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K. Magee-Sauer (Rowan Univ.), M. J. Mumma (NASA GSFC), M. A. DiSanti, N. Dello Russo (NASA GSFC / Catholic Univ.), T. W. Rettig (Notre Dame)
We present results of spectroscopic observations of comets Hale-Bopp and Hyakutake obtained using the CSHELL instrument at the NASA IRTF atop Mauna Kea, HI. The spectral resolution of CSHELL was around 20,000 and spectra were obtained with the slit centered on the nucleus.
HCN was detected in comet Hale-Bopp on four dates between February 24.0 and May 1.0 1997 UT. The average rotational temperature retrieved for the (001) vibrational level on UT 1997 April 29.9 was (95 ± 6) K near the nucleus, increasing to (122 ± 8) K in the intermediate coma (4 – 10 arcseconds off the nucleus), based on eight ro-vibrational lines. The HCN production rate on April 29.9 was (3.09 ± 0.13) x 1028 molecules s-1. When compared with the water production rate obtained from direct measurements of the H2O 100-010 band on common observation dates using the same instrument and data processing algorithms, the weighted average (for the four dates) of the relative abundance (HCN/H2O) was (0.40 ± 0.05)%. The measured spatial distribution for HCN is consistent with its release at the nucleus — no significant contribution from a distributed source is required within 10,000 km of the nucleus.
Observations of HCN in comet Hyakutake were obtained on UT 1996 March 24.5. The average rotational temperature retrieved for the (001) vibrational level for this date was (77 ± 8) K near the nucleus, based on 7 ro-vibrational lines. The HCN production rate on UT 1996 Mar 24.5 was (4.70 ± 0.46) x 1026 molecules s-1. When compared with the water production rate measured Mar 24.5, using the same instrument and data processing algorithms, the relative abundance (HCN/H2O) was (0.22 ± 0.03)%.
Production rates and rotational temperatures for C2H2 emission for both comets are in preparation and the progress of this work will be presented.