Author name code: spruit ADS astronomy entries on 2022-09-14 author:"Spruit, Hendrik C." OR author:"Spruit, Henk C." ------------------------------------------------------------------------ Title: dopmap: Fast Doppler mapping program Authors: Spruit, Hendrik C. Bibcode: 2021ascl.soft06002S Altcode: dopmap constructs Doppler maps from the orbital variation of line profiles of (mass transferring) binaries. It uses an algorithm related to Richardson-Lucy iteration and includes an IDL-based set of routines for manipulating and plotting the input and output data. Title: The formation of (very) slowly rotating stars Authors: Spruit, H. C. Bibcode: 2018arXiv181006106S Altcode: The slow rotation of some young stars and the extreme rotation periods of some Ap stars have so far defied explanation. The absence of sufficiently efficient braking mechanisms for newly formed stars points to the star formation process itself as the origin. I find that a mode of star formation exists by which a protostar can form without accreting angular momentum. It depends on the survival of a magnetic connection between the accreting matter and the birth cloud. The conditions for this process to operate are analyzed, and illustrated with a generic (scale-free) model. Depending on the initial rotation rate of the accreting matter, either a Keplerian disk forms, or the gas ends up rotating with the rotation period of the cloud, even if it is at a large distance. The boundary in parameter space between these two outcomes is sharp. Title: Disruption of a Planet Spiraling into its Host Star Authors: Jia, Shi; Spruit, H. C. Bibcode: 2018ApJ...864..169J Altcode: 2018arXiv180800467J The processes leading to the deformation and destruction of planets spiraling into the convective envelope of their host stars are described. The planet is compressed by ram pressure and deformed into a flattened shape, for which a quantitative model is developed. Compression increases the planet’s density contrast with the envelope and its gravitational binding energy. This increases the survivability, especially of gas planets. An estimate is given for the depth of disruption by ram pressure, and for the subsequent fragmentation of the remnants. We show how the debris of rocky or iron planets, instead of mixing through the convection zone (CZ), sinks below the base of the CZ. The timescale of the entire sequence of events is of the order of a few orbital times of the planet. If spiral-in of (partly) icy, rocky or iron planets has happened to the pre-main sequence Sun, then this could account for the higher opacity below the base of the CZ, as inferred from helioseismology. Title: Electromagnetic deformable mirror for space applications Authors: Kuiper, S.; Doelman, N.; Overtoom, T.; Nieuwkoop, E.; Russchenberg, T.; van Riel, M.; Wildschut, J.; Baeten, M.; Spruit, H.; Brinkers, S.; Human, J. Bibcode: 2017SPIE10562E..30K Altcode: To increase the collecting power and to improve the angular imaging resolution, space telescopes are evolving towards larger primary mirrors. The aerial density of the telescope mirrors needs to be kept low, however, to be compatible with the launch requirements. A light-weight (primary) mirror will introduce additional optical aberrations to the system. These may be caused by for instance manufacturing errors, gravity release and thermo-elastic effects. Active Optics (AO) is a key candidate technology to correct for the resultant wave front aberrations [1]. Title: Flux canceling in three-dimensional radiative magnetohydrodynamic simulations Authors: Thaler, Irina; Spruit, H. C. Bibcode: 2017A&A...601A..88T Altcode: We aim to study the processes involved in the disappearance of magnetic flux between regions of opposite polarity on the solar surface using realistic three-dimensional (3D) magnetohydrodynamic (MHD) simulations. "Retraction" below the surface driven by magnetic forces is found to be a very effective mechanism of flux canceling of opposite polarities. The speed at which flux disappears increases strongly with initial mean flux density. In agreement with existing inferences from observations we suggest that this is a key process of flux disappearance within active complexes. Intrinsic kG strength concentrations connect the surface to deeper layers by magnetic forces, and therefore the influence of deeper layers on the flux canceling process is studied. We do this by comparing simulations extending to different depths. For average flux densities of 50 G, and on length scales on the order of 3 Mm in the horizontal and 10 Mm in depth, deeper layers appear to have only a mild influence on the effective rate of diffusion. Title: Magnetic fields in non-convective regions of stars Authors: Braithwaite, Jonathan; Spruit, Henk C. Bibcode: 2017RSOS....460271B Altcode: 2015arXiv151003198B We review the current state of knowledge of magnetic fields inside stars, concentrating on recent developments concerning magnetic fields in stably stratified (zones of) stars, leaving out convective dynamo theories and observations of convective envelopes. We include the observational properties of A, B and O-type main-sequence stars, which have radiative envelopes, and the fossil field model which is normally invoked to explain the strong fields sometimes seen in these stars. Observations seem to show that Ap-type stable fields are excluded in stars with convective envelopes. Most stars contain both radiative and convective zones, and there are potentially important effects arising from the interaction of magnetic fields at the boundaries between them; the solar cycle being one of the better known examples. Related to this, we discuss whether the Sun could harbour a magnetic field in its core. Recent developments regarding the various convective and radiative layers near the surfaces of early-type stars and their observational effects are examined. We look at possible dynamo mechanisms that run on differential rotation rather than convection. Finally, we turn to neutron stars with a discussion of the possible origins for their magnetic fields. Title: Instability of mass transfer in a planet-star system Authors: Jia, Shi; Spruit, H. C. Bibcode: 2017MNRAS.465..149J Altcode: 2016arXiv160703919J We show that the angular momentum exchange mechanism governing the evolution of mass-transferring binary stars does not apply to Roche lobe filling planets, because most of the angular momentum of the mass-transferring stream is absorbed by the host star. Apart from a correction for the difference in specific angular momentum of the stream and the centre of mass of the planet, the orbit does not expand much on Roche lobe overflow. We explore the conditions for dynamically unstable Roche lobe overflow as a function of planetary mass and mass and radius (age) of host star and equation of state of planet. For a Sun-like host, gas giant planets in a range of mass and entropy can undergo dynamical mass transfer. Examples of the evolution of the mass transfer process are given. Dynamic mass transfer of rocky planets depends somewhat sensitively on equation of state used. Silicate planets in the range 1 < Mp < 10 M⊕ typically go through a phase of dynamical mass transfer before settling to slow overflow when their mass drops to less than 1 M⊕. Title: The growth of helium-burning cores Authors: Spruit, H. C. Bibcode: 2015A&A...582L...2S Altcode: 2015arXiv150900659S Helium burning in the convective cores of horizontal branch and red clump stars appears to involve a process of "ingestion" of unburnt helium into the core, the physics of which has not been clearly identified yet. I show here that a limiting factor controlling the growth is the buoyancy of helium entering the denser C+O core. It yields a growth rate that scales directly with the convective luminosity of the core and agrees with constraints on core size from current asteroseismology. Title: Convective settling in main sequence stars: Li and Be depletion Authors: Andrássy, R.; Spruit, H. C. Bibcode: 2015A&A...579A.122A Altcode: 2015arXiv150604635A The process of convective settling is based on the assumption that a small fraction of the low-entropy downflows sink from the photosphere down to the bottom of the star's envelope convection zone retaining a substantial entropy contrast. We have previously shown that this process could explain the slow Li depletion observed in the Sun. We construct a parametric model of convective settling to investigate the dependence of Li and Be depletion on stellar mass and age. Our model is generally in good agreement with the Li abundances measured in open clusters and solar twins, although it seems to underestimate the Li depletion in the first ~1 Gyr. The model is also compatible with the Be abundances measured in a sample of field stars. Title: Small-scale dynamos on the solar surface: dependence on magnetic Prandtl number Authors: Thaler, I.; Spruit, H. C. Bibcode: 2015A&A...578A..54T Altcode: 2015arXiv150504575T The question of possible small-scale dynamo action in the surface layers of the Sun is revisited with realistic 3D MHD simulations. As in other MHD problems, dynamo action is found to be a sensitive function of the magnetic Prandtl number Pm = ν/η; it disappears below a critical value Pc which is a function of the numerical resolution. At a grid spacing of 3.5 km, Pc based on the hyperdiffusivities implemented in the code (STAGGER) is ≈1, increasing with increasing grid spacing. As in other settings, it remains uncertain whether small scale dynamo action is present in the astrophysical limit where Pm ≪ 1 and magnetic Reynolds number Rm ≫ 1. The question is discussed in the context of the strong effect that external stray fields are observed to have in generating and maintaining dynamo action in other numerical and laboratory systems, and in connection with the type-II hypertransient behavior of dynamo action observed in the absence of such external fields. Title: Overshooting by differential heating Authors: Andrássy, R.; Spruit, H. C. Bibcode: 2015A&A...578A.106A Altcode: 2015arXiv150205628A On the long nuclear time scale of stellar main-sequence evolution, even weak mixing processes can become relevant for redistributing chemical species in a star. We investigate a process of "differential heating", which occurs when a temperature fluctuation propagates by radiative diffusion from the boundary of a convection zone into the adjacent radiative zone. The resulting perturbation of the hydrostatic equilibrium causes a flow that extends some distance from the convection zone. We study a simplified differential-heating problem with a static temperature fluctuation imposed on a solid boundary. The astrophysically relevant limit of a high Reynolds number and a low Péclet number (high thermal diffusivity) turns out to be interestingly non-intuitive. We derive a set of scaling relations for the stationary differential heating flow. A numerical method adapted to a high dynamic range in flow amplitude needed to detect weak flows is presented. Our two-dimensional simulations show that the flow reaches a stationary state and confirm the analytic scaling relations. These imply that the flow speed drops abruptly to a negligible value at a finite height above the source of heating. We approximate the mixing rate due to the differential heating flow in a star by a height-dependent diffusion coefficient and show that this mixing extends about 4% of the pressure scale height above the convective core of a 10 M⊙ zero-age main sequence star.

Appendix A is available in electronic form at http://www.aanda.org Title: The B Fields in OB Stars (BOB) Survey Authors: Kholtygin, A. F.; Castro, N.; Fossati, L.; Hubrig, S.; Langer, N.; Morel, T.; Przybilla, N.; Schöller, M.; Carroll, T.; Ilyin, I.; Irrgang, A.; Oskinova, L.; Schneider, F. R. N.; Díaz, S. S.; Briquet, M.; González, J. F.; Kharchenko, N.; Nieva, M. -F.; Scholz, R. -D.; de Koter, A.; Hamann, W. -R.; Herrero, A.; Maíz Apellániz, J.; Sana, H.; Arlt, R.; Barbá, R.; Dufton, P.; Mathys, G.; Piskunov, A.; Reisenegger, A.; Spruit, H.; Yoon, S. -C. Bibcode: 2015ASPC..494...79K Altcode: The B fields in OB stars (BOB) survey is an ESO large program collecting spectropolarimetric observations for a large number of early-type stars in order to study the occurrence rate, properties, and ultimately the origin of magnetic fields in massive stars. As of July 2014, a total of 98 objects were observed over 20 nights with the FORS2 and HARPSpol. Our preliminary results indicate that the fraction of magnetic OB stars with an organized, detectable field is small. This conclusion, now independently reached by two different surveys, has profound implications for any theoretical model attempting to explain the field formation in these objects. We discuss in this contribution some important issues addressed by our observations (e.g., the lower boundary of the field strength) and the discovery of some remarkable objects. Title: Instability of magnetic equilibria in barotropic stars Authors: Mitchell, J. P.; Braithwaite, J.; Reisenegger, A.; Spruit, H.; Valdivia, J. A.; Langer, N. Bibcode: 2015MNRAS.447.1213M Altcode: 2014arXiv1411.7252M In stably stratified stars, numerical magnetohydrodynamics simulations have shown that arbitrary initial magnetic fields evolve into stable equilibrium configurations, usually containing nearly axisymmetric, linked poloidal and toroidal fields that stabilize each other. In this work, we test the hypothesis that stable stratification is a requirement for the existence of such stable equilibria. For this purpose, we follow numerically the evolution of magnetic fields in barotropic (and thus neutrally stable) stars, starting from two different types of initial conditions, namely random disordered magnetic fields, as well as linked poloidal-toroidal configurations resembling the previously found equilibria. With many trials, we always find a decay of the magnetic field over a few Alfvén times, never a stable equilibrium. This strongly suggests that there are no stable equilibria in barotropic stars, thus clearly invalidating the assumption of barotropic equations of state often imposed on the search of magnetic equilibria. It also supports the hypothesis that, as dissipative processes erode the stable stratification, they might destabilize previously stable magnetic field configurations, leading to their decay. Title: The B Fields in OB Stars (BOB) Survey Authors: Morel, T.; Castro, N.; Fossati, L.; Hubrig, S.; Langer, N.; Przybilla, N.; Schöller, M.; Carroll, T.; Ilyin, I.; Irrgang, A.; Oskinova, L.; Schneider, F. R. N.; Díaz, S. Simon; Briquet, M.; González, J. F.; Kharchenko, N.; Nieva, M. -F.; Scholz, R. -D.; de Koter, A.; Hamann, W. -R.; Herrero, A.; Maíz Apellániz, J.; Sana, H.; Arlt, R.; Barbá, R.; Dufton, P.; Kholtygin, A.; Mathys, G.; Piskunov, A.; Reisenegger, A.; Spruit, H.; Yoon, S. -C. Bibcode: 2015IAUS..307..342M Altcode: 2014arXiv1408.2100M The B fields in OB stars (BOB) survey is an ESO large programme collecting spectropolarimetric observations for a large number of early-type stars in order to study the occurrence rate, properties, and ultimately the origin of magnetic fields in massive stars. As of July 2014, a total of 98 objects were observed over 20 nights with FORS2 and HARPSpol. Our preliminary results indicate that the fraction of magnetic OB stars with an organised, detectable field is low. This conclusion, now independently reached by two different surveys, has profound implications for any theoretical model attempting to explain the field formation in these objects. We discuss in this contribution some important issues addressed by our observations (e.g., the lower bound of the field strength) and the discovery of some remarkable objects. Title: The B Fields in OB Stars (BOB) Survey Authors: Morel, T.; Castro, N.; Fossati, L.; Hubrig, S.; Langer, N.; Przybilla, N.; Schöller, M.; Carroll, T.; Ilyin, I.; Irrgang, A.; Oskinova, L.; Schneider, F. R. N.; Díaz, S. S.; Briquet, M.; González, J. F.; Kharchenko, N.; Nieva, M. -F.; Scholz, R. -D.; de Koter, A.; Hamann, W. -R.; Herrero, A.; Maíz Apellániz, J.; Sana, H.; Arlt, R.; Barbá, R.; Dufton, P.; Kholtygin, A.; Mathys, G.; Piskunov, A.; Reisenegger, A.; Spruit, H.; Yoon, S. -C. Bibcode: 2014Msngr.157...27M Altcode: The B fields in OB stars (BOB) survey is an ESO Large Programme collecting spectropolarimetric observations for a large number of early-type stars in order to study the occurrence rate, properties, and ultimately the origin of magnetic fields in massive stars. A total of 98 objects was observed over 20 nights with FORS2 and HARPSpol to July 2014. Preliminary results indicate that the fraction of magnetic OB stars with an organised, detectable field is low. This conclusion, now independently reached by two different surveys, has profound implications for any theoretical model attempting to explain the field formation in these stars. We also discuss some important issues addressed by our observations (e.g., the lower bound of the field strength) and the discovery of some remarkable objects. Title: Magnetic Fields throughout Stellar Evolution (IAU S302) Authors: Petit, Pascal; Jardine, Moira; Spruit, Hendrik C. Bibcode: 2014IAUS..302.....P Altcode: Foreword; Introduction; 1. Magnetized stellar formation; 2. Magnetic activity in the Sun and main-sequence stars with convective outer layers; 3. Origin and impact of magnetic fields in higher-mass stars with radiative outer layers; 4. Magnetic fields in the ultimate stages of stellar evolution; Author index. Title: Search for Stable Magnetohydrodynamic Equilibria in Barotropic Stars. Authors: Mitchell, J. P.; Braithwaite, J.; Langer, N.; Reisenegger, A.; Spruit, H. Bibcode: 2014IAUS..302..441M Altcode: 2013arXiv1310.2595M It is now believed that magnetohydrodynamic equilibria can exist in stably stratified stars due to the seminal works of Braithwaite & Spruit (2004) and Braithwaite & Nordlund (2006). What is still not known is whether magnetohydrodynamic equilibria can exist in a barotropic star, in which stable stratification is not present. It has been conjectured by Reisenegger (2009) that there will likely not exist any magnetohydrodynamical equilibria in barotropic stars. We aim to test this claim by presenting preliminary MHD simulations of barotropic stars using the three dimensional stagger code of Nordlund & Galsgaard (1995). Title: Brightness of the Sun's small scale magnetic field: proximity effects Authors: Thaler, I.; Spruit, H. C. Bibcode: 2014A&A...566A..11T Altcode: 2014arXiv1404.2871T The net effect of the small scale magnetic field on the Sun's (bolometric) brightness is studied with realistic 3D MHD simulations. The direct effect of brightening within the magnetic field itself is consistent with measurements in high-resolution observations. The high "photometric accuracy" of the simulations, however, reveal compensating brightness effects that are hard to detect observationally. The influence of magnetic concentrations on the surrounding nonmagnetic convective flows (a "proximity effect") reduces the brightness by an amount exceeding the brightening by the magnetic concentrations themselves. The net photospheric effect of the small scale field (≈-0.34% at a mean flux density of 50 G) is thus negative. We conclude that the main contribution to the observed positive correlation between the magnetic field and total solar irradiance must be magnetic dissipation in layers around the temperature minimum and above (not included in the simulations). This agrees with existing inferences from observations. Title: Overshooting by convective settling Authors: Andrássy, R.; Spruit, H. C. Bibcode: 2013A&A...559A.122A Altcode: 2013arXiv1310.8117A We study a process of slow mixing in stars with convective envelopes, which is driven by the settling of cool downward plumes below the base of the convection zone. If a small fraction (of order 10-7) of the material cooled at the surface retains a significant entropy deficit while descending in plumes, it can reach the depth where lithium burning takes place. The model calculates the thermal response and mixing below the convection zone due to the settling process, assuming that the plumes arrive at the base of the convection zone with a broad range of entropy contrasts. We obtain a good fit to the observed lithium depletion in the Sun by assuming that the settling mass flux is distributed with respect to the entropy contrast as a power law with a slope around -2. We find convective settling to have a negligible influence on the stratification below the convection zone, although mixing induced by it could modify the gradient of helium concentration. Title: Semiconvection: numerical simulations Authors: Zaussinger, F.; Spruit, H. C. Bibcode: 2013A&A...554A.119Z Altcode: 2013arXiv1303.4522Z A grid of numerical simulations of double-diffusive convection is presented for the astrophysical case where viscosity (Prandtl number Pr) and solute diffusivity (Lewis number Le) are much lower than the thermal diffusivity. As in laboratory and geophysical cases, convection takes place in a layered form. The proper translation of subsonic flows in a stellar interior and an incompressible (Boussinesq) fluid is given, and the validity of the Boussinesq approximation for the semiconvection problem is checked by comparison with fully compressible simulations. The predictions of a simplified theory of mixing in semiconvection given in a companion paper are tested against the numerical results, and used to extrapolate these to astrophysical conditions. The predicted effective He-diffusion coefficient is nearly independent of the double-diffusive layering thickness d. For a fiducial main sequence model (15 M⊙) the inferred mixing time scale is of the order of 1010 yr. An estimate for the secular increase in d during the semiconvective phase is given. It can potentially reach a significant fraction of the pressure scale height.

Movies associated to Figs. 5 and 7 are available in electronic form at http://www.aanda.org Title: Semiconvection: theory Authors: Spruit, H. C. Bibcode: 2013A&A...552A..76S Altcode: 2013arXiv1302.4005S A model is developed for the transport of heat and solute in a system of double-diffusive layers under astrophysical conditions (where viscosity and solute diffusivity are low compared with the thermal diffusivity). The process of formation of the layers is not part of the model but, as observed in geophysical and laboratory settings, is assumed to be faster than the life time of the semiconvective zone. The thickness of the layers is a free parameter of the model. When the energy flux of the star is specified, the effective semiconvective diffusivities are only weakly dependent on this parameter. An estimate is given of the evolution of layer thickness with time in a semiconvective zone. The model predicts that the density ratio has a maximum for which a stationary layered state can exist, Rρ ≲ Le-1/2. Comparison of the model predictions with a grid of numerical simulations is presented in a companion paper. Title: The Large-scale Magnetic Fields of Thin Accretion Disks Authors: Cao, Xinwu; Spruit, Hendrik C. Bibcode: 2013ApJ...765..149C Altcode: 2013arXiv1301.4543C Large-scale magnetic field threading an accretion disk is a key ingredient in the jet formation model. The most attractive scenario for the origin of such a large-scale field is the advection of the field by the gas in the accretion disk from the interstellar medium or a companion star. However, it is realized that outward diffusion of the accreted field is fast compared with the inward accretion velocity in a geometrically thin accretion disk if the value of the Prandtl number P m is around unity. In this work, we revisit this problem considering the angular momentum of the disk to be removed predominantly by the magnetically driven outflows. The radial velocity of the disk is significantly increased due to the presence of the outflows. Using a simplified model for the vertical disk structure, we find that even moderately weak fields can cause sufficient angular momentum loss via a magnetic wind to balance outward diffusion. There are two equilibrium points, one at low field strengths corresponding to a plasma-beta at the midplane of order several hundred, and one for strong accreted fields, β ~ 1. We surmise that the first is relevant for the accretion of weak, possibly external, fields through the outer parts of the disk, while the latter one could explain the tendency, observed in full three-dimensional numerical simulations, of strong flux bundles at the centers of disk to stay confined in spite of strong magnetororational instability turbulence surrounding them. Title: Essential Magnetohydrodynamics for Astrophysics Authors: Spruit, H. C. Bibcode: 2013arXiv1301.5572S Altcode: This text is intended as an introduction to magnetohydrodynamics in astrophysics, emphasizing a fast path to the elements essential for physical understanding. It assumes experience with concepts from fluid mechanics: the fluid equation of motion and the Lagrangian and Eulerian descriptions of fluid flow. In addition, the basics of vector calculus and elementary special relativity are needed. Not much knowledge of electromagnetic theory is required. In fact, since MHD is much closer in spirit to fluid mechanics than to electromagnetism, an important part of the learning curve is to overcome intuitions based on the vacuum electrodynamics of one's high school days. The first chapter (only 39 pp) is meant as a practical introduction including exercises. This is the `essential' part. The exercises are important as illustrations of the points made in the text (especially the less intuitive ones). Almost all are mathematically unchallenging. The supplement in chapter 2 contains further explanations, more specialized topics and connections to the occasional topic somewhat outside MHD. The emphasis is on physical understanding by the visualization of MHD processes, as opposed to more formal approaches. Title: Accretion discs trapped near corotation Authors: D'Angelo, Caroline R.; Spruit, Hendrik C. Bibcode: 2012MNRAS.420..416D Altcode: 2011arXiv1108.3833D; 2011MNRAS.tmp.1988D We show that discs accreting on to the magnetosphere of a rotating star can end up in a trapped state, in which the inner edge of the disc stays near the corotation radius, even at low and varying accretion rates. The accretion in these trapped states can be steady or cyclic; we explore these states over a wide range of parameter space. We find two distinct regions of instability: one related to the buildup and release of mass in the disc outside corotation, and the other to mass storage within the transition region near corotation. With a set of calculations over long time-scales, we show how trapped states evolve from both non-accreting and fully accreting initial conditions, and also calculate the effects of cyclic accretion on the spin evolution of the star. Observations of cycles such as found here would provide important clues on the physics of magnetospheric accretion. Recent observations of cyclic and other unusual variability in T Tauri stars (EXors) and X-ray binaries are discussed in this context. Title: The relative significance of the H-index Authors: Spruit, H. C. Bibcode: 2012arXiv1201.5476S Altcode: Use of the Hirsch-index ($h$) as measure of an author's visibility in the scientific literature has become popular as an alternative to a gross measure like total citations (c). I show that, at least in astrophysics, $h$ correlates tightly with overall citations. The mean relation is $h=0.5(\sqrt c+1)$. Outliers are few and not too far from the mean, especially if `normalized' ADS citations are used for $c$ and $h$. Whatever the theoretical reasoning behind it, the Hirsch index in practice does not appear to measure something significantly new. Title: Theories of the Solar Cycle and Its Effect on Climate Authors: Spruit, H. Bibcode: 2012PThPS.195..185S Altcode: In the first part, some views of the solar magnetic cycle are discussed and confronted with observations, with focus on two aspects at the core of most models: the role of convective turbulence, and the role of the `tachocline' at the base of the convection zone. The shorter second part discusses the possibility that the magnetic field of the Sun might influence its brightness enough to have an effect on the Earth's climate. The standard view, which treats the solar cycle as a result of the interaction between turbulent convection and magnetic fields is shown to be misplaced. The main ingredient of the solar cycle, apart from differential rotation, is instead buoyant instability of the magnetic field itself. The source of the magnetic field of the solar cycle is usually assumed to be located in the `tachocline': the shear zone at the base of the convection zone. The cycle cannot be powered by the radial shear of the tachocline as assumed in these models, however, since the radiative interior does not support significant shear stresses. Instead, it must be powered by the latitudinal gradient of the rotation rate in the convection zone, as in early models of the solar cycle. The Sun's brightness is known to vary in sync with the sunspot cycle, but our understanding of the mechanisms involved make it unlikely that it has a significant effect on climate, whether on short (decades) or longer time scales. Title: A binary merger origin for inflated hot Jupiter planets Authors: Martin, E. L.; Spruit, H. C.; Tata, R. Bibcode: 2011A&A...535A..50M Altcode: 2011arXiv1102.3336M We hypothesize that hot Jupiters with inflated sizes represent a separate planet formation channel, the merging of two low-mass stars. We show that the abundance and properties of W UMa stars and low mass detached binaries are consistent with their being possible progenitors. The degree of inflation of the transiting hot Jupiters correlates with their expected spiral-in life time by tidal dissipation, and this could indicate youth if the stellar dissipation parameter Q∗' is sufficiently low. Several Jupiter-mass planets can form in the massive compact disk formed in a merger event. Gravitational scattering between them can explain the high incidence of excentric, inclined, and retrograde orbits. If the population of inflated planets is indeed formed by a merger process, their frequency should be much higher around blue stragglers than around T Tauri stars. Title: Magnetically powered jets Authors: Spruit, H. C. Bibcode: 2011AIPC.1381..227S Altcode: This is an update on a previous, more extended conference review text [61], with emphasis on outstanding problem areas. The role of dissipation of magnetic energy in accelerating the flow is discussed, and its importance for explaining high Lorentz factors. The transition between disk and outflow is one of the least understood parts of the magnetic theory; its role in setting the mass flux in the wind, in possible modulations of the mass flux, and the problems in treating it realistically are discussed. Current views on most of these problems are still strongly influenced by the restriction to 2 dimensions (axisymmetry) in previous analytical and numerical work; 3-D effects likely to be important are suggested. An interesting problem area is the nature and origin of the strong, preferably highly ordered magnetic fields known to work best for jet production. The presence or absence of such fields may well be the `second parameter' governing not only the presence of jets but also the X-ray spectra and timing behavior of X-ray binaries. Title: Long-term evolution of discs around magnetic stars Authors: D'Angelo, Caroline R.; Spruit, Hendrik C. Bibcode: 2011MNRAS.416..893D Altcode: 2011arXiv1102.3697D; 2011MNRAS.tmp.1254D We investigate the evolution of a thin viscous disc surrounding a magnetic star, including the spin-down of the star by the magnetic torques it exerts on the disc. The transition from an accreting to a non-accreting state, and the change of the magnetic torque across the corotation radius rc are included in a generic way, the widths of the transition taken in the range suggested by numerical simulations. In addition to the standard accreting state, in which the star gradually moves into spin equilibrium with the disc, two more states are found. An accreting state can develop into a 'dead' disc state, with inner edge rin well outside corotation. More often, a 'trapped' state develops, in which rin stays close to corotation even at very low accretion rates. The long-term evolution of these two states is different. In the dead state the star spins down incompletely, retaining much of its initial spin. In the trapped state the star can asymptotically spin-down to arbitrarily low rates, with its angular momentum transferred to the disc. We identify these outcomes with respectively the rapidly rotating and the very slowly rotating classes of Ap stars and magnetic white dwarfs. Title: Extremely Inflated Hot Jupiters Could Be Extremely Young Authors: Martin, Eduardo L.; Spruit, Henk; Tata, Ramarao Bibcode: 2011ESS.....2.4301M Altcode: Extremely inflated hot Jupiters could have formed recently from the merger of two low-mass stars.

