A Comprehensive Study of Proto-Planetary Disks around Herbig Ae Stars using Long-Baseline Infrared Interferometry.

Planetary systems are born in circumstellar disks around young stellar objects (YSOs) and the disk is thought to play a major role in the evolution of planetary systems. A good understanding of disk structure and its time evolution is therefore essential in comprehending planet formation, planet migration and the diversity of planetary systems. In this thesis, I use high angular resolution observations and state-of-the-art radiative transfer modeling to probe circumstellar disk structure and validate current disk models. First, I discuss models and observations of the gas-dust transition region in YSOs. The dust component in circumstellar disks gets truncated at a finite radius from the central star, inside of which it is too hot for dust to survive. The truncated disk forms an ``evaporation front'' whose shape depends sensitively on dust properties. The possibility of using the front as a probe of the dust physics operating in circumstellar disks is explored. The Center for High Angular Resolution Astronomy~(CHARA) near-infrared~(near-IR) array is used to resolve out the evaporation front in the Herbig Ae stars MWC275 and AB Aur, and the presence of an additional near-IR opacity source within the ``conventional'' dust destruction radius is reported. Second, I describe comprehensive disk models that simultaneously explain the spectral energy distribution (from UV to milli-meter ) and long-baseline interferometry (from near-IR to mm) of Herbig Ae stars. The models are constrained with a wide range of data drawn from the literature as well as new interferometric observations in the K-band with the CHARA array and in the mid-IR with the novel Keck Segment Tilting Experiment. I show that the mid-IR size of MWC275 relative to AB~Aur is small, suggesting that dust grains in the outer disk of MWC275 are significantly more evolved/settled than the grains in the AB~Aur disk. I conclude with a discussion on exciting prospects for measuring the gas-disk morphology on scales of fractions of an AU with the CHARA array, introducing a new powerful tool to understand the ``star-disk connection''.Ph.D.Astronomy and AstrophysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/61678/1/atannirk_1.pd

The Hot Inner Disk of FU Ori

We have constructed a detailed radiative transfer disk model which reproduces the main features of the spectrum of the outbursting young stellar object FU Orionis from ~ 4000 angstrom, to ~ 8 micron. Using an estimated visual extinction Av~1.5, a steady disk model with a central star mass ~0.3 Msun and a mass accretion rate ~ 2e-4 Msun/yr, we can reproduce the spectral energy distribution of FU Ori quite well. With the mid-infrared spectrum obtained by the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope, we estimate that the outer radius of the hot, rapidly accreting inner disk is ~ 1 AU using disk models truncated at this outer radius. Inclusion of radiation from a cooler irradiated outer disk might reduce the outer limit of the hot inner disk to ~ 0.5 AU. In either case, the radius is inconsistent with a pure thermal instability model for the outburst. Our radiative transfer model implies that the central disk temperature Tc > 1000 K out to ~ 0.5 - 1 AU, suggesting that the magnetorotational instability (MRI) can be supported out to that distance. Assuming that the ~ 100 yr decay timescale in brightness of FU Ori represents the viscous timescale of the hot inner disk, we estimate the viscosity parameter (alpha) to be ~ 0.2 - 0.02 in the outburst state, consistent with numerical simulations of MRI in disks. The radial extent of the high mass accretion region is inconsistent with the model of Bell & Lin, but may be consistent with theories incorporating both gravitational instability and MRI.Comment: 32 pages, 10 figures, to appear in the Astrophysical Journa