High-Resolution Thermal-IR Observations of Comet C/1995 O1 Hale-Bopp

We have performed high-resolution (lambda /Delta lambda ~ 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(-1) (10.34 mu m) spectral region, looking for evidence of CO2 and NH3 gas line emission from the coma of the comet. The R(6) and R(8) overtone lines of CO2 (966.250 and 967.707 cm(-1) 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 NH3 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 nu 2 band emission is concentrated on a single line (sQ(3,3) at 967 cm(-1) ) in the same bandpass. We also looked for the nu 9 (R) Q0 branch molecular emission of C2H6 at 822.3 cm(-1) (12.16 mu 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.