The frequency of W UMa stars is not enough

to account for many inflated hot Jupiters,

so low mass detached binaries could also contribute to the progenitor population.

We find that the degree of inflation of the transiting hot Jupiters correlates with their expected spiral-in life time by tidal dissipation, and this

could be an indication of youth if the Q dissipation parameter is sufficiently low, as suggested by the

studies of the Jupiter-Io system.

There is also a correlation between radius anomaly and host star rotational velocity as expected in the merger scenario.

The distribution of rotational velocities among the host stars is statistically similar to that of blue

stragglers in the globular cluster 47 Tuc. A significant challenge to the binary merger hypothesis

is the efficient angular momentum loss required to explain the slow rotation of some inflated hot Jupiter host stars.

As observational tests we point out that if hot Jupiters are mainly formed as a result of binary mergers, the frequency of this kind of planets should be higher around blue stragglers than around T Tauri stars.

The presenter is supported by funding

from the Spanish Ministry of Science

and Technology and the ROPACS european

training network. Title: The brightness of magnetic field concentrations in the quiet Sun Authors: Schnerr, R. S.; Spruit, H. C. Bibcode: 2011A&A...532A.136S Altcode: 2010arXiv1010.4792S In addition to the "facular" brightening of active regions, the quiet Sun also contains a small scale magnetic field with associated brightenings in continuum radiation. We measure this contribution of quiet regions to the Sun's brightness from high spatial resolution (0.16 arcsec-0.32 arcsec) observations of the Swedish 1-m Solar Telescope (SST) and Hinode satellite. The line-of-sight magnetic field and continuum intensity near Fe i 6302.5 Å are used to quantify the correlation between field strength and brightness. The data show that magnetic flux density contains a significant amount of intrinsically weak fields that contribute little to brightness. We show that with data of high spatial resolution a calibration of magnetic flux density as a proxy for brightness excess is possible. In the SST data, the magnetic brightening of a quiet region with an average (unsigned) flux density of 10 G is about 0.15%. In the Hinode data, and in SST data reduced to Hinode resolution, the measured brightening is some 40% lower. With appropriate correction for resolution, magnetic flux density can be used as a reliable proxy in regions of small scale mixed polarity. The measured brightness effect is larger than the variation of irradiance over a solar cycle. It is not clear, however, if this quiet Sun contribution actually varies significantly. Title: Invited Speaker: Hale Prize - How the Cycle Does and Does Not Work Authors: Spruit, Hendrik Bibcode: 2011SPD....42.0501S Altcode: 2011BAAS..43S.0501S The talk will present a personal view of our current understanding of the solar cycle. Central points are the role of magnetic buoyancy as the main driving force (together with differential rotation), and the absence of an essential role for the radial shear in the tachocline. The resulting view is essentially that of Babcock and Leighton of half a century ago, but now supported by much more observational and theoretical evidence. I will show how the traditional interpretation of the cycle as a dynamo driven by convective turbulence is (and has always been) incompatible with the observations, as well as with the numerical results that have accumulated over the past decade. Instead, I will argue that to make progress it will pay off i) to directly confront a couple of currently somewhat neglected theoretical problems, and ii) to address some well-known observational puzzles that have not yet found convincing interpretations in theories of the cycle but may well hold critical clues. Title: The Sun's Magnetic Surface Authors: Spruit, Hendrik Bibcode: 2011AAS...21822301S Altcode: 2011BAAS..43G22301S An overview is given of the history and present status of observations and theory of magnetic fields as observed at the solar surface: their structure, their evolution, and their effect on the Sun's brightness. By a fortunate coincidence, the photosphere is the region most accessible to direct numerical MHD simulations. The spectacular level of qualitative and quantitative realism now possible is illustrated by comparisons with the equally remarkable advances high-resolution observations achieved in recents years. These comparisons are now yielding confident physical interpretations of many of the observed properties of the Sun's surface magnetic fields, including, for example the bewildering detail of sunspot structure. The controversial question of a possible connection between climate and brightness variations over the Sun's magnetic cycle will be discussed in the light of recent observational and numerical results. Title: Inflated Hot Jupiters may not Require Inflated Physics Authors: Martin, Eduardo L.; Spruit, H. Bibcode: 2011AAS...21840608M Altcode: 2011BAAS..43G40608M Due to the Darwin instability, hot Jupiters are expected to spiral in and merge with their host stars. The time scale for this spiral-in can be readily calculated for transiting hot Jupiters, but it is subject to uncertainty in the tidal dissipation parameter Q. Using data available for a sample of over one hundred transiting planets, we calculate the time it takes for hot Jupiters to spiral in from their current distance to their host stars. It is found that the spiral in times are strongly correlated with the excess of the planet's radius relative to its equilibrium radius in the sense that larger radius anomalies correspond to shorter spiral in times. An energy source has to be invoked to keep planets inflated longer than their natural cooling time. Irradiation by the host star has been considered but a plausible mechanism to transport the irradiating flux to the planet interior where it is needed for significant inflation has not yet been identified. A 1 Jupiter mass planet needs an thermal energy excess of the order of its gravitational binding energy in order to inflate it by as much as 50 percent. This rules out a source like dissipation of tides in the planet due to nonsynchronous rotation, since the maximum rotational energy of a planet is only a fraction of its binding energy. We propose that the cause of inflation is that the hot Jupiters are young, typically a few hundred Myr. The reason for this youth is hot Jupiter formation in the merger of a binary. The likely binary populations include W UMa stars (contact binaries) and low mass detached binaries. This scenario also explains other puzzling properties of hot Jupiters, such as their high abundance in orbits close to the host stars and enhanced lithium depletion. Title: The Total Solar Irradiance and Small Scale Magnetic Fields Authors: Schnerr, R. S.; Spruit, H. C. Bibcode: 2011ASPC..437..167S Altcode: “What is the contribution of the weak small scale fields in the quiet Sun to the Total Solar Irradiance (TSI)?.” This is the question we try to answer by investigating the relation between magnetic fields and continuum intensity in SST and Hinode quiet Sun observations. We find that the increase in brightness of the quiet Sun due to weak (B < 30 G) fields could be as large as 0.3%, which would make it a major contributor to TSI variations. We also find that most of the magnetic flux appears to be resolved at the 0.16" resolution of the SST data. Title: High time resolution optical/X-ray cross-correlations for X-ray binaries: anticorrelations and rapid variability Authors: Durant, Martin; Shahbaz, Tariq; Gandhi, Poshak; Cornelisse, Remon; Muñoz-Darias, Teodoro; Casares, Jorge; Dhillon, Vik; Marsh, Tom; Spruit, Hendrik; O'Brien, Kieran; Steeghs, Danny; Hynes, Rob Bibcode: 2011MNRAS.410.2329D Altcode: 2010arXiv1008.4522D; 2010MNRAS.tmp.1581D Using simultaneous observations in X-rays and optical, we have performed a homogeneous analysis of the cross-correlation behaviours of four X-ray binaries: SWIFT J1753.5-0127, GX 339-4, Sco X-1 and Cyg X-2. With high-time-resolution observations using ULTRACAM and RXTE, we concentrate on the short time-scale, δt < 20 s, variability in these sources. Here we present our data base of observations, with three simultaneous energy bands in both the optical and the X-ray, and multiple epochs of observation for each source, all with ∼second or better time resolution. For the first time, we include a dynamical cross-correlation analysis, i.e. an investigation of how the cross-correlation function changes within an observation. We describe a number of trends which emerge. We include the full data set of results, and pick a few striking relationships from among them for further discussion.

We find, that the surprising form of X-ray/optical cross-correlation functions, a positive correlation signal preceded by an anticorrelation signal, is seen in all the sources at least some of the time. Such behaviour suggests a mechanism other than reprocessing as being the dominant driver of the short-term variability in the optical emission. This behaviour appears more pronounced when the X-ray spectrum is hard. Furthermore, we find that the cross-correlation relationships themselves are not stable in time, but vary significantly in strength and form. This all hints at dynamic interactions between the emitting components which could be modelled through non-linear or differential relationships. Title: Theories of the Solar Cycle: A Critical View Authors: Spruit, Hendrik C. Bibcode: 2011sswh.book...39S Altcode: 2010arXiv1004.4545S Some established views of the solar magnetic cycle are discussed critically, with focus on two aspects at the core of most models: the role of convective turbulence, and the role of the `tachocline' at the base of the convection zone. The standard view which treats the solar cycle as a manifestation of the interaction between convection and magnetic fields is shown to be misplaced. The main ingredient of the solar cycle, apart from differential rotation, is instead buoyant instability of the magnetic field itself. This view of the physics of the solar cycle was already established in the 1950s, but has been eclipsed mathematically by mean field turbulence formalisms which make poor contact with observations and have serious theoretical problems. The history of this development in the literature is discussed critically. The source of the magnetic field of the solar cycle is currently assumed to be located in the `tachocline': the shear zone at the base of the convection zone. While the azimuthal field of the cycle is indeed most likely located at the base of the convection zone, it cannot be powered by the radial shear of the tachocline as assumed in these models, since the radiative interior does not support significant shear stresses. Instead, it must be the powered by the latitudinal gradient in rotation rate in the convection zone, as in early models of the solar cycle. Possible future directions for research are briefly discussed. Title: Can a "propelling" disc stay trapped near corotation? Authors: D'Angelo, C.; Spruit, H. C. Bibcode: 2011fxts.confE..39D Altcode: 2011PoS...122E..39D No abstract at ADS Title: Semiconvection Authors: Zaussinger, F.; Spruit, H. C. Bibcode: 2010arXiv1012.5851Z Altcode: A grid of numerical simulations of double-diffusive convection is presented for astrophysical conditions. As in laboratory and geophysical cases convection takes place in a layered form. A translation between the astrophysical fluid mechanics and incompressible (Boussinesq) approximation is given, valid for thin layers. Its validity is checked by comparison of the results of fully compressible and Boussinesq simulations of semiconvection. A fitting formula is given for the superadiabatic gradient as a function of this parameter. The superadiabaticity depends on the thickness $d$ of the double diffusive layers, for which no good theory is available, but the effective He-diffusion coefficient is nearly independent of $d$. For a fiducial main sequence model (15 $M_\odot$) the inferred mixing time scale is of the order $10^{10}$ yr. Title: Modeling the Subsurface Structure of Sunspots Authors: Moradi, H.; Baldner, C.; Birch, A. C.; Braun, D. C.; Cameron, R. H.; Duvall, T. L.; Gizon, L.; Haber, D.; Hanasoge, S. M.; Hindman, B. W.; Jackiewicz, J.; Khomenko, E.; Komm, R.; Rajaguru, P.; Rempel, M.; Roth, M.; Schlichenmaier, R.; Schunker, H.; Spruit, H. C.; Strassmeier, K. G.; Thompson, M. J.; Zharkov, S. Bibcode: 2010SoPh..267....1M Altcode: 2009arXiv0912.4982M; 2010SoPh..tmp..171M While sunspots are easily observed at the solar surface, determining their subsurface structure is not trivial. There are two main hypotheses for the subsurface structure of sunspots: the monolithic model and the cluster model. Local helioseismology is the only means by which we can investigate subphotospheric structure. However, as current linear inversion techniques do not yet allow helioseismology to probe the internal structure with sufficient confidence to distinguish between the monolith and cluster models, the development of physically realistic sunspot models are a priority for helioseismologists. This is because they are not only important indicators of the variety of physical effects that may influence helioseismic inferences in active regions, but they also enable detailed assessments of the validity of helioseismic interpretations through numerical forward modeling. In this article, we provide a critical review of the existing sunspot models and an overview of numerical methods employed to model wave propagation through model sunspots. We then carry out a helioseismic analysis of the sunspot in Active Region 9787 and address the serious inconsistencies uncovered by Gizon et al. (2009a, 2009b). We find that this sunspot is most probably associated with a shallow, positive wave-speed perturbation (unlike the traditional two-layer model) and that travel-time measurements are consistent with a horizontal outflow in the surrounding moat. Title: Striation and convection in penumbral filaments Authors: Spruit, H. C.; Scharmer, G. B.; Löfdahl, M. G. Bibcode: 2010A&A...521A..72S Altcode: 2010arXiv1008.0932S Observations with the 1-m Swedish Solar Telescope of the flows seen in penumbral filaments are presented. Time sequences of bright filaments show overturning motions strikingly similar to those seen along the walls of small isolated structures in the active regions. The filaments show outward propagating striations with inclination angles suggesting that they are aligned with the local magnetic field. We interpret it as the equivalent of the striations seen in the walls of small isolated magnetic structures. Their origin is then a corrugation of the boundary between an overturning convective flow inside the filament and the magnetic field wrapping around it. The outward propagation is a combination of a pattern motion due to the downflow observed along the sides of bright filaments, and the Evershed flow. The observed short wavelength of the striation argues against the existence of a dynamically significant horizontal field inside the bright filaments. Its intensity contrast is explained by the same physical effect that causes the dark cores of filaments, light bridges and “canals”. In this way striation represents an important clue to the physics of penumbral structure and its relation with other magnetic structures on the solar surface. We put this in perspective with results from the recent 3-D radiative hydrodynamic simulations.

4 movies are only available in electronic form at http://www.aanda.org Title: Rapid optical and X-ray timing observations of GX339-4: multicomponent optical variability in the low/hard state Authors: Gandhi, P.; Dhillon, V. S.; Durant, M.; Fabian, A. C.; Kubota, A.; Makishima, K.; Malzac, J.; Marsh, T. R.; Miller, J. M.; Shahbaz, T.; Spruit, H. C.; Casella, P. Bibcode: 2010MNRAS.407.2166G Altcode: 2010arXiv1005.4685G; 2010MNRAS.tmp.1137G A rapid timing analysis of Very Large Telescope (VLT)/ULTRACAM (optical) and RXTE (X-ray) observations of the Galactic black hole binary GX339-4 in the low/hard, post-outburst state of 2007 June is presented. The optical light curves in the r',g' and u' filters show slow (~20s) quasi-periodic variability. Upon this is superposed fast flaring activity on times approaching the best time resolution probed (~50ms in r' and g') and with maximum strengths of more than twice the local mean. Power spectral analysis over ~0.004-10Hz is presented, and shows that although the average optical variability amplitude is lower than that in X-rays, the peak variability power emerges at a higher Fourier frequency in the optical. Energetically, we measure a large optical versus X-ray flux ratio, higher than that seen on previous occasions when the source was fully jet dominated. Such a large ratio cannot be easily explained with a disc alone. Studying the optical-X-ray cross-spectrum in Fourier space shows a markedly different behaviour above and below ~0.2Hz. The peak of the coherence function above this threshold is associated with a short optical time lag with respect to X-rays, also seen as the dominant feature in the time-domain cross-correlation at ~150ms. The rms energy spectrum of these fast variations is best described by distinct physical components over the optical and X-ray regimes, and also suggests a maximal irradiated disc fraction of 20 per cent around 5000Å. If the constant time delay is due to propagation of fluctuations to (or within) the jet, this is the clearest optical evidence to date of the location of this component. The low-frequency quasi-periodic oscillation is seen in the optical but not in X-rays, and is associated with a low coherence. Evidence of reprocessing emerges at the lowest Fourier frequencies, with optical lags at ~10s and strong coherence in the blue u' filter. Consistent with this, simultaneous optical spectroscopy also shows the Bowen fluorescence blend, though its emission location is unclear. However, canonical disc reprocessing cannot dominate the optical power easily, nor explain the fast variability. Title: Local Helioseismology: Three-Dimensional Imaging of the Solar Interior Authors: Gizon, Laurent; Birch, Aaron C.; Spruit, Henk C. Bibcode: 2010ARA&A..48..289G Altcode: 2010arXiv1001.0930G The Sun supports a rich spectrum of internal waves that are continuously excited by turbulent convection. The Global Oscillation Network Group (GONG) network and the SOHO/MDI (Solar and Heliospheric Observatory/Michelson Doppler Imager) space instrument provide an exceptional database of spatially resolved observations of solar oscillations, covering more than an entire sunspot cycle (11 years). Local helioseismology is a set of tools for probing the solar interior in three dimensions using measurements of wave travel times and local mode frequencies. Local helioseismology has discovered (a) near-surface vector flows associated with convection, (b) 250 m s-1 subsurface horizontal outflows around sunspots, (c) ∼50 m s-1 extended horizontal flows around active regions (converging near the surface and diverging below), (d) the effect of the Coriolis force on convective flows and active region flows, (e) the subsurface signature of the 15 m s-1 poleward meridional flow, (f) a ±5 m s-1 time-varying depth-dependent component of the meridional circulation around the mean latitude of activity, and (g) magnetic activity on the farside of the Sun. Title: Episodic accretion on to strongly magnetic stars Authors: D'Angelo, Caroline R.; Spruit, Hendrik C. Bibcode: 2010MNRAS.406.1208D Altcode: 2010arXiv1001.1742D; 2010MNRAS.tmp..908D Some accreting neutron stars and young stars show unexplained episodic flares in the form of quasi-periodic oscillations or recurrent outbursts. In a series of two papers, we present new work on an instability that can lead to episodic outbursts when the accretion disc is truncated by the star's strong magnetic field close to the corotation radius (where the Keplerian frequency matches the star's rotational frequency). In this paper we outline the physics of the instability and use a simple parametrization of the disc-field interaction to explore the instability numerically, which we show can lead to repeated bursts of accretion as well as steady-state solutions, as first suggested by Sunyaev and Shakura. The cycle time of these bursts increases with a decreasing accretion rate. These solutions show that the usually assumed `propeller' state, in which mass is ejected from the system, need not occur even at very low accretion rates. Title: Rapid timing studies of black hole binaries in Optical and X-rays: correlated and non-linear variability Authors: Gandhi, P.; Dhillon, V. S.; Durant, M.; Fabian, A. C.; Makishima, K.; Marsh, T. R.; Miller, J. M.; Shahbaz, T.; Spruit, H. C. Bibcode: 2010AIPC.1248..119G Altcode: In a fast multi-wavelength timing study of black hole X-ray binaries (BHBs), we have discovered correlated optical and X-ray variability in the low/hard state of two sources: GX 339-4 and SWIFT J1753.5-0127. After XTE J1118+480, these are the only BHBs currently known to show rapid (sub-second) aperiodic optical flickering. Our simultaneous VLT/Ultracam and RXTE data reveal intriguing patterns with characteristic peaks, dips and lags down to very short timescales. Simple linear reprocessing models can be ruled out as the origin of the rapid, aperiodic optical power in both sources. A magnetic energy release model with fast interactions between the disk, jet and corona can explain the complex correlation patterns. We also show that in both the optical and X-ray light curves, the absolute source variability r.m.s. amplitude linearly increases with flux, and that the flares have a log-normal distribution. The implication is that variability at both wavelengths is not due to local fluctuations alone, but rather arises as a result of coupling of perturbations over a wide range of radii and timescales. These `optical and X-ray rms-flux relations' thus provide new constraints to connect the outer and inner parts of the accretion flow, and the jet. Title: Accretion disks Authors: Spruit, H. C. Bibcode: 2010arXiv1005.5279S Altcode: In this lecture the basic theory of accretion disks is reviewed, with emphasis on aspects relevant for X-ray binaries and Cataclysmic Variables. The text gives a general introduction as well as a selective discussion of a number of more recent topics. Title: Theory of Magnetically Powered Jets Authors: Spruit, H. C. Bibcode: 2010LNP...794..233S Altcode: 2008arXiv0804.3096S The magnetic theory for the production of jets by accreting objects is reviewed with emphasis on outstanding problem areas. An effort is made to show the connections behind the occasionally diverging nomenclature in the literature, to contrast the different points of view about basic mechanisms and to highlight concepts for interpreting the results of numerical simulations. The role of dissipation of magnetic energy in accelerating the flow is discussed and its importance for explaining high Lorentz factors. The collimation of jets to the observed narrow angles is discussed, including a critical discussion of the role of “hoop stress.” The transition between disk and outflow is one of the least understood parts of the magnetic theory; its role in setting the mass flux in the wind, in possible modulations of the mass flux, and the uncertainties in treating it realistically are discussed. Current views on most of these problems are still strongly influenced by the restriction to two dimensions (axisymmetry) in previous analytical and numerical work; 3-D effects likely to be important are suggested. An interesting problem area is the nature and origin of the strong, preferably highly ordered magnetic fields known to work best for jet production. The observational evidence for such fields and their behavior in numerical simulations is discussed. I argue that the presence or absence of such fields may well be the “second parameter” governing not only the presence of jets but also the X-ray spectra and timing behavior of X-ray binaries. Title: Dissecting the Accretion Environments of X-ray Binaries with High Speed Coordinated Optical and X-ray Timing Observations Authors: Gandhi, Poshak; Durant, M.; Fabian, A. C.; Malzac, J.; Miller, J. M.; Shahbaz, T.; Dhillon, V. S.; Marsh, T. R.; Spruit, H. C.; Makishima, K. Bibcode: 2010HEAD...11.4314G Altcode: 2010BAAS...42..733G We are uncovering significant optical variability in low/hard state observations of several X-ray binaries on the fastest time-scales of just tens of milliseconds typically probed with modern rapid imaging cameras. The optical light curves are remarkable in that they display properties very characteristic of X-ray variations: 1) power spectra with band-limited, red noise over broad time ranges of 10 ms - 1000 s, and in some cases, a low-frequency quasi-periodic oscillation; 2) an instantaneous variability amplitude linearly scaling with source flux; and, 3) log-normal distributions of fluxes. Aperiodic optical variability components can dominate over simple linear X-ray reprocessing expectations, and are much faster than viscous time-scales of the outer accretion disk or flow. Cross-correlated optical vs. X-ray time delays not only constrain emission mechanisms, but can also be used to probe characteristic size scales of the physical components (jet, corona), and to understand how they are coupled. Rapid, multiwavelength timing studies are thus opening a new window on the hearts of accreting sources, though the broad-band spectral plus timing properties remain to be unified consistently. I will briefly review recent results on rapid optical variability, including our new data on black hole and neutron star binary systems. The fact that the sources were all in typical low/hard states (with relatively-bright optical counterparts) suggests that correlated optical/X-ray activity may be a general feature, waiting to be uncovered in more systems. The continuance of RXTE is vital for such work. Title: Solar Surface Magnetism and Irradiance on Time Scales from Days to the 11-Year Cycle Authors: Domingo, V.; Ermolli, I.; Fox, P.; Fröhlich, C.; Haberreiter, M.; Krivova, N.; Kopp, G.; Schmutz, W.; Solanki, S. K.; Spruit, H. C.; Unruh, Y.; Vögler, A. Bibcode: 2009SSRv..145..337D Altcode: The uninterrupted measurement of the total solar irradiance during the last three solar cycles and an increasing amount of solar spectral irradiance measurements as well as solar imaging observations (magnetograms and photometric data) have stimulated the development of models attributing irradiance variations to solar surface magnetism. Here we review the current status of solar irradiance measurements and modelling efforts based on solar photospheric magnetic fields. Thereby we restrict ourselves to the study of solar variations from days to the solar cycle. Phenomenological models of the solar atmosphere in combination with imaging observations of solar electromagnetic radiation and measurements of the photospheric magnetic field have reached high enough quality to show that a large fraction (at least, about 80%) of the solar irradiance variability can be explained by the radiative effects of the magnetic activity present in the photosphere. Also, significant progress has been made with magnetohydrodynamic simulations of convection that allow us to relate the radiance of the photospheric magnetic structures to the observations. Title: The source of magnetic fields in (neutron-) stars Authors: Spruit, Hendrik C. Bibcode: 2009IAUS..259...61S Altcode: Some arguments, none entirely conclusive, are reviewed about the origin of magnetic fields in neutron stars, with emphasis of processes during and following core collapse in supernovae. Possible origins of the magnetic fields of neutron stars include inheritance from the main sequence progenitor and dynamo action at some stage of evolution of progenitor. Inheritance is not sufficient to explain the fields of magnetars. Energetic considerations point to differential rotation in the final stages of core collapse process as the most likely source of field generation, at least for magnetars. A runaway phase of exponential growth is needed to achieve sufficient field amplification during relevant phase of core collapse; it can probably be provided by a some form of magnetorotational instability. Once formed in core collapse, the field is in danger of decaying again by magnetic instabilities. The evolution of a magnetic field in a newly formed neutron star is discussed, with emphasis on the existence of stable equilibrium configurations as end products of this evolution, and the role of magnetic helicity in their existence. A particularly puzzling problem is the large range of field strengths observed in neutron stars (as well as in A stars and white dwarfs). It implies that a single, deterministic process is insufficient to explain the origin of the magnetic fields in these stars. Title: Kink instabilities in jets from rotating magnetic fields Authors: Moll, R.; Spruit, H. C.; Obergaulinger, M. Bibcode: 2008A&A...492..621M Altcode: 2008arXiv0809.3165M We have performed 2.5D and 3D simulations of conical jets driven by the rotation of an ordered, large-scale magnetic field in a stratified atmosphere. The simulations cover about three orders of magnitude in distance to capture the centrifugal acceleration as well as the evolution past the Alfvénsurface. We find that the jets develop kink instabilities, the characteristics of which depend on the velocity profile imposed at the base of the flow. The instabilities are especially pronounced with a rigid rotation profile, which induces a shearless magnetic field. The jet's expansion appears to be limiting the growth of Alfvénmode instabilities. Title: Rapid optical and X-ray timing observations of GX 339-4: flux correlations at the onset of a low/hard state Authors: Gandhi, P.; Makishima, K.; Durant, M.; Fabian, A. C.; Dhillon, V. S.; Marsh, T. R.; Miller, J. M.; Shahbaz, T.; Spruit, H. C. Bibcode: 2008MNRAS.390L..29G Altcode: 2008arXiv0807.1529G; 2008MNRAS.tmpL..87G We present the discovery of optical/X-ray flux correlations on rapid time-scales in the low/hard state of the Galactic black hole GX339-4. The source had recently emerged from outburst and was associated with a relatively faint counterpart with mag V ~ 17. The optical [Very Large Telescope (VLT)/ULTRACAM] and X-ray (Rossi X-ray Timing Explorer) data show a clear positive cross-correlation function (CCF) signal, with the optical peak lagging X-rays by ~150 ms, preceded by a shallow rise and followed by a steep decline along with broad anticorrelation dips. An examination of the light curves shows that the main CCF features are reproduced in superpositions of flares and dips. The CCF peak is narrow and the X-ray autocorrelation function (ACF) is broader than the optical ACF, arguing against reprocessing as the origin for the rapid optical emission. X-ray flaring is associated with spectral hardening, but no corresponding changes are detected around optical peaks and dips. The variability may be explained in the context of synchrotron emission with interaction between a jet and a corona. The complex CCF structure in GX 339-4 has similarities to that of another remarkable X-ray binary XTE J1118+480, in spite of showing a weaker maximum strength. Such simultaneous multiwavelength, rapid timing studies provide key constraints for modelling the inner regions of accreting stellar sources.

Based on observations carried out in ESO programmes 079.D-0535 and 279.D-5021, and Rossi X-ray Timing Explorer proposal number 93119.

