PROTOPLANETARY DISKS, MIDINFRARED DIPS, AND DISK GAPS

The spectral energy distributions (SEDs) of pre-main-sequence (PMS) st ars provide observational constraints on the physical properties of th e protoplanetary disks that dominate PMS emission at wavelengths longe r than a few microns. Many PMS stars have a peculiar signature: a rela tively smooth SED from similar to 1 mu m to similar to 100 mu m, accom panied by a broad dip in the mid-infrared (similar to 10 mu m). These mid-IR dips could be caused by gaps in a protoplanetary disk with a si mple power-law structure, or they could arise in a continuous disk wit h a more realistic structure than the power-law assumption permits. A two-dimensional radiative hydrodynamics code has been used to construc t a suite of detailed protoplanetary disk models, where the thermal st ructure is determined by the balance between radiative cooling and com pressional or viscous heating (Boss 1996). Here we use a two-dimension al ray-tracing code (Yorke 1986) to compute the SEDs of these disk mod els for comparison with the nominal SED of the prototypical PMS star w ith a mid-IR dip, T Tauri. We have reproduced the SED of the same disk as that used by Boss & Yorke (1993), which yielded a good fit to the T Tau spectrum, and have extended the results to include models with v aried disk masses, stellar masses, inclinations, opacities, mass accre tion rates, alpha-viscosities, and midplane density profiles. For disk s undergoing mass accretion from their envelopes at a rate of similar to 10(-6) to 10(-5) M(.) yr(-1), disks with masses of similar to 0.01- 0.02 M(.) orbiting a solar-mass star yield SEDs close to that of T Tau . Variations in the dust grain opacity have relatively little effect, but a steeper midplane profile (rho(0) proportional to r(-2), instead of rho(0) proportional to r(-3/2)) leads to a mid-IR hump rather than a dip. For a disk mass of similar to 0.02 M(.), disk mass accretion ra tes in the range of similar to 10(-7) to 10(-5) M(.) yr(-1) are indica ted for T Tan, When viscous heating is employed, a viscous alpha = 0.0 1 yields a good fit, whereas alpha = 0.1 produces a large mid-IR exces s. These SEDs should be useful for interpreting the Infrared Space Obs ervatory (ISO) and the Space Infrared Telescope Facility (SIRTF) obser vations of protoplanetary disks.