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 H _ { 2 } , pumped by the O vi \lambda 1032 resonance line through the C – X ( 1 – 1 ) Q ( 3 ) \lambda 1031.87 Å transition , are detected by the Far Ultraviolet Spectroscopic Explorer .
These lines are used to derive the H _ { 2 } column density associated with the AU Mic system .
The derived column density is in the range N ( H _ { 2 } ) = 1.9 \times 10 ^ { 17 } – 2.8 \times 10 ^ { 15 } cm ^ { -2 } , roughly two orders of magnitude lower than the upper limit inferred from absorption line studies .
This range of column densities reflects the range of H _ { 2 } excitation temperature consistent with the observations , T ( H _ { 2 } ) = 800 – 2000 K , derived from the presence of emission lines excited by O vi in the absence of those excited by Ly \alpha .
Within the observational uncertainties , the data are consistent with the H _ { 2 } gas residing in the disk .
The inferred N ( H _ { 2 } ) range corresponds to H _ { 2 } -to-dust ratios of \lesssim \frac { 1 } { 30 } :1 and a total M ( H _ { 2 } ) = 4.0 \times 10 ^ { -4 } – 5.8 \times 10 ^ { -6 } M _ { \oplus } .
We use these results to predict the intensity of the associated rovibrational emission lines of H _ { 2 } at infrared wavelengths covered by ground-based instruments , HST -NICMOS , and the Spitzer -IRS .