E-mail: pg@crab.riken.jp Title: Soft X-ray components in the hard state of accreting black holes Authors: D'Angelo, C.; Giannios, D.; Dullemond, C.; Spruit, H. Bibcode: 2008A&A...488..441D Altcode: 2008arXiv0807.1274D Recent observations of two black hole candidates (GX 339-4 and J1753.5-0127) in the low-hard state (L{X}/L{Edd} ≃ 0.003-0.05) suggest the presence of a cool accretion disk very close to the innermost stable orbit of the black hole. This runs counter to models of the low-hard state in which the cool disk is truncated at a much larger radius. We study the interaction between a moderately truncated disk and a hot inner flow. Ion-bombardment heats the surface of the disk in the overlap region between a two-temperature advection-dominated accretion flow and a standard accretion disk, producing a hot (kT{e} ≃ 70 keV) layer on the surface of the cool disk. The hard X-ray flux from this layer heats the inner parts of the underlying cool disk, producing a soft X-ray excess. Together with interstellar absorption these effects mimic the thermal spectrum from a disk extending to the last stable orbit. The results show that soft excesses in the low-hard state are a natural feature of truncated disk models. Title: Rapid Optical/X-ray flux correlations in the low/hard state of GX 339-4 Authors: Gandhi, Poshak; Makishima, K.; Kubota, A.; Fabian, A. C.; Marsh, T.; Dhillon, V.; Durant, M.; Shahbaz, T.; Miller, J.; Spruit, H. Bibcode: 2008xru..confE..33G Altcode: No abstract at ADS Title: Simultaneous Optical And X-ray Flickering Observations Of GX 339-4: Correlations On Sub-second Timescales Authors: Gandhi, Poshak; Makishima, K.; Fabian, A.; Durant, M.; Shahbaz, T.; Dhillon, V.; Marsh, T.; Miller, J.; Spruit, H. Bibcode: 2008HEAD...10.0106G Altcode: We present a rapid timing analysis of VLT (optical) and RXTE data of the Galactic black hole GX 339-4 in the low/hard state in June 2007, soon after emerging from outburst. Both optical and X-ray power density spectra are qualitatively similar with QPO-like feature or a break around 0.05-0.1 Hz. A cross-correlation function (CCF) analysis finds no positive correlation on timescales on 1 s or longer, similar to previous observations in the bright state. Searching for signal on the fastest (50 ms) timescales available, on the other hand, a significant cross-correlation peak is detected, with the optical lagging the X-rays by 150 ms. The CCF is narrow with a shallow rise and sharp decline to an anti-correlation dip. The X-ray auto-correlation function (ACF) is broader than the optical ACF, arguing against re-processing as the emission mechanism for the optical power. Similarities and differences with respect to the complex behaviour of XTE J1118+480 are discussed and the need for further rapid variability observations of this fascinating source are emphasized. Title: Origin of neutron star magnetic fields Authors: Spruit, H. C. Bibcode: 2008AIPC..983..391S Altcode: 2007arXiv0711.3650S Possible origins of the magnetic fields of neutron stars include inheritance from the main sequence progenitor and dynamo action at some stage of evolution of progenitor. Inheritance is not sufficient to explain the fields of magnetars. Energetic considerations point to differential rotation in the final stages of core collapse process as the most likely source of field generation, at least for magnetars. A runaway phase of exponential growth is needed to achieve sufficient field amplification during relevant phase of core collapse; it can probably be provided by a some form of magnetorotational instability. Once formed in core collapse, the field is in danger of decaying again by magnetic instabilities. The evolution of a magnetic field in a newly formed neutron star is discussed, with emphasis on the existence of stable equilibrium configurations as end products of this evolution, and the role of magnetic helicity in their existence. Title: Soft X-ray components in the hard state of accreting black holes Authors: D'Angelo, C.; Giannios, D.; Spruit, H.; Dullemond, C. Bibcode: 2008mqw..confE..10D Altcode: 2008PoS....62E..10D No abstract at ADS Title: OPTIMA: A High Time Resolution Optical Photo-Polarimeter Authors: Kanbach, G.; Stefanescu, A.; Duscha, S.; Mühlegger, M.; Schrey, F.; Steinle, H.; Slowikowska, A.; Spruit, H. Bibcode: 2008ASSL..351..153K Altcode: A high-speed photo-polarimeter, "OPTIMA" short for Optical Pulsar Timing Analyzer, has been designed and developed in the group for gamma-ray astronomy of the Max-Planck-Institut für Extraterrestrische Physik. This sensitive, portable detector is used to observe optical emissions of sources that radiate mainly at X- and gamma-ray energies, like pulsars and other highly variable compact sources. The single photon counting instrument is based on fiber fed avalanche photodiodes (APDs), a GPS timing receiver,a CCD camera for target acquisition and a stand-alone data acquisition and control system. Several configurations are available: for photometry a hexagonal bundle with seven channels and one fiber offset for sky background monitoring; for polarimetry a rotating polarization filter in front of the photometer or a newly developed 4-channel double Wollaston system; and for coarse spectroscopy a 4-colour prism spectrograph. Title: MHD Simulations of Penumbra Fine Structure Authors: Heinemann, T.; Nordlund, Å.; Scharmer, G. B.; Spruit, H. C. Bibcode: 2007ApJ...669.1390H Altcode: 2006astro.ph.12648H We present the results of numerical 3D magnetohydrodynamic (MHD) simulations with radiative energy transfer of fine structure in a small sunspot of about 4 Mm width. The simulations show the development of filamentary structures and flow patterns that are, except for the lengths of the filaments, very similar to those observed. The filamentary structures consist of gaps with reduced field strength relative to their surroundings. Calculated synthetic images show dark cores like those seen in the observations; the dark cores are the result of a locally elevated τ=1 surface. The magnetic field in these cores is weaker and more horizontal than for adjacent brighter structures, and the cores support a systematic outflow. Accompanying animations show the migration of the dark-cored structures toward the umbra, and fragments of magnetic flux that are carried away from the spot by a large-scale ``moat flow.'' We conclude that the simulations are in qualitative agreement with observed penumbra filamentary structures, Evershed flows, and moving magnetic features. Title: Spectral and timing properties of a dissipative γ-ray burst photosphere Authors: Giannios, D.; Spruit, H. C. Bibcode: 2007A&A...469....1G Altcode: 2006astro.ph.11385G We explore the observational appearance of the photosphere of an ultrarelativistic flow with internal dissipation of energy as predicted by the magnetic reconnection model. Previous study of the radiative transfer in the photospheric region has shown that gradual dissipation of energy results in a hot photosphere. There, inverse Compton scattering of the thermal radiation advected with the flow leads to powerful photospheric emission with spectral properties close to those of the observed prompt GRB emission. Here, we build on that study by calculating the spectra for a large range of the characteristics of the flow. An accurate fitting formula is given that provides the photospheric spectral energy distribution in the 10 keV to 10 MeV energy range (in the central engine frame) as a function of the basic physical parameters of the flow. It facilitates the direct comparison of the model predictions with observations, including the variability properties of the lightcurves. We verify that the model naturally accounts for the observed clustering in peak energies of the E \cdot f(E) spectrum. In this model, the Amati relation indicates a tendency for the most luminous bursts to have more energy per baryon. If this tendency also holds for individual GRB pulses, the model predicts the observed narrowing of the width of pulses with increasing photon energy. Title: Gamma-ray Bursts from X-ray Binaries Authors: Spruit, H. C. Bibcode: 2007ralc.conf..407S Altcode: In this contribution I review the mechanism proposed earlier for producing a gamma-ray burst from the rapidly spinning neutron star in an X-ray binary (Spruit 1999), with a discussion of some more recent developments and outstanding issues. Title: Magnetostatic penumbra models with field-free gaps Authors: Scharmer, G. B.; Spruit, H. C. Bibcode: 2006A&A...460..605S Altcode: 2006astro.ph..9130S We present numerical 2D magnetostatic models for sunspot penumbrae consisting of radially aligned field-free gaps in a potential magnetic field, as proposed by Spruit & Scharmer (2006, A&A, 447, 343). The shape of the gaps and the field configurations around them are computed consistently from the condition of magnetostatic pressure balance between the gap and the magnetic field. The results show that field-free gaps in the inner penumbra are cusp-shaped and bounded by a magnetic field inclined by about 70° from the vertical. Here, the magnetic component has a Wilson depression on the order 200-300 km relative to the top of the field-free gap; the gaps should thus appear as noticeably elevated features. This structure explains the large variations in field strength in the inner penumbra inferred from magnetograms and two-component inversions, and the varying appearance of the inner penumbra with viewing angle. In the outer penumbra, on the other hand, the gaps are flat-topped with a horizontal magnetic field above the middle of the gap. The magnetic field has large inclination variations horizontally, but only small fluctuations in field strength, in agreement with observations. Title: Variations in solar luminosity and their effect on the Earth's climate Authors: Foukal, P.; Fröhlich, C.; Spruit, H.; Wigley, T. M. L. Bibcode: 2006Natur.443..161F Altcode: Variations in the Sun's total energy output (luminosity) are caused by changing dark (sunspot) and bright structures on the solar disk during the 11-year sunspot cycle. The variations measured from spacecraft since 1978 are too small to have contributed appreciably to accelerated global warming over the past 30 years. In this Review, we show that detailed analysis of these small output variations has greatly advanced our understanding of solar luminosity change, and this new understanding indicates that brightening of the Sun is unlikely to have had a significant influence on global warming since the seventeenth century. Additional climate forcing by changes in the Sun's output of ultraviolet light, and of magnetized plasmas, cannot be ruled out. The suggested mechanisms are, however, too complex to evaluate meaningfully at present. Title: The role of kink instability in Poynting-flux dominated jets Authors: Giannios, Dimitrios; Spruit, Henk Bibcode: 2006AIPC..848..530G Altcode: The role of kink instability in magnetically driven jets is explored through numerical one-dimensional steady relativistic MHD calculations. The instability is shown to have enough time to grow and influence the dynamics of Poynting-flux dominated jets. In the case of AGN jets, the flow becomes kinetic flux dominated at distances >~1000rg because of the rapid dissipation of Poynting flux. When applied to GRB outflows, the model predicts more gradual Poynting dissipation and moderately magnetized flow at distances of ~ 1016 cm where the deceleration of the ejecta due to its interaction with the external medium is expected. The energy released by the instability can power the compact ``blazar zone'' emission and the prompt emission of GRB outflows with high radiative efficiencies. Title: Magnetic instability in a differentially rotating star Authors: Spruit, H. C. Bibcode: 2006astro.ph..7164S Altcode: Instabilities in magnetic fields wound up by differential rotation as reviewed in Spruit (1999) are discussed with some detail and new developments added. In stellar models which include magnetic torques, the differential rotation tends to accumulate in the gradients in composition. In view of this, instability in a $\mu$-gradient is studied in more detail here, resulting in the detection of a second instability. Its relevance for angular momentum transport is uncertain, however, since it requires high horizontal field gradients and would not operate near the pole. Finally, the possibility is discussed that magnetic instability in a $\mu$-gradient will lead to {\em layer formation}: the gradients breaking up into small steps of uniform composition and rotation rate. This would enhance the angular momentum transport across inhomogeneous zones, and decrease the rotation rates of the end products of stellar evolution. A recent {\tt astro-ph} submission by Dennisenkov and Pinsonneault proposing a modification of the instability conditions is shown to contain a mathematical error. Title: Evolution of the magnetic field in magnetars Authors: Braithwaite, J.; Spruit, H. C. Bibcode: 2006A&A...450.1097B Altcode: 2005astro.ph.10287B We use numerical MHD to look at the stability of a possible poloidal field in neutron stars (Flowers & Ruderman 1977, ApJ, 215, 302), and follow its unstable evolution, which leads to the complete decay of the field. We then model a neutron star after the formation of a solid crust of high conductivity. As the initial magnetic field we use the stable "twisted torus" field which was the result of our earlier work (Braithwaite & Nordlund 2006, A&A, 450, 1077), since this field is likely to exist in the interior of the star at the time of crust formation. We follow the evolution of the field under the influence of diffusion, and find that large stresses build up in the crust, which will presumably lead to cracking. We put this forward as a model for outbursts in soft gamma repeaters. Title: The role of kink instability in Poynting-flux dominated jets Authors: Giannios, D.; Spruit, H. C. Bibcode: 2006A&A...450..887G Altcode: 2006astro.ph..1172G The role of kink instability in magnetically driven jets is explored through numerical one-dimensional steady relativistic MHD calculations. The instability is shown to have enough time to grow and influence the dynamics of Poynting-flux dominated jets. In the case of AGN jets, the flow becomes kinetic flux dominated at distances ⪆1000~rg because of the rapid dissipation of Poynting flux. When applied to GRB outflows, the model predicts more gradual Poynting dissipation and moderately magnetized flow at distances of ~1016 cm where the deceleration of the ejecta due to interaction with the external medium is expected. The energy released by the instability can power the compact "blazar zone" emission and the prompt emission of GRB outflows with high radiative efficiencies. Title: Fine structure, magnetic field and heating of sunspot penumbrae Authors: Spruit, H. C.; Scharmer, G. B. Bibcode: 2006A&A...447..343S Altcode: 2005astro.ph..8504S We interpret penumbral filaments as due to convection in field-free, radially aligned gaps just below the visible surface of the penumbra, intruding into a nearly potential field above. This solves the classical discrepancy between the large heat flux and the low vertical velocities observed in the penumbra. The presence of the gaps causes strong small-scale fluctuations in inclination, azimuth angle and field strength. The field is nearly horizontal in a region around the cusp-shaped top of the gap, thereby providing an environment for Evershed flows. We identify this region with the recently discovered dark penumbral cores. Its darkness has the same cause as the dark lanes in umbral light-bridges, reproduced in numerical simulations by Nordlund & Stein (2005, in preparation). We predict that the large vertical and horizontal gradients of the magnetic field inclination and azimuth in the potential field model will produce the net circular polarization seen in observations. The model also explains the significant elevation of bright filaments above their surroundings. It predicts that dark areas in the penumbra are of two different kinds: dark filament cores containing the most inclined (horizontal) fields, and regions between bright filaments, containing the least inclined field lines. Title: Relativistic jet production from accreting compact objects Authors: Spruit, Hendrik Bibcode: 2006smqw.confE..44S Altcode: 2006PoS....33E..44S The magnetic theory for the production of jets by accreting objects is summarized with attention to some outstanding problem areas. An effort is made to show the connections behind the somewhat diverging nomenclature in the literature, and to contrast the different points of view about basic mechanisms. A key problem is the acceleration of the jet flow to high Lorentz factors. In this context, the role of dissipation of magnetic energy in accelerating the flow is discussed, the importance of which has been realized only recently. A second theme is the ‘collimation problem’, discussion of which is hampered by incorrect ideas about the role of ‘hoop stress’. Current views on these problems are still strongly biased by the restriction to 2 dimensions (axisymmetry) in previous analytical and numerical work. Estimates are given how jets will behave differently in the 3-dimensional world. An interesting problem area is the nature and origin of the strong, preferably highly ordered magnetic fields known to work best for jet production, and observational evidence for such fields. I argue that the presence or absence of such fields may well be the ‘second parameter’ governing the X-ray spectra and timing behavior in X-ray binaries. Title: Magnetic Field Dissipation in GRBs Authors: Topinka, Martin; Spruit, Henk; Karlický, Marián Bibcode: 2005AIPC..801..141T Altcode: We report on the first steps towards the three-dimensional simulation of magnetic field dissipation in gamma-ray burst prompt emission. The model is based on magnetically driven Poynting flux outflow. Title: Mid-infrared emission from cataclysmic variables Authors: Dubus, G.; Campbell, R.; Kern, B.; Taam, R. E.; Spruit, H. C. Bibcode: 2005ASPC..330...55D Altcode: CVs could be surrounded by cold material that has condensed out of the ejecta from the compact binary. The strong braking resulting from tidal interactions between such a circumbinary (CB) disc and the red dwarf might explain several puzzles in our current understanding of CV evolution. The thermal emission from CB discs is expected to peak in the mid-IR. We report on a survey of the mid-IR emission from 8 CVs. AE Aqr and SS Cyg are detected above 5 μm but the observed rapid variability makes CB material unlikely in both cases. We discuss the upper limits on CB discs resulting from this study and the prospects for their detection. Title: Magnetic Flux Captured by an Accretion Disk Authors: Spruit, H. C.; Uzdensky, Dmitri A. Bibcode: 2005ApJ...629..960S Altcode: 2005astro.ph..4429U We suggest a possible mechanism of efficient transport of the large-scale external magnetic flux inward through a turbulent accretion disk. The outward drift by turbulent diffusion that limits the effectiveness of this process is reduced if the external flux passes through the disk in concentrated patches. Angular momentum loss by a magnetocentrifugal wind from these patches of strong field adds to the inward drift. We propose that magnetic flux accumulating in this way at the center of the disk provides the ``second parameter'' determining X-ray spectral states of black hole binaries and the presence of relativistic outflows. Title: Presupernova Evolution of Differentially Rotating Massive Stars Including Magnetic Fields Authors: Heger, A.; Woosley, S. E.; Spruit, H. C. Bibcode: 2005ApJ...626..350H Altcode: 2004astro.ph..9422H As a massive star evolves through multiple stages of nuclear burning on its way to becoming a supernova, a complex, differentially rotating structure is set up. Angular momentum is transported by a variety of classic instabilities and also by magnetic torques from fields generated by the differential rotation. We present the first stellar evolution calculations to follow the evolution of rotating massive stars including, at least approximately, all these effects, magnetic and nonmagnetic, from the zero-age main sequence until the onset of iron-core collapse. The evolution and action of the magnetic fields is as described by Spruit in 2002, and a range of uncertain parameters is explored. In general, we find that magnetic torques decrease the final rotation rate of the collapsing iron core by about a factor of 30-50 when compared with the nonmagnetic counterparts. Angular momentum in that part of the presupernova star destined to become a neutron star is an increasing function of main-sequence mass. That is, pulsars derived from more massive stars rotate faster and rotation plays a more important role in the star's explosion. The final angular momentum of the core has been determined-to within a factor of 2-by the time the star ignites carbon burning. For the lighter stars studied, around 15 Msolar, we predict pulsar periods at birth near 15 ms, though a factor of 2 range is easily tolerated by the uncertainties. Several mechanisms for additional braking in a young neutron star, especially by fallback, are explored. Title: Structure and Evolution of Compact Binary Systems: Infrared Spectroscopy of SS Cygni Authors: Froning, Cynthia; Ciardi, David; Dubus, Guillaume; Hoard, Donald; Howell, Steve; Long, Knox; Spruit, Hendrik; Szkody, Paula; Taam, Ron; Wachter, Stefanie; van Belle, Gerard Bibcode: 2005sptz.prop20074F Altcode: Cataclysmic variables (CVs) are interacting binary systems in which a white dwarf accretes mass from a late-type donor star. In dwarf novae (DN), accretion occurs via a disk around the white dwarf that undergoes semi-regular outbursts caused by thermal-viscous disk instabilities. Because of their proximity and lack of obscuration, CVs are excellent systems in which to study the physics of disk accretion and binary star evolution. Ground-based mid-IR (4 -- 18 micron) photometry of the quiescent DN SS Cygni revealed a flux well in excess of that expected from the accretion disk and the donor star. The IR emission also appeared to vary on 15~min timescales, although the limited sensitivity of the ground-based observations precluded detailed variability study. The source of this emission is not known. Possible IR emission regions in DN include circumbinary material, a remnant of the common envelope evolutionary phase of the binary; a previously unobserved component of the accretion disk; or an outflow from the disk or the donor star. We propose Spitzer IRS staring mode observations to identify and characterize the source of the quiescent IR emission in SS Cyg. We will use the observations to study both the high S/N, time-averaged spectrum of the IR source and to construct time-resolved lightcurves in multiple wavebands. The spectra and lightcurves will allow us to distinguish between circumbinary emission, likely to be cold and dusty, and a disk or outflow source, which will be warmer and possibly show molecular and/or shocked gas emission features. Spitzer is the only instrument capable of obtaining these data, which will enable us to open a new window on the accretion physics and evolution of CVs. Title: Evaporation of ion-irradiated disks Authors: Dullemond, C. P.; Spruit, H. C. Bibcode: 2005A&A...434..415D Altcode: 2005astro.ph..1463D We calculate the evaporation of a cool accretion disk around a black hole due to the ion-bombardment by an ion supported accretion flow (here ISAF, or optically thin ADAF). As first suggested by Spruit & Deufel (2002), this evaporation takes place in two stages: ion bombardment of the cool disk (Shakura-Sunyaev disk: SSD) produces an intermediate-temperature layer on top of the disk (“warm layer”) which constitutes an independent accretion flow on both sides of the SSD. As this warm material accretes inward of the inner radius of the SSD, it becomes thermally unstable by lack of cooling of photons, and evaporates into the ISAF, thereby feeding the latter. Angular momentum conservation forces a certain fraction of the ISAF material to move outward, where it can bombard the SSD with its hot ions. The flow geometry is derived by computing stationary solutions of the continuity- and angular momentum equations for the three components (ISAF, warm flow and SSD). The overall radiative output is dominated by hard X-rays. They are produced mostly from the warm component, rather than the ISAF. The expected time dependence and stability of the flow, not computed here, is discussed briefly. Title: Magnetohydrodynamics at the Interface Between a Disk and a Jet Authors: Spruit, Henk Bibcode: 2005paoa.progE..26S Altcode: No abstract at ADS Title: Physics of Astrophysical Outflows and Accretion Disks Authors: Blaes, O.; Gammie, C.; Spruit, H. Bibcode: 2005paoa.progE....B Altcode: No abstract at ADS Title: Introduction of the Jets and Disks Program Authors: Spruit, Henk Bibcode: 2005kbls.confE..29S Altcode: No abstract at ADS Title: Accretion Disks Authors: Spruit, H. C. Bibcode: 2005ASIB..210..203S Altcode: 2005esns.conf..203S No abstract at ADS Title: Spectra of Poynting-flux powered GRB outflows Authors: Giannios, D.; Spruit, H. C. Bibcode: 2005A&A...430....1G Altcode: 2004astro.ph..1109G We investigate the production of the gamma-ray spectrum of a Poynting-flux dominated GRB outflow. The very high magnetic field strengths (super-equipartition) in such a flow lead to very efficient synchrotron emission. In contrast with internal shocks, dissipation of magnetic energy by reconnection is gradual and does not produce the spectrum of cooling electrons associated with shock acceleration. We show that a spectrum with a break in the BATSE energy range is produced, instead, if the magnetic dissipation heats a small (∼10-4) population of electrons. Title: Comment on ``Variations of Total Solar Irradiance Produced by Structural Changes in the Solar Interior'' Authors: Foukal, P.; Spruit, H. Bibcode: 2004EOSTr..85..524F Altcode: In a recent Eos article, Sofia [2004] argues for the influence on irradiance variation of global changes in the Sun's structure associated with its magnetic dynamo. These changes would act in addition to the relatively well understood modulation by dark sunspots and bright faculae at the surface. His assessment of the present observational evidence for such a global change agrees with our earlier conclusion that it is not widely convincing at the present time [Foukal, 2003]. But Sofia's article also claims (1) that the numerical results obtained by him and his collaborators at Yale disagree with and correct earlier work, and (2) that a hydrostatic approximation is not adequate for variations on the 11-year solar cycle timescale. These surprising claims are based on the results of recently published hydrostatic models [e.g., Sofia and Li, 2004] using the same mixing length approximation for convective heat transport used in earlier work [Spruit, 1982, 1991; Gilliland, 1988]. Title: Excitation of low-frequency QPOs in black hole accretion Authors: Giannios, D.; Spruit, H. C. Bibcode: 2004A&A...427..251G Altcode: 2004astro.ph..7474G We study possible mechanisms of excitation of quasiperiodic oscillations in the accretion flow of black hole accreters in their hard spectral states, in the context of the ``truncated disk'' model. Quasi-spherical oscillations of the inner ion-supported accretion flow (ISAF) can be excited by the interaction of this hot flow with the cool disk extending outward from it. The fundamental mode of (p-mode) oscillation is most easily excited, and has a frequency near the Kepler frequency at the inner edge of the cool disk. The strongest excitation mechanism is a feedback loop involving cooling of the ISAF by soft photons from the cool disk and heating of the cool disk by the ISAF, while synchrotron emission can be a relatively strong damping effect. Growth times are computed by detailed Comptonization calculations of the interaction of an idealized ISAF with a cool disk. Typical growth times as short as a few dynamical times are found, while amplitudes can be as large as 10%. Title: A fossil origin for the magnetic field in A stars and white dwarfs Authors: Braithwaite, Jonathan; Spruit, Hendrik C. Bibcode: 2004Natur.431..819B Altcode: 2005astro.ph..2043B Some main-sequence stars of spectral type A are observed to have a strong (0.03-3tesla), static, large-scale magnetic field, of a chiefly dipolar shape: they are known as `Ap stars', such as Alioth, the fifth star in the Big Dipper. Following the discovery of these fields, it was proposed that they are remnants of the star's formation, a `fossil' field. An alternative suggestion is that they could be generated by a dynamo process in the star's convective core. The dynamo hypothesis, however, has difficulty explaining high field strengths and the observed lack of a correlation with rotation. The weakness of the fossil-field theory has been the absence of field configurations stable enough to survive in a star over its lifetime. Here we report numerical simulations that show that stable magnetic field configurations, with properties agreeing with those observed, can develop through evolution from arbitrary, unstable initial fields. The results are applicable equally to Ap stars, magnetic white dwarfs and some highly magnetized neutron stars known as magnetars. This establishes fossil fields as the natural, unifying explanation for the magnetism of all these stars. Title: The optical and X-ray flickering of XTE J1118+480 Authors: Malzac, J.; Belloni, T.; Spruit, H. C.; Kanbach, G. Bibcode: 2004NuPhS.132..400M Altcode: We study the complex correlations between optical and X-ray variability in the black hole X-ray nova XTE J1118+480. We find that the optical and X-ray fluctuations of very different shapes, amplitudes and time-scales are correlated in a similar mode where the optical light curve is seemingly related to the time derivative of the X-rays. Title: Angular Momentum Transport and Mixing by Magnetic Fields (Invited Review) Authors: Spruit, H. C. Bibcode: 2004IAUS..215..356S Altcode: No abstract at ADS Title: Presupernova Evolution of Rotating Massive Stars and the Rotation Rate of Pulsars (Invited Review) Authors: Heger, A.; Woosley, S. E.; Langer, N.; Spruit, H. C. Bibcode: 2004IAUS..215..591H Altcode: 2003astro.ph..1374H Rotation in massive stars has been studied on the main sequence and during helium burning for decades, but only recently have realistic numerical simulations followed the transport of angular momentum that occurs during more advanced stages of evolution. The results affect such interesting issues as whether rotation is important to the explosion mechanism, whether supernovae are strong sources of gravitational radiation, the star's nucleosynthesis, and the initial rotation rate of neutron stars and black holes. We find that when only hydrodynamic instabilities (shear, Eddington-Sweet, etc.) are included in the calculation, one obtains neutron stars spinning at close to critical rotation at their surface -- or even formally in excess of critical. When recent estimates of magnetic torques (Spruit 2002) are added, however, the evolved cores spin about an order of magnitude slower. This is still more angular momentum than observed in young pulsars, but too slow for the collapsar model for gamma-ray bursts. Title: Magnetically powered prompt radiation and flow acceleration in GRB Authors: Spruit, H. C.; Drenkhahn, G. D. Bibcode: 2004ASPC..312..357S Altcode: 2003astro.ph..2468S The physics of GRB powered by a magnetic energy flux is reviewed. Magnetic fields are natural for transmitting the energy from the central compact object to the small amount of baryons required for a GRB. When dissipation of the flux of magnetic energy by reconnection inside the flow is taken into account, the magnetic model assumes several more convincing properties. For baryon loading typical of observed GRB, most of the dissipation takes place just outside photosphere, so that prompt emission is produced efficiently, and the magnetic field strength in this region is high, resulting in efficient synchrotron emission. Remarkably, the dissipation also causes very efficient acceleration of the bulk flow. This effect is illustrated with a classical hydrodynamic equivalent. In this context, the distinction between the flux of magnetic energy cB^2/8π and the Poynting flux cB^2/4π is important, and an interpretation of the Poynting flux as a `magnetic enthalpy flux' illuminating. Numerical and analytical results for flow acceleration and the relative contribution of photospheric (thermal) and nonthermal emission as functions of the asymptotic bulk Lorentz factor are given. The transition between X-ray flashes and true GRB is predicted at Γ≈ 100. Title: Magnetically powered prompt radiation and flow acceleration in GRB Authors: Drenkhahn, G.; Spruit, H. C. Bibcode: 2004ASPC..312..357D Altcode: Magnetic fields are natural for transmitting the energy from the central compact object to the small amount of baryons required for a GRB. If dissipation of the flux of magnetic energy by reconnection inside the flow is taken into account, the magnetic model has several more convincing properties. For baryon loading typical of observed GRB, most of the dissipation takes place just outside photosphere, so that prompt emission is produced efficiently, and the magnetic field strength in this region is high, resulting in efficient synchrotron emission. The dissipation also causes remarkably efficient acceleration of the bulk flow. In this context, an interpretation of the Poynting flux as a `magnetic enthalpy flux' is illuminating. The transition between X-ray flashes and true GRB is predicted at Γ≈ 100. Title: Local Axisymmetric Diffusive Stability of Weakly Magnetized, Differentially Rotating, Stratified Fluids Authors: Menou, Kristen; Balbus, Steven A.; Spruit, Henk C. Bibcode: 2004ApJ...607..564M Altcode: 2004astro.ph..2150M We study the local stability of stratified, differentially rotating fluids to axisymmetric perturbations in the presence of a weak magnetic field and of finite resistivity, viscosity, and heat conductivity. This is a generalization of the Goldreich-Schubert-Fricke (GSF) double-diffusive analysis to the magnetized and resistive, triple-diffusive case. Our fifth-order dispersion relation admits a novel branch that describes a magnetized version of multidiffusive modes. We derive necessary conditions for axisymmetric stability in the inviscid and perfect-conductor (double-diffusive) limits. In each case, rotation must be constant on cylinders and angular velocity must not decrease with distance from the rotation axis for stability, irrespective of the relative strength of viscous, resistive, and heat diffusion. Therefore, in both double-diffusive limits, solid-body rotation marginally satisfies our stability criteria. The role of weak magnetic fields is essential to reach these conclusions. The triple-diffusive situation is more complex, and its stability criteria are not easily stated. Numerical analysis of our general dispersion relation confirms our analytic double-diffusive criteria but also shows that an unstable double-diffusive situation can be significantly stabilized by the addition of a third, ostensibly weaker, diffusion process. We describe a numerical application to the Sun's upper radiative zone and establish that it would be subject to unstable multidiffusive modes if moderate or strong radial gradients of angular velocity were present. Title: Accretion onto Fast X-Ray Pulsars Authors: Rappaport, S. A.; Fregeau, J. M.; Spruit, H. Bibcode: 2004ApJ...606..436R Altcode: 2003astro.ph.10224R The recent emergence of a new class of accretion-powered, transient, millisecond X-ray pulsars presents some difficulties for the conventional picture of accretion onto rapidly rotating magnetized neutron stars and their spin behavior during outbursts. In particular, it is not clear that the standard paradigm can accommodate the wide range in M (i.e., >~ a factor of 50) over which these systems manage to accrete and the high rate of spin-down that the neutron stars exhibit in at least a number of cases. When the accretion rate drops sufficiently, the X-ray pulsar is said to become a ``fast rotator,'' and in the conventional view, this is accompanied by a transition from accretion to ``propellering,'' in which accretion ceases and the matter is ejected from the system. On the theoretical side, we note that this scenario for the onset of propellering cannot be entirely correct because it is not energetically self-consistent. We show that, instead, the transition is likely to take place through disks that combine accretion with spin-down and terminate at the corotation radius. We demonstrate the existence of such disk solutions by modifying the Shakura-Sunyaev equations with a simple magnetic torque prescription. The solutions are completely analytic and have the same dependence on M and α (the viscosity parameter) as the original Shakura-Sunyaev solutions, but the radial profiles can be considerably modified, depending on the degree of fastness. We apply these results to compute the torques expected during the outbursts of the transient millisecond pulsars and find that we can explain the large spin-down rates that are observed for quite plausible surface magnetic fields of ~109 G. Title: Excess mid-infrared emission in cataclysmic variables Authors: Dubus, G.; Campbell, R.; Kern, B.; Taam, R. E.; Spruit, H. C. Bibcode: 2004MNRAS.349..869D Altcode: 2003astro.ph.12408D We present a search for excess mid-infrared emission due to circumbinary (CB) material in the orbital plane of cataclysmic variables (CVs). Our motivation stems from the fact that the strong braking exerted by a CB disc on the binary system could explain several puzzles in our current understanding of CV evolution. Since theoretical estimates predict that the emission from a CB disc can dominate the spectral energy distribution (SED) of the system at λ > 5 μm, we obtained simultaneous visible to mid-infrared (mid-IR) SEDs for eight systems. We report detections of SS Cyg at 11.7 μm and AE Aqr at 17.6 μm, both in excess of the contribution from the secondary star. In AE Aqr, the IR likely originates from synchrotron-emitting clouds propelled by the white dwarf. In SS Cyg, we argue that the observed mid-IR variability is difficult to reconcile with simple models of CB discs and we consider free-free emission from a wind. In the other systems, our mid-IR upper limits place strong constraints on the maximum temperature of a putative CB disc. The results show that if any sizeable CB discs are present in these systems, they must be self-shadowed or perhaps dust-free, with the peak thermal emission shifted to far-IR wavelengths. Title: Search for fast correlated X-ray and optical variability in Cir X-1 and XTE J1746-321 Authors: Spruit, H. C.; Steinle, H.; Kanbach, G. Bibcode: 2004astro.ph..4524S Altcode: Coordinated observations X-ray+optical observations of two southern X-ray binaries, the black hole candidate XTE J1746-321 and the neutron star accreter Cir X-1 (a `microquasar') are reported. With a photon counting optical photometer on the 1.9m telescope at Sutherland, South Africa and the PCA detector on RXTE, 4h each of simultaneous data were obtained on XTE J1746 and Cir X-1. Cir X-1 showed no X-ray variability at the 2% level, XTE J1746 was variable at 5-7% with a 5Hz QPO. Cross-correlation yielded no correlated signals on either source, to a level of 1%. A problem with a recently published orbital ephemeris of Cir X-1 is pointed out. Title: Gamma-Ray Burst Central Engines Authors: Spruit, H. C. Bibcode: 2004BaltA..13..266S Altcode: 2004OAst...13..266S A brief overview of currently popular central engine models of GRBs is given, with particular emphasis on magnetic models for powering the outflow and the dissipation of energy into radiation. Title: Coordinated Millisecond X-RAY+OPTICAL too for Black Hole Transients Authors: Spruit, Hendrik Bibcode: 2004rxte.prop90019S Altcode: This is a target of opportunity proposal for simultaneous X-ray +optical observations of new soft X-ray transients or new outbursts from known transient sources. Strong X-ray/optical correlations at 0.03--5 s time scales were observed in XTE J1118+480. These turn out to be difficult to fit into a disk reprocessing model. Cyclo-synchrotron emission in the inner accretion regions and/or an outflow may be involved. The high quality of the data obtained on XTE J1118, combined with the puzzling properties of the correlation suggest that observations of this kind provide unique new diagnostics of the accretion proicess in black hole transients. This is a resubmission of a cycle 8 TOO. Title: Coordinated Millisecond RXTE+OPTICAL Observations of Persistent LMXB Authors: Spruit, Hendrik Bibcode: 2004rxte.prop90026S Altcode: In RXTE+optical observatipons of XTE J1118+480 we found the visible light to be strongly correlated with the X-rays on time scales of 0.03 - 5s. The short time scales, and a `premonition dip' in the visible preceding the X-rays, turn out to be difficult to understand in existing models. Simultaneous fast X-ray+optical obervations thus may give new clues on the poorly understood inner regions of the accretion flow in XRB, and their relation to outflows. We propose to observe for this purpose the brightest persistent low-mass black hole accreters, and two well studied neutron star accreters. (Cyg X-1, GX 339-4, Cyg X- 2 and the neutron star jet source Cir X-1). Title: The optical and X-ray flickering of XTE J1118+480 Authors: Malzac, J.; Belloni, T.; Spruit, H. C.; Kanbach, G. Bibcode: 2003A&A...407..335M Altcode: 2003astro.ph..6256M We use both time-domain and Fourier techniques to study the correlated optical/X-rays variability in the black hole X-ray nova XTE J1118+480. Its X-ray timing properties such as the shape of the X-ray power spectrum and cross-correlation functions (CCFs) between X-ray energy bands, the slight decrease of rms variability from 30% in the 2-5.9 keV to 19% in the 15.5-44.4 keV band, as well as the X-ray hardness/flux anti-correlation are very similar to what is found in other black hole binaries in the hard state. The optical/X-ray CCF is virtually independent of the X-ray energies. The optical flux appears to be correlated not only with the X-ray flux but also with the X-ray spectral variability. Both the coherence function and the lags between optical and the X-rays are Fourier frequency dependent. The optical/X-ray coherence function reaches its maximum ( ~ 0.3) in the 0.1-1 Hz range and the time-lags decrease with frequency approximatively like f-0.8. The correlation between X-ray and optical light curves appears to have time-scale-invariant properties. The X-ray/optical CCF maintains a similar but rescaled shape on time-scales ranging at least from 0.1 s to few 10 s. Using the event superposition method we show that the correlation is not triggered by a single type of event (dip or flare) in the light curves. Instead, optical and X-ray fluctuations of very different shapes, amplitudes and time-scales are correlated in a similar mode where the optical light curve is seemingly related to the time derivative of the X-rays. Title: X-ray/Optical Correlations in the Transient Black Hole System KV UMa (XTEJ1118+48) Authors: Kanbach, G.; Steinle, H.; Spruit, H. C.; Straubmeier, C.; Belloni, T. Bibcode: 2003ANS...324Q..23K Altcode: 2003ANS...324..E02K No abstract at ADS Title: Interpretations of Torsional Oscillations Authors: Spruit, H. Bibcode: 2003SPD....34.2608S Altcode: 2003BAAS...35..855S The processes proposed for the torsional oscillation pattern of solar rotation are reviewed. Helioseismology shows that its amplitude declines with depth from the surface, though significant amplitudes survive over most of the convection zone. Its relation with the solar cycle, with largest positive (negative) amplitudes at the leading (trailing) latitudes of the main activity belt, suggests a magnetic cause. The early start of the oscillation, a few years before the start of the new activity cycle is difficult to interpret in magnetic models and has led to suggestions that the primary cause is not magnetic but a hydrodynamic oscillation somehow synchronized with the cycle. `Direct' magnetic models using the Lorentz force of the cycle's magnetic field (likely located at the base of the convection zone) do not fit the depth dependence easily. The latitude dependence, interpreted as a flow around the active latitudes, suggests a strong influence of rotation.

