New Light on Young Stars:
Spitzer's View of Circumstellar Disks

Effects of Accretion Flow on Chemical Structure and Dust Distributions in Protoplanetary Disks

First Author:
Hideko Nomura
Email: nomura AT kusastro.kyoto-u.ac.jp
Kyoto University
Sakyo-ku
Kyoto 606-8502, Japan
Coauthors:
Aikawa, Yuri, Kobe University
Nakagawa, Yoshitsugu, Kobe University
Millar, Tom, Queen's University Belfast

Abstract
The Spitzer Space Telescope have observed the dust continuum emission toward many young circumstellar disks and, in addition, warm molecular lines have been observed toward some disks. Now, accretion flow toward a central star is important for planet formation in the disks, because it is one of the possible mechanisms for the dispersal of the gas, and it will be useful if we can identify observational evidence for the accretion flow in the inner disk. In this work we study the effects of the gas accretion flow on the distribution of molecules in hot inner regions as well as on the spatial and size distribution of dust particles in protoplanetary disks. First, we have performed model calculations of the time-dependent chemical reactions along the accretion flow, which are initiated by ice mantle evaporation, and studied the dependence of the profiles of molecular abundances in the hot (> 100K) inner disks on the accretion flow. Our results have shown that the high C2H2 abundance observed toward a T Tauri star could suggest relatively high accretion velocity in the disks. We have also compared our results with the observed abundances of other warm molecules. Second, we have calculated numerically the coagulation equation for settling and radially moving dust particles, and studied the influence of the accretion flow as well as of the infalling dust particles from the surrounding molecular clouds on the dust continuum emission from the disks. Our results have demonstrated that when the density of the surrounding cloud is high (>10⁴ cm^-3) or the viscous parameter, alpha, is high enough (> 0.001), the observed infrared excess radiation emitted from the dust grains in the disks can be reproduced.
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