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The magnetic field structure of a protostar truncates the accretion disc at some magnetospheric radius greater than the stellar radius. If the stellar rotation axis is not aligned with the disc rotation then the magnetic stresses at this radius transfer misaligned angular momentum and so lead to a tilt in the inner disc. If the configuration and/or strength of the stellar field changes with time then the degree of induced tilt also changes. Under appropriate circumstances such m= 1 tilting motions of the disc can propagate outward through the disc as warp waves. These waves must damp as they approach the corotation radius where their frequency matches the local Keplerian frequency. If the waves propagate sufficiently far, then the disc heating that results from the wave damping can provide a dominant contribution to a local disc heating, and so cause a disc flaring. This flaring can in turn cause changes in the disc spectral energy distribution.

There is evidence for the existence of massive planets at orbital radii of several hundred AU from their parent stars where the timescale for planet formation by core accretion is longer than the disc lifetime. These planets could have formed close to their star and then migrated outwards. We consider how the transfer of angular momentum by viscous disc interactions from a massive inner planet could cause significant outward migration of a smaller outer planet. We find that it is in principle possible for planets to migrate to large radii. We note, however, a number of effects which may render the process somewhat problematic., Accepted for publication in MNRAS

We investigate the migration rates of high-mass protoplanets embedded in accretion discs via two and three-dimensional hydrodynamical simulations. The simulations follow the planet's radial motion and employ a nested-grid code that allows for high resolution close to the planet. We concentrate on the possible role of the coorbital torques in affecting migration rates. We analyse two cases: (a) a Jupiter-mass planet in a low-mass disc and (b) a Saturn-mass planet in a high-mass disc. Planet migration in case (b) is much more susceptible to coorbital torques than in case (a). We find that the coorbital torques in both cases do not depend sensitively on whether the planet is allowed to migrate through the disc or is held on a fixed orbit. We also examine the dependence of the planet's migration rate on the numerical resolution near the planet. For case (a), numerical convergence is relatively easy to obtain, even when including torques arising from deep within the planet's Hill sphere. The migration rate in this case is numerically on order of the Type II migration rate and much smaller than the Type I rate, if the disc has 0.01 solar-masses inside 26 AU. Torques from within the Hill sphere provide a substantial opposing contribution to the migration rate. In case (b), the gas mass within the Hill sphere is larger than the planet's mass and convergence is more difficult to obtain. If the torques within the Hill sphere are ignored, convergence is more easily achieved but the migration rate is artificially large. At our highest resolution, the migration rate for case (b) is much less than the Type I rate, but somewhat larger than the Type II rate., Comment: 20 pages, 17 figures, 3 tables. To appear in the Monthly Notices of the Royal Astronomical Society

We analyse the non-linear, three-dimensional response of a gaseous, viscous protoplanetary disc to the presence of a planet of mass ranging from one Earth mass (1 M$_e$) to one Jupiter mass (1 M$_J$) by using the ZEUS hydrodynamics code. We determine the gas flow pattern, and the accretion and migration rates of the planet. The planet is assumed to be in a fixed circular orbit about the central star. It is also assumed to be able to accrete gas without expansion on the scale of its Roche radius. Only planets with masses $M \gsim 0.1$ M$_J$ produce significant perturbations in the disc's surface density. The flow within the Roche lobe of the planet is fully three-dimensional. Gas streams generally enter the Roche lobe close to the disc midplane, but produce much weaker shocks than the streams in two-dimensional models. The streams supply material to a circumplanetary disc that rotates in the same sense as the planet's orbit. Much of the mass supply to the circumplanetary disc comes from non-coplanar flow. The accretion rate peaks with a planet mass of approximately 0.1 M$_J$ and is highly efficient, occurring at the local viscous rate. The migration timescales for planets of mass less than 0.1 M$_J$, based on torques from disc material outside the planets' Roche lobes, are in excellent agreement with the linear theory of Type I (non-gap) migration for three-dimensional discs. The transition from Type I to Type II (gap) migration is smooth, with changes in migration times of about a factor of 2. Starting with a core which can undergo runaway growth, a planet can gain up to a few M$_J$ with little migration. Planets with final masses of order 10 M$_J$ would undergo large migration, which makes formation and survival difficult., Comment: Accepted by MNRAS, 18 pages, 13 figures (6 degraded resolution). Paper with high-resolution figures available at http://www.astro.ex.ac.uk/people/mbate/

