Far Ultraviolet Spectroscopic Explorer Observations of CO and H2 Emission in Comet C/2001 A2 (LINEAR)
Observations of comet C/2001 A2 (LINEAR) were made with the Far Ultraviolet Spectroscopic Explorer beginning 2001 July 12.58 and coinciding with a photometric increase of ~1.5 mag. Spectra were obtained in the 905-1180 Å range at 0.25 Å spectral resolution using the 30''×30'' aperture. Several new cometary emissions were identified, particularly the (0,0) bands of the CO Hopfield-Birge systems C-X and B-X at 1088 and 1151 Å, respectively, O I (1D-1D) at 1152 Å, and three lines of the H2 Lyman system at 1071.6, 1118.6, and 1166.8 Å, pumped by solar Lyβ fluorescence. Also detected were O I multiplets at 989, 1027, and 1040 Å and several lines of the H I Lyman series. The rotational envelopes of the CO bands are resolved and appear to consist of both ``cold'' and ``hot'' components, the cold component accounting for 70% of the flux and with a rotational temperature of 55+/-5 K. The hot component may be indicative of a CO2 source. The CO bands, H2 lines, and O I λ1152 all decreased by a factor of 2 over the 7.5 hr observation. The derived time-averaged production rates are Q(CO)=1.3×1027 molecules s-1 and Q(H2O)=2.1×1029 molecules s-1. These values may be uncertain by as much as a factor of 2 because of uncertainties in the solar flux and the electron impact contribution to the excitation.