An `indirect' magnetic model is proposed, in which the flow is driven by the thermal effects of the surface magnetic fields. The additional radiative loss in the small scale magnetic field (which exceeds the reduced radiation from spots) causes a small temperature decrease below active regions. The associated pressure effect causes a geostrophic flow with the correct amplitude, latitude dependence and depth dependence. The early start of the oscillation may indicate that the cycle starts with small scale mixed fields that do not show up on synoptic magnetograms.

It is pointed out that any long-lived temperature fluctuations near the surface must be associated with systematic flows. These flows are much easier to detect with current technology than the surface brightness variations due to the temperature effects themselves. Title: Origin of the torsional oscillation pattern of solar rotation Authors: Spruit, H. C. Bibcode: 2003SoPh..213....1S Altcode: 2002astro.ph..9146S A model is presented that explains the `torsional oscillation' pattern of deviations in the solar rotation rate as a geostrophic flow. The flow is driven by temperature variations near the surface due to the enhanced emission of radiation by the small-scale magnetic field. The model explains the sign of the flow, its amplitude and the fact that the maxima occur near the boundaries of the main activity belts. The amplitude of the flow decreases with depth from its maximum at the surface but penetrates over much of the depth of the convection zone, in agreement with the data from helioseismology. It predicts that the flow is axisymmetric only on average, and in reality consists of a superposition of circulations around areas of enhanced magnetic activity. It must be accompanied by a meridional flow component, which declines more rapidly with depth. Title: Design and results of the fast timing photo-polarimeter OPTIMA Authors: Kanbach, Gottfried; Kellner, Stefan; Schrey, Fritz Z.; Steinle, Helmut; Straubmeier, Christian; Spruit, Henk C. Bibcode: 2003SPIE.4841...82K Altcode: A high-speed photometer, "OPTIMA" short for Optical Pulsar Timing Analyzer, has been designed as a sensitive, portable detector to observe optical pulsars and other highly variable sources. The detector contains eight fiber fed avalanche photodiode single photon counters, a GPS timing receiver, a CCD camera for target acquisition and a computerized control unit. The central fibers are configured as a hexagonal bundle around the target fiber, while one fiber is located at a distance of ~1' as a monitor for the night sky background. Recently a rotating polarization filter and a 4-color prism spectrograph have been added to the system as optional equipment. Since January 1999 OPTIMA has been used on different telescopes to measure detailed lightcurves and polarization of the Crab Pulsar, in a search for optical emission from the Geminga pulsar, and for the timing of cataclysmic variables and X-ray transients. Title: Disks Surrounding Cataclysmic Binaries Authors: Spruit, Henk C.; Taam, Ronald E. Bibcode: 2003ASPC..308..323S Altcode: No abstract at ADS Title: Coordinated Millisecond X-RAY+OPTICAL too for Black Hole Transients Authors: Spruit, Hendrik Bibcode: 2003rxte.prop80144S Altcode: This is a target of opportunity proposal for simultaneous X-ray +optical observations of new soft X-ray transients. Strong X- ray/optical correlations at 0.03--5 s time scales were observed in XTE J1118+480. These turn out to be difficult to fit into a disk reprocessing model. Cyclo-synchrotron emission in the inner accretion regions and/or an outflow may be involved. The high quality of the data obtained on XTE J1118, combined with the puzzling properties of the correlation suggest that observations of this kind provide unique new diagnostics of the accretion proicess in black hole transients. This is a resubmission of a cycle 7 TOO (not triggered). Title: A search for excess mid-IR emission from cataclysmic variables Authors: Dubus, G.; Campbell, R.; Kern, B.; Taam, R.; Spruit, H. Bibcode: 2003sf2a.conf..589D Altcode: 2003sf2a.confE.244D Cataclysmic variables (CVs) regularly eject matter through winds and novae outbursts. A small fraction of this material may stay bound and accumulate around the binary in a disk. The strong braking exerted on the red dwarf by such a circumbinary (CB) disk would explain several puzzles in our current understanding of CV evolution. Thermal emission from CB disks could easily have gone unnoticed since the mid-IR emission of CVs remains largely unexplored. We present the results of a near- and mid-IR survey of 8 CVs obtained at the Keck I in Sep. 2002. Title: Orbital Bulk-flow Comptonization in Accretion Disks around Black Holes Authors: Kylafis, Nikolaos D.; Reig, Pablo; Spruit, Henk C. Bibcode: 2003ASPC..308..153K Altcode: No abstract at ADS Title: Coordinated Millisecond RXTE+OPTICAL Observations of Persistent LMXB Authors: Spruit, Hendrik Bibcode: 2003rxte.prop80114S Altcode: We propose to search for correlated X-ray/optical emission from black hole candidates and neutron star accreters. The unexpectedly fast and complex correlations between visible light and X-rays seen in our previous observations of XTE J1118+480 indicate that the visible light may be be an important source of information on the nature of the accretion flow in the inner regions of the accretion flow, and its relation with outfl;ows. Targets to be observed are the persistent black hole candidates GX 339-4 and Cyg X-1, and the neutron star accreters Cyg X-2 and the jet-source Cir X-1. Title: Efficient acceleration and radiation in Poynting flux powered GRB outflows Authors: Drenkhahn, G.; Spruit, H. C. Bibcode: 2002A&A...391.1141D Altcode: 2002astro.ph..2387D We investigate the effects of magnetic energy release by local magnetic dissipation processes in Poynting flux-powered GRBs. For typical GRB parameters (energy and baryon loading) the dissipation takes place mainly outside the photosphere, producing non-thermal radiation. This process converts the total burst energy into prompt radiation at an efficiency of 10-50%. At the same time the dissipation has the effect of accelerating the flow to a large Lorentz factor. For higher baryon loading, the dissipation takes place mostly inside the photosphere, the efficiency of conversion of magnetic energy into radiation is lower, and an X-ray flash results instead of a GRB. We demonstrate these effects with numerical one-dimensional steady relativistic MHD calculations. Title: Correlated X-ray and optical variability in KV UMa Authors: Spruit, H. C.; Kanbach, G. Bibcode: 2002A&A...391..225S Altcode: 2002astro.ph..5431S We present coordinated X-ray and optical observations of the X-ray transient KV UMa (=XTE J1118+480) at high time resolution. The optical variation associated with spikes in the X-rays consist of a dip preceding the spike by 0-5 s, and a peak following it. The cross-correlation function between X-rays and optical has the same shape. It rises sharply, within 30 ms of the peak of the X-rays. The shape of the cross correlation shows significant intrinsic variability on time scales as short as 30 s. Analyzing this variability with principal component analysis (calibrated with simulated data), we find two statistically independent components. The first is similar to the average cross-correlation. The second also consists of a dip followed by a peak, but on a 3x shorter time scale. These properties of the optical light, together with the high optical brightness of the transient, are not easily explained by reprocessing of the X-rays. Title: Hydrodynamical simulations of the stream-core interaction in the slow merger of massive stars Authors: Ivanova, N.; Podsiadlowski, Ph.; Spruit, H. Bibcode: 2002MNRAS.334..819I Altcode: 2001astro.ph..9524I We present detailed simulations of the interaction of a stream emanating from a mass-losing secondary with the core of a massive supergiant in the slow merger of two stars inside a common envelope. The dynamics of the stream can be divided into a ballistic phase, starting at the L1 point, and a hydrodynamical phase, where the stream interacts strongly with the core. Considering the merger of a 1- and 5-Msolar star with a 20-Msolar evolved supergiant, we present two-dimensional hydrodynamical simulations using the PROMETHEUS code to demonstrate how the penetration depth and post-impact conditions depend on the initial properties of the stream material (e.g. entropy, angular momentum, stream width) and the properties of the core (e.g. density structure and rotation rate). Using these results, we present a fitting formula for the entropy generated in the stream-core interaction and a recipe for the determination of the penetration depth based on a modified Bernoulli integral. Title: A New Class of High-Mass X-Ray Binaries: Implications for Core Collapse and Neutron Star Recoil Authors: Pfahl, Eric; Rappaport, Saul; Podsiadlowski, Philipp; Spruit, Hendrik Bibcode: 2002ApJ...574..364P Altcode: 2001astro.ph..9521P We investigate an interesting new class of high-mass X-ray binaries (HMXBs) with long orbital periods (Porb>30 days) and low eccentricities (e<~0.2). The orbital parameters suggest that the neutron stars in these systems did not receive a large impulse, or ``kick,'' at the time of formation. After considering the statistical significance of these new binaries, we develop a self-consistent phenomenological picture wherein the neutron stars born in the observed wide HMXBs receive only a small kick (<~50 km s-1), while neutron stars born in isolation, in the majority of low-mass X-ray binaries, and in many of the well-known HMXBs with Porb<~30 days receive the conventional large kicks, with a mean speed of ~300 km s-1. Assuming that this basic scenario is correct, we discuss a physical process that lends support to our hypothesis, whereby the magnitude of the natal kick to a neutron star born in a binary system depends on the rotation rate of its immediate progenitor following mass transfer-the core of the initially more massive star in the binary. Specifically, the model predicts that rapidly rotating precollapse cores produce neutron stars (NSs) with relatively small kicks, and vice versa for slowly rotating cores. If the envelope of the NS progenitor is removed before it has become deeply convective, then the exposed core is likely to be a rapid rotator. However, if the progenitor becomes highly evolved prior to mass transfer, then a strong magnetic torque, generated by differential rotation between the core and the convective envelope, may cause the core to spin down to the very slow rotation rate of the envelope. Our model has important implications for the dynamics of stellar core collapse, the retention of neutron stars in globular clusters, and the formation of double neutron star systems in the Galaxy. Title: X-ray spectra from accretion disks illuminated by protons Authors: Deufel, B.; Dullemond, C. P.; Spruit, H. C. Bibcode: 2002A&A...387..907D Altcode: 2001astro.ph..8496D The X-ray spectrum from a cool accretion disk heated by virialized protons is computed. The cool disk is either embedded in a magnetically heated accretion disk corona or partly extends into an ion supported torus (or ADAF). We calculate the stationary equilibrium between proton heating, electron thermal conduction and the radiative losses by bremsstrahlung and Compton scattering. A heated surface layer on top of the accretion disk is produced with temperatures between 60-90 keV above a cool layer with temperatures of 0.01 keV (AGN) and 1 keV (galactic black hole candidates). The spectra produced by the surface layer are reminiscent of hard state spectra, but a bit too steep, especially for AGN's. Near the inner edge of the disk, where the optical depth of the disk tau <~ 1, we find that the cool component of the disk disappears. Instead, the hot protons from the corona/ADAF heat the disk, on a dynamical time-scale, to temperatures of several 100 keV, limited by pair production. This region, here called a ``warm disk'', could contribute significantly to the hard X-ray spectra and could be important for feeding material into an ADAF. Title: The transition from a cool disk to an ion supported flow Authors: Spruit, H. C.; Deufel, B. Bibcode: 2002A&A...387..918S Altcode: 2001astro.ph..8497S We show that the inner regions of a cool accretion disk in an X-ray binary can transform into an advective, ion supported accretion flow (an optically thin ADAF, here called ISAF), through events involving only the known properties of the Coulomb interaction in a two-temperature plasma, standard radiation processes, and viscous heating. The optically thin inner edge of the disk is heated to a few 100 keV by the strong flux of hot ions from the surrounding hot ISAF. We show that he resident ions in this ``warm'' disk are thermally unstable due to internal viscous heating, and heat up to their virial temperature. The innermost disk regions thus evaporate and feed the ISAF. These processes are demonstrated with time dependent calculations of a two-temperature plasma in vertical hydrostatic equilibrium, including heating by external ions, internal proton-electron energy exchange, and viscous heating. The process complements the ``coronal'' evaporation mechanism which operates at larger distances from the central object. Title: Investigation of the Sources of Irradiance Variation on the Sun (ISIS) Authors: LaBonte, B. J.; Bernasconi, P. N.; Rust, D.; Foukal, P.; Hudson, H.; Spruit, H. Bibcode: 2002AAS...200.5608L Altcode: 2002BAAS...34..736L There is a persistent correlation of the longterm climate change and solar irradiance. ISIS is designed to understand the physical basis of this correlation. ISIS combines an innovative bolometric imager and a multiband CCD imager. The bolometric imager has uniform response from 200 nm to 3000 nm, spatial resolution < 5 arcseconds, and precision of < 0.1% in a one minute integration. The multiband imager records ultraviolet irradiance variation in the band from 200 to 350 nm, measures photospheric temperature structure, and provides chromospheric structure in Ca II K and H-alpha, with spatial resolution <1.0 arcsecond. Designed for flight on the Solar Dynamics Observatory, ISIS will provide the comprehensive photometric measurements needed to characterize the irradiance variation from identifiable structures and challenge theoretical models of convection and the solar dynamo. Title: The Structure and Evolution of Circumbinary Disks in Cataclysmic Variable Systems Authors: Dubus, Guillaume; Taam, Ronald E.; Spruit, H. C. Bibcode: 2002ApJ...569..395D Altcode: 2001astro.ph.12389D We investigate the structure and evolution of a geometrically thin, viscous, Keplerian circumbinary (CB) disk, using detailed models of their radiative/convective vertical structure. We use a simplified description for the evolution of the cataclysmic binary and focus on cases where the circumbinary disk causes accelerated mass transfer (>~10-8 Msolar yr-1). The inner edge of the disk is assumed to be determined by the tidal truncation radius, and the mass input rate into the disk is assumed to be a small fraction (10-5 to 0.01) of the mass transfer rate. Under the action of the viscous stresses in the disk the matter drifts outward with the optically thick region extending to several AU. The inner part of the disk is cool with maximum effective temperatures <~3000 K, while the outermost parts of the disk are <~30 K and optically thin. We calculate the effects of thermal instability on a sufficiently massive CB disk. It leads to outbursts reminiscent of those in thermally unstable accretion disks, with the instability remaining confined to the inner regions of the CB disk. However, for most of the evolutionary sequences, the surface densities required to trigger instability are not reached. The spectral energy distributions from circumbinary disks are calculated, and the prospects for the detection of such disks in the infrared and submillimeter wavelength regions are discussed. Title: Instability of an accretion disk with a magnetically driven wind Authors: Cao, X.; Spruit, H. C. Bibcode: 2002A&A...385..289C Altcode: 2001astro.ph..8484C We present a linear analysis of the stability of accretion disks in which angular momentum is removed by the magnetic torque exerted by a centrifugally driven wind. The effects of the dependence of the wind torque on field strength and inclination, the sub-Keplerian rotation due to magnetic forces, and the compression of the disk by the field are included. A WKB dispersion relation is derived for the stability problem. We find that the disk is always unstable if the wind torque is strong. The growth time scale of the instability can be as short as the orbital time scale. The instability is mainly the result of the sensitivity of the mass flux to changes in the inclination of the field at the disk surface. Magnetic diffusion in the disk stabilizes if the wind torque is small. Title: ORGANIZED DISCUSSION: What can the Sun teach us about black hole accretion disks? Authors: Spruit, Hendrik Bibcode: 2002bhty.confE..38S Altcode: No abstract at ADS Title: Do Sunspots Influence the Earth's Climate? Authors: Spruit, Hendrik Bibcode: 2002kbls.confE..28S Altcode: No abstract at ADS Title: Evaporation of the Inner Disk in Black Hole Candidates Authors: Spruit, Henk C.; Deufel, Bernhard Bibcode: 2002luml.conf..479S Altcode: One of the promising models for the inner accretion flow in black hole accreters is a combination of an ion supported flow (ADAF) inside, and red by, a cool standard disk extending to larger distances. This model is suggested by the observed correlations between the reflection components in the X-ray spectrum (Fe line and Compton bump) with the slope of the power law component in the same spectrum and with the frequency of quasiperiodic oscillations in the X-rays (Churazov et al. 2001, Zdziarski et al. 1999). Title: Coordinated Millisecond Rxte/optical too for Black Hole Transients Authors: Spruit, Hendrik Bibcode: 2002rxte.prop70129S Altcode: This a target of opportunity proposal for simultaneous X-ray+optical observations of new soft X-ray transients. Strong X-ray/optical correlations at 0.02--3 sec time scales were observed in last year's transient XTE J1118+480 (RXTE+OPTIMA July 2000). These turn out to be difficult to fit into a disk reprocessing model. Cyclo-synchrotron emission in the inner accretion regions and/or an outflow may be involved. The high quality of the data data obtained on XTE J1118, combined with the puzzling properties of the correlation suggest that observations of this kind provide unique new diagnostics of the accretion process in black hole transients. Title: X-ray/optical correlations in the transient black hole system KV UMa (XTE J1118+48). Authors: Kanbach, G.; Steinle, H.; Spruit, H. C.; Straubmeier, C.; Belloni, T. Bibcode: 2002AGAb...19Q..34K Altcode: No abstract at ADS Title: Coordinated Millisecond Rxte/optical Observations of XRB Authors: Spruit, Hendrik Bibcode: 2002rxte.prop70104S Altcode: We propose simultaneous X-ray+optical observations of Cyg X-1 and Cyg X-2 to follow up on the puzzling X-ray/optical correlations at 0.02--3 sec time scales observed in XTE J1118+480 (RXTE+OPTIMA July 2000). These turn out to be difficult to fit into a disk reprocessing model, thus reviving an older suggestion (made for GX 339-4) that black holes in their hard spectral states produce significant amounts of optical/UV radiation by cyclo-synchrotron emission in the inner accretion regions. If optical emission like that in XTE J1118 exists in Cyg X-1, it will be easily detectable through its correlation with the X-rays, in spite of the bright O-star secondary. The second target Cyg X-2 is proposed to see if/how optical emission from neutron star accreters differs. Title: Dynamo action by differential rotation in a stably stratified stellar interior Authors: Spruit, H. C. Bibcode: 2002A&A...381..923S Altcode: 2001astro.ph..8207S Magnetic fields can be created in stably stratified (non-convective) layers in a differentially rotating star. A magnetic instability in the toroidal field (wound up by differential rotation) replaces the role of convection in closing the field amplification loop. Tayler instability is likely to be the most relevant magnetic instability. A dynamo model is developed from these ingredients, and applied to the problem of angular momentum transport in stellar interiors. It produces a predominantly horizontal field. This dynamo process is found to be more effective in transporting angular momentum than the known hydrodynamic mechanisms. It might account for the observed pattern of rotation in the solar core. Title: A New Class of High-Mass X-ray Binaries: Implications for Core Collapse and Neutron-Star Recoil Authors: Pfahl, E.; Rappaport, S.; Podsiadlowski, Ph.; Spruit, H. Bibcode: 2001AAS...199.9508P Altcode: 2001BAAS...33.1446P We investigate the statistical significance and implications of a new class of high-mass X-ray binaries (HMXBs), with Porb > 30 days and e < 0.2. The orbital parameters suggest that the neutron stars in these systems did not receive a very large kick at the time of formation. We develop a scenario wherein the neutron stars born in the new class of HMXBs receive kick speeds of only < 50 km/s, while neutron stars born in isolation and in low-mass X-ray binaries receive the conventional large kicks of 200-300 km/s. This apparent dichotomy may be explained if the kick speed depends on the rotation rate of the pre-collapse core, such that rapidly rotating core yield small kicks, and vice versa for slowly rotating cores. Title: Disks surrounding Cataclysmic Binaries Authors: Spruit, H. C.; Taam, R. E. Bibcode: 2001astro.ph.11420S Altcode: Observations of Cataclysmic Variables show a number of phenomena that do not fit easily into the standard magnetic braking scenario. These include the large spread in mass transfer rates, the low surface temperatures of many of the companion stars, and evidence for material at low velocities. We propose that these anomalies have a common cause: the presence of a circumbinary (CB) disk. This may be a significant component of mass transferring binaries in general. Direct detection of such CB disks may be possible but difficult, because of their low luminosity Disks surrounding Cataclysmic Binaries and spectral energy distribution peaking in the mid-IR. Title: Correlated fast X-ray and optical variability in the black-hole candidate XTE J1118+480 Authors: Kanbach, G.; Straubmeier, C.; Spruit, H. C.; Belloni, T. Bibcode: 2001Natur.414..180K Altcode: Black holes become visible when they accrete gas, a common source of which is a close stellar companion. The standard theory for this process (invoking a `thin accretion disk') does not explain some spectacular phenomena associated with these systems, such as their X-ray variability and relativistic outflows, indicating some lack of understanding of the actual physical conditions. Simultaneous observations at multiple wavelengths can provide strong constraints on these conditions. Here we report simultaneous high-time-resolution X-ray and optical observations of the transient source XTE J1118+480, which show a strong but puzzling correlation between the emissions. The optical emission rises suddenly following an increase in the X-ray output, but with a dip 2-5 seconds in advance of the X-rays. This result is not easy to understand within the simplest model of the optical emission, where the light comes from reprocessed X-rays. It is probably more consistent with an earlier suggestion that the optical light is cyclosynchrotron emission that originates in a region about 20,000km from the black hole. We propose that the time dependence is evidence for a relatively slow (<0.1c), magnetically controlled outflow. Title: The Evolution of Cataclysmic Variable Binary Systems with Circumbinary Disks Authors: Taam, Ronald E.; Spruit, H. C. Bibcode: 2001ApJ...561..329T Altcode: The effect of a circumbinary (CB) disk on the evolution of a binary consisting of a low-mass secondary star with a white dwarf primary is investigated, taking into account the viscous spreading of matter within the circumbinary disk and the response of the secondary to mass loss. The CB disk is assumed to be fed a constant fraction δ of the mass transfer from the secondary to the primary through a wind such as those observed in nova-like systems. The CB disk is effective in draining orbital angular momentum from the system provided that δ exceeds about 0.01. In this case, the mass transfer rates are elevated and the evolution accelerated in comparison with an evolution with the standard angular momentum loss processes of magnetic braking and gravitational radiation. The mass transfer rates for a given system can vary by more than an order of magnitude during its evolution. With a CB disk, a binary can thus evolve between the various subclasses of cataclysmic variables. A large spread in transfer rates at a given orbital period results, reflecting a range of ages of the systems, the possible presence of a CB disk already at the beginning of the evolution of the binary, and the mass and evolutionary state of the donor. For high mass input rates into the circumbinary disk (δ>~0.015), the secondary can be completely dissolved in less than a Hubble time. Title: X-ray spectra from protons illuminating a neutron star Authors: Deufel, B.; Dullemond, C. P.; Spruit, H. C. Bibcode: 2001A&A...377..955D Altcode: 2001astro.ph..8438D We consider the interaction of a slowly rotating unmagnetized neutron star with a hot (ion supported, ADAF) accretion flow. The virialized protons of the ADAF penetrate into the neutron star atmosphere, heating a surface layer. Detailed calculations are presented of the equilibrium between heating by the protons, electron thermal conduction, bremsstrahlung and multiple Compton scattering in this layer. Its temperature is of the order 40-70 keV. Its optical depth increases with the incident proton energy flux, and is of the order unity for accretion at 10-2-10-1 of the Eddington rate. At these rates, the X-ray spectrum produced by the layer has a hard tail extending to 100 keV, and is similar to the observed spectra of accreting neutron stars in their hard states. The steep gradient at the base of the heated layer gives rise to an excess of photons at the soft end of the spectrum (compared to a blackbody) through an ``inverse photosphere effect''. The differences with respect to previous studies of similar problems are discussed, they are due mostly to a more accurate treatment of the proton penetration process and the vertical structure of the heated layer. Title: Orbital Comptonization in accretion disks around black holes Authors: Reig, P.; Kylafis, N. D.; Spruit, H. Bibcode: 2001hell.confE.114R Altcode: We have performed Monte Carlo simulations of Compton upscattering of low-energy photons in an accretion disk around a Schwarzschild black hole. The photons gain energy from the rotational motion of the electrons in the disk. The upscattering occurs near the black hole horizon, where the flow velocity of the electrons approaches the speed of light. We show that this type of bulk-flow Comptonization can produce power-law X-ray spectra similar to the ones observed in black-hole X-ray transients in the high/soft state, i.e., a soft bump dominating the spectrum below ∼ 10 keV and a power-law tail with photon index in the range 2--3. In order to reproduce the observed hard to soft flux ratio the disk has to have vertical optical depth above 3 at the last stable orbit.We conclude that the power-law component of the high/soft state of black-hole transients may be due to an intrinsically cool disk extending all the way to the hole, without a separate hot plasma component. Title: Comptonization in rotating flows Authors: Reig, Pablo; Kylafis, N. D.; Spruit, H. C. Bibcode: 2001ESASP.459..255R Altcode: 2001egru.conf..255R Although the thermal Comptonization theory has been quite successful in describing the high energy spectra of black hole candidates there are still a number of unresolved issues. Due to the turbulent nature of the accretion disk and high radiation pressure in the vicinity of the black hole, it seems "natural" for the disk to possess a corona. However, neither the geometry nor the heating mechanism of such a corona are known. In bulk Comptonization models a purely radial flow of plasma has been considered, making the assumption that the radiation force on the accreting matter is negligible. On the other hand, a general agreement on the presence of an accretion disk exists. In this work we show that Comptonization in a rotating flow can also give rise to power-law spectra. Soft photons get upscattered by collisions with fast-moving electrons, which follow Keplerian trajectories in the accretion disk. Title: Orbital Comptonization in accretion disks around black holes Authors: Reig, P.; Kylafis, N. D.; Spruit, H. C. Bibcode: 2001A&A...375..155R Altcode: 2001astro.ph..4389R We have performed Monte Carlo simulations of Compton upscattering of low-energy photons in an accretion disk around a Schwarzschild black hole. The photons gain energy from the rotational motion of the electrons in the disk. The upscattering occurs near the black hole horizon, where the flow velocity of the electrons approaches the speed of light. We show that this type of bulk-flow Comptonization can produce power-law X-ray spectra similar to the ones observed in black-hole X-ray transients in the high/soft state, i.e., a soft bump dominating the spectrum below ~ 10 keV and a power-law tail with photon index in the range 2-3. In order to reproduce the observed hard to soft flux ratio the disk has to have vertical optical depth above ~ 3 at the last stable orbit. We conclude that the power-law component of the high/soft state of black-hole transients may be due to an intrinsically cool disk extending all the way to the hole, without a separate hot plasma component. Title: Correlated msec X-ray and optical emission from XTE J1118+480 Authors: Kanbach, G.; Straubmeier, C.; Spruit, H. C.; Belloni, T. Bibcode: 2001astro.ph..8199K Altcode: We report on fast (msec) simultaneous observations in X-rays and optical light of the black hole candidate XTE J1118+480, showing correlation on time scales from 20msec to several seconds. The shape of the correlation is not