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P. D. Feldman, H. A. Weaver (JHU), E. B. Burgh (U. Wisconsin)
We report observations of comet C/2001 A2 (LINEAR) with the Far Ultraviolet Spectroscopic Explorer beginning July 12.58 coinciding with a photometric increase of ~1.5 magnitudes. Spectra were obtained in the 905--1180~Å\ range at 0.3 Å\ spectral resolution using the 30'' \times 30'' aperture. Several new cometary emissions were identified, particularly the (0,0) bands of the CO Birge-Hopfield systems (C-X and B-X) at 1088 and 1151 Å, respectively, \ion{O}{1} (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 Lyman-\beta fluorescence. Also detected were \ion{O}{1} multiplets at 989, 1027, and 1040 Å, and several lines of the \ion{H}{1} Lyman series. The rotational envelopes of the CO bands are resolved and appear to consist of both cold and warm components, the cold component accounting for 80% of the flux and with a rotational temperature of 60 K. The warm component may be indicative of a CO2 source. Both the CO bands and the \ion{O}{1} \lambda1152 emission (an indicator of H2O production) decreased by a factor of two over the 7.5 hr observation. Preliminary estimates of the production rates at the beginning of the observation are Q({\rm CO}) = 4 \times 1027 molecules s-1 and Q({\rm H2O}) = 3 \times 1029 molecules s-1 (vectorial model). These values may be uncertain by as much as a factor of two due to uncertainties in the solar flux. No emission is detected from Ar I at 1048 and 1067 Å\ and \ion{He}{1} at 584~Å\ (in second order). We derive Q({\rm Ar}) \le 6 \times 1025 atoms s-1 (5\sigma upper limit), which implies that Ar/O is more than a factor of ten less than solar. In addition to the features listed above, there are about two dozen other emissions that have not yet been definitively identified, although some appear to be from \ion{N}{1}, \ion{C}{1}, and \ion{S}{1}. We do not detect any emission from \ion{O}{6}, which may constrain models that seek to explain cometary X-ray emission as being produced by charge exchange of solar wind ions with cometary neutrals.
This work is based on data obtained for the Guaranteed Time Team by the NASA-CNES-CSA FUSE mission operated by The Johns Hopkins University. Financial support has been provided by NASA contract NAS5-32985.
The author(s) of this abstract have provided an email address for comments about the abstract: pdf@pha.jhu.edu