Title: Phoenix Observations of CH4, HCN and CO in the Planet Formation Zone
Abstract: The structure and composition of envelopes and disks surrounding protostars are key factors in planet and comet formation. The region at 1-10s of AU is of particular interest, but often highly obscured in young systems and therefore not accessible at optical wavelengths. High resolution infrared spectroscopy provides a powerful probe of the gas and dust close to young stars, however. Indeed, in our large 4.7 um CO survey of protostars line widths of several 10s of km/s have been observed both in emission and absorption. These velocities tie the gas to the inner few AU of the disk/envelope. We propose to follow up on this survey and observe the fundamental C-H stretching bands of HCN at 3.1 um and CH4 at 3.3 um using the R=60,000 (5 km/s) mode of Phoenix at Gemini-South. As gas and dust travel toward the protostar, CH4 will first sublimate from icy grain mantles and is expected to 'burn' rapidly into CO. However, at high pressures and moderate temperatures CO can be converted back into CH4, and the CH4/CO abundance ratio may thus be a unique tracer of protoplanetary condensations within disks. On the other hand, HCN is a tracer of high temperature `hot core'-type chemistry in the very inner regions of protostellar disks. At high spectral resolution provided by Phoenix, the line profiles will localize the absorbing/emitting regions, and thus the disk composition in the potential planet formation zone.
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