We determine the torque exerted in a steady state by an external potential on a three-dimensional gaseous disk at a non-coorbital corotation resonance. Our model accounts for the feedback of the torque on the surface density and vorticity in the corotation region, and assumes that the disk has a barotropic equation of state and a nonzero effective viscosity. The ratio of the torque to the value given by the formula of Goldreich & Tremaine depends essentially on a single dimensionless parameter, which quantifies the extent to which the resonance is saturated. We discuss the implications for the eccentricity evolution of young planets., 23 pages, 1 figure, revised version, to be published in the Astrophysical Journal

In a recent paper, Elmegreen (2000) has made a cogent case, from an observational point of view, that the lifetimes of molecular clouds are comparable to their dynamical timescales. If so, this has important implications for the mechanisms by which molecular clouds form. In particular we consider the hypothesis that molecular clouds may form not by {\it in situ} cooling of atomic gas, but rather by the agglomeration of the dense phase of the interstellar medium (ISM), much, if not most, of which is already in molecular form., 6 pages, no figures, accepted on 20 June 2001 for publication in MNRAS

The outer layers of a gas giant planet in a close orbit are isothermal because of heating by the star, and therefore these layers are convectively stable. A resonant tidal torque is exerted at the outer boundary of the interior convection zone. Tidal dissipation occurs through nonlinear damping. This process is similar to that previously considered for high-mass binary stars. Two novel aspects of the planet case are (1) the torque is exerted in a region where H/rp 1, for density scale height H and planet radius rp, and (2) at high spin rates, effects of Coriolis forces are critical in permitting a resonance to occur. Gravity waves carrying angular momentum are launched outward from the resonant region. As these waves damp by nonlinear processes, an oversynchronous planet is spun down toward synchronism with the orbit. The torques are powerful enough to have spun down planet 51 Peg B below Jupiter's spin rate in about 102 yr and to approximate synchronism (ωspin ~ ωorbit) in about 105 yr. These estimates ignore effects of tidal heating, which are likely important at the spin rate of Jupiter or higher. Fast-rotating Jupiter-mass planets with orbital periods of up to about 40 days can be spun down to less than one-tenth the spin rate of Jupiter within 1010 yr. A planet's photosphere would be spun down even more effectively than is estimated above if its outer layers can rotate differentially. A crude estimate suggests that eccentricity is damped on timescales of about 1010 yr, so a more detailed analysis is required to determine the significance of this effect.

Motivated by recent observations of short-timescale variations in the infrared emission of circumstellar disks, we propose that coronal mass ejections can remove dust grains on timescales as short as a few days. Continuous monitoring of stellar activity, coupled to infrared observations, can place meaningful constraints on the proposed mechanism., Comment: accepted to Astrophysical Journal Letters

Standard, planar accretion discs operate through a dissipative mechanism, usually thought to be turbulent, and often modelled as a viscosity. This acts to take energy from the radial shear, enabling the flow of mass and angular momentum in the radial direction. In a previous paper we discussed observational evidence for the magnitude of this viscosity, and pointed out discrepancies between these values and those obtained in numerical simulations. In this paper we discuss the observational evidence for the magnitude of the dissipative effects which act in non--planar discs, both to transfer and to eliminate the non--planarity. Estimates based on the model by Ogilvie (1999), which assumes a small--scale, isotropic viscosity, give alignment timescales for fully ionized discs which are apparently too short by a factor of a few compared with observations, although we emphasize that more detailed computations as well as tighter observational constraints are required to verify this conclusion. For discs with low temperature and conductivity, we find that the timescales for disc alignment based on isotropic viscosity are too short by around two orders of magnitude. This large discrepancy suggests that our understanding of viscosity in quiescent discs is currently inadequate., Comment: 7 pages, no figures. MNRAS, in press

