Observation and Analysis of High Resolution Optical Line Profiles in Comet Hyakutake (C/1996 B2)

M. R. Combi (SPRL/U. Michigan), A. L. Cochran (U. Texas)

Very high resolution (R=200,000) and high signal-to-noise echelle spectra were obtained of comet C/Hyakutake 1996 B2 using the 2DCoude spectrograph on the 2.7 m telescope at McDonald Observatory during late March and early April 1996. Doppler resolved profiles are presented for individual lines of most of the major optical neutral species: CN, C tex2html_wrap_inline16 , O( tex2html_wrap_inline18 D) at 6300 tex2html_wrap_inline20 , O( tex2html_wrap_inline18 S) at 5577 tex2html_wrap_inline20 , NH tex2html_wrap_inline16 , and H Balmer-alpha at 6563 tex2html_wrap_inline20 . These may be the first ever to be published for CN, C tex2html_wrap_inline16 , and O( tex2html_wrap_inline18 S). In all cases the instrument spectral function is smaller than the intrinsic line widths of the individual cometary lines, so the observations provide clear signatures of lines which are Doppler broadened by different combinations of the coma expansion, exothermic photochemical ejection speeds, and collisional thermalization. For modeling analysis of these data we have used a hybrid fluid/kinetic Monte Carlo approach which can realistically include all of the relevant physical/chemical processes important for shaping the spectral lines. Because of the very short lifetime of the NH tex2html_wrap_inline16 parent (NH tex2html_wrap_inline36 ), the NH tex2html_wrap_inline16 is collisionally thermalized and provides an excellent probe of the outflow of the expanding coma. Because O( tex2html_wrap_inline18 D) atoms in the region sampled are produced mainly by the photodissociation of water and the resulting photon is a prompt emission, the line retains signatures of both the basic coma expansion velocity and the 1.6 km s tex2html_wrap_inline42 ejection speed of the O( tex2html_wrap_inline18 D) atoms. The O( tex2html_wrap_inline18 S) profile is consistent with that for the O( tex2html_wrap_inline18 D). The profiles of CN and C tex2html_wrap_inline16 are somewhat broadened (CN more so than C tex2html_wrap_inline16 ), compared with NH tex2html_wrap_inline16 , and seem to require a combination of coma expansion and the exothermic ejection speed they receive upon their production. Although the H Balmer-alpha line is complicated by a chance coincidence of an H tex2html_wrap_inline16 O tex2html_wrap_inline58 line and optical depth effects in the solar Lyman-beta which pumps the Balmer-alpha emission, the spread of the wings of the line is consistent with other observations of past comets and the production by dissociation of H tex2html_wrap_inline16 O and OH, and subsequent partial thermalization.