize the principal advantages to be gained by astronomical observations from the Moon. We then give a brief outline of the working hypothesis that present-day star formation in the Galaxy involves four conceptually distinct stages. We conclude by emphasizing that the greatest strides forward in understanding will probably come from interferometric investigations that can be carried out with at least 10 milliarcsecond angular resolution at submillimeter and far infrared wavelengths, and 10 microarcsecond resolution at ultraviolet, optical, and near infrared wavelengths. Title: The Formation of Sunlike Stars Authors: Lada, Charles J.; Shu, Frank H. Bibcode: 1990Sci...248..564L Altcode: Understanding how stars like the sun formed constitutes one of the principal challenges confronting modern astrophysics. In recent years, advances in observational technology, particularly at infrared and millimeter wave-lengths, have produced an avalanche of critical data and unexpected discoveries about the process of star formation, which is blocked from external view at optical and shorter wavelengths by an obscuring blanket of interstellar dust. Fueled by this new knowledge, a comprehensive empirical picture of stellar genesis is beginning to emerge, laying the foundations for a coherent theory of the birth of sunlike stars. Title: The Dense Interstellar Medium and the Birthplaces of Ob-Stars Authors: Shu, Frank H. Bibcode: 1990arpf.symp..179S Altcode: 1990arpf.conf..179S A review is presented of Stromgren's research on the relationships among atomic, ionized, and molecular hydrogen in the Galaxy and the spatial distribution and kinematics of OB stars. Particular attention is given to the development of density-wave theory, the implication that the pattern of the formation of OB stars in a spiral galaxy should rotate at a constant angular speed, which differs generally from the mean material speed of the interstellar gas. Other theories involving the mechanical support of molecular clouds; triggered star formation; and the relationships among H2, H I, and H II are discussed. Title: Eccentric Gravitational Instabilities in Nearly Keplerian Disks Authors: Adams, Fred C.; Ruden, Steven P.; Shu, Frank H. Bibcode: 1989ApJ...347..959A Altcode: The growth of global gravitational instabilities in young stellar objects (YSOs) with associated circumstellar disks is studied. The possibility that the accretion ultimately owes its origin to the growth of spiral gravitational instabilities is explored. The results indicate that YSO disks will be unstable to the growth of eccentric distortions which have growth rates comparable to the orbital frequency at the outer edge of the disk. Thus, the distortions grow on nearly a dynamical time scale. Perturbations with m = 1 force the star to move from the center of mass and thereby transfer angular momentum to the stellar orbit. Depending on whether or not an axisymmetric stability parameter Q barrier exists near the corotation radius of the disturbance, this coupling may lead to mass accretion or to the formation of a binary companion from the disk, or both. Title: Molecular Cloud Cores and Bimodal Star Formation Authors: Lizano, Susana; Shu, Frank H. Bibcode: 1989ApJ...342..834L Altcode: The phenomenon of bimodal star formation is reviewed in the context of supercritical and subcritical states for molecular clouds that are supported against their self-gravitation by magnetic fields. The governing set of equations is derived subject to the quasi-static and axisymmetric approximations. The method of numerical solution and tests of the resultant computer code are outlined. The results of the evolutionary calculations are discussed, emphasizing time scales, masses, and typical sizes of modeled cores that can be compared with observations. For a fixed mass, it is found that the level or turbulent support determines whether a dense core forms or not. This is used to generalize the concept of a critical mass to account for the contributions of turbulence and thermal pressures to the support of a cloud. Title: The Role of Dust in Star Formation - Part Four - Nebular Disks around Young Stellar Objects Authors: Shu, F. H.; Adams, F. C.; Lizano, S. Bibcode: 1989eidr.proc..262S Altcode: No abstract at ADS Title: Eccentric gravitational instabilities in nearly Keplerian discs. Authors: Ruden, S. P.; Adams, F. C.; Shu, F. H. Bibcode: 1989dad..conf..119R Altcode: There is growing evidence for circumstellar discs associated with young stellar objects. Motivated by observational evidence suggesting that these discs produce significant luminosity, LD≡L*, and have moderate masses, MD≡M*, the authors explore the possibility that the accretion mechanism ultimately owes its origin to the growth of spiral gravitational instabilities. As a start, they study the growth and structure of linear, global, gravitational disturbances in star/disc systems. Title: The Role of Dust in Star Formation - Part Two - Radiative Transfer in Dust Envelopes Authors: Shu, F. H.; Adams, F. C.; Lizano, S. Bibcode: 1989eidr.proc..233S Altcode: No abstract at ADS Title: The Role of Dust in Star Formation - Part Three - Spectral Evolution of Protostars Authors: Shu, F. H.; Adams, F. C.; Lizano, S. Bibcode: 1989eidr.proc..242S Altcode: No abstract at ADS Title: The Role of Dust in Star Formation - Part One - Overview of the Problem of Star Formation Authors: Shu, F. H.; Adams, F. C.; Lizano, S. Bibcode: 1989eidr.proc..213S Altcode: No abstract at ADS Title: Eccentric Gravitational Instabilities in Nearly Keplerian Disks Authors: Adams, F. C.; Ruden, S. P.; Shu, F. H. Bibcode: 1988BAAS...20.1069A Altcode: No abstract at ADS Title: Thermal Structure of Neutral Winds from Young Stellar Objects Authors: Ruden, S. P.; Shu, F. H.; Glassgold, A. E. Bibcode: 1988BAAS...20.1068R Altcode: No abstract at ADS Title: Mass Loss from Rapidly Rotating Magnetic Protostars Authors: Shu, Frank H.; Lizano, Susana; Ruden, Steven P.; Najita, Joan Bibcode: 1988ApJ...328L..19S Altcode: The authors propose that bipolar outflows from young stellar objects originate from a protostar rotating at breakup at its equator because it is being spun up by an adjoining accretion disk. Mass outflow at an appreciable fraction of the infall rate from a surrounding molecular cloud core onto the star and disk can be driven centrifugally if the protostar has a sufficiently strong magnetic field. The expansion of the flow toward the rotational poles may provide a collimation mechanism for focusing an ordinary stellar wind into optical jets. Title: Neutral Stellar Winds That Drive Bipolar Outflows in Low-Mass Protostars Authors: Lizano, Susana; Heiles, Carl; Rodriguez, Luis F.; Koo, Bon-Chul; Shu, Frank H.; Hasegawa, T.; Hayashi, S.; Mirabel, I. F. Bibcode: 1988ApJ...328..763L Altcode: Using the Arecibo radio telescope at the 21 cm line of atomic hydrogen, the authors detected a neutral atomic wind in the bipolar flow source HH 7-11. The H I gas reaches velocities up to ≡170 km s-1 and has an associated mass-loss rate from the protostar of ≡3×10-6M_sun;yr-1, which suffices to drive the observed CO bipolar flow. The 21 cm line profile indicates the H I wind to be decelerating. The deceleration of the atomic wind may arise from interaction with the surrounding molecular cloud. Evidence exists that ambient molecular material has been entrained in the decelerating H I wind.At the Nobeyama 45 m telescope the authors detected HCO+ at velocities intermediate between the H I wind and the CO bipolar outflow. The authors also discuss more ambiguous evidence in H I and in OH for a decelerating neutral stellar wind from L1551 IRS 5. Title: Book-Review - Galactic Dynamics Authors: Binney, J.; Tremaine, S.; Shu, F. Bibcode: 1988Natur.333..219B Altcode: No abstract at ADS Title: Stellar systems in theory Authors: Shu, Frank Bibcode: 1988Natur.333..219S Altcode: No abstract at ADS Title: The Disks of T Tauri Stars with Flat Infrared Spectra Authors: Adams, Fred C.; Lada, Charles J.; Shu, Frank H. Bibcode: 1988ApJ...326..865A Altcode: The authors model the energy distributions of T Tauri stars with flat infrared spectra by assuming that they have spatially thin disks with unorthodox radial gradients of temperature. These sources differ from more common T Tauri stars in that the extreme shallowness of their spectral slope cannot be plausibly explained by conventinal ideas (e.g., Keplerian accretion or reprocessing of starlight by a slightly flared disk). Derived model parameters show that the disks associated with the flat-spectrum sources must contain intrinsic luminosity in addition to the energy intercepted and reprocessed from the central star; i.e., the disks must be active. Self-gravity may provide one possible source for this activity - the spectral limits and estimates of the disk masses derived for three systems (T Tau, DG Tau, and HL Tau) are close to the theoretical values that would make the self-gravity of the disks dynamically important. Title: Applied mathematics, fluid mechanics, astrophysics. Proceedings. Authors: Benney, D. J.; Shu, F. H.; Chi, Yuan Bibcode: 1988amfm.conf.....B Altcode: Contents: 1. Applied mathematics. 2. Stability & turbulence. 3. General fluid mechanics. 4. Plasma physics. 5. Galactic astrophysics. 