easily explained with existing models. We suggest that is is caused by optical cyclosynchrotron emission in a dense, relatively slow outflow from the inner regions of the accretion. Title: Stability of accretion discs threaded by a strong magnetic field Authors: Stehle, R.; Spruit, H. C. Bibcode: 2001MNRAS.323..587S Altcode: 2001astro.ph..3409S We study the stability of poloidal magnetic fields anchored in a thin accretion disc. The two-dimensional hydrodynamics in the disc plane is followed by a grid-based numerical simulation including the vertically integrated magnetic forces. The three-dimensional magnetic field outside the disc is calculated in a potential field approximation from the magnetic flux density distribution in the disc. For uniformly rotating discs we confirm numerically the existence of the interchange instability as predicted by Spruit, Stehle & Papaloizou. In agreement with predictions from the shearing sheet model, discs with Keplerian rotation are found to be stabilized by the shear, as long as the contribution of magnetic forces to support against gravity is small. When this support becomes significant, we find a global instability which transports angular momentum outwardly and allows mass to accrete inwardly. The instability takes the form of a m=1 rotating `crescent', reminiscent of the purely hydrodynamic non-linear instability previously found in pressure-supported discs. A model where the initial surface mass density Σ(r) and Bz(r) decrease with radius as power laws shows transient mass accretion during about six orbital periods, and settles into a state with surface density and field strength decreasing approximately exponentially with radius. We argue that this instability is likely to be the main angular momentum transport mechanism in discs with a poloidal magnetic field sufficiently strong to suppress magnetic turbulence. It may be especially relevant in jet-producing discs. Title: Structure and Evolution of Circumbinary Disks in Cataclysmic Variable Systems Authors: Taam, R. E.; Dubus, G.; Spruit, H. C. Bibcode: 2001AAS...198.1101T Altcode: 2001BAAS...33..802T Circumbinary disks have been proposed as an additional means by which angular momentum can be lost from a cataclysmic variable binary system. Provided that the mass input rate is sufficiently high, the evolution of the system can be significantly accelerated. The structure and evolution of such disks is investigated based on radiative/convective vertical structure models combined with time dependent radial diffusion models. The results of the numerical calculations will be presented and the possible observational consequences of such disks will be discussed. This research is supported by NSF under grant AST-9727875, by NASA under grants NAG5-7007 and NAG5-70034, and by the European Commission under grant TMR grant ERBFMRX-CT98-0195. Title: Overview of Solar Luminosity Variation Mechanisms Authors: Spruit, H. C. Bibcode: 2001AGUSM..SP31B01S Altcode: The theory of stellar structure can be used to put constraints on proposed mechanisms for the irradiance variations associated with the solar cycle. Changes in the emissivity of the solar surface turn out to be particularly effective. Such changes are produced by sunspots and the small scale magnetic field, either directly (spots being dark, for example) or indirectly through their effect on the convective flow in their immediate vicinity. They can account for most of the observed variation. Sources of variation deeper below the solar surface either have much smaller effects, or require ad-hoc new physics. Helioseismology may soon be able to distinguish between surface emissivity changes and interior sources, since their predicted effects on p-mode frequencies and travel times tend to have opposite sign. Title: Large scale magnetic fields and their dissipation in GRB fireballs} Authors: Spruit, H. C.; Daigne, F.; Drenkhahn, G. Bibcode: 2001A&A...369..694S Altcode: 2000astro.ph..4274S We consider possible geometries of magnetic fields in GRB outflows, and their evolution with distance from the source. For magnetically driven outflows, with an assumed ratio of magnetic to kinetic energy density of order unity, the field strengths are sufficient for efficient production of gamma -rays by synchrotron emission in the standard internal shock scenario, without the need for local generation of small scale fields. In these conditions, the MHD approximation is valid to large distances (ga 1019 cm). In outflows driven by nonaxisymmetric magnetic fields, changes of direction of the field cause dissipation of magnetic energy by reconnection. Much of this dissipation takes place outside the photosphere of the outflow, and can convert a significant fraction of the magnetic energy flux into radiation. Title: Circumbinary Disks and Cataclysmic Variable Evolution Authors: Spruit, H. C.; Taam, Ronald E. Bibcode: 2001ApJ...548..900S Altcode: 2000astro.ph.10194S The influence of a circumbinary (CB) disk on the evolution of cataclysmic variable (CV) binary systems is investigated. We show that CB mass surface densities sufficient to influence the evolution rate are plausibly provided by the outflows observed in CVs, if the net effect of these winds is to deliver 10-4 to 10-3 of the mass transfer rate to the CB disk. The torque exerted by the CB disk provides a positive feedback between mass transfer rate and CB disk mass that can lead to mass transfer rates of ~10-8 to 10-7 Msolar yr-1. This mechanism may be responsible for causing the range of variation of mass transfer rates in CVs. In particular, it may explain rates inferred for the nova-like variables and the supersoft X-ray binary systems observed near the upper edge of the period gap (P~3-4 hr), as well as the spread in mass transfer rates above and below the period gap. Consequences and the possible observability of such disks are discussed. Title: Common-Envelope Evolution: the Nucleosynthesis in Mergers of Massive Stars Authors: Ivanova, Natalia; Podsiadlowski, Philipp; Spruit, Henk Bibcode: 2001ASPC..229..261I Altcode: 2001ebms.conf..261I; 2001astro.ph..2141I We study the merging of massive stars inside a common envelope for binary systems consisting of a red supergiant with a mass of 15-20 Msun and a main-sequence companion of 1-5 Msun. We are particularly interested in the stage when the secondary, having overfilled its Roche lobe inside the common envelope, starts to transfer mass to the core of the primary at a very high mass-transfer rate and the subsequent nucleo-synthesis in the core-impact region. Using a parametrized model for the structure of the envelope at this stage, we perform 2-dimensional hydrodynamical calculations with the Munich Prometheus code to calculate the dynamics of the stream emanating from the secondary and its impact on the core of the primary. We find that, for the lower end of the estimated mass-transfer rate, low-entropy, hydrogen-rich material can penetrate deep into the primary core where nucleosynthesis through the hot CNO cycle can take place and that the associated neutron exposure may be sufficiently high for significant s-processing. For mass-transfer rates at the high end of our estimated range and higher densities in the stream, the stream impact can lead to the dredge-up of helium, but the neutron production is too low for significant s-processing. Title: Accretion Discs Authors: Spruit, H. C. Bibcode: 2001nsbh.conf..111S Altcode: No abstract at ADS Title: Origin of the Rotation Rates of Single White Dwarfs and Neutron Stars Authors: Spruit, Henk C. Bibcode: 2001ASPC..229...43S Altcode: 2001ebms.conf...43S No abstract at ADS Title: Gamma-ray Bursts from Neutron Stars Spun up in X-ray Binaries Authors: Spruit, H. C. Bibcode: 2001nsbh.conf..467S Altcode: No abstract at ADS Title: Radiatively Inefficient Accretion Discs Authors: Spruit, H. C. Bibcode: 2001nsbh.conf..141S Altcode: No abstract at ADS Title: Circumbinary Disks and Cataclysmic Variable Evolution Authors: Taam, R. E.; Spruit, H. C. Bibcode: 2000AAS...19712601T Altcode: 2000BAAS...32.1612T The influence of a circumbinary (CB) disk on the evolution of cataclysmic variable (CV) binary systems is investigated. The torque exerted by the CB disk can provide a positive feedback on the mass transfer rate between the stellar components of the system and the CB disk. Such a mechanism may be responsible for producing a variation of mass transfer rates in CV's. The results of evolutionary calculations will be presented and the consequences of such disks on CV evolution will be discussed. This research is supported by NSF under grant AST-9727875 and by the European Commission under grant TMR grant ERBFMRX-CT98-0195. Title: Theory of solar irradiance variations Authors: Spruit, H. Bibcode: 2000SSRv...94..113S Altcode: The theory of stellar structure can be used to identify the most plausible mechanisms for the irradiance variations associated with the solar cycle. Changes in surface emissivity, i.e. the reduced cooling in spots and enhanced emission by small scale magnetic fields, are the most effective mechanisms and account for most of the observed variation. New views of stellar surface convection developed from realistic numerical simulations changes the physical description of thermal perturbations of the solar envelope. Helioseismology will soon be able to test the consequences of changes in surface emissivity, and distinguish them from other scenarios for irradiance variability. Title: Accretion Disks Authors: Spruit, H. Bibcode: 2000eaa..bookE2124S Altcode: 2000astro.ph..3144S By accretion one understands in astrophysics the accumulation of matter onto a heavy object, under the influence of its pull of gravity. The object can be, for example, a star, a black hole or a neutron star. Most of the objects of these types are quite isolated, without much gas around that could be pulled in by their gravity. Accordingly, the vast majority of stars do not show any evidence of... Title: Solar Photosphere Authors: Spruit, H. Bibcode: 2000eaa..bookE2015S Altcode: The photosphere (Gr φως, light) of the Sun are those layers in its atmosphere that one observes in white light (broad band visible light). An image in such light (figure 1) shows a sharp edge, as if the light were emitted from a single layer, the `visible surface' of the Sun. Since stars are gaseous, they do not have sharp surfaces, however. Moreover, the radiation at different wavelengths is gen... Title: Comptonization in an accretion disk illuminated by protons Authors: Deufel, B.; Spruit, H. C. Bibcode: 2000A&A...362....1D Altcode: We compute the X-ray spectrum from a cool, optically thick, geometrically thin accretion disk embedded in a hot ion supported torus (or ADAF). Most of the gravitational power is assumed to be dissipated in the torus, where the protons are near their local virial temperature. The protons are slowed down in the accretion disk via Coulomb interactions, producing a hot surface layer with an optical thickness tau_T ~1-2. The heating of this layer is balanced by cooling through Comptonization of soft photons. The soft photons are produced by the reprocessing (treated in an approximate way) of hard photons penetrating into the cool disk. Solving the energy balance problem of the heated layer together with the Comptonization by a Monte-Carlo simulation, we find X-ray spectra reminiscent of observed spectra of AGN and black hole candidates in their hard states. Title: Disks around Mass Transferring Binaries Authors: Spruit, Henk Bibcode: 2000smyn.progE..44S Altcode: No abstract at ADS Title: Gamma Ray Bursts from Rapidly Rotating Neutron Stars Authors: Spruit, Hendrik Bibcode: 2000smyn.progE..43S Altcode: No abstract at ADS Title: The sign of temperature inhomogeneities deduced from time-distance helioseismology Authors: Brüggen, M.; Spruit, H. C. Bibcode: 2000SoPh..196...29B Altcode: 2000astro.ph..4140B Inhomogeneities in wave propagation conditions near and below the solar surface have been detected by means of time-distance helioseismology. Here we calculate the effect of temperature inhomogeneities on the travel times of sound waves. A temperature increase, e.g., in active regions, not only increases the sound speed but also lengthens the path along which the wave travels because the expansion of the heated layers shifts the upper turning of the waves upward. Using a ray-tracing approximation we find that in many cases the net effect of a temperature enhancement is an increase of the travel times. We argue that the reduced travel times that are observed are caused by a combination of magnetic fields in the active region and reduced subsurface temperatures. Such a reduction may be related to the increased radiative energy loss from small magnetic flux tubes. Title: Gamma-ray bursts from rapidly spinning neutron stars Authors: Spruit, H. C. Bibcode: 2000AIPC..526..589S Altcode: 2000hgrb.symp..589S A neutron star in an X-ray binary (XRB) is spun up by mass transfer from its companion to rotation periods of the order of a millisecond. Evidence for this are the existence of ms pulsars, believed to descend from X-ray binaries. Direct evidence of such rotation rates in XRB is the of 2.5 ms rotation in SAX 1808. The rotation energy in a neutron star spinning at 1 ms is 1052 erg, in the range of that required for GRB. The environment also satisfies the baryon loading constraint, since the accretion disk contains only of the order 10-9 Msolar, and the companion star blocks only of the order 1% of the sky as seen from the neutron star. . Title: The magnetosphere of an oscillating neutron star. Non-vacuum treatment Authors: Timokhin, A. N.; Bisnovatyi-Kogan, G. S.; Spruit, H. C. Bibcode: 2000MNRAS.316..734T Altcode: 1999astro.ph..9354T We generalize a formula for the Goldreich-Julian charge density (ρGJ), originally derived for a rotating neutron star, for arbitrary oscillations of a neutron star with an arbitrary magnetic field configuration under the assumption of low current density in the inner parts of the magnetosphere. As an application, we consider the toroidal oscillation of a neutron star with a dipole magnetic field and calculate the energy losses. For some oscillation modes, the longitudinal electric field cannot be cancelled by putting charged particles in the magnetosphere without the presence of a strong electric current (j~=ρGJc×cωr). It is shown that the energy losses are strongly affected by plasma in the magnetosphere, and cannot be described by vacuum formulae. Title: The X-ray spectrum of a disc illuminated by ions Authors: Spruit, H. C.; Haardt, F. Bibcode: 2000MNRAS.315..751S Altcode: 2000astro.ph..1458S The X-ray spectrum from a cool disc embedded in an ion-supported torus is computed. The interaction of the hot ions with the disc increases the hard X-ray luminosity of the system. A surface layer of the disc is heated by the protons from the torus. The Comptonized spectrum produced by this layer has a shape that depends only weakly on the incident energy flux and the distance from the accreting compact object. It consists of a `blue bump' of un-Comptonized soft photons and a flat high-energy tail, reminiscent of the observed spectra. The hard tail becomes flatter as the thermalization depth in the cool disc is increased. Further evidence for ion illumination is the Li abundance in the secondaries of low-mass X-ray binaries and the 450-keV lines sometimes seen in black hole transient spectra. Title: Jets from Compact Objects Authors: Spruit, H. C. Bibcode: 2000IAUS..195..113S Altcode: 2000astro.ph..3043S Some topics in the theory of jets are reviewed. These include jet precession, unconfined jets, the origin of knots, the internal shock model as a unifying theme from protostellar jets to gamma-ray bursts, relations between the Blandford-Znajek and MHD disk-wind models, and jet collimation in magnetic acceleration models. Title: Spirals and the size of the disk in EX Dra Authors: Joergens, V.; Spruit, H. C.; Rutten, R. G. M. Bibcode: 2000A&A...356L..33J Altcode: 2000astro.ph..2302J Observations at high spectral and temporal resolution are presented of the dwarf nova EX Dra in outburst. The disk seen in the He I line reconstructed by Doppler tomography shows a clear two-armed spiral pattern pointing to spiral shocks in the disk. The Balmer and He Ii maps also give evidence for the presence of spirals. The eclipse as seen in the red continuum indicates a disk radius of 0.31 times the orbital separation, which might be large enough to explain the observed spiral shocks through excitation by the tidal field of the secondary. The eclipse in the Balmer line profiles, well resolved in our observations, indicates a somewhat smaller disk size (0.25). We discuss the possibility that this is related to an optical depth effect in the lines. Title: Internal shock model for microquasars Authors: Kaiser, C. R.; Sunyaev, R.; Spruit, H. C. Bibcode: 2000A&A...356..975K Altcode: 2000astro.ph..1501K We present a model for the radio outbursts of microquasars based on the assumption of quasi-continuous jet ejection. The jets are `lit up' by shock fronts traveling along the jets during outbursts. The shocks accelerate relativistic particles which emit the observed synchrotron radiation. The observed comparatively flat decay light curves combined with gradually steepening spectral slopes are explained by a superposition of the radiation of the aging relativistic particle population left behind by the shocks. This scenario is the low energy, time-resolved equivalent to the internal shock model for GRBs. We show that this model predicts energy contents of the radiating plasma similar to the plasmon model. At the same time, the jet model relaxes the severe requirements on the central source in terms of the rate at which this energy must be supplied to the jet. Observations of `mini-bursts' with flat spectral slopes and of infrared emission far from the source centre suggest two different states of jet ejections: (i) A `mini-burst' mode with relatively stable jet production and weak radio emission with flat spectra and (ii) an outburst mode with strong variations in the jet bulk velocities coupled with strong radio emission with steeper spectra. We also show that the continuous jets in microquasars should terminate in strong shocks and possibly inflate radio lobes similar to extragalactic jet sources. We investigate the possibility of testing the predictions of this model with resolved radio observations. Finally, we suggest that Doppler-shifted X-ray iron lines, and possibly H-alpha lines, may be emitted by the jet flow of microquasars if thermal instabilities analogous to those in SS433 exist in their jets. Title: Radiatively inefficient accretion disks Authors: Spruit, H. C. Bibcode: 2000astro.ph..3143S Altcode: Radiatively inefficient (or advection dominated) disks are discussed at an introductory level. Ion supported and radiation supported flows are discussed, the different consequences of advection dominated flows onto black holes vs. solid surfaces (neutron stars, white dwarfs), hydrodynamics, the role of the ratio of specific heats, and the possible connection between ADAFs and outflows. Title: Theory of solar luminosity variations Authors: Spruit, H. C. Bibcode: 2000ASIC..558..289S Altcode: 2000astro.ph..3044S; 2000asre.conf..289S The theory of stellar structure can be used to identify the most plausible mechanisms for the irradiance variations associated with the solar cycle. Changes in surface emissivity, i.e. the reduced cooling in spots and enhanced emission by small scale magnetic fields, are the most effective mechanisms and account for most of the observed variation. Helioseismology will soon be able to test the consequences of changes in surface emissivity, and distinguish them from other scenarios for irradiance variability. Title: Max-Planck-Institut für Astrophysik. Annual report 1999. Authors: Spruit, H.; Hillebrandt, W.; White, S. D. M. Bibcode: 2000mfaa.book.....S Altcode: No abstract at ADS Title: Magnetosphere of oscillating neutron star Authors: Timokhin, A. N.; Bisnovatyi-Kogan, G. S.; Spruit, H. C. Bibcode: 2000NuPhS..80C1117T Altcode: No abstract at ADS Title: Theory of Solar Irradiance Variations Authors: Spruit, H. Bibcode: 2000svc..book..113S Altcode: No abstract at ADS Title: Differential rotation and magnetic fields in stellar interiors Authors: Spruit, H. C. Bibcode: 1999A&A...349..189S Altcode: 1999astro.ph..7138S The processes contributing to the evolution of an initially weak magnetic field in a differentially rotating star are reviewed. These include rotational smoothing (akin to convective expulsion) and a list of about 5 instabilities, among them magnetorotational instability, buoyancy instability, and pinch-type instabilities. The important effects of thermal and magnetic diffusion on these instabilities are analyzed in some detail. The first instability to set in is a pinch-type instability. It becomes important in modifying the field configuration before magnetic buoyancy-driven instabilities set in. The evolution of an initially strong field remains a more open question, including the old problem whether dynamically stable magnetic equilibria exist in stars. Title: Hydrodynamics of accretion discs of variable thickness Authors: Stehle, R.; Spruit, H. C. Bibcode: 1999MNRAS.304..674S Altcode: We present a new model for the hydrodynamics of thin accretion discs. In the (r, phi ) plane full hydrodynamics is taken into account, while the time evolution of the disc thickness H is followed by assuming uniform expansion and contraction in the vertical direction. This amounts to a `one-zone' approximation for the vertical equation of motion. We show that the model correctly incorporates the fundamental mode of vertical oscillation of a disc for waves long compared with the disc thickness. A numerical scheme to solve these equations on an Eulerian grid is introduced and tested for validity. As an application we compute the excitation of disc thickness oscillations by the tidal field of a companion. Strong excitation occurs at the 2{:}1 resonance, at r ~= 0.32 a, where a is the binary separation. Title: Gamma-ray bursts from X-ray binaries Authors: Spruit, H. C. Bibcode: 1999A&A...341L...1S Altcode: 1998astro.ph.11007S A weakly magnetized ( ~ 10(7) G) neutron star, slowly spun up by accretion in an X-ray binary, crosses the instability boundary for r-mode instability at P=1-2 msec. The amplitude of the oscillation, which initially increases only at the spinup time scale, is secularly unstable due to the negative temperature dependence of the viscosity in neutron star matter, and diverges after a few hundred years. Angular momentum loss by the gravitational wave causes strong differential rotation, in which the magnetic field is wound up to 10(17) G on a time scale of a few months. When this field becomes unstable to buoyancy instability, a surface field strength of a few 10(16) G is produced on a time scale of seconds, which then powers a GRB with energies of 10(51) -10(52) and duration of 1-100 sec. Title: The stream impact region in the disc of WZ SGE Authors: Spruit, H. C.; Rutten, R. G. M. Bibcode: 1998MNRAS.299..768S Altcode: We report the observation of new features in the spectrum of the cataclysmic variable WZ Sge. The disc eclipse is seen as a well-defined structure in the Hα line. From phases 0.25 to 0.5 an absorption feature of the same shape as the emission S wave is seen in this line, but redshifted by ~200 kms^-1. Two possible interpretations of this feature are given, both of which imply that it originates at the impact point of the stream on the disc edge. In addition, evidence is found for substructure in the velocity map of the spot. Emission from line-emitting post-shock material, extending to about 60 deg downstream from the continuum hotspot, is seen in the Doppler map in the form of a tail extending from the hotspot. A theoretical estimate shows that such a tail is to be expected as a consequence of the post-impact hydrodynamics of the stream. A new determination of the system parameters is made. They agree with those of Gilliland et al.; in particular, the data support a high primary mass. The variation of Hα surface brightness with distance from the primary is flatter, in the inner regions, than the r^-1.5 dependence found by Horne for other cataclysmic variables in quiescence. Title: Fast maximum entropy Doppler mapping Authors: Spruit, H. C. Bibcode: 1998astro.ph..6141S Altcode: A numerical code is described for constructing Doppler maps from the orbital variation of line profiles of (mass transfering) binaries. It uses an algorithm related to Richardson-Lucy iteration, and is much faster than the standard algorithm used for ME problems. The method has been tested on data of cataclysmic variables (including WZ Sge and SS Cyg), producing maps of up to 300X300 points. It includes an IDL-based set of routines for manipulating and plotting the input and output data, and can be downloaded from http://www.mpa-garching.mpg.de/~henk Title: Bolometric Light Curves of Supernovae and Postexplosion Magnetic Fields Authors: Ruiz-Lapuente, P.; Spruit, H. C. Bibcode: 1998ApJ...500..360R Altcode: 1997astro.ph.11248R The various effects leading to diversity in the bolometric light curves of supernovae are examined; nucleosynthesis, kinematic differences, ejected mass, degree of mixing, and configuration and intensity of the magnetic field are discussed. In Type Ia supernovae, a departure in the bolometric light curve from the full-trapping decline of 56Co can occur within the 2.5 years after the explosion, depending on the evolutionary path followed by the white dwarf (WD) during the accretion phase. If convection has developed in the WD core during the presupernova evolution, starting several thousand years before the explosion, a tangled magnetic field close to the equipartition value should have grown in the WD. Such an intense magnetic field would confine positrons where they originate from the 56Co decays and preclude a strong departure from the full-trapping decline as the supernova expands. This situation is expected to occur in C + O Chandrasekhar WDs as opposed to edge-lit detonated sub-Chandrasekhar WDs. If the preexplosion magnetic field of the WD is less intense than 105-108 G, a lack of confinement of the positrons emitted in the 56Co decay and a departure from full trapping of their energy would occur. The time at which the departure takes place can provide estimates of the original magnetic field of the WD, its configuration, and also of the mass of the supernova ejecta. In SN 1991BG, the bolometric light curve suggests an absence of a significant tangled magnetic field: its intensity is estimated to be lower than 103 G. Chandrasekhar-mass models do not reproduce the bolometric light curve of this supernova. For SN 1972E, on the contrary, there is evidence for a tangled configuration of the magnetic field and its light curve is well reproduced by a Chandrasekhar WD explosion. A comparison is made for the diagram of absolute magnitude and rate of decline in Type Ia supernovae coming from different explosion mechanisms. The effects of mixing and ejected mass in the bolometric light curve of Type Ib, Ic supernovae are also discussed. Title: Birth kicks as the origin of pulsar rotation Authors: Spruit, H.; Phinney, E. S. Bibcode: 1998Natur.393..139S Altcode: 1998astro.ph..3201S Radio pulsars are thought to born with spin periods of 0.02-0.5s and space velocities of 100-1,000kms-1, and they are inferred to have initial dipole magnetic fields of 1011-1013G (refs 1-5). The average space velocity of their progenitor stars is less than 15kms-1, which means that pulsars must receive a substantial `kick' at birth. Here we propose that the birth characteristics of pulsars have a simple physical connection with each other. Magnetic fields maintained by differential rotation between the core and envelope of the progenitor would keep the whole star in a state of approximately uniform rotation until10 years before the explosion. Such a slowly rotating core has 1,000 times less angular momentum than required to explain the rotation of pulsars. The specific physical process that `kicks' the neutron star at birth has not been identified, but unless its force is exerted exactly head-on it will also cause the neutron star to rotate. We identify this process as the origin of the spin of pulsars. Such kicks may cause a correlation between the velocity and spin vectors of pulsars. We predict that many neutron stars are born with periods longer than 2s, and never become radio pulsars. Title: Origin of the rotation rates of single white dwarfs Authors: Spruit, H. C. Bibcode: 1998A&A...333..603S Altcode: 1998astro.ph..2141S I argue that the rotation of white dwarfs is not a remnant of the angular momentum of their main sequence progenitors but a result of the mass loss process on the AGB. Weak magnetic fields, if present in stellar interiors, are likely to maintain approximately uniform rotation in stars, both on the main sequence and on the giant branches. The nearly uniform rotation of the core of the Sun is evidence for the existence of such fields. Exactly axisymmetric mass loss on the AGB from uniformly rotating stars would lead lead to white dwarfs with very long rotation periods (> 10 yr). Small random non-axisymmetries ( ~ 10(-3) ) in the mass loss process, on the other hand, add sufficient angular momentum to explain the observed rotation periods around one day. The process illustrated with a computation of the probability distribution of the rotation periods under the combined influence of random forcing by weak nonaxisymmetries and angular momentum loss in the AGB superwind. Such asymmetries can in principle be observed by proper motion studies of the clumps in interferometric images of SiO maser emission. Title: Origin of the rotation rates of single white dwarfs and neutron stars Authors: Spruit, H. C. Bibcode: 1998ASPC..138..335S Altcode: 1998stas.conf..335S No abstract at ADS Title: Solar Irradiance Variations: Theory Authors: Spruit, H. C. Bibcode: 1998IAUS..185..103S Altcode: The theory of stellar structure can be used to make definite predictions about the effect disturbances in the solar interior or atmosphere on the observed irradiance. For stars like the sun, the extended convective envelope plays an important role as a heat storage reservoir. The irradiance is easily modulated on