We describe a model for the long term evolution of a circumplanetary disk that is fed mass from a circumstellar disk and contains regions of low turbulence (dead zones). We show that such disks can be subject to accretion driven outbursts, analogous to outbursts previously modeled in the context of circumstellar disks to explain FU Ori phenomena. Circumplanetary disks around a proto-Jupiter can undergo outbursts for infall accretion rates onto the disks in the range ~10^{-9} to 10^{-7} M_sun/yr, typical of accretion rates in the T Tauri phase. During outbursts, the accretion rate and disk luminosity increases by several orders of magnitude. Most of the planet mass growth during planetary gas accretion may occur via disk outbursts involving gas that is considerably hotter than predicted by steady state models. For low infall accretion rates less than ~10^{-10} M_sun/yr that occur in late stages of disk accretion, disk outbursts are unlikely to occur, even if dead zones are present. Such conditions are favorable for the formation of icy satellites., Accepted for publication in ApJ Letters

We present high spatial resolution maps of ro-vibrational molecular hydrogen emission from the environment of the GG Tau A binary component in the GG Tau quadruple system. The H2 v= 1-0 S(1) emission is spatially resolved and encompasses the inner binary, with emission detected at locations that should be dynamically cleared on several hundred-year timescales. Extensions of H2 gas emission are seen to ~100 AU distances from the central stars. The v = 2-1 S(1) emission at 2.24 microns is also detected at ~30 AU from the central stars, with a line ratio of 0.05 +/- 0.01 with respect to the v = 1-0 S(1) emission. Assuming gas in LTE, this ratio corresponds to an emission environment at ~1700 K. We estimate that this temperature is too high for quiescent gas heated by X-ray or UV emission from the central stars. Surprisingly, we find that the brightest region of H2 emission arises from a spatial location that is exactly coincident with a recently revealed dust "streamer" which seems to be transferring material from the outer circumbinary ring around GG Tau A into the inner region. As a result, we identify a new excitation mechanism for ro-vibrational H2 stimulation in the environment of young stars. The H2 in the GG Tau A system appears to be stimulated by mass accretion infall as material in the circumbinary ring accretes onto the system to replenish the inner circumstellar disks. We postulate that H2 stimulated by accretion infall could be present in other systems, particularly binaries and "transition disk" systems which have dust cleared gaps in their circumstellar environments., Comment: 18 pages, including 4 figures. Accepted for publication in ApJ

We consider a thin accretion disc of half-thickness H, vertically threaded by a magnetic field. The field is due to contributions from both the disc current and an external current (giving rise to a uniform external field). We derive an integro-differential equation for the evolution of the magnetic field, subject to magnetic diffusivity η and disc accretion with radial velocity v r . The evolution equation is solved numerically, and a steady state is reached. The evolution equation depends upon a single, dimensionless parameter D=2η/(3H|v r |)=(R/H)(η/ν), where the latter equality holds for a viscous disc having viscosity ν. At the disc surface, field lines are bent by angle i from the vertical, such that tan i=1.52D −1

We consider the linear stability of an isothermal, pressure-bounded, self-gravitating gas slab. Such a configuration is unstable at sufficiently long wavelengths and grows at rate ∼ √Gρ, where ρ is the density of the gas slab, but at high external pressure the nature of this instability is quite different from that of the usual Jeans instability. We develop an analytical model that reproduces the behaviour of the instability found numerically by Elmegreen & Elmegreen. They found, surprisingly, that the critical wavenumber for the onset of the instability is always of the order of the layer thickness, independently of the level of self- gravity and the external pressure

We present the results of a coronagraphic scattered-light imaging survey of six young disk candidate stars using the Hubble Space Telescope Advanced Camera for Surveys. The observations made use of the 1.8" occulting spot through the F606W (broad V) filter. Circumstellar material was imaged around HD 97048, a Herbig Ae/Be star located in the Chamaeleon I dark cloud at a distance of 180 pc. The material is seen between ~2" (360 AU) and ~4" (720 AU) from the star in all directions. A V-band azimuthally averaged radial surface brightness profile peaks at r = 2" with a value of 19.6 ± 0.2 mag arcsec^(-2) and smoothly decreases with projected distance from the star as I ∝ r^(-3.3±0.5). An integrated flux of 16.8 ± 0.1 mag is measured between 2" and 4", corresponding to a scattered-light fractional luminosity lower limit of L_(sca)/L_* > 8.4 × 10^(-4). Filamentary structure resembling spiral arms similar to that seen in Herbig Ae/Be disks is observed. Such structure has been attributed to the influence of orbiting planets or stellar encounters. Average surface brightness upper limits are determined for the five nondetections: HD 34282, HD 139450, HD 158643, HD 159492, and HD 195627. Possible reasons for the nondetections are disks that are too faint or disks hidden by the occulter.

