AAS 205th Meeting, 9-13 January 2005
Session 11 Brown Dwarfs and Exoplanets
Poster, Monday, January 10, 2005, 9:20am-6:30pm, Exhibit Hall

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[11.22] Limits on Primordial Gas in the AU Microscopii Disk from Far-UV Spectroscopy

A. Roberge, A. J. Weinberger (DTM-CIW), P. D. Feldman (JHU), S. Redfield (U. of Texas)

AU~Mic (GJ 803) recently became the first circumstellar disk imaged around an M-type star (Kalas, Liu, & Matthews 2004). AU~Mic is a member of the \beta~Pictoris moving group, indicating it is about 12~Myr old (Zuckerman et al. 2001). This disk is extraordinarily well-suited for comparison to the \beta~Pic debris disk and provides a unique opportunity to examine the dependence of disk evolution on spectral type. The low inclination of the AU~Mic disk (< 1\circ within 50~AU; Krist et al. 2004) permits investigation of the disk gas with line-of-sight absorption spectroscopy, as has been done in the case of \beta~Pic (i = 3\circ; Heap et al. 2000). In particular, far-UV absorption spectroscopy of molecular hydrogen is sensitive to small amounts of cold gas, since the far-UV electronic transitions of molecular hydrogen are very strong (unlike the far-IR pure rotational transitions). We present an upper limit on H2 gas in the disk using the Far Ultraviolet Spectroscopic Explorer satellite. This limit is much lower than that obtained from non-detection of mm-wavelength CO emission, and does not require extrapolation from an assumed CO/H2 ratio (Liu et al. 2004). We discuss the implications of this limit for the depletion of primordial gas in the AU~Mic disk and compare it to the limit for \beta~Pic.

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