6. General astrophysics. Title: Infrared Spectra of Young Stellar Objects Authors: Adams, F. C.; Shu, F. H. Bibcode: 1988LNP...297..164A Altcode: 1988ctc..conf..164A The authors present models of the spectral energy distributions for an evolutionary sequence of young stellar objects, from protostars to pre-main-sequence stars. The protostellar theory, characterized by a central star and disk embedded within an infalling envelope of dust and gas, can explain the observed infrared spectra of embedded IRAS sources associated with molecular cloud cores. Next it is shown that T Tauri stars with near- and mid-infrared excesses can be understood as young stars surrounded by nebular disks. The disks in T Tauri systems are found in two varieties: passive disks which have no intrinsic luminosity and merely intercept and re-radiate stellar photons, and active disks which have appreciable intrinsic luminosity. Title: SIRTF and Star Formation Authors: Shu, Frank H. Bibcode: 1988ApL&C..27..105S Altcode: 1988ApL....27..105S The author discusses four problems in the field of star formation that can be attacked to advantage with SIRTF: (1) the patterns of star formation in spiral galaxies, (2) the physical mechanism for bimodal star formation, (3) the nature of bipolar outflows from young stellar objects, and (4) the birth of brown dwarfs. In each case, SIRTF can provide the crucial combination of high angular resolution with great sensitivity over a broad range of wavelengths that is needed to address the relevant issues. Title: The evolution of molecular clouds. Authors: Shu, F. H.; Lizano, S. Bibcode: 1988imgh.conf...65S Altcode: This paper is also published in Astrophys. Lett. Commun. See 45.131.143. Title: The Evolution of Molecular Clouds Authors: Shu, Frank H.; Lizano, Susana Bibcode: 1988ApL&C..26..217S Altcode: 1988ApL....26..217S The problem of the structure and evolution of molecular clouds is reviewed, with particular emphasis given to the relationship with star formation. The basic hypothesis is that magnetic fields are the primary agents for supporting molecular clouds, although damped Alfven waves may play an important role in the direction parallel to the field lines. This picture naturally leads to a conception of 'bimodal star formation'. It is proposed that high-mass stars form from the overall gravitational collapse of a supercritical cloud, whereas low-mass stars form from small individual cores that slowly condense by ambipolar diffusion from a more extended envelope until they pass the brink of graviational instability and begin to collapse dynamically from 'inside-out'. The evidence that the infall stage of protostellar evolution is terminated by the development of a powerful stellar wind is reviewed. Title: Winds from Low Mass Protostars Authors: Shu, Frank H.; Lizano, Susana; Adams, Fred C.; Ruden, Steven P. Bibcode: 1988ASSL..148..105S Altcode: 1988pmls.conf..105S In its last stages, star formation in molecular clouds includes the onset of a stellar wind that helps to clear away the surrounding placenta of gas and dust, thereby making the young stellar object optically visible. The authors discuss new observational evidence that the emerging wind is largely neutral and atomic in low-mass protostars. They then suggest a simple theoretical mechanism for the generation of such powerful neutral winds. Title: Beginning and End of a Low-Mass Protostar Authors: Shu, Frank H.; Lizano, Susana; Adams, Fred C.; Ruden, Steven P. Bibcode: 1988ASIC..241..123S Altcode: 1988felm.conf..123S No abstract at ADS Title: The Evolution of Molecular Clouds Authors: Shu, Frank H.; Lizano, Susana Bibcode: 1988inma.conf...65S Altcode: This paper is also published in Astrophys. Lett. Commun. See 45.131.143. Title: The Disks of T Tauri Stars with Flat Infrared Spectra Authors: Adams, F. C.; Shu, F. H.; Lada, C. J. Bibcode: 1987BAAS...19.1096A Altcode: No abstract at ADS Title: Voyager Photometry of Saturn's A Ring Authors: Dones, L.; Cuzzi, J. N.; Shu, F. H. Bibcode: 1987BAAS...19..881D Altcode: No abstract at ADS Title: Low Luminosity Sources Authors: Shu, Frank H. Bibcode: 1987NASCP2466..743S Altcode: 1987sfig.conf..743S The author summarized certain aspects of the conference. He shares this task with another colleague thereby breaking the task into more manageable proportions. The author covers the low luminosity sources. He begins his review with a summary of some major themes of the conference and ends with a few speculations on possible theoretical mechanisms. Title: On Statistical Mechanical Theories of Violent Relaxation Authors: Shu, Frank H. Bibcode: 1987ApJ...316..502S Altcode: The criticisms of Madsen (1987) directed against the author's particulate description for the statistical mechanics of violent relaxation are answered. To Madsen's argument that the particulate approach violates Liouville's theorem, it is answered that the theorem is necessarily violated on a macroscopic level if the concept of violent relaxation is to have sensible meaning. The use of macrocells and microcells in the particulate approach to describe stars and elementary particles is discussed, and the argument against the particulate approach based on constraints on maximum phase-space density are considered. Title: Planetary ring dynamics and morphology. Authors: Cuzzi, Jeffrey N.; Durisen, Richard H.; Shu, Frank H. Bibcode: 1987pggp.rept..125C Altcode: Evidence for a moonlet belt in the region between Saturn's close-in moonrings Pandora and Prometheus is discussed. It is argued that little-known observations of magnetospheric electron density by Pioneer 11 imply substantial, ongoing injections of mass into the 2000 km region which surrounds the F ring. A hypothesis is presented that these events result naturally from interparticle collisions between the smaller members of an optically thin belt of moonlets. Also discussed is work on Uranus ring structure and photometry, image processing and analysis of the Jonian ring strucure, photometric and structural studies of the A ring of Saturn, and improvements to an image processing system for ring studies. Title: Heating of molecular cloud cores. Authors: Lizano, Susana; Shu, Frank H. Bibcode: 1987RMxAA..14..587L Altcode: Following the work of Chu (1983), the present treatment of the role of ambipolar diffusion in the heating of molecular cloud cores calculates the quasistatic evolution of a plane-parallel self-gravitating slab of slightly ionized gas due to the ambipolar diffusion of the field. The temperature at each point of the cloud is computed in light of the most important heating and cooling processes; the results obtained are found to partially explain the thermal differences between the cores forming low- and high-mass stars. Title: Star formation in molecular clouds: observation and theory. Authors: Shu, Frank H.; Adams, Fred C.; Lizano, Susana Bibcode: 1987ARA&A..25...23S Altcode: Star-formation (SF) processes occurring on the scale of giant molecular clouds (10 to the 6th solar masses and 10 to the 20th cm) or smaller are discussed, reviewing the results of recent theoretical and observational investigations. Topics examined include the origin of stellar masses; bimodal SF; initial mass functions; binary stars, bound clusters, and hierarchical fragmentation; and the efficiency of SF. The properties of molecular clouds and the origin of substructures in molecular clumps are explored in detail, and consideration is given to gravitational collapse and protostars, bipolar outflows from young stellar objects, visible young stellar objects, and the implications for binary-star and planetary-system formation. Title: Star formation in molecular cloud cores Authors: Shu, Frank H.; Lizano, Susana; Adams, Fred C. Bibcode: 1987IAUS..115..417S Altcode: A scenario for star formation is described based on the assumption that magnetic fields play an important dynamical role in the interstellar medium. Particular attention is given to the evolution of a molecular cloud which is initially subcritical but which has a mass (supported by a combination of magnetic fields and sub-Alfvenic 'turbulence') that much exceeds the Jeans mass computed on the basis of the average density and temperature. It is argued that the magnetic support of such an object will automatically lead to the production of many small cores. Title: Formation and heating of molecular cloud cores. Authors: Lizano, Susana; Shu, Frank H. Bibcode: 1987ASIC..210..173L Altcode: 1987ppic.proc..173L The origin of molecular cloud cores and their evolution into star-forming regions are discussed, reviewing the results of recent theoretical investigations. Numerical results are presented graphically, and it is argued that the existence and properties of self-gravitating clouds can be explained most easily by the action of magnetic fields. Supercritical and subcritical cloud regimes (where the cloud mass is greater or less, respectively, than the mass at which cloud self-gravity can overcome magnetic-field support) are defined and related to two types of star formation: (1) highly efficient star formation and (2) inefficient formation of low-mass stars. Mechanism (1) is shown to favor the formation of high-mass stars whenever significant core heating occurs (e.g., via ambipolar diffusion). Title: Spectral Evolution of Young Stellar Objects Authors: Adams, Fred C.; Lada, Charles J.; Shu, Frank H. Bibcode: 1987ApJ...312..788A Altcode: An evolutionary sequence, from protostars to pre-main sequence stars, for the classification of young stellar objects is derived by comparing the predictions of the theoretical protostar models of Adams and Shu (AS, 1986) with the morphological classification scheme of Lada and Wilking (1984). It is shown that the AS models adequately explain the emergent spectral energy distributions of unidentified objects with negative spectral indices in the mid-IR and near-IR in both Taurus and Ophiuchus. If the infalling dust envelope is then completely removed, the spectra of the underlying stars and nebular disks used by AS provide a natural explanation for the near-IR and mid-IR excesses and the positive spectral indices of embedded T Tauri stars. It is found that the addition of a simple physical model for residual dust envelopes can reproduce the far-IR excesses found in some of these T Tauri stars. Title: Star formation and the circumstellar matter of young stellar objects. Authors: Shu, Frank H.; Adams, Fred C. Bibcode: 1987IAUS..122....7S Altcode: The authors propose that the formation of low mass stars in molecular clouds takes place in four stages. The first stage is the formation of slowly rotating cloud cores through the slow leakage of magnetic (and turbulent) support by ambipolar diffusion. The second