The migration of a planet through a gaseous disc causes the locations of their resonant interactions to drift and can alter the torques exerted between the planet and the disc. We analyse the time-dependent dynamics of a non-coorbital corotation resonance under these circumstances. The ratio of the resonant torque in a steady state to the value given by Goldreich & Tremaine (1979) depends essentially on two dimensionless quantities: a dimensionless turbulent diffusion time-scale and a dimensionless radial drift speed. When the drift speed is comparable to the libration speed and the viscosity is small, the torque can become much larger than the unsaturated value in the absence of migration, but is still proportional to the large-scale vortensity gradient in the disc. Fluid that is trapped in the resonance and drifts with it acquires a vortensity anomaly relative to its surroundings. If the anomaly is limited by viscous diffusion in a steady state, the resulting torque is inversely proportional to the viscosity, although a long time may be required to achieve this state. A further, viscosity-independent, contribution to the torque comes from fluid that streams through the resonant region. In other cases, torque oscillations occur before the steady value is achieved. We discuss the significance of these results for the evolution of eccentricity in protoplanetary systems. We also describe the possible application of these findings to the coorbital region and the concept of runaway (or type III) migration. [Abridged], 15 pages, 6 figures, to be published in MNRAS

We consider the alignment torque between a spinning black hole and an accretion disc whose angular momenta are misaligned. This situation must hold initially in almost all gas accretion events on to supermassive black holes, and may occur in binaries where the black hole receives a natal supernova kick. We show that the torque always acts to align the hole's spin with the total angular momentum without changing its magnitude. The torque acts dissipatively on the disc, reducing its angular momentum, and aligning it with the hole if and only if the angle theta between the angular momenta J_d of the disc and J_h of the hole satisfies the inequality cos theta > -J_d / 2 J_h. If this condition fails, which requires both theta > pi/2 and J_d < 2 J_h, the disc counteraligns., MNRAS, in press

We study the dynamical origin of the structures observed in the scattered-light images of the resolved debris disk around HD 141569A. We explore the roles of radiation pressure from the central star, gas drag from the gas disk, and the tidal forces from two nearby stars in creating and maintaining these structures. We use a simple one-dimensional axisymmetric model to show that the presence of the gas helps confine the dust and that a broad ring of dust is produced if a central hole exists in the disk. This model also suggests that the disk is in a transient, excited dynamical state, as the observed dust creation rate applied over the age of the star is inconsistent with submillimeter mass measurements. We model in two dimensions the effects of a fly-by encounter between the disk and a binary star in a prograde, parabolic, coplanar orbit. We track the spatial distribution of the disk's gas, planetesimals, and dust. We conclude that the surface density distribution reflects the planetesimal distribution for a wide range of parameters. Our most viable model features a disk of initial radius 400 AU, a gas mass of 50 M_earth, and beta = 4 and suggests that the system is being observed within 4000 yr of the fly-by periastron. The model reproduces some features of HD 141569A's disk, such as a broad single ring and large spiral arms, but it does not reproduce the observed multiple spiral rings or disk asymmetries nor the observed clearing in the inner disk. For the latter, we consider the effect of a 5 M_Jup planet in an eccentric orbit on the planetesimal distribution of HD 141569A., Comment: Accepted to ApJ

We analyze the gas accretion flow through a planet-produced gap in a protoplanetary disk. We adopt the alpha disk model and ignore effects of planetary migration. We develop a semi-analytic, one-dimensional model that accounts for the effects of the planet as a mass sink and also carry out two-dimensional hydrodynamical simulations of a planet embedded in a disk. The predictions of the mass flow rate through the gap based on the semi-analytic model generally agree with the hydrodynamical simulations at the 25% level. Through these models, we are able to explore steady state disk structures and over large spatial ranges. The presence of an accreting Jupiter-mass planet significantly lowers the density of the disk within a region of several times the planet's orbital radius. The mass flow rate across the gap (and onto the central star) is typically 10% to 25% of the mass accretion rate outside the orbit of the planet, for planet-to-star mass ratios that range from 5e-5 to 1e-3., Comment: 10 pages, 3 figures, 1 table. To appear in The Astrophysical Journal