observable time scales by changes in heat transport efficiency close to the surface, such as those due to spots and the small scale magnetic field. The nonmagetic part of the surface is predicted to be unaffected, to high accuracy, by such perturbations. The enhanced overall irradiance at solar maximum is interpreted as due to the high emissivity of the small scale magnetic field, which dominates over the lower emissivity of spots. The meachnisms causing these emissivity changes are known and have been recovered in numerical simulations. Quantititative predictions, however, are still hampered limits in our knowledge of the sub-arcsecond structure of the surface magnetic fields. Title: Is stellar granulation turbulence? Authors: Nordlund, A.; Spruit, H. C.; Ludwig, H. -G.; Trampedach, R. Bibcode: 1997A&A...328..229N Altcode: We show that power spectra of granulation images or velocity fields cannot be compared meaningfully with spectra from theoretical models based on turbulent cascades. The small scale power in these images is due almost entirely to the sharp edges between granules and intergranular lanes, not to turbulence in the usual sense. This is demonstrated with a number of experiments with result from numerical simulations, and with simpler synthetic data with power spectra similar to that of granulation. The reason for the seemingly laminar behavior of the granulation flow, in spite of the high Reynolds numbers involved, is the influence of stratification on the local ratio of turbulence to bulk flow. The rapid expansion of upflows, their deep origin and near-adiabatic stratification lead to low levels of turbulence in the overturning fluid at the surface. Higher levels of turbulence are expected in the converging flows near downdrafts, but mostly at scales that are below current observational resolution limits, and contributing relatively little to the total convective flux and to spectral line broadening. Title: Collimation of magnetically driven jets from accretion discs Authors: Spruit, H. C.; Foglizzo, T.; Stehle, R. Bibcode: 1997MNRAS.288..333S Altcode: We argue that the toroidal fields developing in a magnetically accelerated jet are sufficiently unstable that they cannot contribute much to collimation. We show how an initially collimated jet is decollimated by decay of the toroidal field and by the build-up of internal pressure due to kink instability. We propose that most of the collimation of observed jets is due to the poloidal field anchored in the disc. We show how the collimation by this mechanism depends on the distribution of poloidal field strength in the disc. We find that the maximum achievable collimation increases with the ratio of outer to inner disc radius, and can be of the order of 1 deg. This dependence is found to be consistent with the available data, in particular the absence of collimated outflows from cataclysmic variables. Because of the decay of the toroidal field a new characteristic distance plays a role: the collimation distance, z_c, of the order of the disc radius or less. Beyond z_c the jet is entirely ballistic and only weakly magnetic. No external medium is needed beyond this distance to explain observed narrow opening angles of jets, and no interaction with an external medium is necessary to explain the parallel field orientation observed in fast jets. Title: X-ray spectrum of a disk illuminated by ions Authors: Spruit, H. C. Bibcode: 1997LNP...487...67S Altcode: 1997adna.conf...67S The X-ray spectrum from a cool disk inside an ion supported torus is computed. A surface layer of the disk is heated by the protons from the torus. It produces a comptonized spectrum with a shape that is only very weakly dependent on the incident energy flux and the distance from the accreting compact oject, and is similar to observed spectra. Further evidence for 'ion illuminated' disks are the Li abundance in the secondaries of low mass X-ray binaries and the 450 keV lines sometimes seen in black-hole transient spectra. Title: Convection in stellar envelopes: a changing paradigm. Authors: Spruit, H. C. Bibcode: 1997MmSAI..68..397S Altcode: 1996astro.ph..5020S Progress in the theory of stellar convection over the past decade is reviewed. The similarities and differences between convection in stellar envelopes and laboratory convection at high Rayleigh numbers are discussed. Direct numerical simulation of the solar surface layers, with no other input than atomic physics, the equations of hydrodynamics and radiative transfer is now capable of reproducing the observed heat flux, convection velocities, granulation patterns and line profiles with remarkably accuracy. These results show that convection in stellar envelopes is an essentially non-local process, being driven by cooling at the surface. This differs distinctly from the traditional view of stellar convection in terms of local concepts such as cascades of eddies in a mean superadiabatic gradient. The consequences this has for our physical picture of processes in the convective envelope are illustrated with the problems of sunspot heat flux blocking, the eruption of magnetic flux from the base of the convection zone, and the Lithium depletion problem. Title: Accretion Disks - New Aspects Authors: Meyer-Hofmeister, Emmi; Spruit, Henk Bibcode: 1997LNP...487.....M Altcode: 1997adna.conf.....M The most luminous compact objects are powered by accretion of mass. Accretion disks are the one common and fundamental element of these sources on widely different scales, ranging from close stellar binaries, galactic black holes and X-ray pulsars to active galactic nuclei (AGN). Key new developments in theory and observations, reviewed by experts in the field, are presented in this book. The contributions to the workshop cover the puzzles presented by the X-UV spectra of AGN and their variability, the recent numerical simulations of magnetic fields in disks, the remarkable behavior of the superluminal source 1915+105 and the "bursting pulsar" 1744-28, to mention a few of the topics. Title: Mechanisms of p-mode absorption by active regions Authors: Spruit, H. C. Bibcode: 1996BASI...24..211S Altcode: Proposed mechanisms for explaining the observed absorption of p-mode waves by active regions are briefly reviewed. Absorption by conversion into downward propagating slow mode waves is identified as particularly promising. The observed dependence of absorption and scattering amplitudes on wavenumber allows in principle the determination of the field strength in active region and sunspot fields down to a depth of about 20000 km. Title: A `curve of growth' of astronomers on the Citation Index Authors: Spruit, H. C. Bibcode: 1996QJRAS..37....1S Altcode: The average citation career of a sample of 72 astronomers who received their PhD between 1977 and 1988, and who were employed in astronomy in 1994, is analysed using data from the Science Citation Index. The citation rates n_i(t) are fairly well represented by the assumption n_i(t)=alpha_i f(t) where f(t) is a `universal' function and alpha_i an individual `amplitude factor'. The shape of f shows that the citation rate 2 years after PhD is, on average, already one half of what it will be 12 years after PhD. Probability intervals are given for the later citation rate, for given rates early in the career. Early citation rates are found to have a modest but measurable predictive value. Title: Differential rotation, circulation and turbulence in radiative zones of stars. Authors: Urpin, V. A.; Shalybkov, D. A.; Spruit, H. C. Bibcode: 1996A&A...306..455U Altcode: We examine the steady state circulation and differential rotation in the radiative zones of early-type stars, following Zahn's assumption of latitude-independent rotation. The formulation includes in a consistent way the turbulence produced by shear, and the rotational and baroclinic forces driving the circulation. The angular momentum transfer by the combined action of meridional circulation and turbulence results in a differential rotation in the radiative zone. In its turn, the differential rotation causes the circulation (due to deviations from the spherical symmetry) and turbulence (due to instabilities associated with the shear stresses). Numerical results are given for the steady state in a 20Msun_ main sequence star. The circulation flow is very weak near the surface, and mixing between the internal and surface layers is practically absent even at high rotation rates. However, during the initial stage of evolution before the steady state is reached, a fraction of matter from the deep layers can be transported to the surface. Title: Magnetohydrodynamic winds and jets from accretion disks Authors: Spruit, H. C. Bibcode: 1996astro.ph..2022S Altcode: The theory of magnetically accelerated outflows and jets from accretion disks is reviewed at an introductory level, with special attention to problem areas like the launching conditions of the flow at the disk surface, stability of the magnetic field, and collimation mechanisms. This text will appear in R.A.M.J. Wijers, M.B. Davies and C.A. Tout, eds., Physical processes in Binary Stars, Kluwer Dordrecht, 1996 (NATO ASI series). Title: Magnetohydrodynamic jets and winds from accretion disks Authors: Spruit, H. C. Bibcode: 1996ASIC..477..249S Altcode: 1996epbs.conf..249S The theory of magnetically accelerated outflows and jets from accretion disks is reviewed at an introductory level, with special attention to problem areas like the launching conditions of the flow at the disk surface, stability of the magnetic field, and collimation mechanisms. Title: Eclipse Mapping of the Cataclysmic Variable GS Pav Authors: Barziv, O.; Augusteijn, T.; Spruit, H.; Kuulkers, E.; Berger, M.; van Paradijs, J.; Seiradakis, J. H. Bibcode: 1996hell.conf..246B Altcode: No abstract at ADS Title: Strategies for detecting Thorne-Zytkow objects. Authors: van Paradijs, J.; Spruit, H. C.; van Langevelde, H. J.; Waters, L. B. F. M. Bibcode: 1995A&A...303L..25V Altcode: Thorne-Zytkow objects (TZO) are likely to have high mass loss rates, such that their optical photospheres are obscured by a dusty envelope. Indentification of massive TZO through abundance anomalies associated with the irp process in the core is more likely to be possible by observing molecular transitions in the 4-8 μm region and at mm wavelengths than in the optical. A promising molecule is SiO, with anomalously high abundances predicted for ^29^SiO and ^30^SiO. An upper limit for ^30^SiO is reported for the unusually bright galactic center SiO maser OH359.762+0.120. Title: Switch-off conditions for radio pulsars Authors: Tsygan, A. I.; Spruit, H. Bibcode: 1995AstL...21..790T Altcode: 1995PAZh...21..877T No abstract at ADS Title: Interchange instability in and accretion disc with a poloidal magnetic field Authors: Spruit, H. C.; Stehle, R.; Papaloizou, J. C. B. Bibcode: 1995MNRAS.275.1223S Altcode: 1995astro.ph..4043S We investigate the stability to nonaxisymmetric perturbations of an accretion disc in which a poloidal magnetic field provides part of the radial support against gravity. Interchange instability due to radial gradients in the magnetic field are strongly stabilized by the shear flow in the disc. For smooth field distributions this instability is restricted to discs in which the magnetic energy is comparable to the gravitational energy. An incompressible model for the instability akin to the Boussinesq approximation for convection is given which predicts the behaviour of the instability accurately. Global axisymmetric disturbances are also considered and found to be stable for a certain class of models. The results indicate that accretion discs may be able to support poloidal fields which are strong enough to suppress other forms of magnetic instability. These strong and stable field distributions are likely to be well suited for the magnetic acceleration of jets and winds. Title: Magnetic Interchange Instability in Accretion Disks Authors: Lubow, Stephen H.; Spruit, Hendrik C. Bibcode: 1995ApJ...445..337L Altcode: We investigate the stability of a disk to magnetic interchange in the disk plane, when a poloidal magentic field provides some radial support of the disk. The disk is assumed to be geometrically thin and may possess rotation and shear. We assume the unperturbed magnetic field vertically threads the disk and has a comparable radial component at the disk surface. We formulate the linear stability problem as an initial value problem in shearing coordinates and ignore any effects of winds. Shear stabilizes the interchange instability strongly compared to the uniformly rotating case studied previously and makes the growth algebraic rather than exponential. A second form of instability with long wavelengths is identified, whose growth appears to be transient. If the field strength is measured by the travel time tauA of an Alfven wave across the disk thickness, significant amplification for both forms of instability requires (tauA Omega)-2 greater than or approximately equal to L/H, where L is the radial length scale of the field gradient and H is the disk thickness. Field strengths such that 1 less than or approximately equal (tauA Omega)-2 less than or approximately equal L/H are stable to these instabilities as well as the instability recently investigated by Balbus & Hawley (1991). The results suggest that in dark environments in which the magnetic energy density is greater than the thermal energy density, disks are stable over a substantial range of parameter space, with radial advection of magnetic flux limited by the interchange instability possibly near the disk center. Such environments may be relevant for the production of magnetic winds or jets in young stars or active galactic nuclei. Title: Cyclic Accretion from a Disk onto a Neutron Star Magnetosphere Authors: Spruit, H. C. Bibcode: 1995ASIC..450..377S Altcode: 1995lns..conf..377S; 1996lns..proc..377S No abstract at ADS Title: Accretion Disks Authors: Spruit, H. C. Bibcode: 1995ASIC..450..355S Altcode: 1995astro.ph..2098S; 1995lns..conf..355S In this lecture the basic concepts used in accretion disk theory are reviewed, with emphasis on aspects relevant for X-Ray Binaries and Cataclysmic Variables. Title: Energy implications of Li production in X-ray transients. Authors: Martin, E. L.; Spruit, H. C.; van Paradijs, J. Bibcode: 1994A&A...291L..43M Altcode: The Li-abundances recently observed in the low mass secondaries of several soft X-ray transients have been proposed to come from nuclear reactions in the vicinity of the compact objects during the outbursts of these systems. This scenario implies a large flux of energetic particles in X-ray transients. We show that Li newly produced in an outburst is mixed into the secondary very quickly, so that the observed abundance must represent the average production over many outbursts. With known limits on the Li-depletion rates in low mass stars, we derive minimal energy fluxes in energetic protons and/or α-particles, assuming that either the secondary or the accretion disk is the main target of this flux. In both cases, at least a few per cent of the accretion energy must be converted to fast particles with energies exceeding 10MeV/nucleon. Title: Fast eclipse mapping. Authors: Spruit, H. C. Bibcode: 1994A&A...289..441S Altcode: A recent algorithm by Lucy is used for the problem of mapping the intensity distribution of an object elipsed by a companion star, and is found to be an efficient way of obtaining solutions of the maximum entropy type. More general default maps than the standard axisymmetric one are explored; they allow some reduction of the artefacts in eclipse maps. It is shown that reconstruction of the intensity distribution on a surface parallel to the orbital plane is a special case in which the forward and backward projection steps can be done much more efficiently. Title: Magnetically driven wind from an accretion disk with low-inclination field lines. Authors: Cao, Xinwu; Spruit, H. C. Bibcode: 1994A&A...287...80C Altcode: The inclination of the field lines at the surface of an accretion disk has a strong influence on the nature of the resulting magnetically driven wind. If the field lines are steep, a hot corona must be present to feed the wind, at low inclinations the wind is fed directly from the surface. With a simple model for the field distribution in a disk we show how the transition between the two regimes takes place. At high inclinations an ordinary wind of the stellar type results. At low inclinations, a high- ˙(m) wind results with very low terminal speed and highly wound-up field lines. We argue that the latter type of wind will be highly unstable to nonaxisymmetric (Parker type) instabilities, and that it gives rise to a magnetically driven circulation along the disk surface rather than a high density wind. Title: Theoretical interpretation of solar and stellar irradiance variations Authors: Spruit, H. C. Bibcode: 1994svsp.coll..270S Altcode: 1994IAUCo.143P.270S The main cause of variability of solar type stars are their varying magnetic fields. To compute irradiance variations one has to compute the magnetic field (the dynamo problem), and from this the irradiance effects. The second problem is considered here. The theoretical work of the past decade has shown that the dominant effect of magnetic fields is a surface effect: a change of effective emissivity of the magnetic parts of the surface while the nonmagnetic part of the surface contributes very little to the irradiance variation on almost all time scales. No other processes have yet been found that would cause variations exceeding (at the current level of magnetic activity) the observed 0.1% irradiance fluctuation of the Sun. This implies that a knowledge of the surface magnetic fields [separated into its bright small scale (faculae, network) and dark large scale (spots) components] is sufficient for pre- or post-dicting the solar irradiance. It is hypothesized that the discrepancy remaining between the measured irradiance variations and values reconstructed from proxies is due to the difficulty of finding a proxy that accurately correlates with the continuum contrast of a dispersed small scale magnetic field. Stellar structure theory predicts that the variations in the solar radius associated with magnetic activity are quite small. For stars, color and brightness variations should primarily be interpreted in terms of variations in the fraction of the surface covered by magnetic patches. Their (long term) displacement from the main sequence is not very large. Title: Theories of radius and luminosity variations Authors: Spruit, H. C. Bibcode: 1994seit.conf..107S Altcode: The main cause of variability of the Sun is its magnetic field. The theoretical work of the past decade has shown that the dominant effect of magnetic fields is a surface effect: a change of effective emissivity of the magnetic parts of the surface while the nonmagnetic part of the surface contributes very little to the irradiance variation on almost all time scales. No other processes have yet been found that would cause variations exceeding (at the current level of magnetic activity) the observed 0.1% irradiance fluctuation of the Sun. This implies that a knowledge of the surface magnetic fields [separated into its bright small scale (faculae, network) and dark large scale (spots) components] is sufficient for pre- or post-dicting the solar irradiance. It is hypothesized that the discrepancy remaining between the measured irradiance variations and values reconstructed from proxies is due to the difficulty of finding a proxy that accurately correlates with the continuum contrast of a dispersed small scale magnetic field. Stellar structure theory predicts that the variations in the solar radius associated with magnetic activity are quite small, of the order of a few times 10^{-7} Observations of radius variations much above this value would constitute a serious challenge for stellar structure theory. Title: Magnetic winds from stars and disks Authors: Spruit, H. C. Bibcode: 1994ASIC..422...33S Altcode: 1994coma.conf...33S Some current problems in magnetic wind theory are discussed: i) the uncertainties in quantitative theories of magnetic braking of stars and how these influence the theory of the evolution of Cataclysmic Variables, ii) the instability of the toroidal field component in winds and jets from accretion disks, iii) the importance of poloidal vs toroidal collimation of jets, and iv) what determines the distribution of field strength in the disk from which the wind originates. It is argued, in particular, that the geometrical properties of the poloidal field of a disk may well be the main contributor to the observed collimation of jet. Title: Numerical Simulations of Shock-driven Accretion Authors: Rozyczka, M.; Spruit, H. C. Bibcode: 1993ApJ...417..677R Altcode: We calculate how accretion in a mass transferring binary system takes place if shock waves are the only means of angular momentum transport and energy dissipation. Cooling by radiation from the disk is included. In the absence of a mass transferring stream, with shocks excited by the tidal force only, the disk quickly settles into a quasi-stationary shock pattern. The presence of a stream impacting on the disk has a profound effect by keeping the flow very nonsteady. From simulations covering several hundred binary orbits, we find the following sequence of events. After an initial transient (which lasts on the order of 20 orbits) most of the mass transferred accumulates in a ring while a lower level accretion takes place from the ring onto the central object. For disk temperatures of a few percent of the local virial temperature, the effective alpha-viscosity, as measured by the accretion rate, during this phase is of the order 10-3. The size of the disk and the shape of the brightness distribution across it agree well with observations of quiescent CV disks. The rotation profile in the ring approaches a constant angular momentum distribution and then becomes violently unstable by a process observed earlier by Blaes and Hawley. During the instability, the accretion rate onto the central object is enhanced. Storage of mass in a ring alternating with accreting phases due to instability of the torus is expected to take place in general at low disk viscosity, whatever the process responsible for the viscosity. This provides a new mechanism for soft X-ray transients and the superoutburst cycle in cataclysmic variables. Title: An Instability Associated with a Magnetosphere-Disk Interaction Authors: Spruit, H. C.; Taam, Ronald E. Bibcode: 1993ApJ...402..593S Altcode: The evolution of a thin accretion disk surrounding a rapidly rotating magnetosphere is considered. By taking account of the variations of the magnetospheric boundary in response to the conditions at the inner edge of the disk, we find from linear analysis and numerical computation that the accretion disk can become unstable. Mass can be accreted by the central object in a cyclic fashion, with the cycle involving the storage and release of mass in the inner parts of the disk. The physical origin of the instability is associated with the variations of the magnetospheric boundary about corotation. The recurrence time scale of the cycle can vary by several orders of magnitude depending on the details of the conditions at the magnotosphere. The possible applicability of this instability process to the 'rapid burster' MXB 1730-335 is briefly discussed. Title: Physical Processes and Conditions Associated with the Formation of Protoplanetary Disks Authors: Morfill, G.; Spruit, H.; Levy, E. H. Bibcode: 1993prpl.conf..939M Altcode: Stars and planetary systems are thought to develop more or less contemporaneously from extended disk-shaped nebulae. Because the dynamical states of such disks are far from their final equilibrium configurations, the nebulae can dissipate large amounts of energy, as matter accumulates in the center and angular momentum moves to the peripheries. Experience with cosmical systems that are far from equilibrium indicates that such rapid dissipative evolution frequently occurs through a variety of collective behaviors, sometimes producing phenomena of surprising violence. This chapter reviews the range of types of collective processes that may occur in protostellar disks and play significant roles in speeding the evolution of the disks, as well as affecting the physical state of the disk and altering the state of protoplanetary matter. The processes considered here include collective angular momentum transport processes, electrostatic lightning and magnetic flares. Title: Unstable accretion from a disk onto a neutron star magnetosphere Authors: Spruit, H. Bibcode: 1993heac.conf...60S Altcode: No abstract at ADS Title: The Conversion of p-Modes to Slow Modes and the Absorption of Acoustic Waves by Sunspots Authors: Spruit, H. C.; Bogdan, T. J. Bibcode: 1992ApJ...391L.109S Altcode: The study considers the possibility that the acoustic absorption by sunspots and the surrounding plage, reported by Braun et al. (1988, 1990), is a consequence of the conversion of p-modes to slow modes (s-modes) by the principally vertical magnetic fields within these structures. It is found that for the f-mode, the absorption coefficient increases monotonically from small to large horizontal wavenumbers, and, along the nth p-mode ridge, this same general trend is modulated by the presence of n localized absorption minima. These characteristic signatures of acoustic absorption by p-mode/s-mode conversion distinguish this mechanism from other competing processes and afford the diagnostic possibility of determining the sunspot magnetic field strength from the location in wavenumber of the predicted absorption minima. Title: The Fate of the Heat Flux Blocked by Sunspots Authors: Spruit, H. C. Bibcode: 1992ASIC..375..163S Altcode: 1992sto..work..163S A sunspot blocks a part of the heat flux from the solar surface, causing a thermal adjustment in the convection zone. Two very different time scales are involved, and the adjustments are different on the two time scales. On time scales longer than the thermal time scale tau(t) of the convection zone, a significant fraction of the heat flux blocked by spots can reappear elsewhere at the surface. The average spot coverage over this time scale leads to a small change in position of the sun in the HRD. On time scales short compared with tau(t), almost all changes in the heat flux are absorbed by the convection zone. The surface temperature outside spots and the solar radius do not change on such time scales. Thus for all time scales of observational interest, the irradiance variations directly reflect the instantaneous surface coverage by spots (and faculae if these are present). Title: Influence of Starspots on Internal Stellar Structure (Invited) Authors: Spruit, H. C. Bibcode: 1992LNP...397...78S Altcode: 1992sils.conf...78S No abstract at ADS Title: The rate of mixing in semiconvective zones. Authors: Spruit, H. C. Bibcode: 1992A&A...253..131S Altcode: The present theory for mixing in semiconvective zones, under the assumption that the layered convection observed in similar laboratory circumstances occurs, takes into account the development of boundary layers with steep gradients in entropy and mean weight at the interface of adjacent layers. The most important property of the derived mixing rate is that it is proportional to the square root of the microscopic diffusion coefficient, reflecting the essential role played by both diffusion and advection of overturning cells in the transport mechanism. The diffusion coefficient depends only on the energy flux, the thermodynamic variables, and the chemical composition. Title: On the mechanism of angular momentum transport in accretion disks. Authors: Livio, M.; Spruit, H. C. Bibcode: 1991A&A...252..189L Altcode: Recently, a new mechanism for angular momentum transport in accretion disks has been proposed. The mechanism is based on tidally induced perturbations which develop into coherent spiral shocks. Angular momentum transfer by the spiral shocks was shown to be quite effective for hot, two-dimensional disks. Observational data on the mean recurrence time-scale of dwarf nova eruptions are used in conjunction with the disk instability model, to obtain a relation between the viscosity parameter alpha and the mass ratio of the system. This relation can be compared to the effective 'viscosity' obtained from the spiral-shocks scheme. The data show a weaker dependence on the mass ratio than predicted. Possible causes for the discrepancy are discussed. Title: Li Depletion in F Stars by Internal Gravity Waves Authors: Garcia Lopez, Ramon J.; Spruit, Hendrik C. Bibcode: 1991ApJ...377..268G Altcode: It is argued that internal gravity waves generated by the convective envelope of a star may be effective in producing a weak mixing in its radiative interior. The spectrum of wavelengths and frequencies generated is estimated. Only the largest horizontal wavelengths and the lowest frequencies contribute significantly to the wave energy flux and the mixing at the Li-burning depth. At this layer, the combination of the dependence on spectral type, both for the wave energy flux and the shear rate induced by the waves, would produce a distribution of Li abundances similar to that of the Li gap found among F-type stars of several open clusters, and the field. Quantitative agreement with the observed location and time scale of the gap is obtained, but only by increasing the intensity of the waves generated by a factor 15 above straightforward mixing-length estimates. The ratio of mixing length to scale height needed to get the gap in the right spectral range is 1.6. The blue edge of the gap is predicted to be sharp. Title: Filigree and flux tube physics. Authors: Spruit, H. C.; Schuessler, M.; Solanki, S. K. Bibcode: 1991sia..book..890S Altcode: This review covers the properties of the small-scale (outside sunspots) magnetic field from observational and theoretical points of view. Special emphasis is put on the physics of small isolated magnetic concentrations (flux tubes). Topics discussed include the basic observational properties, the origin and disappearance of the small-scale field, the properties of individual magnetic elements, their influence on the solar irradiance and their interaction with solar oscillations. Title: Absorption of p-Mode Waves