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M. A. DiSanti (NASA-GSFC / Catholic U.), M. J. Mumma (NASA-GSFC), N. Dello Russo (NASA-GSFC / Catholic U.), K. Magee-Sauer (Rowan U.), R. Novak (Iona Coll.), T. W. Rettig (Notre Dame U.)
We present results from high resolution spectroscopic observations of comet C/1995 O1 Hale-Bopp on 12 dates between June 1996 (4.1 AU pre-perihelion) and September 1997 (3.1 AU post-perihelion). The observations used the cryogenic echelle grating spectrometer (CSHELL) which has sensitivity from 1 - 5 microns, and incorporates a 256x256 InSb detector with 0.2" pixels. CSHELL combines excellent spatial (frequently 1" or better) and spectral resolution (R ~ 20,000), enabling detailed studies of line-by-line excitation along the 30" slit length.
Study of the nature and evolution of cometary CO production is an integral part of our program of cometary composition through remote sensing. Our observations yield spatially-resolved information on the distribution of CO molecules in the coma, through emission in the v = 1 - 0 fundamental band near 4.7 microns. This has permitted discrimination between a source stored as ice in the nucleus (the "native" source), and a source of CO produced in the coma (the "distributed" source). The onset of distributed CO emission occurred between 2.0 and 1.5 AU pre-perihelion, and accounted for approximately half of the total CO production for comet Hale-Bopp from January - May 1997.
Comparison with the apparent evolution in continuum intensity suggests that the observed increase (nearly 4-fold) in Q(CO) between 2.0 and 1.5 AU cannot be explained in terms of increased dust production alone. Rather, it is plausible that a threshold (likely thermal) is reached for release of species giving rise to additional CO in the coma. In addition to production rates, rotational temperatures derived by comparison of multiple CO line intensities will be presented.
This work was supported by grants NAG5-4362 to M.A. DiSanti and RTOP 344-32-03 to M.J. Mumma through the NASA Planetary Astronomy Program.