phase begins when a condensing cloud core passes the brink of instability and collapses dynamically from "inside-out", building up a central protostar and nebular disk. The emergent spectral energy distributions of theoretical models in the infall stage are in close agreement with those of recently found infrared sources with steep spectra. As the rotating protostar gains mass, deuterium will eventually ignite in the central regions and drive the star nearly completely convective if its mass is less than about 2 M_sun;. This initiates the next step of evolution - the bipolar outflow phase - in which a stellar wind pushes outward and breaks through the infalling envelope. The initial breakout is likely to occur along the rotational poles, leading to collimated jets and bipolar outflows. The intense stellar wind eventually widens to sweep out gas in nearly all 4π steradian, revealing the fourth stage - a T Tauri star with a surrounding remnant nebular disk. Radiation from a disk adds an infrared excess to the expected spectral energy distribution of the revealed source. Title: Selected papers of C. C. Lin. Volume 1: Fluid mechanics and applied mathematics. Volume 2: Astrophysics. Authors: Benney, D. J.; Shu, F. H.; Chi, Yuan Bibcode: 1987spcc.book.....B Altcode: These two volumes of the Selected papers of C. C. Lin are compiled on the occasion of his retirement from active service. The introducing sections entitled "Recollections and comments" which portray the development of C. C. Lin's thoughts on various subjects and place events in historical perspective are given by himself. The selected papers are grouped according to subject. In Vol. 1, the papers belonging to fluid mechanics and applied mathematics are divided into five topics: stability theory, turbulence, general fluid dynamics and aerodynamics, mathematical physics, and essays on applied mathematics. Vol. 2 contains Lin's work in astrophysics, largely concerning the development of the density wave theory for the spiral structure in galaxies. A complete list of C. C. Lin's publications is given in Vol. 2. Title: Why do Molecular Clouds Give Birth to Objects like Stars Authors: Shu, F. H. Bibcode: 1986PASP...98.1099S Altcode: No abstract at ADS Title: Studies of Star Formation with SIRTF Authors: Shu, F. H. Bibcode: 1986BAAS...18.1033S Altcode: No abstract at ADS Title: Infrared Spectra of Rotating Protostars Authors: Adams, F. C.; Shu, F. H. Bibcode: 1986ApJ...308..836A Altcode: Earlier calculations of the infrared emission expected from stars in the process of being made are corrected to include the most important observable effects of rotation and generalized. An improved version of the spherical model of a previous paper is developed, and the corresponding emergent spectral energy distributions are calculated for the theoretically expected mass infall rate in the cores of cool and quiescent molecular clouds. The dust grain opacity model and the temperature profile parameterization are improved. It is shown that the infrared spectrum of the IRAS source 04264+2426, which is associated with a Herbig-Haro object, can be adequately represented in terms of a rotating and accreting protostar. This strengthens the suggestion that collimated outflows in young stellar objects originate when a stellar wind tries to emerge and reverse the swirling pattern of infall which gave birth to the central star. Title: Nonlinear Density Waves in Saturn's Rings: Theory Authors: Dones, L.; Shu, F. H.; Cuzzi, J. N.; Lissauer, J. J.; Lane, A. L.; Yuan, C. Bibcode: 1986BAAS...18..839D Altcode: No abstract at ADS Title: Molecular Cloud Cores and Star Formation Authors: Shu, Frank H. Bibcode: 1986copr.conf..219S Altcode: The author examines the problem of how stars form from the great reservoirs of cold matter in giant molecular clouds. Title: Theory of Protostellar Objects Authors: Shu, F. H. Bibcode: 1986LNP...255...10S Altcode: 1986rhsc.conf...10S; 1986IAUCo..89...10S Many problems in the theory of star formation are amenable to a complementary attack in which the analytical approach is used to reduce the governing equations to a form amenable to efficient numerical solution. The author concentrates here on the semi-analytical methods which can be used to reduce the governing equations to a form amenable to quick and accurate numerical solution. Title: Nonlinear spiral density waves - Viscous damping Authors: Shu, F. H.; Dones, L.; Lissauer, J. J.; Yuan, C.; Cuzzi, J. N. Bibcode: 1985ApJ...299..542S Altcode: The authors calculate the viscous damping of nonlinear density waves in Saturn's rings using various kinetic formulations to estimate the collisional change (δP/δt)c of the vertically integrated pressure tensor P in a particulate disk. The pressure tensor is computed as a function of two parameters: (1) the ratio of the average collision frequency to the epicyclic frequency, and (2) a measure of how close streamlines are to crossing when perturbed by nonaxisymmetric forces. When parameters approximately correct for the density waves excited by Mimas's 5:3 resonance in the A ring are used, the authors obtain a satisfactory fit with the observed profile, providing one uses a coefficient of restitution that is appropriate to crystalline ice and providing one uses an extreme Krook formulation to evaluate (δP/δt)c. When a similar calculation is made for the density waves in the B ring excited by the comparably strong 2:1 resonance of the so-called co-orbital satellites, the results are more mixed. It is speculated that the densities in the actual peaks of the density waves in Saturn's B ring are limited, not by viscosity, but by the condition of excluded volumes when ice balls are packed tightly against one another. Finally, the authors discuss the problems of mass and angular momentum transport in a disk with externally deposited resonant torques, and of opening gaps and maintaining sharp edges in planetary rings. Title: Infrared emission from protostars. Authors: Adams, F. C.; Shu, F. H. Bibcode: 1985ApJ...296..655A Altcode: The authors use an approximate technique to calculate the temperature profiles, and thereby the emergent spectral energy distributions, of protostars which derive all their luminosity by infall of a gas and dust envelope. The formulation is general, but the application is to protostars with spherical symmetry. It is shown that the emergent spectrum in the infrared from the dust envelope is insensitive to the details assumed for the temperature profile as long as allowance is made for a transition from optically thick to optically thin conditions and luminosity conservation is enforced at an inner shell and an outer shell. An atlas of emergent spectral energy distributions is presented for a grid of values of the instantaneous mass M of the protostar and the mass infall rate M. The theoretical spectra consistently produce too little mid-infrared radiation in comparison with the observed sources, especially those which are in a fairly late stage of protostellar evolution. Title: Infrared Spectra of Rotating Protostars Authors: Adams, F. C.; Shu, F. H. Bibcode: 1985BAAS...17..863A Altcode: No abstract at ADS Title: The collisional dynamics of particulate disks Authors: Shu, F. H.; Stewart, G. R. Bibcode: 1985Icar...62..360S Altcode: We use a Krook equation, modified to allow collisions to be inelastic, to describe the dynamics of a particulate disk. By a simple heuristic argument, we compute the effective collision rate in a disk of spherical particles with a power-law distribution of sizes. For Saturn's rings, the effective collision rate for momentum transport is substantially lower than that conventionally estimated on the basis of an observed optical depth at visual wavelengths. We then discuss how the vertically integrated set of moment equations may be closed without the need to discard the third-order moments at the outset; our formulation allows for the possibility of a bent disk. In the limit that the collision frequency is much larger than the orbit frequency, we recover the usual Navier-Stokes equations of viscous hydrodynamics for a thin disk, with an explicit expression for the shear viscosity. For an unperturbed disk, we can solve the krook equation directly, without any assumptions about the magnitude of the collision frequency. Our analytical results, for an unperturbed disk, are in good agreement with the treatments of Hämeen-Anttila, of Goldreich and Tremaine, and of Borderies, Goldreich, and Tremaine, using a Boltzmann description for a collection of identical spheres (assumed to be smooth so that the rotational and translational degrees of freedom do not couple). As a final application of the method, we generalize the formation to include the effects of gravitational scattering. This generalization is not crucial for many applications in planetary rings, but it may be important for the discussion of gas clouds in the disk of a spiral galaxy, and it is probably central to the accumulation of planets from smaller bodies in the primitive solar nebula. Title: Density Waves in Saturn's Rings: Nonlinear Damping Theory and Photometry Authors: Dones, L.; Cuzzi, J. N.; Shu, F. H.; Lissauer, J. J.; Lane, A. L.; Yuan, C. Bibcode: 1985BAAS...17..717D Altcode: No abstract at ADS Title: The Formation of preplanetary disks from the collapse of rotating molecular cloud cores Authors: Cassen, P.; Shu, F. H.; Terebey, S. Bibcode: 1985pggp.rept..143C Altcode: Solutions that describe the collapse of a molecular cloud core that is initially in unstable equilibrium, embedded within an envelope of uniform density, and rotating at the same rate as the envelope are given. Hydrodynamic equations, including self gravity, are deduced to a set of ordinary differential equations, which are solved by the method of matched asymptotic expansions. Results of these calculations are: (1) the range of stellar masses derived seems to correspond to realistic ranges of observed stellar masses and interstellar cloud parameters, (2) the