THE STRUCTURE AND APPEARANCE OF PROTOSTELLAR ACCRETION DISKS - LIMITSON DISK FLARING

Vertical structure models are used to investigate the structure of pro tostellar, a-law, accretion disks. Conditions investigated cover a ran ge of mass fluxes (10(-9) to 10(-5) M. yr(-1)), viscous efficiencies ( alpha = 10(-2) and 10(-4)), and stellar masses (0.5-3 M.). Analytic fo rmulae for midplane temperatures, optical depths, and volume and surfa ce densities are derived and are shown to agree well with numerical re sults. The temperature dependence of the opacity is shown to be the cr ucial factor in determining radial trends. We also consider the effect on disk structure of illumination from a uniform field of radiation s uch as might be expected of a system immersed in a molecular cloud cor e or other star-forming environment: T-amb = 10, 20, and 100 K. Model results are compared to Hubble Space Telescope observations of HH30 an d the Orion proplyds. Disk shape is derived in both the Rosseland mean approximation and as viewed at particular wavelengths (lambda lambda = 0.66, 2.2, 60, 100, 350, and 1000 mu m) In regions where the opacity is an increasing function of temperature (as in the molecular regions where kappa proportional to T-2), th, disk does not flare, but decrea ses in relative thickness with radius under both Rosseland mean and si ngle wavelength approximations. The radius at which the disk becomes s hadowed from central object illumination depends on radial mass how an d varies from a few tenths to about 5 au over the range of mass fluxes tested. This suggests that most planet formation occurred in environm ents unheated by stellar radiation. Viewing the system at any single w avelength increases the apparent flaring of the disk but leaves the sh adow radius essentially unchanged. External heating further enhances f laring at large radii, but, except under extreme illumination (100 K), the inner disk will shield the planet-forming regions of all but the lowest mass flux disks from radiation originating near the origin such as from the star or from an FU Orionis outburst.