CO Emission in Comets C/1995 O1 (Hale-Bopp) and C/1996 B2 (Hyakutake): Evidence for a Distributed Source

M. A. DiSanti, M. J. Mumma, N. R. Dello Russo (NASA-GSFC), K. Magee-Sauer (Rowan Univ.), R. Novak (Iona College), T. W. Rettig (Notre Dame Univ.), M. N. Fomenkova (U. Cal.-La Jolla)

We used high resolution spectroscopy to target CO emission in C/1995 O1 on 14 days between June 1996 (R tex2html_wrap_inline16 = 4.1 AU pre-perihelion) and May 1997 (R tex2html_wrap_inline16 = 1.06 post-perihelion). We present examples of these observations, and compare with our pre-perihelion measurements of C/1996 B2 in March/April 1996. The observations are part of our comprehensive program of compositional studies of comets, using the Cryogenic Echelle Spectrometer (CSHELL) at the NASA-IRTF 3-m telescope atop Mauna Kea. The instrument operates in the tex2html_wrap_inline20 m region, incorporating a 256x256 InSb detector array having 0.2 tex2html_wrap_inline22 pixels. CSHELL combines excellent spatial and spectral resolution ( tex2html_wrap_inline24 ), enabling detailed studies of line-by-line excitation over the 30 tex2html_wrap_inline22 length of the slit. For C/1996 B2, we sampled lower state rotational levels J = 0 through 6, while for C/1995 O1, levels up to J = 10 were sampled. We thereby have maps of CO excitation in the coma for these two comets and, in the case of C/1995 O1, over a large range in heliocentric distance.

The majority of spectra were obtained with the cometary nucleus centered in the (east-west) slit. Spatial profiles of the CO emission exhibit a central r tex2html_wrap_inline28 brightness distribution, indicative of a native source, superimposed upon a relatively flat underlying profile representing a distributed source of CO. We also mapped CO over a range of J-values in C/1995 O1, by offsetting the slit from the nucleus, both along and transverse to the slit length. Examples of CO profiles will be presented for these two comets, and compared with the continuum, and with volatiles having a seemingly purely native source, such as ethane.

This work is supported by the NASA Planetary Astronomy Program, under CPN 075-20757 to M. A. DiSanti, and RTOP 344-32-03 to M. J. Mumma.