proper measure of dissipation rate is the ratio of accretion time to viscous diffusion time, and (3) the pressure distribution on the surface of an accreting protostar is nonuniform in a way that favors the channeling of a stellar wind into a bipolar flow directed along the rotation axis. Title: Nonlinear spiral density waves - an inviscid theory Authors: Shu, F. H.; Yuan, C.; Lissauer, J. J. Bibcode: 1985ApJ...291..356S Altcode: It is pointed out that the theory of spiral density waves, invented to explain the spiral structure of disk galaxies, has also been found useful for the study of planetary rings. The linear theory is by now well developed, while the nonlinear theory is less complete. Analytical calculations which include self-gravitation have, so far, obtained results only in the slightly nonlinear regime, or have concentrated on partial effects which are not of primary importance to the physical problem at hand. In the present paper, it is attempted to remedy these shortcomings. The simplest asymptotic ordering which can still yield useful results is adopted. Attention is given to the reduction to a nonlinear integral equation in a single variable, the use of the Wentzel-Kramers-Brillouin-Jeffreys theory, and the replacement of an equation by another which is easier to handle numerically. Title: Star formation in molecular clouds Authors: Shu, F. H. Bibcode: 1985IAUS..106..561S Altcode: The problem of star formation in spiral galaxies is addressed, with attention to the length scales involved. The phenomenon spans at least twelve orders of magnitude, from 10 to the 23rd cm in size to 10 to the 11th cm. It is concluded that: (1) The distribution of newly born stars in the Galaxy depends on the origin of giant-molecular-cloud complexes. For individual complexes, the mechanism of Parker's instability behind galactic shocks is favored. (2) Magnetic fields help to support the clumps of molecular gas making up a complex against gravitation collapse. On a timescale of 10 million years, these fields slip by ambipolar diffusion relative to the neutral gas, leading to the formation of dense cloud cores. (3) When to cores collapse, they usually give rise to low-mass stars. (4) A powerful stellar wind shuts off the accretion flow and determines the mass of the new star. The stellar wind is driven in low-mass stars by a release of energy of differential rotation when the entire protostar becomes convectively unstable at the onset of deuterium burning. Title: Protostellar disks and star formation. Authors: Cassen, P.; Shu, F. H.; Terebey, S. Bibcode: 1985prpl.conf..448C Altcode: The status of theoretical work on protostellar disks is reviewed. Accretion disk theory and its application to models of the solar nebula and protostellar disks are discussed. A unified view of the process of star formation is presented, starting from the evolution of molecular clouds, and leading naturally to the formation of protostellar disks. The models used to describe this process are idealized, but are believed to provide good prototypes that well represent the essential hydromagnetic phenomena involved in star and disk formation. Several possible evolutionary paths and final configurations are qualitatively discussed, showing how the outcomes depend on the relative efficiencies of various angular momentum transport processes. Title: The collapse of the cores of slowly rotating isothermal clouds Authors: Terebey, S.; Shu, F. H.; Cassen, P. Bibcode: 1984ApJ...286..529T Altcode: The authors present here a semianalytic perturbational solution for the collapse of a slowly rotating cloud core. The initial equilibrium state is exact and corresponds to the uniformly rotating analogue of the singular isothermal sphere. Star (and disk) formation proceeds because the equilibrium is unstable to core collapse. The evolution in time can be followed by performing a perturbational analysis on the known similarity solution for the nonrotating case. The hydrodynamic equations, including self-gravity, can be reduced to a set of linear ordinary differential equations, which is solved by the method of matched asymptotic expansions. These calculations provide a self-consistent description of the dynamical collapse of rotating molecular cloud cores and a framework for the study of the formation of stars and nebular disks. Title: Infrared Emission from Protostars Authors: Shu, F. H.; Adams, F. Bibcode: 1984BAAS...16..973S Altcode: No abstract at ADS Title: Damping of Nonlinear Density Waves in Saturn's Rings Authors: Dones, L.; Shu, F. H.; Lissauer, J. J.; Cuzzi, J. N.; Yuan, C. Bibcode: 1984BAAS...16R.922D Altcode: No abstract at ADS Title: Book-Review - the Physical Universe Authors: Shu, F. H.; Smith, R. C. Bibcode: 1984Obs...104..101S Altcode: No abstract at ADS Title: Viscosity in Saturn's Rings Authors: Lissauer, J. J.; Shu, F. H.; Cuzzi, J. N. Bibcode: 1984plri.coll..385L Altcode: 1984prin.conf..385L; 1984IAUCo..75..385L The authors discuss the technique of estimating the viscosity in Saturn's rings from the damping rate of waves observed to be propagating within the rings. They present a new method that considers the damping of spiral bending waves, a diagnostic which is largely free of major complications. The authors deduce a kinematic viscosity of 260 (+150, -100) cm2/s for the middle of the A ring where bending waves have been excited by the 5:3 vertical resonance with Mimas. Title: Waves in planetary rings Authors: Shu, F. H. Bibcode: 1984prin.conf..513S Altcode: The Voyager spacecraft revealed the rings of Saturn to have an unexpected richness of structure. Many of the observed features have now been identified as collective effects arising from the self-gravity of the ring material. These effects include spiral density waves and spiral bending waves, the main topics of this review. Both kinds of waves were first discussed in the astronomical literature in connection with the dynamics and structure of spiral galaxies, and the present discussion contrasts the similarities and differences between the disks of galaxies and planetary rings. After developing the theory of free and forced waves of both types, the way in which the observed waves can be used as diagnostics to obtain crucial parameters that characterize the physical state of the rings is discussed. Title: POST-USE REVIEW: The Physical Universe: An Introduction to Astronomy Authors: Shu, Frank; Edwards, Suzan; Greenstein, George Bibcode: 1984AmJPh..52...91S Altcode: No abstract at ADS Title: The Formation of Cool Stars from Cloud Cores Authors: Shu, Frank H.; Terebey, Susan Bibcode: 1984LNP...193...78S Altcode: 1984csss....3...78S We consider models of the processes by which a molecular cloud acquires dense cores, a (magnetized) rotating core collapses to give a protostar plus nebular disk, and a powerful stellar wind sets in to reverse the accretion flow and reveal the central object as a pre-main-sequence star. At each stage, we rely on a combination of theory and observation to fix the basic parameters of the model. We show that core formation in a molecular cloud is an inevitable byproduct of ambipolar diffusion in a magnetized self-gravitating medium of low fractional ionization. We find that the gravitational collapse of a uniformly-rotating isothermal core, which possesses a 1/r2 density profile in its inner parts, has simple analytic properties. And we propose that strong stellar winds in T Tauri stars represent a phase of readjustment in the angular momentum distribution after deuterium burning drives convection throughout a strongly differentially-rotating protostar. We conclude that the major missing link in this picture is the evolutionary behavior of massive nebular disks that may accumulate around protostars. Otherwise, there seems to be a satisfying connection between the cloud cores observed by molecular-line radio astronomers and the active stellar atmospheres of young stars studied by optical and x-ray astronomers. Title: Ambipolar diffusion in self-gravitating isothermal layers Authors: Shu, F. H. Bibcode: 1983ApJ...273..202S Altcode: Quasi-magnetohydrostatic and local ionization equilibria are assumed in the present formulation and solution of the drift of magnetic field and ions embedded in a self-gravitating layer of neutral isothermal gas. The introduction of Lagrangian coordinates referred to the neutral gas allows this problem to be reduced to a nonlinear diffusion equation for the magnetic field, whose dimensionless form involves no parameters other than those introduced by the initial values. In the shape-invariant solution, the magnetic field at each surface density point in the neutral fluid decays as the inverse square root of the elapsed time. Explicit estimates are given, as a function of the initial magnetic to neutral gas pressure in a natural family of cases, for the amount of time that must pass before the shape-invariant solution becomes a good approximation for actual behavior. The results obtained are interpreted physically. Title: Nonlinear Density Waves in Saturn's Rings Authors: Shu, F. H.; Yuan, C.; Lissauer, J. J. Bibcode: 1983BAAS...15..959S Altcode: No abstract at ADS Title: Rotating Cloud Collapse: 1018 cm to 1011 cm Authors: Terebey, S.; Shu, F. H.; Cassen, P. M. Bibcode: 1983BAAS...15..922T Altcode: No abstract at ADS Title: Book-Review - the Physical Universe - an Introduction to Astronomy Authors: Shu, F.; Hughes, D. W. Bibcode: 1983Natur.302..186S Altcode: No abstract at ADS Title: Book-Review - the Physical Universe - an Introduction to Astronomy Authors: Shu, F. H.; Miles, H. Bibcode: 1983JBAA...93...97S Altcode: No abstract at ADS Title: Bending waves in Saturn's rings Authors: Shu, F. H.; Cuzzi, J. N.; Lissauer, J. J. Bibcode: 1983Icar...53..185S Altcode: We investigate certain brightness variations seen in Saturn's A ring and find them to be due to vertical corrugations of the local ring plane caused by a spiral bending wave. This wave is resonantly excited by Mimas and propagates inward via the collective gravity of the ring particles. B. A. Smith et al. [ Science212, 163-191 (1981)] had previously associated vertical relief with this feature due to its observed azimuthal variations and its proximity to an inclination resonance with Mimas. We develop the theory of forced bending waves, some aspects of which have been treated in the galactic context by C. Hunter and A. Toomre [ Astrophys. J.155, 747-776 (1969)] and by G. Bertin and J.W.