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S. D. Brittain, T. W. Rettig (University of Notre Dame), T. Simon (Univerisy of Hawaii), C. Kulesa (Unviersity of Arizona)
To determine the nature of the disk environments around protostars, it is critical to obtain high resolution spectral information that can be used to constrain the abundance and location of gas phase molecules prior to and during planetesimal formation. Theoretical models of disks predict that dust preferentially settles to the midplane, leaving behind a predominantly gaseous atmosphere at higher vertical scale heights. These theoretical predictions set the stage for models of planet formation -- but have not been observationally tested. The vertical distribution of dust and gas in disks can be assessed by simultaneous comparison of infrared CO absorption lines with infrared extinction. Both extinction and CO line absorption probe the same 'pencil-beam' (~0.01 arcsecond) column of gas. We will present results for 12CO, 13CO, and C18O line absorption in two young disks; their chemical abundances demonstrate closer parity to solar system abundances than their ISM values. The absorbing gas is determined to be cool and located at larger radii, where the line of sight apparently intersects the flared portion of the disk(s). Interestingly, the dust extinction is insufficient to explain the very large columns of absorbing gas. The most straightforward interpretation of the existing data would confirm the stratification of dust and gas in circumstellar disks as predicted by theoretical models.
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Bulletin of the American Astronomical Society, 35 #3
© 2003. The American Astronomical Soceity.