AAS 205th Meeting, 9-13 January 2005
Session 56 Planets and Solar System Objects
Poster, Tuesday, January 11, 2005, 9:20am-6:30pm, Exhibit Hall

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[56.11] The Fourth Positive System of CO in the HST Spectrum of Comet C/2002 C1 (Ikeya-Zhang)

R.E. Lupu, P.D. Feldman (JHU), H.A. Weaver (JHU/APL), G.-P. Tozzi (Oss.Arcetri)

The HST-STIS spectrum of comet C/2002 C1 (Ikeya-Zhang) was obtained on 2002 April 20 (rh = 0.89 AU, vh = 29.1 km s-1) in the wavelength range 1150-1730 Åwith a resolution of 4 Å. The spectrum shows the rich structure of the Fourth Positive Group of carbon monoxide. The variation of relative intensities of the A-X bands along the slit is consistent with a spatial distribution of CO which is optically thick close to the nucleus and becomes optically thin as the distance to the nucleus increases. A model for the CO A-X system including the latest values for the oscillator strengths and the effects of self-absorption on a line-by-line basis is constructed. The model enables us to evaluate the CO column density along the slit by simultaneously fitting all the bands of the 4PG seen in the STIS spectrum. To improve the fit, contributions from H2, C I, C II, O I and S I are included in the model spectrum. The STIS spectrum of comet C/2002 Q4 (NEAT) taken on 2004 April 25 (rh = 1.03 AU, vh = -10 km s-1) also shows emission from the Fourth Positive Group of CO. For this comet a FUSE spectrum was taken at about the same time. This dataset deserves further attention due to the possibility of comparing FUSE observations of CO emission in the B-X (1150 Å) and C-X (1088 Å) bands with predictions based on the analysis of the Fourth Positive System.

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