DPS 2001 meeting, November 2001
Session 20. Comet Posters
Displayed, 9:00am Tuesday - 3:00pm Saturday, Highlighted, Wednesday, November 28, 2001, 10:30am-12:30pm, French Market Exhibit Hall

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[20.09] Infrared Observations of HCN and C2H2 in Five Oort-cloud Comets

K. Magee-Sauer (Rowan Univ.), M. J. Mumma (NASA GSFC), M. A. DiSanti, N. Dellow Russo (Catholic U./ GSFC)

Previous to comet Hyakutake, measurements of HCN were limited to detections at millimeter wavelengths. With improved infrared detector technology, HCN was first observed at infrared wavelengths in comet Hyakutake (Mumma et al. 1996, Tokunaga et al. 1996). Since then, HCN has been observed at infrared wavelengths in comets C/1995 O1 Hale-Bopp, C/1999 H1 Lee, C/1999 S4 Linear, C/1999 T1 McNaught-Hartley, C/2001 A2 Linear, and will be targeted in comet C/2000 WM1 Linear. While millimeter observations are more sensitive and can detect HCN at lower production rates and larger heliocentric distance, infrared observations of HCN provide high spatial resolution and rotational temperature analysis, as well as abundance information. Moreover, infrared measurements are less sensitive to destruction lifetimes. For the five Oort-cloud comets measured to date, spatial profiles indicate that HCN is a natal ice. The abundance of HCN relative to H2O is ~ 0.2% in four of them, but in Hale-Bopp it was twice as large. These relative abundances are typically a factor of two larger than millimeter observations.

Symmetric hydrocarbons (C2H2, C2H6, and CH4) were discovered in comet Hyakutake (Mumma et al. 1996, Brooke et al. 1996), and were detected in four more Oort-cloud comets since then. C2H4 was also searched, but was not detected. The emission of the \nu3 band of C2H2 lies in the region of the \nu3 band of HCN and can be observed within the same grating settings. Here, we present spectra, rotational temperatures, spatial profiles, production rates, and relative abundances of HCN and C2H2 detected in comets from comet Hyakutake to the present. The significant gains obtained through use of the NIRSPEC instrument at the Keck Observatory will be demonstrated.

This research is sponsored by the National Science Foundation, Research at Undergraduate Institutions Program through grant AST-0098411 to KM-S.

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