A Low-Mass H₂ Component to the AU Microscopii Circumstellar Disk^*

We present a determination of the molecular gas mass in the AU Microscopii circumstellar disk. Direct detection of a gas component to the AU Mic disk has proven elusive, with upper limits derived from ultraviolet absorption line and submillimeter CO emission studies. Fluorescent emission lines of H₂, pumped by the O VI λ1032 resonance line through the C-X (1-1) Q(3) λ1031.87 Å transition, are detected by the Far Ultraviolet Spectroscopic Explorer. These lines are used to derive the H₂ column density associated with the AU Mic system. The derived column density is in the range N(H₂) = 1.9 × 10¹⁷ to 2.8 × 10¹⁵ cm^-2, roughly 2 orders of magnitude lower than the upper limit inferred from absorption line studies. This range of column densities reflects the range of H₂ excitation temperature consistent with the observations, T(H₂) = 800-2000 K, derived from the presence of emission lines excited by O VI in the absence of those excited by Lyα. Within the observational uncertainties, the data are consistent with the H₂ gas residing in the disk. The inferred N(H₂) range corresponds to H₂-to-dust ratios of ≲  : 1 and a total M(H₂) = 4.0 × 10^-4 to 5.8 × 10^-6 M_⊕. We use these results to predict the intensity of the associated rovibrational emission lines of H₂ at infrared wavelengths covered by ground-based instruments, HST NICMOS, and the Spitzer IRS.