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 other do not. We also found ones which can be understood as passive disks in the transient phase of particle settling. We found almost the same number of candidate objects (several for each) for passive disks with and without dust particle settling; thus, we can conclude that the timescale for particle settling will be comparable to the lifetime of T Tauri stars (10(6)-10(7) yr). Contrary to such indicators of disk accretion as UV and optical veiling or near-infrared colors or excess, which reflect the disk accretion in the innermost part (< 0.1 AU) or even the boundary layer between the disk and the central star, our analysis of IRAS fluxes will provide an independent probe for disk accretion in the regions relevant to planet formation. We found general correlations between the presence of disk accretion in innermost parts and that in outer regions relevant to planet formation. However, through quantitative discussion, we found some objects which show mass accretion rates higher in innermost parts than in outer parts of the disks. These different mass accretion rates in the inner and outer parts of the disks could produce inner holes in the disks, whose existence has been expected from the deficit in near-infrared fluxes for some objects; this hypothesis on inner hole formation seems to be an interesting alternative to unseen planets clearing the inner-part material. 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.