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K. H. Hinkle, S. D. Brittain (NOAO)
We report a detection of the 4.6 micron CO vibration-rotation lines in absorption in the spectrum of \beta Pic. From the R(1), R(2), and R(3) lines we find an excitation temperature of 19±5 K and a CO column density of (2.5±0.5) x 1015 cm-2. The observations were made with the NOAO Phoenix high-resolution, near-infrared spectrograph and the Gemini South telescope. The infrared CO column density is a factor of four larger than the CO column density derived by Roberge et al. (2000 ApJ 538 904) from ultraviolet observations of CO A-X lines.
Assuming a solar carbon abundance, our observed CO column density corresponds to an H2 column density of 3 x 1020 cm-2. There is considerable disagreement in the literature over the \beta Pic disk gas mass. Brandeker et al. (2004 AA 413 681) observed various optical atomic emission lines. These imply a column density of H2 of at least 3 x 1018 cm-2. This column density is also in agreement with an upper limit placed by Lecavelier et al. (2001 Nature 412 706) based on the non-detection of ultraviolet H2 lines. A column density of 3 x 1018 cm-2 implies a total gas mass of ~ 0.1 Mearth. However, this is inconsistent with results by Thi et al.~(2001 ApJ 561 1074) based on the ISO detection of H2, which require approximately 50 times more gas.
Our results will be discussed in light of this and other spectroscopy reported in the literature as well as recent theoretical work.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.