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

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

Feb 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999apj...512..961c&link_type=abstract

The Astrophysical Journal, Volume 512, Issue 2, pp. 961-968.

Astronomy and Astrophysics

Astronomy

17

Comets: Individual (Hyakutake [C/1996 B2]), Hydrodynamics, Line: Profiles, Molecular Processes

Scientific paper

Very high resolution (R~200,000) and high signal-to-noise echelle spectra were obtained of comet Hyakutake (C/1996 B2) using the 2D coudé spectrograph on the 2.7 m telescope at W. J. McDonald Observatory during 1996 in late March and early April. Doppler resolved profiles are presented for individual lines of the major optical neutral species: CN, C_2, NH_2, O(^1D) at 6300 Å, and H Balmer-alpha at 6563 Å. They are consistent with the expected expansion of a water-dominated cometary coma. Because of the small aperture and the small geocentric distance of the comet, the profiles of CN, C_2, and NH_2 are totally shaped by the outflowing hydrodynamic coma. The NH_2 is the narrowest of the group because of the very short lifetime of the NH_2 parent (NH_3). The profile of C_2 is somewhat broadened, compared with NH_2, because of the relatively larger contribution of C_2 radicals farther from the nucleus where the bulk outflow speed is larger. Any exothermic ejection speed they receive upon their production would be quenched. Although the profile of CN is broader than C_2, this is due to the fact that most of the CN lines are blends of two closely spaced but nearly equal strength components. Because O(^1D) atoms in the region samples are produced mainly by the photodissociation of water which results in a prompt emission photon, the line retains contributions of both the basic coma expansion velocity and the 1.6 km s^-1 exothermal ejection speed of the O(^1D) atoms. The H Balmer-alpha line is complicated by a chance coincidence of a line of H_2O^+ and/or NH_2 and by optical depth effects in solar Lybeta which are primarily responsible for the Balmer-alpha excitation. However, the width of the line wings is consistent with other comet observations, including Hyakutake itself, and the production by dissociation of H_2O and OH and partial thermalization. We have successfully reproduced these data using a hybrid kinetic/hydrodynamic and Monte Carlo approach which include the important physical/chemical processes which can shape the spectral lines. Because the model includes no adjustable fitting parameters, the model-data agreement indicates that the underlying assumptions and calculated coma outflow speeds are quite reasonable.

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