ITAL]Far Ultraviolet Spectroscopic Explorer[/ITAL] Observations of CO and H[TINF]2[/TINF] Emission in Comet C/2001 A2 (LINEAR)

Abstract

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 A range at 0.25 A 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 A, respectively, O I (1D-1D) at 1152 A, and three lines of the H2 Lyman system at 1071.6, 1118.6, and 1166.8 A, pumped by solar Lyβ fluorescence. Also detected were O I multiplets at 989, 1027, and 1040 A and several lines of the H I Lyman series. The rotational envelopes of the CO bands are resolved and appear to consist of both and 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.