-K. Mark [ Astron. Astrophys.88, 289-297 (1980)]. Our theory is in good agreement with the observations. In particular, the presence of these bending waves may resolve the conflict between ground-based estimates of 1-2 km for the global ring thickness [e.g., A. Brahic and B. Sicardy, Nature289, 447-450 (1981)] and Voyager stellar occultation measurements of <200 m for the local ring thickness [A. L. Lane et al., Science215, 537-543 (1982); E. A. Marouf and G. L. Tyler, Science217, 243-245 (1982)]. Title: Book-Review - the Physical Universe Authors: Shu, F.; Fraknoi, A. Bibcode: 1983Mercu..12S..88S Altcode: No abstract at ADS Title: The Expanding Universe and the Largescale Geometry of Spacetime Authors: Shu, F. Bibcode: 1983Mercu..12..162S Altcode: No abstract at ADS Title: The Role of Rotation in the Formation of Stars and Planetary Systems Authors: Terebey, S.; Shu, F. H.; Cassen, P. M. Bibcode: 1982BAAS...14..639T Altcode: No abstract at ADS Title: Book-Review - the Physical Universe Authors: Shu, F. H. Bibcode: 1982JBAA...93R..45S Altcode: No abstract at ADS Title: The Physical Universe Authors: Shu, Frank H. Bibcode: 1982phyn.book.....S Altcode: 1982puai.book.....S; 1982phun.book.....S This is a truly astonishing book, invaluable for anyone with an interest in astronomy and surely the bargain of the year.--Physics Bulletin Title: Book-Review - the Physical Universe - Introduction to Astronomy Authors: Shu, F. H. Bibcode: 1982Natur.298R.310S Altcode: No abstract at ADS Title: Erratum: 'Mass, angular momentum and energy transfer in close binary stars' [Ann. Rev. Astron. Astrophys., Vol. 19, 277 - 293 (1981)]. Authors: Lubow, S. H.; Shu, F. H. Bibcode: 1982ARA&A..20.....L Altcode: No abstract at ADS Title: Book-Review - the Physical Universe Authors: Shu, F. H.; Trimble, V. Bibcode: 1982S&T....64..342S Altcode: No abstract at ADS Title: Book-Review - the Physical Universe - a Introduction to Astronomy Authors: Shu, F. H. Bibcode: 1982S&T....64Q.152S Altcode: No abstract at ADS Title: The evolution of protostars. III - The accretion envelope Authors: Stahler, S. W.; Shu, F. H.; Taam, R. E. Bibcode: 1981ApJ...248..727S Altcode: The radiation gas dynamics of the rapidly inflowing material during the main accretion phase of protostellar evolution is considered. The momentum and energy transfer by radiation and matter, the dissociation of the molecular gas, and the thermochemical destruction of graphite grains are followed in detail. Considerable physical insight is gained by taking advantage of the vastly disparate time and length scales associated with the various processes. Different computational techniques are used in different portions of the accretion flow: the optically thick dust envelope, the thermalization layer of stellar photons, the opacity gap, and the radiative precursor (if one exists). The evolution of the envelope of a protostellar model whose hydrostatic core was described in an earlier communication is summarized. Title: Density waves in Saturn's rings Authors: Cuzzi, J. N.; Lissauer, J. J.; Shu, F. H. Bibcode: 1981Natur.292..703C Altcode: Certain radial brightness variations in the outer Cassini division of Saturn's rings may be spiral density waves driven by Saturn's large moon Iapetus, in which case a value of ~16 g cm-2 for the surface density is calculated in the region where the waves are seen. The kinematic viscosity in the same region is ~170 cm2 s-1 and the vertical scale height of the ring is estimated to be a maximum of ~40 m. Title: Moonlets in Saturn's rings? Authors: Lissauer, J. J.; Shu, F. H.; Cuzzi, J. N. Bibcode: 1981Natur.292..707L Altcode: The brightness structure within Cassini's division in Saturn's rings is explained in terms of perturbations produced by moonlets embedded within an optically thin disk of smaller ring particles. The moonlets exert gravitational torques on neighbouring ring particles and create gaps; diffusion acts to fill the gaps. A new explanation is offered for the inner edge of the Cassini division being located at the 2:1 resonance with Mimas. Title: Mass, angular momentum, and energy transfer in close binary stars Authors: Shu, F. H.; Lubow, S. H. Bibcode: 1981ARA&A..19..277S Altcode: The paper surveys some aspects of the dynamics and energetics of close binary stars which have heretofore not received a detailed review. Attention is directed to those processes of mass, angular momentum, and energy transfer that are thought to have potentially important consequences for the evolutionary history of close binary systems. The presentation, which proceeds mainly along theoretical lines, considers in turn detached binaries, semidetached binaries, and contact binaries. Title: The evolution of protostars. II - The hydrostatic core Authors: Stahler, S. W.; Shu, F. H.; Taam, R. E. Bibcode: 1980ApJ...242..226S Altcode: A computational scheme is developed for the evolution of an accreting protostellar core. Novel techniques are introduced to account for the dynamical addition of mass and to follow the disparate thermal responses in different parts of the star. Numerical results are presented from two evolutionary calculations that begin with very different distributions of the specific entropy. The two sequences converge rapidly in time, justifying a posteriori the assumption that the final results are insensitive to the detailed mechanism for the formation of the core. Title: Why extended radio doubles are found in elliptical galaxies Authors: Sparke, L. S.; Shu, F. H. Bibcode: 1980ApJ...241L..65S Altcode: Elliptical galaxies rotate very slowly, and this rotation affects the way in which gas lost from stars finds its way into the nucleus. It is suggested that the dynamical nature of this flow is the reason why extended double radio sources occur in elliptical galaxies, rather than in spirals. A specific computed example is presented to illustrate the principle, and some observational consequences are discussed. Title: The evolution of protostars. I - Global formulation and results Authors: Stahler, S. W.; Shu, F. H.; Taam, R. E. Bibcode: 1980ApJ...241..637S Altcode: A new formulation for the problem of protostar evolution is developed on the basis of a method that involves the division of the global problem into a set of more manageable subproblems. Jump conditions of the radiative accretion shock which joins the hydrostatic mass-gaining core to the dynamic inner cloud envelope are derived. Standard integration schemes yield the high accuracy needed to resolve complex spatial structures which span many orders of magnitude in density and temperature. It is shown that a 1-solar-mass protostar ends its main accretion phase moderately high up in the H-R diagram; this star begins its pre-main-sequence phase of quasi-static contraction on a convective Hayashi track. Title: On various criticisms of the contact discontinuity model Authors: Shu, F. H.; Lubow, S. H.; Anderson, L. Bibcode: 1980ApJ...239..937S Altcode: We discuss various criticisms raised recently by Lucy and Wilson, by Papaloizou and Pringle, and by Smith, Robertson, and Smith against the contact discontinuity model for contact binaries. In the process we discover a promising means by which the filled fraction occupied by the common envelope could, in principle, be determined mechanistically for a contact binary of given total mass, angular momentum, initial chemical composition, and age. Title: The origin and lifetime of giant molecular cloud complexes Authors: Blitz, L.; Shu, F. H. Bibcode: 1980ApJ...238..148B Altcode: From the available observational and theoretical evidence it is argued that the mass of molecular gas in the Galaxy has been considerably overestimated and that the ages of the giant molecular complexes do not exceed a few times 10-million years. An expression derived for the collisional time scale for clumps in a complex has a maximum value of 10-million yr. It is argued that the formation of giant complexes by random collisional agglomeration of small molecular clouds is incompatible with several firm observational results. The Parker instability is discussed as a possible formation mechanism which can explain many of the observed properties of the complexes. Title: Rotation broadening and the shapes of W Ursae Majoris Stars Authors: Anderson, L.; Raff, M.; Shu, F. H. Bibcode: 1980IAUS...88..485A Altcode: Rotation broadening functions are extracted from the spectra of W Ursae Majoris Stars. A fast Fourier transform is used to deconvolve photographic spectra, covering some 500 A including dozens of strong lines, with equivalent spectra from non-rotating stars of similar spectral type. The resulting rotation functions contain information about global features such as the shape of the stellar surface (e.g. mass ratio and degree of contact), gravity brightening and limb darkening. Preliminary data on the stars VW Cep and ER Vul are presented. The rotation function of the former reveals the presence of the third component found visually by Heintz (1975), while that of the latter shows it to be detached and have mass ratio 0.9. Title: Protostars - a New Approach Authors: Stahler, S. W.; Shu, F. H. Bibcode: 1980BAAS...12..439S Altcode: No abstract at ADS Title: Theories of contact binary stars Authors: Shu, F. H. Bibcode: 1980IAUS...88..477S Altcode: We review and contrast two current theories for the structure of contact binary stars: discontinuity theory and thermal relaxation oscillation theory. We find that the two theories are complementary with the crucial theoretical issue to be resolved being