We consider the evolution of a warped disc around a Kerr black hole, under conditions such that the warp propagates in a wavelike manner. This occurs when the dimensionless effective viscosity, alpha, that damps the warp is less than the characteristic angular semi-thickness, H/R, of the disc. We adopt linearized equations that are valid for warps of sufficiently small amplitude in a Newtonian disc, but also account for the apsidal and nodal precession that occur in the Kerr metric. Through analytical and time-dependent studies, we confirm the results of Demianski & Ivanov, and of Ivanov & Illarionov, that such a disc takes on a characteristic warped shape. The inner part of the disc is not necessarily aligned with the equator of the hole, even in the presence of dissipation. We draw attention to the fact that this might have important implications for the directionality of jets emanating from discs around rotating black holes., 8 pages, 6 figures, to be published in MNRAS

We analyse the non-linear propagation and dissipation of axisymmetric waves in accretion discs using the ZEUS-2D hydrodynamics code. The waves are numerically resolved in the vertical and radial directions. Both vertically isothermal and thermally stratified accretion discs are considered. The waves are generated by means of resonant forcing and several forms of forcing are considered. Compressional motions are taken to be locally adiabatic ($\gamma = 5/3$). Prior to non-linear dissipation, the numerical results are in excellent agreement with the linear theory of wave channelling in predicting the types of modes that are excited, the energy flux by carried by each mode, and the vertical wave energy distribution as a function of radius. In all cases, waves are excited that propagate on both sides of the resonance (inwards and outwards). For vertically isothermal discs, non-linear dissipation occurs primarily through shocks that result from the classical steepening of acoustic waves. For discs that are substantially thermally stratified, wave channelling is the primary mechanism for shock generation. Wave channelling boosts the Mach number of the wave by vertically confining the wave to a small cool region at the base of the disc atmosphere. In general, outwardly propagating waves with Mach numbers near resonance ${\cal M}_{\rm r} \ga 0.01$ undergo shocks within a distance of order the resonance radius., Comment: 28 pages, 21 figures - 8 as GIF, 13 embedded postscript, Accepted for publication in MNRAS. Full postscript version available from http://www.astro.ex.ac.uk/people/mbate

We investigate the migration of massive extrasolar planets due to gravitational interaction with a viscous protoplanetary disc. We show that a model in which planets form at 5 AU at a constant rate, before migrating, leads to a predicted distribution of planets that is a steeply rising function of log (a), where a is the orbital radius. Between 1 AU and 3 AU, the expected number of planets per logarithmic interval in orbital radius roughly doubles. We demonstrate that, once selection effects are accounted for, this is consistent with current data, and then extrapolate the observed planet fraction to masses and radii that are inaccessible to current observations. In total, about 15 percent of stars targeted by existing radial velocity searches are predicted to possess planets with masses 0.3 M_Jupiter < M_p sin (i) < 10 M_Jupiter, and radii 0.1 AU < a < 5 AU. A third of these planets (around 5 percent of the target stars) lie at the radii most amenable to detection via microlensing. A further 5-10 percent of stars could have planets at radii of 5 AU < a < 8 AU that have migrated outwards. We discuss the probability of forming a system (akin to the Solar System) in which significant radial migration of the most massive planet does not occur. About 10-15 percent of systems with a surviving massive planet are estimated to fall into this class. Finally, we note that a smaller fraction of low mass planets than high mass planets is expected to survive without being consumed by the star. The initial mass function for planets is thus predicted to rise more steeply towards small masses than the observed mass function., MNRAS, in press

A planet of low mass orbiting in a two-dimensional gaseous disc generates a one-armed spiral wake. We explain this phenomenon as the result of constructive interference between wave modes in the disc, somewhat similar to the Kelvin wedge produced in the wake of a ship. The same feature is not expected in a three-dimensional disc with thermal stratification., 5 pages, 5 figures, to be published in MNRAS

