We have been studying the structure of circumstellar dust disks, one of the principal goals being to obtain measurements of the gas-to-dust (G/D) mass ratio. This ratio is of pivotal importance for the processes of planet formation. Dust settling in the midplane of the disks and dust growth are conditional for the formation of Earth-like planets, while the gas phase is the main driver of the dust dynamics, but also of complex chemistry.
Disk models require numerous parameters describing grain size distribution, disk flaring, disk size, inclination angle, etc, requiring a commensurate number of input parameters. We have performed a detailed modeling of the near-edge-on massive disk of IRAS 04158+2805 in Taurus, using near-infrared images in three bands (left figure), Spitzer and 2MASS photometry (right figure), Spitzer IRS spectroscopy (right figure), optical imaging polarimetry, and millimeter measurements to confine the interesting parameters (work led by graduate student A. Glauser; Glauser et al. 2007).
Several parameters were tightly constrained, and the dust column toward the central star was derived from the model. The gas column density toward the star was derived from Chandra X-ray spectral modeling. The resulting G/D = 220+/-160 indicates, although poorly constrained toward low values, that dust depletion has not occurred by orders of magnitude along the line of sight. This being the first time that this IR/X-ray methodology has been applied, we are planning a systematic study for various disk inclination angles in order to probe dust settling and therefore the first steps to planet formation.
We have identified anomalous gas column densities in strongly accreting T Tauri stars such as DG Tau (see below). Here, a G/D mass ratio of ~1000 (ten times the interstellar value) is interpreted as being due to dust evaporation before the mass streams enter the accretion funnel flows. The latter absorb the stellar X-ray emission but lead to little additional visual extinction (Güdel et al. 2007b).