the secular stability of the temperature inversion layer. Critical observational tests remain to be performed. Title: A theoretical atlas of light curves and rotation broadening functions for contact binary stars. Authors: Anderson, L.; Shu, F. H. Bibcode: 1979ApJS...40..667A Altcode: Bolometric light curves and rotation broadening functions of contact binaries are computed for a grid of values of mass ratio, filled fraction, and orbital inclination, assuming three different extreme combinations of gravity brightening and limb darkening. The results are presented in a standard graphical format which is readily usable for comparisons with observations. Title: On the structure of contact binaries. IV. High-mass models. Authors: Lubow, S. H.; Shu, F. H. Bibcode: 1979ApJ...229..657L Altcode: Interior models for zero-age contact binaries of high total mass are constructed on the basis of the contact discontinuity hypothesis. The contact discontinuity in systems with a total mass greater than about 4 solar masses is shown to occur on the side of the primary rather than the secondary. Results for two models, one with masses of 4 + 2 solar masses and the other with 8 + 4 solar masses, are presented, along with the properties of the corresponding single stars. These results indicate that the most significant change introduced by the contact configuration is the appearance of a convection zone beneath the contact discontinuity at the Roche lobe of the primary. A proof is given that zero-age main-sequence radiative stars in thermal equilibrium cannot be expanded to fill their Roche lobes in a contact binary. Title: On the structure of contact binaries. III. Mass and energy flow. Authors: Shu, F. H.; Lubow, S. H.; Anderson, L. Bibcode: 1979ApJ...229..223S Altcode: A variety of topics dealing with the mass and energy flow in contact binaries is examined. It is argued that the contact discontinuity hypothesis proposed in earlier communications is both necessary and sufficient to resolve Kuiper's (1941) paradox concerning the existence of contact binaries with unequal components. A detailed qualitative picture of the maintenance by fluid flow of the contact and weak discontinuities of zero-order models in presented. This discussion supplies a description of the mechanism by which the interior luminosities are redistributed to give common envelope luminosities. It also provides order-of-magnitude estimates for the thickness of the transition layer and mixing region that constitute the actual structure of the contact and weak discontinuities. The gas dynamics of mass loss that occurs if stellar evolution should cause a synchronously rotating common surface to try to expand beyond the outer critical surface is calculated. The role of various processes in the problem of the evolution of contact binaries is considered. Title: Rotation Broadening Functions of Selected W Ursae Majoris Stars. Authors: Anderson, L.; Raff, M.; Shu, F. H. Bibcode: 1979BAAS...11..438A Altcode: No abstract at ADS Title: The extrapolated central surface brightness of galaxies. Authors: Allen, R. J.; Shu, F. H. Bibcode: 1979ApJ...227...67A Altcode: Disney's (1976) suggestion is reexamined that selection effects dominate Fish's (1964) law concerning the potential energy-mass relation for elliptical galaxies and Freeman's (1970) law concerning the extrapolated central surface brightness of disk galaxies. It is found that selection effects may indeed have influenced the samples discussed by Fish and Freeman, but not in the sense described by Disney. In particular, it is argued that the data set does not show a bias toward large galaxies, but there is a bias against the faint end of the distribution of central surface brightnesses. It is therefore suggested that the observed paucity of ellipticals and spirals with very high extrapolated central surface brightnesses is probably real. Hence Fish's and Freeman's laws should be reformulated to state that very few elliptical and spiral galaxies have extrapolated central isophotes in B which are brighter than, respectively, 12.0 mag per square arcsec and 19.5 mag per square arcsec. Title: On the statistical mechanics of violent relaxation. Authors: Shu, F. H. Bibcode: 1978ApJ...225...83S Altcode: This paper reexamines the foundations of Lynden-Bell's (1967) statistical-mechanical discussion of violent relaxation in collisionless stellar systems. It is argued that Lynden-Bell's formulation in terms of a continuum description introduces unnecessary complications, and a more conventional formulation in terms of particles is considered. The exclusion principle discovered by Lynden-Bell is found to be quantitatively important only at phase densities where two-body encounters are no longer negligible. Since the dynamical basis for the exclusion principle vanishes in such cases anyway, Lynden-Bell statistics always reduces in practice to Maxwell-Boltzmann statistics when applied to stellar systems. Lynden-Bell also found the equilibrium distribution function generally to be a sum of Maxwellians with velocity dispersions dependent on the phase density at star formation. It is shown that this difficulty vanishes in the particulate description for an encounterless stellar system as long as stars of different masses are initially well mixed in phase space. Title: On the role of photospheric convection in W Ursae Majoris stars. Authors: Anderson, L.; Shu, F. H. Bibcode: 1978ApJ...221..926A Altcode: We correct the derivation of the source function in the atmospheres of contact binaries given in an earlier communication by Anderson and Shu. This correction affects the cases when convection is present in the photosphere. In our new treatment photospheric convection is more efficient for reducing limb darkening. This result does not, however, modify the numerical examples considered in our earlier paper. Subject headings: convection - stars: W Ursae Majoris Title: Hot Gas in the Galaxy: how Extensive is It? Authors: Shu, F. H. Bibcode: 1978IAUS...77..139S Altcode: Observational and theoretical evidence is reviewed concerning the possible extent of hot gas at a temperature of the order of 1 million K in the Galaxy. Consequences of hot gas filling a large fraction of interstellar space for galactic shocks are considered. H I radio observations are discussed along with O VI observations in the UV, soft X-ray observations, and theoretical estimates of the filling factor of hot gas at 1 million K. A series of future radio and soft X-ray observations of external spiral galaxies is proposed. Title: On the structure of contact binaries. II. Zero-age models. Authors: Lubow, S. H.; Shu, F. H. Bibcode: 1977ApJ...216..517L Altcode: Zero-age models of contact binaries of roughly solar composition are constructed on the basis of contact discontinuity hypothesis. With this formulation, systems with common radiative envelopes can be constructed as well as systems with common convective envelopes. Two models with total masses, respectively, of 1.5 and 3 solar masses are presented explicitly; both binary models have a mass ratio chosen equal to 0.5. The properties of the interior structure of these models are compared with the properties of zero-age single stars which have masses corresponding to the individual components. The predictions of the theory are compared with the empirical period-color relationship found by Eggen (1961, 1967) for W Ursae Majoris stars. The agreement with observations is satisfactory. Title: On the light curves of W Ursae Majoris stars. Authors: Anderson, L.; Shu, F. H. Bibcode: 1977ApJ...214..798A Altcode: We develop a physical theory for the light curves of contact binaries based on the assumption that the dynamical time scale is very short in comparison with the thermal time scale at the base of the common envelope. In contrast with the case for common radiative envelopes, the flux distribution in common convective envelopes does not exhibit any effect of gravity brightening. Combined with a unified treatment of reflection and limb darkening, this result produces Wtype light curves for W UMa stars of spectral type later than F5 if the orbit is inclined by less than approximately 70 90o. The sign of the effect is in rough accord with the observations, but some discrepancy remains concerning the magnitude of the effect. We speculate that the interaction between rapid rotation and convection may contribute to the remaining discrepancy; it may also produce the asymmetry and time variability which are observed in some light curves. Subject headings: stars: binaries - stars: W Ursae Majoris Title: Self-similar collapse of isothermal spheres and star formation. Authors: Shu, F. H. Bibcode: 1977ApJ...214..488S Altcode: Similarity solutions are obtained which describe the gravitational collapse of isothermal spheres that originated as gas clouds not far removed from the condition of marginal stability. It is argued that the similarity solution of Larson and Penston (1969) for the stages before core formation is physically artificial, but the gasdynamic flow subsequent to core formation exhibits self-similar properties. Similarity solutions are determined for the collapse of singular isothermal spheres, minus solutions without critical points are obtained by imposing the condition that the fluid velocities are negligible at the 'initial instant', and an expansion-wave collapse solution is evaluated. The results are illustrated with a numerical example roughly corresponding to conditions appropriate for Bok globules or the central regions of a nonmagnetic molecular cloud. Two possible applications of the solutions are discussed: analyzing the stability to gravitational fragmentation of collapsing pressure-free gas spheres and determining the amount of energy radiated away during protostar formation. Title: Theory of Close Binary Stars. Authors: Shu, F. H. Bibcode: 1977BAAS....9..318S Altcode: No abstract at ADS Title: On the structure of contact binaries. I. The contact discontinuity. Authors: Shu, F. H.; Lubow, S. H.; Anderson, L. Bibcode: 1976ApJ...209..536S Altcode: The problem of the interior structure of contact binaries is reviewed, and a simple resolution of the difficulties which plague the theory is suggested. It is proposed that contact binaries contain a contact discontinuity between the lower surface of the common envelope and the Roche lobe of the cooler star. This discontinuity is maintained against thermal diffusion by fluid flow, and the transition layer is thin to the extent that the dynamical time scale is short in comparison with the thermal time scale. The idealization that the transition layer has infinitesimal thickness allows a simple formulation of the structure equations which are closed by appropriate jump conditions across the discontinuity. The further imposition of the standard boundary conditions suffices to define a unique model for the system once the chemical composition, the masses of the two stars, and the orbital separation are specified. Title: Zero-Age Models of Contact Binaries. Authors: Lubow, S. H.; Shu, F. H. Bibcode: 1976BAAS....8Q.520L Altcode: No abstract at ADS Title: Self-Similar Collapse of Isothermal Spheres and Star Formation. Authors: Shu, F. H. Bibcode: 1976BAAS....8Q.547S Altcode: No abstract at ADS Title: On the Light Curves of W Ursae Majoris Stars. Authors: Anderson, L.; Shu, F. H. Bibcode: 1976BAAS....8R.521A Altcode: No abstract at ADS Title: Gas dynamics of semidetached binaries. II. The vertical structure of the stream. Authors: Lubow, S. H.; Shu, F. H. Bibcode: 1976ApJ...207L..53L Altcode: The letter considers the three-dimensional dynamics of the stream in semidetached binaries undergoing Roche-lobe overflow. Results of an earlier communication are generalized to include the dynamic effects in the direction perpendicular to the orbital plane. It is found that the scale height of the stream characteristically exceeds its corresponding hydrostatic value by a significant factor because the inertia of the gas prevents it from responding instantaneously to the changing gravitational field. This effect is important for the interpretation of observations relating to stream-disk impacts in cataclysmic variables and in binary X-ray sources of low total mass. Title: Mass Transfer in Semi-Detached Binaries Authors: Shu, F. H. Bibcode: 1976IAUS...73..253S Altcode: No abstract at ADS Title: Gas dynamics of semidetached binaries. Authors: Lubow, S. H.; Shu, F. H. Bibcode: 1975ApJ...198..383L Altcode: The gas dynamics of semidetached binary systems is analyzed in the context of the Roche model. It is shown that the steady flow can be formulated in terms of a problem with multiple length scales if it is assumed that the contact component rotates synchronously and the flow occurs isothermally with the thermal speed being a small fraction of the relative orbital speed of the two stars. This concept and semianalytical methods are used to demonstrate that material escapes from the surface of the contact component in a highly anisotropic stellar wind, that the wind throttles into a narrow stream of material whose width is a function of the orbital separation and whose density is a function of the mass transfer rate and the relative orbital speed of the two stars, and that the stream results in the formation of a disk of material of prescribed size orbiting the detached component if the latter is smaller than a specified size. Title: Density wave theory and the classification of spiral galaxies. Authors: Roberts, W. W., Jr.; Roberts, M. S.; Shu, F. H. Bibcode: 1975ApJ...196..381R Altcode: Axisymmetric models of disk galaxies taken together with the density wave theory allow one to distinguish and categorize spiral galaxies by means of two fundamental galactic parameters: the total mass of the galaxy, divided by a characteristic dimension, and the degree of concentration of mass toward the galactic center. These two parameters govern the strength of the galactic shocks in the interstellar gas and the geometry of the spiral wave pattern. In turn, the shock strength and the theoretical pitch angle of the spiral arms play a major role in determining the degree of development of spiral structure in a galaxy and its Hubble type. The application of these results to 24 external galaxies demonstrates that the categorization of galaxies according to this theoretical framework correlates well with the accepted classification of these galaxies within the observed sequences of luminosity class and Hubble type. Title: Star Formation in Spiral Galaxies (Review) Authors: Shu, Frank H. Bibcode: 1975dgs..conf..309S Altcode: No abstract at ADS Title: The Parker Instability in Differentially-rotating Disks Authors: Shu, F. H. Bibcode: 1974A&A....33...55S Altcode: Summary. We investigate Parker's instability for a differentially rotating system comprised of thermal gas, magnetic field, and cosmic-ray particles. The rotation axis coincides with the direction of the vertical gravity, and the rotation is modelled to occur with linear shear. The general initial-value problem is formulated, and the condition for normal modes is obtained from this formulation. A dispersion relation is obtained for the limiting case when the growth rate (or frequency) of the wave is large in comparison with the kinematic shear rate. This dispersion relation suffices to show, in the absence of dissipation, that no finite amount of shear and rotation can ever completely stabilize Parker's mode although the growth rate of certain Fourier components can be materially reduced. Eigenvalues and eigenfunctions are computed analytically for a number of limiting cases of interest, and a few numerical examples are given. The effect of rotation is to suppress waves which are very long in the horizontal directions; the full effects of shear are more difficult to assess - numerical methods are suggested for future work. Subsidiary issues examined in this study are (i) the derivation of an alternative equation, valid in the nonlinear regime and for arbitrary geometries, for the usual fluid equation adopted to describe the behavior of the cosmic-ray pressure, (ii) the distinction between environments under which Parker's mode of instability is likely to lead to convection, to cosmic-ray inflation, or to gas drainage downward to form dense clumps of matter, (iii) an explanation of the physical reasonableness of normal mode solutions with finite energy densities at infinity and the relation of such solutions to the initial-value problem. Key words: interstellar medium - instability - magnetic field - cosmic-rays - differential rotation Title: On the Formation of Interstellar Cloud Complexes, OB Associations and Giant H II Regions Authors: Mouschovias, T. Ch.; Shu, F. H.; Woodward, P. R. Bibcode: 1974A&A....33...73M Altcode: Summary. We propose that large cloud complexes, OB associations, and giant H ii regions form as a result of the initiation of the magnetic Rayleigh-Taylor instability in the interstellar medium by the passage of a galactic shock. Total masses of about 106 M0, formation of unbound systems, and alignment along spiral arms with typical separation of about 1 kpc are natural consequences of this point of view. In addition, we argue that the enhancement of synchrotron radiation to be expected in the compression zones of spiral galaxies has hitherto been considerably overestimated. Key words: instability - spiral arms - cloud complexes - giant H ii regions Title: On the Strength of the Galactic Shock Wave and the Degree of Development of Spiral Structure Authors: Roberts, W. W., Jr.; Roberts, M. S.; Shu, F. H. Bibcode: 1974IAUS...58..439R Altcode: 1974fdg..symp..439R No abstract at ADS Title: Density-Wave Theory of Spiral Structure Authors: Shu, F. H. Bibcode: 1974ASIC....6..219S Altcode: 1974inme.conf..219S No abstract at ADS Title: Nonlinear Gaseous Density Waves and Galactic Shocks Authors: Shu, Frank H.; Milione, Vincenzo; Roberts, William W., Jr. Bibcode: 1973ApJ...183..819S Altcode: We follow the perturbations in the flow of interstellar gas which result from steady forcing by spiral gravitational fields of various strengths. The density response is quite nonlinear even if the amplitude of the spiral field maintained by the disk stars is only a small fraction of the basic axisymmetric field. An analytical study of the properties of slightly nonlinear flows yields certain results which are qualitatively similar to those found numerically for fully nonlinear flows. Galactic shocks arise naturally, indeed necessarily, if the strength of the underlying spiral gravitational field exceeds a certain critical value. The breadth of the zone of high gas compression depends critically on whether the Dopplershifted phase-velocity of the stellar density wave is greater than or less than the "effective acoustic speed" of the gas. In the former case, very narrow compression zones result; in the latter, quite broad zones. This distinction may explain why some galaxies have narrow optical arms while others have broad optical arms. In addition, a certain range of values for the intrinsic frequency of the wave gives rise to ultraharmonic resonances which can introduce secondary compressions of the interstellar gas. This result may relate directly to the origin of the Carina spiral feature in our own Galaxy as well as to the phenomena of branches, spurs, and feathers which are often seen in external spiral galaxies. Subject headings: galactic structure - interstellar matter Title: Spiral structure, dust clouds, and star formation. Authors: Shu, F. H. Bibcode: 1973AmSci..61..524S Altcode: 1974AmSci..61..524S No abstract at ADS Title: On the Genetic Relation Between Interstellar Clouds and Dust Clouds Authors: Shu, F. H. Bibcode: 1973IAUS...52..257S Altcode: No abstract at ADS Title: The Role of Gaseous Dissipation in Density Waves of Finite Amplitude Authors: Roberts, W. W., Jr.; Shu, F. H. Bibcode: 1972ApL....12...49R Altcode: No abstract at ADS Title: Galactic Shocks in an