In a planetary system, a secular particle resonance occurs at a location where the precession rate of a test particle (e.g. an asteroid) matches the frequency of one of the precessional modes of the planetary system. We investigate the secular interactions of a system of mutually inclined planets with a gaseous protostellar disk that may contain a secular nodal particle resonance. We determine the normal modes of some mutually inclined planet-disk systems. The planets and disk interact gravitationally, and the disk is internally subject to the effects of gas pressure, self-gravity, and turbulent viscosity. The behavior of the disk at a secular resonance is radically different from that of a particle, owing mainly to the effects of gas pressure. The resonance is typically broadened by gas pressure to the extent that global effects, including large-scale warps, dominate. The standard resonant torque formula is invalid in this regime. Secular interactions cause a decay of the inclination at a rate that depends on the disk properties, including its mass, turbulent viscosity, and sound speed. For a Jupiter-mass planet embedded within a minimum-mass solar nebula having typical parameters, dissipation within the disk is sufficient to stabilize the system against tilt growth caused by mean-motion resonances., 30 pages, 6 figures, to be published in The Astrophysical Journal

We consider the dynamics of a protostellar disc in a binary system where the disc is misaligned with the orbital plane of the binary, with the aim of determining the observational consequences for such systems. The disc wobbles with a period approximately equal to half the binary's orbital period and precesses on a longer timescale. We determine the characteristic timescale for realignment of the disc with the orbital plane due to dissipation. If the dissipation is determined by a simple isotropic viscosity then we find, in line with previous studies, that the alignment timescale is of order the viscous evolution timescale. However, for typical protostellar disc parameters, if the disc tilt exceeds the opening angle of the disc, then tidally induced shearing within the disc is transonic. In general, hydrodynamic instabilities associated with the internally driven shear result in extra dissipation which is expected to drastically reduce the alignment timescale. For large disc tilts the alignment timescale is then comparable to the precession timescale, while for smaller tilt angles $\delta$, the alignment timescale varies as $(\sin \delta)^{-1}$. We discuss the consequences of the wobbling, precession and rapid realignment for observations of protostellar jets and the implications for binary star formation mechanisms., Comment: MNRAS, in press. 10 pages. Also available at http://www.ast.cam.ac.uk/~mbate

We extend our analysis of the three-dimensional response of a vertically polytropic disk to tidal forcing at Lindblad resonances by including the effects of a disk atmosphere. The atmosphere is modeled as an isothermal layer that joins smoothly on to an underlying polytropic layer. The launched wave progressively enters the atmosphere as it propagates away from the resonance. The wave never propagates vertically, however, and the wave energy rises to a (finite) characteristic height in the atmosphere. The increase of wave amplitude associated with this process of wave channeling is reduced by the effect of the atmosphere. For waves of large azimuthal mode number m generated by giant planets embedded in a disk, the increase in wave amplitude is still substantial enough to be likely to dissipate the wave energy by shocks for even modest optical depths (tau greater than about 10) over a radial distance of a few times the disk thickness. For low-m waves generated in circumstellar disks in binary stars, the effects of wave channeling are less important and the level of wave nonlinearity increases by less than a factor of 10 in going from the disk edge to the disk center. For circumbinary disks, the effects of wave channeling remain important, even for modest values of optical depth., Comment: 11 pages, 4 figures, submitted to the Astrophysical Journal

Observations suggest that accretion disks are eccentrically unstable during superout-burst of close binary star systems. We review the theory of eccentricity growth of such disks by the m = 3 component of the tidal field due to the 3:1 resonance

It is now widely recognized that mass transfer in close binaries has important implications to binary structure and evolution on a wide range of length and time scales. Many of the earliest and also subsequent studies concentrated on the evolutionary significance of mass transfer (see e.g., reviews by Paczynski 1971, Plavec 1973, De Greve 1986). Observations indicate that close binaries with degenerate or nondegenerate component stars demonstrate a variety of phenomena (see reviews by Plavec 1990; White 1989). A more detailed analysis of various processes in such systems requires considerations of gas dynamics. Accordingly, this paper is largely devoted to such processes. No attempt is made to provide a comprehensive review of recent theoretical developments. Instead, attention is given to a few major problems which are of broad interest. Issues requiring further analysis are described.