by Magnetic Fields Authors: Spruit, H. C. Bibcode: 1991ctsm.conf..121S Altcode: No abstract at ADS Title: Absorption of p-Mode Waves by Magnetic Fields Authors: Spruit, H. C. Bibcode: 1991LNP...388..121S Altcode: Vertical magnetic flux tubes can leak energy out of the p-mode wave guide in the form of transversal and longitudinal tube waves. A procedure is developed for calculating this energy loss for tubes whose diameter is much smaller than the wavelength of the exciting p-mode. Title: Theory of Luminosity and Radius Variations Authors: Spruit, H. C. Bibcode: 1991suti.conf..118S Altcode: Theoretical calculations of luminosity and radius variations are reviewed. As sources for these variations, two kinds of effects are considered: (1) temporary conversion of thermal energy inside the convection zone into a different energy form (such as magnetic fields); and (2) the effect of surface magnetic fields on the energy emissivity of the solar surface. Because strength, extent, and location of the subsurface fields are uncertain, emphasis is on upper limits, derived from general thermodynamic considerations, and from the buoyancy of strong fields. If the toroidal fields from which active regions erupt are located at a depth greater than 10,000 km, their effect on the solar radius is likely to be below 3 x 10 exp -6 during a solar cycle and their efect on the luminosity below 10 exp -4. The vertical fields seen at the surface itself have effects on the luminosity that compare well with observations. The dominant effect in this case is a modulation of the surface emissivity (reduction in spots, enhancement in small-scale fields). The radius changes associated with this effect are negligible. Title: Shock Waves in Accretion Disks. Authors: Spruit, H. C. Bibcode: 1991RvMA....4..197S Altcode: No abstract at ADS Title: The influence of internal gravity waves on the light elements depletion among F-type stars. Authors: Garcia Lopez, Ramon J.; Spruit, H. C. Bibcode: 1991MmSAI..62..183G Altcode: It is proposed that the Li abundances in F-type stars can be affected by the weak mixing generated by internal gravity waves since the absence of Li is so strong among this spectral type. The mixing mechanism caused by the waves is describe theoretically and is applied to the F-stars because of the inherent similarity of the Li gap. The wave-energy flux is found to grow with age, the flux values of the Li and Be burning layers are similar, and the resulting predictions for young stellar systems are compatible with observational data. Title: Shock Waves in Accretion Disks Authors: Spruit, H. C. Bibcode: 1991heac.conf..333S Altcode: No abstract at ADS Title: Mass transfer by tidally induced spiral shocks in an accretion disk Authors: Matsuda, T.; Sekino, N.; Shima, E.; Sawada, K.; Spruit, H. Bibcode: 1990A&A...235..211M Altcode: Numerical simulations are presented of the evolution of disks in binary systems under the influence of accretion through spiral shock waves. The calculations are followed for at least one radial drift time scale, thereby improving upon previous results. The accretion time scale (radial drift time from the outer edge of the disk) decreases from about 1000 orbital periods at a mass ratio of 1000 to about 15 orbits at q = 0.1. Due to the neglect of radiative losses the disks studied are still significantly hotter than realistic disks, however. Title: Mass transport in a neutron star magnetosphere Authors: Spruit, H. C.; Taam, R. E. Bibcode: 1990A&A...229..475S Altcode: The interaction between a thin Keplerian accretion disk and a magnetosphere surrounding a central object is investigated within the framework of an analytical description for the magnetic field configuration. The commonly held assumption that all accreting plasma flows from the magnetospheric boundary to the stellar surface is shown to be overly restrictive. If the magnetospheric boundary is defined as the distance where the rotation starts deviating significantly from the Kepler rate, it is found that there is an extensive region inside this boundary where gas, nearly corotating with the star, drifts inward across the field by an interchange instability. The linear analysis of this instability is presented. It is also found that gas tied to field lines can be in equilibrium at positions off the midplane, and that gas can plausibly flow from the midplane to these positions, in certain circumstances. The observational consequences of such a picture are briefly discussed. Title: Angular momentum transport and magnetic fields in the solar interior Authors: Spruit, H. C. Bibcode: 1990ASSL..159..415S Altcode: 1990insu.conf..415S; 1990IAUCo.121..415S The possible mechanisms of angular momentum transport in convectively stable regions of a star are reviewed, with emphasis on transport by magnetic torques. The strength and configuration of the fields in such layers is quite uncertain, because it is not known if the field strength is obtained by assuming that the field can reach a dynamically stable configuration. A lower limit to the field strength is obtained by assuming that the field is always dynamically unstable, and decaying at the (rotation modified) dynamical time scale. The present field in the sun would then be of the order 1 G, with poloidal and toroidal components of similar strength. The differential rotation in the core, if due only to the solar wind torque, would be very small for this field strength, and instead would more likely be governed by magnetic coupling to the differential rotation of the convection zone. If smnall scale hydrodynamic transport mechanisms are present, their properties would also be influenced by a field of this strength. Title: Will a Magnetic Field Inhibit Turbulent Transport? Authors: Spruit, H. C. Bibcode: 1990LNP...366..151S Altcode: 1990rmsi.conf..151S No abstract at ADS Title: Solar convection. Authors: Spruit, H. C.; Nordlund, A.; Title, A. M. Bibcode: 1990ARA&A..28..263S Altcode: The current understanding of solar convection is examined in connection with optical observations of the surface, helioseismological observations of the interior, and theories and simulations of compressible convection. Recent progress in these fields has been documented in workshops on solar granulation, the solar photosphere, and helioseismology. Title: The Disrupted Magnetic Braking Hypothesis and the Period Gap of Cataclysmic Variables Authors: Taam, Ronald E.; Spruit, H. C. Bibcode: 1989ApJ...345..972T Altcode: The consequences of a rearrangement of the surface magnetic field of a late-type star on the rate of angular momentum loss associated with a stellar wind are explored. The rearrangement of the field from a low-order to a higher order multipole may plausibly reflect the changing character of a stellar dynamo as a rapidly rotating mass-losing star in a short-period binary system enters into a fully convective state. Upon generalizing the results of Mestel and Spruit (1987) to higher order magnetic field configurations, it is found that the fraction of open field lines decreases by a factor ranging from 2 to 200 (depending on which multipole order is assumed) for rotational periods near 3 hr. The angular momentum loss rate corresponding to this reduction can decrease by a factor ranging from 3 to 2000. This loss in effectiveness of the stellar wind can be accomplished without a sudden decline in magnetic activity. It is suggested that the operation of a mechanism of this type may be responsible for the origin of the period gap of cataclysmic variable binary systems. Title: Stability of Sunspots to Convective Motions. I. Adiabatic Instability Authors: Moreno-Insertis, F.; Spruit, H. C. Bibcode: 1989ApJ...342.1158M Altcode: For determining the adiabatic stability of a uniform vertical field in an arbitrary stratification it is sufficient to consider the limit of infinitesimal horizontal wavelength. It is shown how the behavior of the instability can be estimated qualitatively from the dependence of the equipartition field strength on depth. Modes are calculated numerically for analytic stratification models and for a detailed sunspot stratification, including the effects of partial ionization. It is concluded that for the observed field strengths of umbrae the stratification is indeed unstable, with a growth time of about 18 minutes. The unstable eigenfunctions have a maximum at about 2300 km below the surface of the umbra and are about 3900 km deep. Deeper layers may also be unstable depending on unknown details of the stratification. A connection between fluting instability and convective instability is noted. Title: Accretion and particle acceleration by spiral shock waves Authors: Spruit, H. C. Bibcode: 1989ASSL..156...59S Altcode: 1989admf.proc...59S The physics of accretion through a system of stationary spiral shaped shock waves is reviewed. The angular momentum transport by such waves can be described in terms of the negative angular momentum carried by trailing waves. Excitation processes are discussed. In `large' disks (inner radius r_i &lt&lt&lt outer radius r_o) there is a range of distances r_i &lt&lt r &lt&lt r_o where the shock-accretion solution is insensitive to conditions at both r_i and r_o. For a simple equation of state and opacity this part of the solution has a selfsimilar (scale invariant) form. These properties make spiral shocked accretion especially attractive in protostellar, neutron star and AGN disks, where the traditional `angular momentum problem' is most severe. It is shown that spiral shocks are also good locations for producing energetic protons, and I propose that this is the way in which the energetic particles inferred in many AGN's are produced. Title: Mass transfer by tidally induced spiral shocks in an accretion disc Authors: Matsuda, T.; Sekino, N.; Shima, T.; Sawada, K.; Spruit, H. C. Bibcode: 1989ASIC..290..355M Altcode: 1989tad..conf..355M Numerical simulations of two-dimensional adiabatic inviscid flow in an accretion disc in a binary system or a proto-planetary system are performed using a second-order accurate implicit Roe upwind scheme. A purpose of the work is to estimate quantitatively the amount of mass accretion rate due to spiral shocks produced by a less massive component. The mass ratio of the binary system, q, is varied to see the tidal effect of the less-massive component on the formation of spiral shocks. Five cases, namely q=0.001, 0.01, 0.1 and 1 are examined. In the case q &gt=0.01, steady spiral shocks persist until the end of the calculations, i.e. about 15-20 rotation periods. If q is a small as 0.001, i.e. the case of the Jupiter, the amplitude of the spiral waves is not very strong. On the basis of calculated mass accretion rate, we can determine an effective alpha parameter in the standard accretion disc model. It depends on q, and we found that 4e-4 &ltalpha&lt0.04 for 1e-3 &ltq&lt1. Title: Spiral shocks in accretion disks: a preliminary numerical study Authors: Rozyczka, M.; Spruit, H. C. Bibcode: 1989ASIC..290..341R Altcode: 1989tad..conf..341R The response of accretion disks to tidal forces is studied numerically with the help of a 2-D, second order hydrodynamical code. The code is tested on analytical self-similar models of spiral shocks and proven capable of maintaining them stationary in the grid. The disks are assumed to reside in potential wells of primary components of binary systems. The models are not structured perpendicularly to the orbital plane, and only the flow in the orbital plane is studied. A uniform inflow condition applied to the outer boundary of the disk invariably leads to the formation of two-armed spiral waves resembling the analytical solutions. Provided the disk approaches the Roche lobe, fairly strong shocks are observed even for mass ratios (secondary to primary) as small as 0.01. Stopping the inflow at the outer boundary modifies the spiral pattern and causes the disk to shrink rapidly. Title: Physics of accretion by spiral shock waves Authors: Spruit, H. C. Bibcode: 1989ASIC..290..325S Altcode: 1989tad..conf..325S The properties of disks accreting by global spiral shocks are reviewed. The discussion includes the formation of these waves, the way they transport angular momentum, a selfsimilar model for spiral shocked accretion and recent numerical simulations. These simulations suggest that at least in disks produce by mass overflow in close binaries the efficiency of accretion (in terms of an effective alpha-value) can be substantial. Title: Polar observatories Authors: Spruit, H. C.; Title, A. M.; Peterson, R. C. Bibcode: 1988Natur.334..466S Altcode: No abstract at ADS Title: Particle acceleration in a flow accreting through shock waves Authors: Spruit, H. C. Bibcode: 1988A&A...194..319S Altcode: It was shown recently (Donner, 1979; Sawada et al., 1986; Spruit, 1987) that accretion with angular momentum onto a compact object is possible by a process in which the dissipation and angular momentum transport are due to a system of stationary shock waves. The accreting mass spirals many times through the shocks before reaching the central object. In this kind of shocked flow diffusive particle acceleration works well even for low shock strengths, in contrast to the case of an isolated shock. The process is illustrated with analytical examples for diffusion of relativistic particles in a periodically shocked flow, and with a numerical solution for diffusion in a detailed two-dimensional flow model. Flat particle spectra (spectral index around 2) are found under rather mild assumptions on the diffusion coefficient and the probability for escape of particles from the accreting flow. It is proposed that this process is involved in the production of energetic particles in active galactic nuclei. Title: Influence of magnetic activity on the solar luminosity and radius Authors: Spruit, H. C. Bibcode: 1988srov.proc..254S Altcode: Theoretical calculations of the effect of surface magnetic fields and fields inside the convection zone on the solar radius and luminosity are reviewed. Since the strength, extent and location of the subsurface fields is uncertain, emphasis is on upper limits, derived from i) general thermodynamic considerations, and ii) the buoyancy of strong fields. In either case fields located at the base of the convection zone do not produce luminosity or radius changes of observational interest. Fields closer to the surface, but still embedded entiurelyt inside the convection zone can in principle have measurable effects through their influence on the efficiency of convective transport. The vertical fields seen at the surface itself have effects on the luminosity that compare well with observations. The dominating effect in this case is a modulation of the surface emissivity (reduction in spots, enhancements in small scale fields). Title: Spiral shocks and accretion in discs. Authors: Spruit, H. C.; Matsuda, T.; Inoue, M.; Sawada, K. Bibcode: 1987MNRAS.229..517S Altcode: Recent numerical and analytical results on disc-like accretion with shock waves as the only dissipation mechanism are compared. The global properties of the process are similar to those of the viscous (α) disc model, but precise values of the effective α value as a function of the accretion rate can be calculated. At low values of the ratio of specific heats (γ < 1.45) accretion is possible without radiative losses. Such adiabatic accretion can occur in practice at high accretion rates on to low mass objects and may be important in the formation of planets. Following Donner, and Lynden-Bell, it is pointed out that non-axisymmetric perturbations in the outer parts of a disc increase in amplitude as they propagate in and cause spiral shocks more easily in a disc than perturbations originating in the inner parts. It is suggested for this reason that the cause of spiral structure in normal spiral galaxies lies in moderate non-axisymmetries in their gaseous outer discs. Title: Stationary shocks in accretion disks Authors: Spruit, H. C. Bibcode: 1987A&A...184..173S Altcode: Special solutions of the equations of motion and continuity near the midplane of a thin accretion disk are obtained by analytic and numerical means. They have the form of stationary, self-similar flows containing two or more spiral shaped shock waves. They represent shocks initiated by a disturbance at the outer edge of the disk. Self-similar shocks excited at the center of the disk do not seem to exist. No solutions were found containing only one shock. Adiabatically accreting solutions exist in which there is a unique relation between the ratio of specific heats γ and the opening angle of the shock. When radiative losses from the disk are included, solutions exist for γ above a certain minimum value. The strength of the shock decreases with decreasing disk thickness. An effective "α parameter" can be defined, it varies with disk thickness like (h/r)1.5. An analogy between the present problem and the case of spherically symmetric accretion is discussed. Title: What Determines the Temperature of a Sunspot? Authors: Spruit, H. C.; Simon, G. W. Bibcode: 1987BAAS...19..943S Altcode: No abstract at ADS Title: On magnetic braking of late-type stars Authors: Mestel, L.; Spruit, H. C. Bibcode: 1987MNRAS.226...57M Altcode: The paper reports on a preliminary study of the variation with angular velocity of the rate of braking of a late-type star. The strength of the dynamo-built field B will normally increase with rotation, so that the surface where the wind speed becomes Alfvénic moves further out from the star and the angular momentum transported per unit mass is correspondingly higher. The stronger field tends to trap more gas within a 'dead zone' which does not contribute to the braking, but the extent of the dead zone is limited by the pressure required to balance the higher centrifugal force. These effects are illustrated by a simply parametrized field model. Braking rates are estimated for the two cases with the coronal base density independent of B and proportional to B, respectively. Title: Is there a weak mixed polarity background field? Theoretical arguments Authors: Spruit, H. C.; Title, A. M.; van Ballegooijen, A. A. Bibcode: 1987SoPh..110..115S Altcode: A number of processes associated with the formation of active regions produce `U-loops': fluxtubes having two ends at the photosphere but otherwise still embedded in the convection zone. The mass trapped on the field lines of such loops makes them behave in a qualitative different way from the `omega-loops' that form active regions. It is shown that U-loops will disperse though the convection zone and form a weak (down to a few gauss) field that covers a significant fraction of the solar surface. This field is tentatively identified with the inner-network fields observed at Kitt Peak and Big Bear. The process by which these fields escape through the surface is described; a remarkable property is that it can make active regions fields apparently disappear in situ. The mixed polarity moving magnetic features near sunspots are interpreted as a locally intense form of this disappearance by escape of U-loops. Title: Is there a weak mixed polarity background field? Theoretical arguments. Authors: Spruit, H. C.; Title, A. M.; van Ballegooijen, A. A. Bibcode: 1987MPARp.271.....S Altcode: No abstract at ADS Title: How is the Penumbra formed? Authors: Spruit, H. C. Bibcode: 1987rfsm.conf..199S Altcode: Simple theoretical arguments are reviewed that restrict the directions in which the answer should be sought. They make plausible that the field of the penumbra is in quasistatic equilibrium except with respect to motions along the field, that penumbral filaments are unlikely to be elevated with respect to the photosphere or their bright neighbors, that the boundary between the field and the underlying convection zone is at a shallow depth of the order of a pressure scale height and that convective processes in the penumbra take the form of flows nearly parallel to the field rather than Danielson's (1961) rolls. Why there should be a penumbra at all, i.e. why a spot does not look like a giant pore, seems to be a more difficult question. Title: Angular momentum transport in the radiative interior of the sun. Authors: Spruit, H. C. Bibcode: 1987MPARp.273.....S Altcode: No abstract at ADS Title: How is the penumbra formed? Authors: Spruit, H. C. Bibcode: 1987MPARp.274.....S Altcode: No abstract at ADS Title: Angular Momentum Transport in the Radiative Interior of the Sun Authors: Spruit, H. C. Bibcode: 1987ASSL..137..185S Altcode: 1987isav.symp..185S The known mechanisms for transporting angular momentum in the core of the Sun are reviewed. All hydrodynamic mechanisms are insufficient to account for the present low rotation rate with the possible exception of internal gravity waves. Several mechanisms are likely to be far too effective in depleting lithium, however. It is argued that weak magnetic fields can easily solve both problems. Their stability properties are reviewed and a scenario is presented for the evolution of such fields. It predicts that the present field in the Sun has poloidal and toroidal components of similar strengths, on the order of a few Gauss. The differential rotation in the core, if due to such a field alone, would be extremely small. Title: Mixing and transport of angular momentum in the solar interior. Authors: Spruit, H. C. Bibcode: 1987ppcs.work...78S Altcode: The observational indicators of mixing in the solar radiative interior, and the known mechanisms for mixing are reviewed critically. It is concluded that the three most important indicators, Li-depletion, the neutrino discrepancy and the internal rotation, probably each have a different, rather than a common origin. The low internal rotation is interpreted as a strong evidence for the presence of a weak magnetic field in the core of the sun. Title: Spiral shocks and accretion in disks. Authors: Spruit, H. C.; Matsuda, T.; Inoue, M.; Sawada, K. Bibcode: 1987MPARp.301.....S Altcode: No abstract at ADS Title: Particle acceleration in a flow accreting through shock waves. Authors: Spruit, H. C. Bibcode: 1987MPARp.304.....S Altcode: No abstract at ADS Title: Colors and luminosities of stars with spots. Authors: Spruit, H. C.; Weiss, A. Bibcode: 1986A&A...166..167S Altcode: Theoretical zero-age main sequence models from 0.2 to 1.9 solar masses have been made for stars with large sunspot-like spots, and their black body UBVRIJK colors calculated. The effect of spots is different on long and short time scales (compared with the thermal time scale of the convection zone) and differs between mainly convective and mainly radiative stars. During a short term increase in spot area the stars always become redder in all color indices, but the long term change can be either to the blue or the red, depending on stellar mass and the color index used. The anomalies in color diagrams predicted for Hyades stars are much smaller than those discussed by Campbell (1984), unless a spot coverage factor close to unity is assumed. It is suggested that these color anomalies may reflect the brightening in short wavelength bands due to chromospheric emission rather than the effect of spots themselves. Title: Baroclinic waves in a vertically stratified thin accretion disk Authors: Knobloch, E.; Spruit, H. C. Bibcode: 1986A&A...166..359K Altcode: The conditions for steady equilibrium and energy transport in general require a thin accretion disk to be baroclinic, i.e. the rotation speed to vary with height in the disk. A necessary condition for baroclinic instability is derived which is analogous to that encountered in geophysics. The instabilty can occur only if the radial scale length is locally comparable to the vertical scale height H, or if the vertical stratification is close to adiabatic. The maximum level of turbulence expected is limited by the dominant effect of the shear in the Kepler flow. The role of the thermal wind and of meridional circulations in establishing the mean state is elucidated. Attention is drawn to the role of buoyancy resonances which may play an important role in many wave phenomena in disks. Title: Energy transport in sunspot penumbrae. Authors: Schmidt, H. U.; Spruit, H. C.; Weiss, N. O. Bibcode: 1986A&A...158..351S Altcode: It is proposed that the magnetic field in the outer penumbra of a sunspot is almost horizontal and that the penumbra itself is very shallow, with a sharp lower boundary. A simple model of energy transport in the outer penumbra predicts that there is a Wilson depression of about 100 km, below which the penumbra extends for only 80 km. A two-component model with bright and dark filaments suggests that the associated differences in observed field strength and corrugations of the visible surface of the penumbra will be small. The authors argue that flows along the field are needed to explain the existence of bright and dark filaments while the Evershed flow is driven by pressure differences along flux tubes. Title: Stationary shocks in accretion disks. Authors: Spruit, H. C. Bibcode: 1986MPARp.232.....S Altcode: No abstract at ADS Title: Colors and luminosities of stars with spots. Authors: Spruit, H. C.; Weiss, A. Bibcode: 1986MPARp.225.....S Altcode: No abstract at ADS Title: Baroclinic waves in a vertically stratified thin accretion disk. Authors: Knobloch, E.; Spruit, H. C. Bibcode: 1986MPARp.217.....K Altcode: No abstract at ADS Title: No Title Provided Authors: Spruit, H. C. Bibcode: 1986rfsm.proc..199S Altcode: Instead of answering the question posed by the title, simple theoretical arguments are revuewd that restrict the directions in which the answer should be sought. They make plausible that the field of the penumbra is in quasistatic equilibrium except to motions along the field, that the penumbral filaments are unlikely to be elevated with respect to the photosphere or their bright neighbors, that the boundary between the field and the underlying convection zone is at a shallow depth of the order of a scale height and that convective processes in the penumbra take the form of flows nearly parallel to the field rather than Danielson's (1961) rolls. Why there should be a penumbra at all, i.e. why a spot does not look like a giant pore, seems to be a more difficult question. Title: Observational Signature of Tube Waves Authors: Spruit, H. C. Bibcode: 1985tphr.conf..158S Altcode: No abstract at ADS Title: Propagation of nonlinear, radiatively damped longitudinal waves along magnetic flux tubes in the solar atmosphere Authors: Herbold, G.; Ulmschneider, P.; Spruit, H. C.; Rosner, R. Bibcode: 1985A&A...145..157H Altcode: For solar magnetic flux tubes the authors compare three types of waves: longitudinal MHD tube waves, acoustic tube waves propagating in the same tube geometry but with rigid walls and ordinary acoustic waves in plane geometry. They find that the effect of distensibility of the tube is small and that longitudinal waves are essentially acoustic tube waves. Due to the tube geometry there is considerable difference between longitudinal waves or acoustic tube waves and ordinary acoustic waves. Longitudinal waves as well as acoustic tube waves show a smaller amplitude growth, larger shock formation heights, smaller mean chromospheric temperature but a steeper dependence of the temperature gradient on wave period. Title: Baroclinic instability in the presence of a strong horizontal shear Authors: Knobloch, E.; Spruit, H. C. Bibcode: 1985GApFD..32..197K Altcode: The theory of baroclinic instability is extended to include strong horizontal shears. The f-plane and anelastic approximations are used, but the assumption that the Rossby number is small is relaxed. Although this problem is nongeostrophic, much of the standard theory, including necessary conditions for instability and a semicircle theorem, generalize readily for disturbances of low zonal wavenumber. Title: Energy transport in sunspot penumbrae. Authors: Schmidt, H. U.; Spruit, H. C.; Weiss, N. O. Bibcode: 1985MPARp.182.....S Altcode: No abstract at ADS Title: Theorie des solaren Magnetfeldes. Authors: Spruit, H. C. Bibcode: 1985Sonne...9....6S Altcode: No abstract at ADS Title: Observational signature of tube waves. Authors: Spruit, H. C. Bibcode: 1985MPARp.212..158S Altcode: No abstract at ADS Title: Mixing in the solar interior. Authors: Spruit, H. C. Bibcode: 1984ESASP.220...21S Altcode: 1984ESPM....4...21S It seems appropriate to study in detail which mixing processes can occur in a star, and why the effective diffusivity resulting from these processes should be a weakly varying multiple of the kinematic viscosity. The author reviews the extent to which these questions can be answered at present. Title: Baroclinic instability in stars Authors: Spruit, H. C.; Knobloch, E. Bibcode: 1984A&A...132...89S Altcode: The conditions for the occurrence of adiabatic baroclinic instability in differentially rotating stars are studied using the quasi-geostrophic approximation on a β-plane. In rapidly rotating stars (near breakup) the instability can occur throughout the star. In slowly rotating stars, it can only occur in very narrow regions located near (1) the center of the star, (2) the boundary of a convection zone, (3) a jump in the molecular weight or in the buoyancy frequency, or (4) a jump in the gradient of the rotation rate. It is concluded that the instability cannot be responsible for the mixing needed to explain the Li abundance anomalies in main sequence stars or the low neutrino flux of the Sun. It could however, be important for mixing near the cores of red giants. Title: Evolution of the Sun with Mixing by Hydrodynamic Instabilities Authors: Law, W. Y.; Knobloch, E.; Spruit, H. C. Bibcode: 1984IAUS..105..523L Altcode: Following Schatzman and Maeder (1981) the authors compute the evolution of the sun with partial mixing by hydrodynamic instabilities. Instead of simply assuming a turbulent diffusion coefficient which is a constant multiple of the viscosity, they incorporate some of the properties of hydrodynamic instabilities. This puts limits on the amount of diffusion that can be obtained, and makes it dependent on time and position in the star. Title: Interaction of Fluxtubes With Convection (Keynote) Authors: Spruit, H. C. Bibcode: 1984ssdp.conf..249S Altcode: The author discusses the interaction of flux tubes with convection from two points of view: (1) the formation of flux tubes by interaction of a weak field with