The NH and CH bands of Comet C/1996 B2 (Hyakutake)

Comet Hyakutake was an ideal target for studying the near-nuclear region of a bright comet. On 1996 March 26.3-26.5 (UT) we observed the comet with an echelle spectrometer at the 4-m telescope atop Kitt Peak. High-resolution spectra of NH and CH are presented and compared with fluorescence models, At a spectral resolution of Delta lambda less than or equal to 0.2 Angstrom two multiplets of ND are not blended by the overlying NH line forest. From these lines we derived a 3 sigma upper limit for the D/H ratio in NH, (D/H)(NH) less than or equal to 0.006. We reevaluated the g-factors for the NH (0-0) band and present g-factors for the strongest ND multiplets as a function of heliocentric velocity. Based on our measurements of the OH and NH (0-0) band we inferred a relative production rate Q(NH3)/Q(H2O) = (3.5 +/- 1.0) x 10(-3). We demonstrate that single-cycle fluorescence models can be used to describe the observed CH bands, if we introduce different rotational temperatures for the two spin states, T(F-1) = 100 Ii and T(F-2) = 130 K. These temperatures are believed to be remnants from a previous dissociative excitation. Obviously collisions or radiative processes cannot completely relax CH before the radical decays. The rotational temperature may thus provide clues to the identity of the CH parent(s). A theoretical analysis of CD suggests that several lines of the CD A-X system are not masked by CH lines. (C) 1998 Academic Press.