The 4.2-day orbit of the newly discovered planet around 51~Pegasi is formally unstable to tidal dissipation. However, the orbital decay time in this system is longer than the main-sequence lifetime of the central star. Given our best current understanding of tidal interactions, a planet of Jupiter's mass around a solar-like star could have dynamically survived in an orbit with a period as short as $\sim10\,$hr. Since radial velocities increase with decreasing period, we would expect to find those planets close to the tidal limit first and, unless this is a very unusual system, we would expect to find many more. We also consider the tidal stability of planets around more evolved stars and we re-examine in particular the question of whether the Earth can dynamically survive the red-giant phase in the evolution of the Sun., Comment: AAS LaTeX macros v.4, 14 pages, 2 postscript figures, also available from http://ensor.mit.edu/~rasio/, to appear in ApJ

The issues addressed at a workshop on the nature and observability of resonances in spiral galaxies are summarized. The need for studying these resonances is discussed.

We study the propagation properties of three-dimensional oscillating modes in a differentially rotating gaseous disk, without self-gravity. We consider mainly axisymmetric waves in a locally vertically isothermal disk and show that the wave structure can be determined analytically. Low-frequency axisymmetric g-modes propagate in a region that lies somewhere inside the wave resonance radius defined by ω=Ω(r), for wave frequency ω and disk angular speed Ω(r). High-frequency axisymmetric p-modes propagate in a region that lies somewhere outside this resonance. Waves exist that cause the disk midplane to oscillate vertically (corrugation waves), as well as waves that keep the disk midplane fixed

Using smoothed particle hydrodynamics, the quasi-stationary structure of a circumbinary disk is found and its gravitational interaction with the central binary system is evaluated. The resulting rate of change of the orbital elements of the binary is examined. It is demonstrated that, in general, the semimajor axis decreases, the eccentricity grows, and the orbit precesses in the prograde sense. Numerical results for one particular model and a qualitative explanation for the effects found, mainly that of eccentricity driving, are provided. A protobinary or newly formed binary star embedded in a disk may undergo a significant orbital evolution before the disk dispersal. The application of the results to the interpretation of the observed statistics of binary eccentricities is discussed. 32 refs.

The results of new calculations of solar-neutrino fluxes are presented; the fluxes are obtained from detailed solar models that make use of improved opacities and nuclear-physics cross sections. By evaluating known uncertainties in the predicted capture rate for the $^{37}\mathrm{Cl}$ solar-neutrino experiment, we find that the ratio of theoretical to (best-estimate) observed capture rate lies in the range 4.0 to 2.6. These results constitute a strong constraint on models of neutrino oscillations if the entire discrepancy is ascribed to neutrino oscillations.

The solar five minute oscillation has been shown to consist of a large number of nonradial acoustic modes. Observations of these modes provide a probe of the solar interior; specifically tests on models of the solar convection zone.

Analyse de l'amortissement des ondes de densite etoiles-gaz lineaires. Pour la Galaxie, les collisions de nuages interstellaires conduisent a des temps d'amortissement < 10 9 ans, ce qui doit suffire pour limiter la croissance des instabilites WASER

The letter considers the three-dimensional dynamics of the stream in semidetached binaries undergoing Roche-lobe overflow. Results of an earlier communication are generalized to include the dynamic effects in the direction perpendicular to the orbital plane. It is found that the scale height of the stream characteristically exceeds its corresponding hydrostatic value by a significant factor because the inertia of the gas prevents it from responding instantaneously to the changing gravitational field. This effect is important for the interpretation of observations relating to stream-disk impacts in cataclysmic variables and in binary X-ray sources of low total mass.

Zero-age models of contact binaries of roughly solar composition are constructed on the basis of contact discontinuity hypothesis. With this formulation, systems with common radiative envelopes can be constructed as well as systems with common convective envelopes. Two models with total masses, respectively, of 1.5 and 3 solar masses are presented explicitly; both binary models have a mass ratio chosen equal to 0.5. The properties of the interior structure of these models are compared with the properties of zero-age single stars which have masses corresponding to the individual components. The predictions of the theory are compared with the empirical period-color relationship found by Eggen (1961, 1967) for W Ursae Majoris stars. The agreement with observations is satisfactory.