Accretion Disks around Young Objects. II. Tests of Well‐mixed Models with ISM Dust

Abstract

We construct detailed vertical structure models of irradiated accretion disks around T Tauri stars with interstellar medium dust uniformly mixed with gas. The dependence of the structure and emission properties on mass accretion rate, viscosity parameter, and disk radius is explored using these models. The theoretical spectral energy distributions (SEDs) and images for all inclinations are compared with observations of the entire population of classical T Tauri stars (CTTSs) and class I objects in Taurus. In particular, we Ðnd that the median near-infrared Ñuxes can be explained within the errors with the most recent values for the median accretion rates for CTTSs. We further show that the majority of the class I sources in Taurus cannot be class II sources viewed edge-on because they are too luminous and their colors would be consistent with disks seen only in a narrow range of inclinations. Our models appear to be too geometrically thick at large radii, as suggested by (1) larger far-infrared disk emission than in the typical SEDs of T Tauri stars, (2) wider dark dust lanes in the model images than in the images of HH 30 and HK Tau/c, and (3) a larger predicted number of stars extincted by edge-on disks than consistent with current surveys. The large thickness of the model is a consequence of the assumption that dust and gas are well mixed, suggesting that some degree of dust settling may be required to explain the observations.

High‐Resolution Near‐Infrared Images and Models of the Circumstellar Disk in HH 30

“…Recent detailed models of disk structure Ðnd that the scale height exponent varies with radius in the inner regions but follows a similar power law beyond a few AU (DÏAlessio et al 1999 ;Chiang & Goldreich 1997 ;Bell et al 1997). The scattered-light models of Burrows et al (1996) and demonstrated that high-resolution imaging enables the determination of the disk scale height at large radii.…”

Formation of simple organic molecules in inner T Tauri disks

“…The gas density decreases radially outward as a power law of r, and in the vertical direction it decreases as z increases in roughly an exponential way. The gas heating across the disk Article published by EDP Sciences is dominated by the stellar irradiation of the surface layers at large radii (r ≥ 10 AU), thus, the kinetic temperature increases with z, while at small radii (r ∼ a few AU) viscous dissipation may become an important heating mechanism in the midplane regions (D'Alessio et al 1998(D'Alessio et al , 1999. A flared disk is significantly exposed to the strong UV field from the central star.…”

New Infrared Veiling Measurements and Constraints on Accretion Disk Models for Classical T Tauri Stars

“…These models assumed that a single temperature characterizes any given radius in the disk. Vertical temperature structure in the disk was Ðrst considered by Calvet et al (1991Calvet et al ( , 1992, with subsequent analysis by DÏAlessio et al (1998DÏAlessio et al ( , 1999, CG97, and CG99. In these models, absorption of stellar radiation occurs only in surface layers of the disk, while any heating due to viscous dissipation (accretion) occurs mainly in the midplane.…”

“…In these models, absorption of stellar radiation occurs only in surface layers of the disk, while any heating due to viscous dissipation (accretion) occurs mainly in the midplane. DÏAlessio et al (1998DÏAlessio et al ( , 1999 compute disk emission spectra assuming identical gas and dust temperature structures, whereas CG9799 essentially ignore the gas. The key issue is whether dust opacity and emissivity at the disk surface overwhelms the gas contribution at all wavelengths, in which case the gas may be ignored.…”

“…As expected (CG97 ; CG99), models without a superheated dust layer produced less K-band emission than our model, exacerbating the discrepancy with veiling measurements that are generally higher than expected in any model. DÏAlessio et al (1998DÏAlessio et al ( , 1999 computed detailed disk structure models, generally matching an average CTTS SED from optical to millimeter wavelengths, assuming a disk that extends to the stellar surface. Separate star and disk SEDs are presented only in Figure 13 of DÏAlessio et al (1998) Figure 13 of DÏAlessio et al (1998) implies r K D 0.4.…”