Ultraviolet CO Emission in Comet C/1996 B2 (Hyakutake)

Details Ultraviolet CO Emission in Comet C/1996 B2 (Hyakutake) Ultraviolet CO Emission in Comet C/1996 B2 (Hyakutake)

Sep 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996dps....28.0929m&link_type=abstract

American Astronomical Society, DPS meeting #28, #09.29; Bulletin of the American Astronomical Society, Vol. 28, p.1093

Astronomy and Astrophysics

Astronomy

4

Scientific paper

The pre-perihelion, near Earth passing of comet C/1996 B2 (Hyakutake) in March--April 1996 gave an unprecedented opportunity for detailed study of the inner coma of a comet with Hubble Space Telescope. To follow up our April observations, we observed Hyakutake post-perihelion at roughly the same heliocentric distance (June 2, 1996, r = 0.935 AU, Delta = 1.12 AU) using the Faint Object Spectrograph with a smaller aperture to achieve higher spectral resolution. Spectra were taken with the G270H and G190H gratings using the 0.43" radius aperture (350 km at the comet.) Pointing analysis showed the comet to be offset 0.3" (240 km) from the center of the aperture during the G190H observations. The G190H spectra showed emission from CO in both the Fourth Positive (A(1Pi ) --X(1Sigma ) (+) ) and Cameron (a(3Pi ) --X(1Sigma ) (+) ) systems. These emissions were fit simultaneously with synthetic spectra via a least squares method. After applying a correction for optical depth effects, we derive {Q}CO = 2.3x1028 s^{-1} or ~ 14% relative to {Q}H_2O (derived from the G270H spectrum.) The shape of the CO Cameron bands is consistent with a primarily ``cold'' (T ~ 200 K) rotational distribution indicating the dominant source of this emission is electron impact on CO as photodissociation of CO_2 produces Cameron emission with a ``hot'' (T ~ 1000 K) rotational distribution. Our best fit to the data indicates that 80% of the Cameron emission is due to electron impact on CO and 20% is due to photodissociation of CO_2. This leads to {Q}CO = 1.8x1028 s^{-1} and {Q}CO_2 = 6.3x1027 s^{-1} ( ~ 11% and ~ 4% relative to H_2O\ respectively.) Given the difficulty inherent in determining the rate coefficient for production of CO(a(3Pi ) ) by electron impact on CO, the close agreement between the CO production rates derived from the Cameron and Fourth Positive emissions is encouraging. It should be noted that {Q}CO has been derived assuming a point source and effects of a distributed source have not been considered. After removal of the CO synthetic spectra there are residual features that we have identified with the CO+ First Negative (B(2Sigma ^+) --X(2Sigma ^+) ) system indicating a CO+ column density of 9.9x1011 cm(-2) .

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