36th DPS Meeting, 8-12 November 2004
Session 23 Comets Coma I
Oral, Wednesday, November 10, 2004, 1:30-3:00pm, Clark

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[23.05] Observational constraints on excitation of the 18-cm OH lines in comets

A.J. Lovell (Agnes Scott College), E.S. Howell (NAIC), F. P. Schloerb (UMass Amherst)

We have observed 18-cm OH lines in 11 comets from Arecibo Observatory. These observations provide useful constraints on the gas production and velocity in the coma, the distribution of outgassing from the nucleus, and the excitation conditions in the coma. Monte Carlo simulations of velocities and asymmetries suggest enhanced dayside emissions in some observations of comets C/2001 A2(b) LINEAR, 153P/Ikeya-Zhang, and C/2002 T7 LINEAR. Enhanced nightside emissions appear to be required for C/1999 T1 McNaught-Hartley on 22 January 2001 and 153P/Ikeya Zhang on 11 March 2002.

In a few circumstances, data were taken when the OH excitation was near zero for the heliocentric velocity of the nucleus. Observations of comets 153P/Ikeya Zhang and of C/2002 T7 LINEAR suggest that the zero crossing in the excitation may be occurring at a different heliocentric velocity than predicted in the OH inversion models of either Despois et al. (A&A, 99, 320, 1981) or of Schleicher and A'Hearn (ApJ 331, 1058, 1988). We discuss these results in comparison to 1P/Halley and suggest that, when possible, observations at heliocentric velocities around -11 and +9 km/s be targeted for future comets.

Spatially-resolved observations at a beam spacing of 4.1 arcminutes were made for 5 of the comets, and may provide constraints on the excitation conditions in the coma. As the coma density increases, the OH population thermalizes and inner-coma OH emission is quenched. We employ a Monte Carlo simulation that includes the cometocentric radius for quenching as a free parameter, and estimate either a best fit radius or an upper limit. These radii and limits are lower than the predictions (Gerard, A&A 230, 489, 1990) based on previous data. We present mapping data with the model fits for quenching and compare to predictions from the literature.

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