Interstellar Medium with Two Stable Phases Authors: Shu, Frank H.; Milione, Vincenzo; Gebel, William; Yuan, C.; Goldsmith, D. W.; Roberts, W. W. Bibcode: 1972ApJ...173..557S Altcode: Quasi-steady flows of interstellar gas in a spiral gravitational field are followed for the purpose of investigating galactic shocks and the resultant processes of the formation of stars and interstellar clouds. \N'7e model the interstellar medium with two stable phases in which thermal balance is maintained through heating by low-energy cosmic rays The problem, including transitions between the two phases, is given a general formulation but is solved in an approximation which ignores the difference in fluid velocities of the two phases. We also assume that the cosmic-ray flux is uniform in circles about the center of the Galaxy and that the relative abundances of the chemical elements are "normal." For a spiral gravitational field with strength equal to 5 percent that of the axisymmetric field at 10 kpc from the galactic center, the density rat at and minimum compressions is 9: 1 for the inter- cloud medium while it is 40:1 for the gas in a typical cloud During the decompression phase of the flow, a small percentage of the mass of the clouds evaporates to become intercloud material, but this small amount is recovered in the shock As a by-product of phase , the properties of the clouds in the regions between spiral arms are such as to make their detection in 21-cm absorption very difficult. In the absence of the cloud phase, we determine the thickness of the shock layer in intercloud medium to be typically 50 pc. An interstellar cloud immersed as a test particle in the intercloud medium experiences a dynamic rather than a quasi-static compression as it passes through the shock layer. The critical mass for the gravitational collapse of a cloud is ieduced by a large factor because of the compression in the shock. Title: On the Density-Wave Theory of Galactic Spirals. III. Comparisons with External Galaxies Authors: Shu, Frank H.; Stachnik, Robert V.; Yost, Jonathan C. Bibcode: 1971ApJ...166..465S Altcode: We undertake a semiempirical study of the spiral patterns of three model galaxies constructed from the observed rotation curves of I33, M51, and M8t. Consistent with the proposal that density waves are initiated in the outer regions of a galaxy, we find a good fit to result for the observed spiral structure if we choose a pattern speed equal to the rotation speed where the distribution of H ii regions is seen to end. The circumferential bands containing the most prominent H ii regions are located in qualitative agreement with a mechanism in which star formation is triggered by spiral galactic shocks. A suggestion is made for M51 for the coexistence of short and long trailing waves traveling with the same pattern speed. Title: On a Mass-Energy Angular Momentum Relation for Spiral Galaxies. Authors: Gezari, D.; Mongillo, M.; Shu, F. H. Bibcode: 1971BAAS....3..238G Altcode: No abstract at ADS Title: Galactic Shocks in an Interstellar Medium with Two Stable Phases. Authors: Shu, F. H.; Gebel, W.; Milione, V.; Goldsmith, D. W. Bibcode: 1971BAAS....3..246S Altcode: No abstract at ADS Title: Density wave theory of spiral structure. Authors: Lin, C. C.; Shu, F. H. -S. Bibcode: 1971agr.....2..235L Altcode: No abstract at ADS Title: On the Spiral Structure of M33, M51, and M81. Authors: Shu, Frank H.; Stachnik, Robert V.; Yost, Jonathan C. Bibcode: 1970BAAS....2Q.344S Altcode: No abstract at ADS Title: On the Density-Wave Theory of Galactic Spirals. I. Spiral Structure as a Normal Mode of Oscillation Authors: Shu, Frank H. Bibcode: 1970ApJ...160...89S Altcode: An exact formulation of the linearized problem, including appropriate boundary conditions, is de- veloped to explore whether extensive galactic density waves of spiral form are permissible normal modes of oscillation for a stellar disk. An "anti-spiral theorem," of the type reported previously by Lynden- Bell and Ostriker for neutral modes in a gaseous disk, holds here with limited validity-namely, when- ever the effects of stellar resonances can be ignored Title: On the Density-Wave Theory of Galactic Spirals. II. The Propagation of the Density of Wave Action Authors: Shu, Frank H. Bibcode: 1970ApJ...160...99S Altcode: The properties of galactic density waves are studied in the WKBJ approximation. In the lowest order of approximation, we reproduce the dispersion relation reported by Lin and Shu in an earlier communi- cation. In the next order, we demonstrate explicitly that the density of "wave action" is transported with the group velocity derived by Toomre. Some general implications are drawn for mechanisms proposed for the origin of spiral structure Title: The Propogation and Absorption of Spiral Density Waves Authors: Shu, F. H. Bibcode: 1970IAUS...38..323S Altcode: 1970ssg..conf..323S No abstract at ADS Title: Models of Partially Relaxed Stellar Disks Authors: Shu, Frank H. Bibcode: 1969ApJ...158..505S Altcode: The proposal by Lynden~Be11 that violently changing gravitational fields serve as the primary mecha- nism of relaxation in a galaxy of stars is reexamined for disk galaxies. The point of view adopted is that the oniy relaxation mechanism operative for stars in the early life of such galaxies is an axisymmetric form of the Jeans instability discussed by Toomre. The most probable form of the distribution function which results for a disk of infinitesimal thickness is obtained from statistical considerations. An asymptot- ic method of solution based on this distribution is developed for the construction of galactic models from observed rotation curves. The distribution of stars with low peculiar velocities follows Schwarzschild's law whereas that of stars with high velocities shows the asymmetry in galactic longitude discussed by Oort Title: Erratum: on the Spiral Structure of Disk Galaxies. III. Comparison with Observations Authors: Lin, C. C.; Yuan, C.; Shu, Frank H. Bibcode: 1969ApJ...156..797L Altcode: No abstract at ADS Title: On the Spiral Structure of Disk Galaxies. III. Comparison with Observations Authors: Lin, C. C.; Yuan, C.; Shu, Frank H. Bibcode: 1969ApJ...155..721L Altcode: The density-wave theory of galactic spirals is developed in a form slightly more general than that outlined by Lin and Shu in an earlier short communication. Only self-sustained waves are studied in this paper, and the problem of the origin of the spiral structure is barely discussed. The implications of the theory are examined, both in general terms and in detail. The conclusions are compared with observa- tions. Specifically, we consider (1) the distribution of atomic hydrogen in the Galaxy, (2) the systematic motion of the gas, (3) the distribution of young stars and other optical objects, and (4) the migration of moderately young stars. Good agreements are obtained in all cases if we adopt a pattern speed of about 13 km sec' kpc', and a spiral gravitational field equal to about 5 per cent of the symmetrical field. General discussions are also given on (a) the structure of the magnetic field and its role on the systematic motion of the gas, (b) the role of the density wave in the process of star formation, and (c) the distribution of liii regions as revealed by the 109a radio observations Title: On the Effects of Finite Disk Thickness and Gas Content on Spiral Structure. Authors: Shu, Frank H. Bibcode: 1968AJS....73R.201S Altcode: The density wave theory of spiral structure in disk galaxies outlined previously by Lin and Shu (Astrophys. J. 140, 646, 1964; Proc. Nati. Acad. Scj. 55, 229,1966; Proc. IAU-URSI, Symp. No. 31,1967) is extended to include the effects of finite disk thickness and the presence of a fair amount of interstellar gas. In the theory based on a model of infinitesimal thickness, there exists a decided discrepancy between the theoretically predicted values of the stellar velocity dispersion required for stability and those actually observed in our own Galaxy. With the consideration of the effect of the finite thickness of the disks of stars and gas, this discrepancy disappears. The numerical form of the resultant dispersion relation between wave frequency and wavelength for spiral~waves does not differ appreciably from that obtained on the basis of the simple theory. The relative participation of gas and stars in the spiral structure of our own Galaxy is found to be about the same in the solar vicinity. In the interior parts, stars will play the more important role. Frank H. Shu is currently at the State University of New York, Stony Brook. Title: The Dynamics and Large-Scale Structure of Spiral Galaxies. Authors: Shu, Frank Hsia-San Bibcode: 1968PhDT.........1S Altcode: No abstract at ADS Title: Density waves in disk galaxies. Authors: Lin, C. C.; Shu, F. H. Bibcode: 1967IAUS...31..313L Altcode: No abstract at ADS Title: On the Spiral Structure of Disk Galaxies, II. Outline of a Theory of Density Waves Authors: Lin, C. C.; Shu, Frank H. Bibcode: 1966PNAS...55..229L Altcode: No abstract at ADS Title: The Gravitational Collapse of a Uniform Spheroid. Authors: Lin, C. C.; Mestel, L.; Shu, F. H. Bibcode: 1965ApJ...142.1431L Altcode: A uniform, non-rotating, pressure-free spheroid is supposed to collapse gravitationally from rest. It is shown that the initial eccentricity is steadily increased by the anisotropic gravitational field: an initially oblate spheroid tends toward a disk, and an initially prolate spheroid toward a spindle. Numerical results are computed for a series of initial eccentricities. Title: On the Spiral Structure of Disk Galaxies. Authors: Lin, C. C.; Shu, Frank H. Bibcode: 1964ApJ...140..646L Altcode: It is shown that gravitational instability is a plausible basis for the formation of the spiral pattern in disk galaxies An explicit asymptotic formula is obtained for the form of the spiral. It gives reasonable numerical results for the galaxy, and qualitatively satisfactory trends for normal spirals of various types.