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H. A. Weaver (JHU/APL), P. D. Feldman (JHU), M. F. A'Hearn (UMd), C. Arpigny (U. Liège), M. R. Combi (U. Michigan), M. C. Festou (Obs. Midi-Pyrénées), V. Krasnopolsky (CUA), C. M. Lisse (UMd), D. E. Shemansky (USC), G.-P. Tozzi (Oss. Astro. Arcetri)
The long-period comet C/2000~WM1 (LINEAR) was exceptionally bright at the time of its discovery in November 2000 at a heliocentric distance of ~6~AU. Although its performance near perihelion fell short of expectations, the comet was still an excellent target because it passed within 0.3~AU of the Earth in December 2001. During that time we organized a campaign of ultraviolet observations using both the Hubble Space Telescope (HST) and the Far Ultraviolet Spectroscopic Explorer (FUSE) and obtained spectra covering a wavelength range from 910-3566~Å. The spatial brightness distributions of OH and CS were measured using the HST data. Emissions in the CO Fourth Positive Group near 1500~Å\ were marginally detected in the HST spectra, but emissions from S2 and the CO Cameron system are apparently absent. Over 90 emission features were detected in the FUSE spectra, including the C-X, B-X, and E-X bands of CO, at least 3 lines of H2, a multiplet of N~I, several multiplets of O~I and C~I, 10 members of the H~I Lyman series, and possibly emission from O~VI. However, most of the lines remain unidentified. The estimated CO abundance was only ~0.4%, which is among the smallest value ever measured. We searched for, but did not detect, argon in the FUSE spectra and estimate that [Ar/O] is depleted by at least a factor of 13 relative to the solar value. The low CO abundance and the absence of S2 and argon implies that the nucleus of C/2000~WM1 (LINEAR) has been warmed to \geq60~K for a substantial period of time, which suggests a formation region at a relatively small heliocentric distance.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.