The Formation of Protostellar Disks. III. The Influence of Gravitationally Induced Angular Momentum Transport on Disk Structure and Appearance

Abstract

Hydrodynamical two-dimensional calculations are presented for the evolution of collapsing, rotating protostars, including the effects of radiative acceleration and angular momentum transport. The initial cloud is assumed to be a uniformly rotating centrally condensed sphere with ρ ∝ r-2. Results are presented for masses of 1, 2, and 10 M☉, over times comparable to protostellar lifetimes. The calculations show how a warm, quasi-hydrostatic disk surrounding a central unresolved core forms, grows in mass and size, and accretes onto the central object. As a result of the accretion of material from the parent cloud, the disk is encased in one or two accretion shock fronts, located several scale heights above the equatorial plane. During accretion, the disks grow radially due to the effects of angular momentum transport, which according to our model arises from tidally induced gravitational torques. In this manner, quasi-static disks in excess of several thousand AU in radius can be produced. Accretion onto the central object slows down, however, and rather long timescales (M/ 107 yr) are reached while an appreciable fraction of the total mass (≈35%) is still in the disk. In order to further reduce the disk mass on a shorter timescale, processes not considered here must be invoked. Alternatively, if the initially selected angular momentum is significantly lower, smaller disk sizes would result. Frequency-dependent radiative transfer calculations at selected ages, including the effects of scattered radiation in the infrared and optical spectral regimes, show how the continuum spectra of the structure depend on the disk's orientation and age and how the observed isophotal contours vary with wavelength and viewing angle. Because of the strong dependence on viewing angle, continuum spectra alone should not be used to estimate the evolutionary stage of development of these objects. The infrared flux at ~10 μm can vary by orders of magnitude between pole-on and edge-on views, and the inferred total bolometric luminosity will vary by up to a factor of 30 as a function of viewing angle during much of the lifetime of accreting circumstellar disks. We conclude that near- and mid-infrared searches for disks will be strongly biased toward pole-on orientations because of this flashlight effect.