High-Resolution Thermal-IR Observations of Comet C/1995 O1 Hale-Bopp
P. V. Sada (CAM), D. E. Jennings, D. C. Reuter, and G. H. McCabe (NASA-GSFC)
We have performed high-resolution ( / 10,000) spectroscopic observations of comet C/1995 O1 Hale-Bopp at selected thermal-infrared wavelengths on February 18-20, 1997. We used CELESTE, a Goddard-developed cryogenic echelle spectrometer, in conjunction with the McMath-Pierce Solar Telescope (NSO/NOAO) at Kitt Peak National Observatory.
In particular we observed the 966-968 cm (10.34 m) spectral region, looking for evidence of CO and NH gas line emission from the coma of the comet. The R(6) and R(8) overtone lines of CO (966.250 and 967.707 cm respectively) are good detection candidates because they are well populated level transitions at 100 K, and because the Doppler velocity line shift (about -31 km/s at the time) separates comet transitions from weak telluric absorption lines. Although NH is no longer considered an abundant species in cometary nuclei, we searched for it because non-LTE calculations (Weaver and Mumma 1984, ApJ 276, 782) predict that 10% of the entire band emission is concentrated on a single line (sQ(3,3) at 967 cm ) in the same bandpass. We also looked for the Q branch molecular emission of C H at 822.3 cm (12.16 m). This molecule has recently been detected in the near-IR on comet C/1996 B2 Hyakutake (Mumma et al. 1996, Science 272, 1310).
In both instances a strong dust emission continuum from the comet's coma was detected. However, no molecular emission nor absorption features were detected superimposed on this continuum to our sensitivity limits. These observations place constraints on the temperature and composition of the gas within the coma.