convection and (2) the interaction of already established flux tubes with the convective flow. The first part describes the current understanding of the processes that produce flux tubes of the observed high field strength. The second gives a classification of the types of motion that a (thin) flux tube can execute, the ways in which these can be excited by granulation, and describes a model for the propagation of these motions into the higher atmosphere. Title: No Title Provided Authors: Spruit, H. C. Bibcode: 1984ssdp.proc..249S Altcode: In this contribution I discuss the interaction of flux tubes with convection from two points of view: i) the formation of flux tubes by interaction of a weak field with convection and ii) the interaction of already established flux tubes with the convective flow. The first part describes current understanding of the processes that produce flux tubes of the observed high field strength. The second gives a classification to the types of motion that a (thin) flux tube can execute, the way in which these can be excited by granulation, and describes a model for the propagation of these motions into the higher atmosphere. Title: Baroclinic instability in the presence of a strong horizontal shear. I. Instability conditions. Authors: Knobloch, E.; Spruit, H. C. Bibcode: 1984MPARp.170.....K Altcode: No abstract at ADS Title: Mixing in the solar interior. Authors: Spruit, H. C. Bibcode: 1984MPARp.161.....S Altcode: No abstract at ADS Title: Stellar activity and the period gap in cataclysmic variables Authors: Spruit, H. C.; Ritter, H. Bibcode: 1983A&A...124..267S Altcode: The evolution of a cataclysmic binary is computed under the assumption of angular momentum loss due primarily to a magnetic wind from the secondary star, in order to investigate the possible relation between the period gap in cataclysmic variables and the sudden decrease in magnetic activity observed in main sequence stars at B-V values of about 1.65. A decrease in the angular momentum loss rate at the point where the secondary becomes fully convective can temporarily switch off mass transfer. A period gap of the correct width and location is obtained if the decrease is rapid and large enough, and in addition if the initial relative angular momentum loss rate lies between about 4 and 7 x 10 to the -9th/year. The calculation is based on homologous stellar models calibrated with the Grossman et al. (1974) main sequence. Attention is given to the influence of star spots, by way of their effect on the radius of the secondary star. Title: Magnetic flux tubes on the Sun Authors: Spruit, H. C.; Roberts, B. Bibcode: 1983Natur.304..401S Altcode: Magnetic fields are the cause of almost all forms of solar activity. Near the solar surface, and possibly in the entire convection zone, these fields occur in the form of isolated flux tubes. In recent years, new views have been developed (and older ones revived) in which this property plays a central role. Here we review these ideas, dealing with the nature of the solar cycle, sunspot structure, the origin of spicules and the source of mechanical heating in the solar atmosphere. The ideas are illustrated with the aid of a simple mathematical model for the behaviour of thin magnetic flux tubes. The properties of inhomogeneities in the corona (coronal loops) are also discussed. Title: The molecular weight barrier and angular momentum transport in radiative stellar interiors. Authors: Knobloch, E.; Spruit, H. C. Bibcode: 1983A&A...125...59K Altcode: The stability of a differentially rotating fluid with respect to axisymmetric instabilities is investigated in the presence of finite viscosity (nu), thermal diffusion (kappa-T) and a gradient of chemical composition, of diffusivity kappa-S. The results confirm that the Goldreich-Schubert-Fricke instability is suppressed already at very small stabilizing composition gradients of the order N(S) approximately Omega, where N(S) is the buoyancy frequency due to the chemical gradient and Omega the rotation rate. This excludes the GSF instability as a mixing agent in chemically inhomogeneous stellar interiors. However, two other modes of instability exist, one of which is stabilized only by much stronger composition gradients of the order N(S) approximately N(T) where N(T) is the thermal buoyancy frequency. It is an unstable inertial oscillation, operating on the baroclinicity of the stratification. It is argued that this instability might produce significant mixing in radiative stellar interiors. The other instability is triply diffusive in nature and is probably not very important since like the GSF instability it is limited to composition gradients with N(S) approximately equal to or less than Omega. Title: Internal rotation of the Sun Authors: Spruit, H. C.; Knobloch, E.; Roxburgh, I. W. Bibcode: 1983Natur.304..520S Altcode: The low surface rotation rate of the Sun and other main-sequence stars is believed to be the result of angular momentum loss due to a stellar wind1. This loss also leads to a differential rotation, the interior spinning more rapidly than the surface. The rate of increase with depth of the rotation speed is limited by hydrodynamic instabilities, which cause an outward diffusion of angular momentum2. The conditions for the occurrence of hydrodynamic instabilities in a radiative stellar interior are reviewed here assuming that the rotation is constant on spheres. The instability with the lowest threshold is a double diffusive one, the axisymmetric baroclinic diffusive (ABCD) instability. A minimum rotation curve for the present Sun is calculated using the assumption that the efficiency of this instability is sufficiently high that the rotation of the Sun is close to marginal stability. This lower limit to the internal rotation rate is roughly consistent with present observations of the rotational splitting of solar oscillations. Title: The Sun's Magnetic Field. (Book Reviews: Solar Magnetohydrodynamics) Authors: Spruit, H. C. Bibcode: 1983Sci...220..191S Altcode: No abstract at ADS Title: The Sun's Magnetic Field. (Book Reviews: Solar Magnetohydrodynamics) Authors: Spruit, H. C. Bibcode: 1983Sci...220..191P Altcode: No abstract at ADS Title: Theory of photospheric magnetic fields Authors: Spruit, H. C. Bibcode: 1983IAUS..102...41S Altcode: The magnetic fields at the solar (stellar) surface are discussed from a theoretical point of view. A description of the general structure of the magnetic field near the photosphere is presented, and the formation of flux tubes is discussed. The characteristics of flux tubes in a stratified atmosphere are examined, taking into account the stability of a tube and convective collapse, aspects of buoyancy, and the energy balance of tubes. Attention is given to the wave modes of a cylindrical flux tube of finite diameter, nonlinear tube waves, the energy transport from the photosphere to higher layers, the 'missing flux' problem, the eruption of new flux, and the distribution of fields over the surface. Title: Magnetic Activity of Stars: Theoretical Aspects Authors: Spruit, H. C. Bibcode: 1983MitAG..60...83S Altcode: Theories of the magnetic activity of stars derived from studies of solar activity are reviewed. The structure of the magnetic field of a star, the formation of flux tubes and the strength of the small scale field, magnetic heating of stellar atmospheres, dynamo action, and star spots are discussed. Extensive references are provided. Title: Erratum - Stability of Toroidal Flux Tubes in Stars Authors: Spruit, H. C.; van Vallegooijen, A. A. Bibcode: 1982A&A...113..350S Altcode: No abstract at ADS Title: Stability of differential rotation in stars Authors: Knobloch, E.; Spruit, H. C. Bibcode: 1982A&A...113..261K Altcode: Consideration of the conditions for the occurrence of hydrodynamic instabilities in differentially rotating, stably stratified stars leads to the suggestion that under normal stellar conditions, buoyancy is strong enough to suppress baroclinic instabilities. Shear instability is important for angular momentum transport across constant pressure surfaces in rapidly rotating stars, as well as the thin shear layers of slow rotators. The evolution of the angular momentum distribution through the Golreich-Schubert-Fricke instability is sketched for the case of a star which is slowly spun down by a stellar wind torque. The evolution is toward a state in which angular momentum is nearly constant on equipotentials. Title: Effect of spots on a star's radius and luminosity Authors: Spruit, H. C. Bibcode: 1982A&A...108..348S Altcode: The effects of the sudden appearance of spots on a star's radius and luminosity are calculated. Variations in luminosity associated with the birth and disappearance of spots and with the activity cycle are studied on time scales much less than the Kelvin-Helmholtz time scale, and luminosity changes are calculated for a simple model of the distribution of spots on the stellar surface. An analytic model for the stratification of the convection envelope is used to show how the diffusive time scale and the Kelvin-Helmholtz time scale follow from an energy equation, and how they relate to each other. Changes in surface luminosity and radius due to changes in efficiency of convection in the envelope are also calculated, and radius changes, in particular, are very small. Title: The flow of heat near a starspot Authors: Spruit, H. C. Bibcode: 1982A&A...108..356S Altcode: The thermal disturbance due to a single starspot of radius R and depth d in a polytropic convective envelope is calculated using a turbulent diffusion approximation. The effect of systematic fluid flows is neglected. Times large compared with the turbulent diffusion time scale and short compared with the Kelvin-Helmholtz time scale are considered so that the heat flow is quasi-static. Of the heat flux blocked by the spot, a fraction is stored in the deeper layers of the envelope and the remaining fraction reappears at the surface. It is concluded that most of the blocked heat flux is stored in the convection zone, in agreement with recent observations of the solar luminosity variation. The results imply that the light variations (in the visual band) in spotted stars can be interpreted directly in terms of the fraction of the projected surface area covered by spots. Title: Stability of toroidal flux tubes in stars Authors: Spruit, H. C.; van Ballegooijen, A. A. Bibcode: 1982A&A...106...58S Altcode: A thin tube approximation is used to study the stability of a magnetic flux tube in the equatorial plane of a star, where (1) only adiabatic disturbances are considered, (2) the tubes are unstable to poleward motion and to (3) poleward motions within the equatorial plane, if the superadiabaticity of the stratification is large enough. While the curvature of the tube in a spherical geometry has a stabilizing effect, it is not strong enough to stabilize flux tubes in the convective envelopes of main sequence stars. The longer wavelengths are favored by the instability and, for the case of the sun, modes m equals 0-4 are unstable. It is suggested in view of this instability that toroidal fields, in a stellar dynamo, occur at the interface between convection zone and radiative interior rather than within the convection zone. Title: Propagation Speeds and Acoustic Damping of Waves in Magnetic Flux Tubes Authors: Spruit, H. C. Bibcode: 1982SoPh...75....3S Altcode: Propagation speeds are derived for the wave modes of a thin magnetic tube in an otherwise homogeneous magnetized or unmagnetized fluid. These results generalize results obtained by previous authors. There are three types of wave, a (torsional) Alfvén wave and two waves which are specific for the thin tube. These are named the longitudinal and transversal tube waves, according to their polarization properties. They can be camped by radiating an MHD or acoustic wave into the surroundings of the tube. Conditions for occurrence of this acoustic damping, and the damping rates, are derived. The behavior of the waves in the solar convection zone and corona is discussed. Title: Magnetohydrodynamics of sunspots Authors: Spruit, H. C. Bibcode: 1981SSRv...28..435S Altcode: Current theories are reviewed concerning the pressure equilibrium and thermal balance of sunspots, their hydromagnetic stability, energy transport mechanisms, and propagation of waves. Title: Equations for thin flux tubes in ideal MHD Authors: Spruit, H. C. Bibcode: 1981A&A...102..129S Altcode: A Hamilton-Lagrange formalism is used to derive the equation of motion for an isolated magnetic flux tube embedded in a static gravitating fluid. An equation of motion is derived from the Lagrangian density, and a procedure analogous to that in Berstein et al. (1958) is used to show that a system of non-interacting flux tubes has a self-adjoint force operator so that the system is either stable or monotonically unstable. Title: Magnetic flux tubes. Authors: Spruit, H. C. Bibcode: 1981NASSP.450..385S Altcode: 1981suas.nasa..385S The magnetohydrodynamics of flux tubes are considered. The sections on equilibrium of flux tubes, and stability and waves deal with sunspots, the largest members of the general class of photospheric flux tubes. Title: Motion of magnetic flux tubes in the solar convection zone and chromosphere. Authors: Spruit, H. C. Bibcode: 1981A&A....98..155S Altcode: General equations of motion are derived for a thin untwisted magnetic flux tube embedded in a non-magnetic compressible fluid, in the presence of gravity. Special equations governing small perturbations in purely horizontal or vertical flux tubes are derived from these. There is in general a longitudinal and a transversal wave mode. The transversal wave in a vertical tube is studied in detail. This wave propagates under the combined influences of buoyancy and magnetic tension along the tube. In an exponential atmosphere it has a cutoff frequency which is at least twice as low as the acoustic cutoff frequency. It can transport a significant amount of energy from the convection zone to the chromosphere. The transversal motions seen in the magnetic field of the chromosphere are interpreted as transversal tube waves generated in the convection zone. Title: The Size Dependence of Contrasts and Numbers of Small Magnetic Flux Tubes in an Active Region Authors: Spruit, H. C.; Zwaan, C. Bibcode: 1981SoPh...70..207S Altcode: Intensity contrasts and number densities of bright points, knots and pores ranging in size between 0″.15 and 4″ are studied using high resolution pictures in Mg b1 of a young active region. On the average, the contrast in the wing of the line increases very strongly with decreasing size, while the continuum contrast increases more slowly. The ratio of contrast in the line to contrast in the continuum increases rapidly with decreasing size. The possibility is explored of using this contrast ratio as an indicator of size. The distribution of the contrast ratio in a part of the active region is used in this way to derive a size distribution of facular points. The resulting distribution has a limited accuracy, but is free from systematic distortion due to selection effects. Validity checks on the method are presented. We measure the size distribution of the pores in the same area, and combine the result with that for the facular points. The combined distribution shows that the surface area covered by magnetic elements with diameter δ has a maximum near δ = 0″.8. It increases roughly proportional to δ for δ < 0″.3 and falls off as δ for δ > 1″.5. It is inferred that elements with 0″.5 < δ < 1″.6, which show no conspicuous contrast in the line wing or in the continuum, occupy as much area as the pores, and twice as much as the facular points. We suggest that the changing appearance of a facular area with increasing height of formation reflects at least as much the increasing weight of the small elements in the contrast as a real change in intrinsic properties (such as the diameter) of individual elements. A spatial resolution better than 0″.1 may be needed to resolve the individual elements in plages and the chromospheric network. Title: Magnetohydrodynamics of thin flux tubes. Authors: Spruit, H. C. Bibcode: 1981ASIC...68..289S Altcode: 1981spss.conf..289S In this contribution a formalism is presented for studying the magnetohydrodynamics of a magnetic field consisting of isolated flux tubes. Applications are given for the following problems: (a) the value of the field strength in magnetic elements at the stellar surface, (b) the transfer of energy from the convection zone to the chromosphere via the magnetic field, and (c) the instability of toroidal magnetic fields in a convection zone and its effect on the dynamo process. Title: A cluster model for sunspots Authors: Spruit, H. C. Bibcode: 1981phss.conf...98S Altcode: A flux tube model for sunspots is presented, including a critical review of its deficiencies and an analysis of the points of accuracy. The umbral field is assumed to be divided into thin flux tubes with diameters around 500 km, beginning some distance below the umbral photosphere. The tube model was chosen from evidence that sunspots are an agglomeration of smaller elements. The stability of the flux tubes is examined qualitatively, suggesting that they are rooted in a field of 10,000 gauss, thereby being kept stable by the field lines. The tubes are modeled as having a fixed field strength at the sunspot cluster, and buoyancy keeps them clustered. Their repellant force is quantified as that between two half-monopoles, and conditions are defined for buoyancy overcoming the repulsion in order to confine the tube tops as a spot. Suggestions are offered for further development of the model to accurately describe the field strength variability across the spot, the merging depth, and the right field strength. Title: Small scale phenomena in umbras and penumbras - The role of convective processes Authors: Spruit, H. C. Bibcode: 1981phss.conf..359S Altcode: Small scale phenomena in sunspots that are related to convection are examined. Topics discussed include umbral dots and umbral granulations, umbral flashes, small scale motions, mechanisms of energy transport in the umbra, the structure and dynamics of penumbral filaments, and the Evershed flow. For all of these topics, the available observational data are compared with the various theoretical ideas that have been advanced. Title: Effect of spots on a star's radius and luminosity Authors: Spruit, H. C. Bibcode: 1981phss.conf..480S Altcode: The effects of sunspot size variations on the temperature fields of the unspotted surface fraction and the solar luminosity are examined. Thermal relaxation of a convective envelope is discussed in a polytropic model, showing that a small temperature fluctuation is a sufficient force to drive a large-scale flux disturbance. The Kelvin-Helmholtz time scale is demonstrated to be relevant only in the convection zone and not the entire sun. The heat flux disturbances caused by many small sunspots or a single sunspot are quantified, taking into account the increase in thermal conductivity with depth below the surface. The single spot is assumed to have a zero heat conductivity, and changes in the heat flux are calculated in terms of changes in the spot's size. It is shown that changes in the size of sunspots do not affect the stellar radius, and that luminosity variations are proportional to the size of the surface which is blocked by sunspots. Title: A Formalism for Differential Rotation Authors: Durney, B. R.; Spruit, H. C. Bibcode: 1980HiA.....5..121D Altcode: No abstract at ADS Title: On the Dynamics of the Solar Convection Zone Authors: Durney, B. R.; Spruit, H. C. Bibcode: 1980LNP...114...15D Altcode: 1980IAUCo..51...15D; 1980sttu.coll...15D No abstract at ADS Title: On the dynamics of stellar convection zones - The effect of rotation on the turbulent viscosity and conductivity Authors: Durney, B. R.; Spruit, H. C. Bibcode: 1979ApJ...234.1067D Altcode: We derive expressions for the turbulent viscosity and turbulent conductivity applicable to convection zones of rotating stars. We assume that the relative dimensions of the dominant convective cell are known and derive a simple distribution function for the turbulent convective velocities under the influence of rotation. From this distribution function (which includes, in particular, the stabilizing effect of rotation on convection) we calculate in the mixing-length approximation: (i) the turbulent Reynolds stress tensor and (ii) the expression for the heat flux in terms of the superadiabatic gradient. The contributions of the turbulent convective motions to the mean momentum and energy equation (which determine the large-scale motions in stellar convection zones) are treated consistently, and assumptions about the turbulent viscosity and heat transport are replaced by assumptions about the turbulent flow itself. The free parameters in our formalism are the relative cell dimensions and their dependence on depth and latitude. Title: Convective instability of thin flux tubes. Authors: Spruit, H. C.; Zweibel, E. G. Bibcode: 1979SoPh...62...15S Altcode: The stability of magnetic flux tubes embedded vertically in a convection zone is investigated. For thin tubes, the dominant instability is of the convective type, i.e. it is driven by buoyancy forces associated with displacements along the tube. The stability is determined by β = 8πP/B2; if β ≤ βc the tube is convectively stable, otherwise it is unstable, where the critical value βc depends on the stratification of the convection zone. For a solar convection zone model, βc = 1.83, corresponding to a magnetic field strength of 1350 G at the surface of the Sun. It is concluded that the flux tubes making up the small scale field of the Sun are probably hydrodynamically stable. Title: Convective collapse of flux tubes. Authors: Spruit, H. C. Bibcode: 1979SoPh...61..363S Altcode: Flux tubes of constant β extending vertically through the solar convection zone are unstable to a convective instability if the surface field strength is less than 1270 G. By downward displacement of matter along the tube an unstable tube can transform into a new equilibrium state with lower energy which has a higher field strength. Numerical calculations of these `collapsed' states are presented. If the collapse starts in a field with a strength corresponding to equipartition with kinetic energy in the convection zone, it yields a surface field strength of about 1650 G. It is proposed that the small scale magnetic field in active regions consists of such tubes. The collapsed state is not in thermal equilibrium. In the deeper layers the heat exchange following the collapse is very slow but the surface layers return rapidly to temperature equilibrium. It is argued that during the gradual thermal evolution of the collapsed state its surface layers may start an overstable oscillation. A brightness-velocity correlation in this oscillation could account for the observed downdraft in the tubes. Title: Convection instability of flux tubes in the solar photosphere. Authors: Spruit, H. C. Bibcode: 1978BAAS...10Q.729S Altcode: No abstract at ADS Title: Heat flow near obstacles in the solar convection zone. Authors: Spruit, H. C. Bibcode: 1977SoPh...55....3S Altcode: Disturbances in the heat flow in the solar convection zone are calculated with a turbulent thermal diffusion coefficient based on a mixing length approximation. As a consequence of the radiative boundary condition at the surface and the strong increase of the diffusion coefficient with depth, the convection zone resembles a thermally superconducting shell enclosed between a thin surface layer and an interior core of low thermal conductivity. Thermal disturbances originating in the convection zone do not penetrate into the interior, and penetrate only weakly through the solar surface. A thermally isolating obstacle buried entirely in the convection zone casts a `shadow' of reduced temperature at the solar surface; the brightening surrounding this shadow is undetectable. The shadow is weak unless the object is located close to the surface (less than 2000 km). Assuming a sunspot to be an area of reduced thermal conductivity which extends a finite depth into the convection zone, the heat flow around this obstacle is calculated. The heat flux blocked below the spot (`missing flux') spreads over a very extended area surrounding the spot. The brightening corresponding to this `missing flux' is undetectable if the reduction of the thermal conductivity extends to a depth greater than 1000 km. It is concluded that no effect other than a decrease of the convective efficiency is needed to explain the temperature change observed at the solar surface in and around a sunspot. The energy balance is calculated between magnetic flux tubes, oriented vertically in the solar surface, (magnetic elements in active regions and the quiet network) and their surroundings. Near the visible surface radiation enters the tube laterally from the surrounding convection zone. The heating effect of this influx is important for small tubes (less than a few arcseconds). Due to this influx tubes less than about 1″ in diameter can appear as bright structures irrespective of the amount of heat conveyed along the tube itself. Through the lateral influx, small tubes such as are found in the quiet network act as little `leaks' in the solar surface through which an excess heat flux escapes from the convection zone. Title: Magnetic flux tubes and transport of heat in the convection zone of the Sun. Authors: Spruit, H. C. Bibcode: 1977PhDT.......237S Altcode: This thesis consists of five papers dealing with the transport of heat in the solar convection zone on te one hand, and with the structure of magnetic flux tubes in the top of the convection zone on the other hand. Included is a table of the solar convection zone (improved and more detailed version of the model published in Solar physics 34 p.270). Title: Appearance at the solar surface of disturbances in the heat flow associated with differential rotation. Authors: Spruit, H. C. Bibcode: 1977A&A....55..151S Altcode: Surnrnary It is shown that the variation of the solar surface flux due to large disturbances in the deeper layers is usually very small (depending on the horizontal scale and on the depth of origin). This is due to the high efficiency of turbulent diffusion of heat in the deeper layers of the convection zone, and to the behaviour of the solar surface, which acts as an isolating layer with respect to flux disturbances. It is suggested that the observed homogeneity of the surface flux is a much less severe constraint on theories of solar rotation than has been assumed. The large disturbances in the heat flux in the deeper layers of the convection zone found in these theories are compatible with observed fluxes, if their horizontal length scale is less than a solar radius. Stars with efficient convective envelopes will show a negligible variation of the heat flux at their surface unless the variation is due to a disturbance just below the surface, or is due to a strong magnetic field. Key words: Differential rotation, turbulent diffusion, convection zones. Title: Small magnetostatic flux tubes. Authors: Spruit, H. C. Bibcode: 1977IAUS...62..265S Altcode: No abstract at ADS Title: Magnetic flux tubes and transport of heat in the convection zone of the sun. Authors: Spruit, H. C. Bibcode: 1977mftt.book.....S Altcode: No abstract at ADS Title: Pressure equilibrium and energy balance of small photospheric fluxtubes. Authors: Spruit, H. C. Bibcode: 1976SoPh...50..269S Altcode: Field configurations and temperature distributions of axially symmetric fluxtubes are determined on the basis of pressure equilibrium and energy balance of the tubes. The description concentrates on layers below ≈ 600 km above the photosphere; a magnetostatic field, and energy transport by a diffusion process are assumed. It is assumed also that the magnetic field of the tubes prevents convective flow across the field lines, so that only radiative energy exchange between the tube and the convection zone is present. A set of model tubes is presented ranging in size from facular points (150 km) to small pores (1000 km), for different values of the field amplitude and the asymptotic energy flux F0 flowing along the tube from the deeper layers. Radial influx of heat into the tube at the photospheric level influences the temperature in the tube strongly for all these models. For a pore-like tube f0 = 0.25 (similar to the flux from a spot umbra) seems appropriate (F0 in units of the normal photospheric flux). If in the smallest fluxtubes F0 is also 0.25, a comparison of the intensity contrast with observations of facular points indicates that the radius of tubes corresponding to facular points is 50-100 km. In the continuum the structure looks like a depression in the photosphere (similar to the Wilson depression of spots). The magnitude of this depression is ≈ 200 km for pores of 1000 km diameter and ≈ 100 km for facular points. The walls of the hole created by the depression contribute considerably to the contrast of structures observed near the solar limb. It is shown how this contribution may explain the centre to limb behaviour of facular contrast as seen in the continuum, and why the continuum CLV differs so strongly from that in line cores. Over the first 400 km above the photosphere the tube expands by a factor of ≈ 2 for all the tubes calculated. Title: A model of the solar convection zone Authors: Spruit, H. C. Bibcode: 1974SoPh...34..277S Altcode: A model of the convection zone is presented which matches an empirical model atmosphere (HSRA) and an interior model. A mixing length formalism containing four adjustable parameters is used. Thermodynamical considerations provide limits on two of these parameters. The average temperature-pressure relation depends on two or three combinations of the four parameters. Observational information on the structure of the outermost layers of the convection zone, and the value of the solar radius limit the range of possible parameter combinations. It is shown that in spite of the remaining freedom of choice of the parameters, the mean temperature-pressure relation is fixed well by these data. Title: Magnetische velden en convectie in de zon. Authors: Spruit, H. C.; Zwaan, C. Bibcode: 1974NTNA...40.....S Altcode: No abstract at ADS