Dust Particle Settling in Passive Disks around T Tauri Stars: Models and IRAS Observations

We present intensive comparisons of the observed IRAS fluxes at mid- to far-infrared wavelength regions for a sample of 62 T Tauri stars (including both classical and weak-lined T Tauri stars) in the Taurus-Auriga cloud complex with theoretical emergent thermal fluxes from simple passive reprocessing disk models. The fluxes at IRAS wavelengths are sensitive to the parts of a disk with temperature approximately 30-300 K, which correspond to the parts with distance from the central star of 0.1-100 AU -- the regions of planet formation, at least in our solar system. Thus, detailed comparisons of the IRAS observations and theoretical disk models provide direct probes for the temperature distributions in the probable planet-forming regions. Indeed, we can probe not only for the presence of mass accretion through the disk in the planet-forming regions, but also, among purely passive reprocessing disks without intrinsic luminosity, for the occurrence of dust particle settling toward the midplane, which will change the distribution of the height of the absorbing surface of the disk against the stellar light and thus the efficiency of central stellar heating. As a result of the intensive comparisons, 14 objects among the 46 classical T Tauri stars of our sample are found to be almost passive or to have no intrinsic disk luminosity, while 12 objects among the 16 weak-lined T Tauri stars are found to be so. Furthermore, among passive disk candidates, we found that several disks show evidence of dust settling while others do not. We also found ones which can be understood as passive disks in the transient phase of particle settling. Particle settling, the occurrence of which has been suggested by the present study, is also expected theoretically by the standard scenario of planetary system formation and is inevitable in quiescent disks where turbulence has ceased. It will lead to the formation of a dust-rich layer around the midplane of the disk, which may provide an environment for subsequent planetesimal formation and planetary accretion.