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THE DISKS OF T-TAURI STARS WITH FLAT INFRARED-SPECTRA
No full text[[abstract]]The authors model the energy distributions of T Tauri stars with flat infrared spectra by assuming that they have spatially thin disks with unorthodox radial gradients of temperature. These sources differ from more common T Tauri stars in that the extreme shallowness of their spectral slope cannot be plausibly explained by conventinal ideas (e.g., Keplerian accretion or reprocessing of starlight by a slightly flared disk). Derived model parameters show that the disks associated with the flat-spectrum sources must contain intrinsic luminosity in addition to the energy intercepted and reprocessed from the central star; i.e., the disks must be active. Self-gravity may provide one possible source for this activity - the spectral limits and estimates of the disk masses derived for three systems (T Tau, DG Tau, and HL Tau) are close to the theoretical values that would make the self-gravity of the disks dynamically important.
--------------------------------------------------------------------------------[[fileno]]2010118010083[[department]]物理
STAR FORMATION IN MOLECULAR CLOUD CORES
No full text[[abstract]]A scenario for star formation is described based on the assumption that magnetic fields play an important dynamical role in the interstellar medium. Particular attention is given to the evolution of a molecular cloud which is initially subcritical but which has a mass (supported by a combination of magnetic fields and sub-Alfvenic 'turbulence') that much exceeds the Jeans mass computed on the basis of the average density and temperature. It is argued that the magnetic support of such an object will automatically lead to the production of many small cores.[[fileno]]2010118010090[[department]]物理
STAR FORMATION AND THE CIRCUMSTELLAR MATTER OF YOUNG STELLAR OBJECTS
No full text[[abstract]]The authors propose that the formation of low mass stars in molecular clouds takes place in four stages. The first stage is the formation of slowly rotating cloud cores through the slow leakage of magnetic (and turbulent) support by ambipolar diffusion. The second phase begins when a condensing cloud core passes the brink of instability and collapses dynamically from "inside-out", building up a central protostar and nebular disk. The emergent spectral energy distributions of theoretical models in the infall stage are in close agreement with those of recently found infrared sources with steep spectra. As the rotating protostar gains mass, deuterium will eventually ignite in the central regions and drive the star nearly completely convective if its mass is less than about 2 M_sun;. This initiates the next step of evolution - the bipolar outflow phase - in which a stellar wind pushes outward and breaks through the infalling envelope. The initial breakout is likely to occur along the rotational poles, leading to collimated jets and bipolar outflows. The intense stellar wind eventually widens to sweep out gas in nearly all 4π steradian, revealing the fourth stage - a T Tauri star with a surrounding remnant nebular disk. Radiation from a disk adds an infrared excess to the expected spectral energy distribution of the revealed source.[[fileno]]2010118010091[[department]]物理
NONLINEAR SPIRAL DENSITY WAVES - VISCOUS DAMPING
No full text[[abstract]]The authors calculate the viscous damping of nonlinear density waves in Saturn's rings using various kinetic formulations to estimate the collisional change (δP/δt)c of the vertically integrated pressure tensor P in a particulate disk. The pressure tensor is computed as a function of two parameters: (1) the ratio of the average collision frequency to the epicyclic frequency, and (2) a measure of how close streamlines are to crossing when perturbed by nonaxisymmetric forces. When parameters approximately correct for the density waves excited by Mimas's 5:3 resonance in the A ring are used, the authors obtain a satisfactory fit with the observed profile, providing one uses a coefficient of restitution that is appropriate to crystalline ice and providing one uses an extreme Krook formulation to evaluate (δP/δt)c. When a similar calculation is made for the density waves in the B ring excited by the comparably strong 2:1 resonance of the so-called co-orbital satellites, the results are more mixed. It is speculated that the densities in the actual peaks of the density waves in Saturn's B ring are limited, not by viscosity, but by the condition of excluded volumes when ice balls are packed tightly against one another. Finally, the authors discuss the problems of mass and angular momentum transport in a disk with externally deposited resonant torques, and of opening gaps and maintaining sharp edges in planetary rings.
--------------------------------------------------------------------------------[[fileno]]2010118010094[[department]]物理
THE FORMATION OF COOL STARS FROM CLOUD CORES
No full text[[abstract]]We consider models of the processes by which a molecular cloud acquires dense cores, a (magnetized) rotating core collapses to give a protostar plus nebular disk, and a powerful stellar wind sets in to reverse the accretion flow and reveal the central object as a pre-main-sequence star. At each stage, we rely on a combination of theory and observation to fix the basic parameters of the model. We show that core formation in a molecular cloud is an inevitable byproduct of ambipolar diffusion in a magnetized self-gravitating medium of low fractional ionization. We find that the gravitational collapse of a uniformly-rotating isothermal core, which possesses a 1/r2 density profile in its inner parts, has simple analytic properties. And we propose that strong stellar winds in T Tauri stars represent a phase of readjustment in the angular momentum distribution after deuterium burning drives convection throughout a strongly differentially-rotating protostar. We conclude that the major missing link in this picture is the evolutionary behavior of massive nebular disks that may accumulate around protostars. Otherwise, there seems to be a satisfying connection between the cloud cores observed by molecular-line radio astronomers and the active stellar atmospheres of young stars studied by optical and x-ray astronomers.[[fileno]]2010118010100[[department]]物理
THE EVOLUTION OF PROTOSTARS .3. THE ACCRETION ENVELOPE
No full text[[abstract]]The radiation gas dynamics of the rapidly inflowing material during the main accretion phase of protostellar evolution is considered. The momentum and energy transfer by radiation and matter, the dissociation of the molecular gas, and the thermochemical destruction of graphite grains are followed in detail. Considerable physical insight is gained by taking advantage of the vastly disparate time and length scales associated with the various processes. Different computational techniques are used in different portions of the accretion flow: the optically thick dust envelope, the thermalization layer of stellar photons, the opacity gap, and the radiative precursor (if one exists). The evolution of the envelope of a protostellar model whose hydrostatic core was described in an earlier communication is summarized.
--------------------------------------------------------------------------------[[fileno]]2010118010106[[department]]物理
MASS, ANGULAR-MOMENTUM, AND ENERGY-TRANSFER IN CLOSE BINARY STARS
No full text[[abstract]]The paper surveys some aspects of the dynamics and energetics of close binary stars which have heretofore not received a detailed review. Attention is directed to those processes of mass, angular momentum, and energy transfer that are thought to have potentially important consequences for the evolutionary history of close binary systems. The presentation, which proceeds mainly along theoretical lines, considers in turn detached binaries, semidetached binaries, and contact binaries.[[fileno]]2010118010107[[department]]物理
ON THE STRUCTURE OF CONTACT BINARIES .2. ZERO-AGE MODELS
No full text[[abstract]]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.
--------------------------------------------------------------------------------[[fileno]]2010118010119[[department]]物理
SPIRAL STRUCTURE, DUST CLOUDS, AND STAR FORMATION
No full text[[fileno]]2010118010128[[department]]物理
COSMIC-RAY VARIATION DUE TO SOLAR MOTION IN GALAXY
No full text[[fileno]]2010118010133[[department]]物理