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I. Kamp (STScI ESA), W. Freudling (ESO), J.N. Chengalur (NCAR, India)
Even though it is generally believed that planets form in protoplanetary disks, little is known about the process itself. While IR imaging and spectral energy distributions of such disks reveal significant grain growth even in the earliest stages of disk evolution, little is known about the evolution of the gas in those disks. Standard methods of gas detection such as CO submm and H2 near IR lines either trace only one component of the disk (the cold or warm molecular gas) or fail entirely in later stages of disk evolution. The interpretation of such observations and the identification of more suitable gas tracers makes it necessary to develop detailed chemical models of these disks, including all the relevant physics.
It turns out that the surfaces of optically thick disks as well as a large fraction of the tenuous optically thin disks are atomic, thus making HI a viable probe for the disk structure models. We model here the HI 21cm line emission from our 2D disk models and compare them to observed upper limits; this helps to constrain the gas masses of young protoplanetary and old debris disks. It turns out that the debris disk systems, where the predicted HI/total mass fraction is very large (10-100%), are the best candidates for detecting 21cm flux. For example, the maximum total gas mass for beta Pictoris (10 Myr), that is still consistent with the HI 21cm line upper limits, is below 5 MEarth; this mass is much lower than the one necessary to explain the Na scattered light observations of Olofsson et al. (2004), 40 MEarth. Hence we suggest that the disk might be hydrogen deficient, that is composed primarily of gas evaporated in asteroid collisions and/or from comets.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.