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E. S. Howell (NAIC), A. J. Lovell (Agnes Scott College), F. P. Schloerb (U. Mass. Amherst)
Observations of the 18-cm OH lines can provide much useful information about comets: the total number of OH molecules and gas velocity in the coma, and the rate and distribution of sublimating water ice from the comet nucleus. Taken together, these observations provide information on the heating and sublimation of ice in the nucleus, and on the heating of water vapor in the coma. In comets, excitation of the 18-cm OH line involves absorption of solar UV photons and a subsequent radiative cascade to the ground state. This process generally leads to a strong inversion or anti-inversion of the \Lambda-doublet population, and the comet amplifies or absorbs background radiation to produce a strong observed line. However, in regions of larger density in the inner coma, the maser emission may be quenched by collisions, yielding a weaker line than otherwise expected. Collisional quenching of the maser plays a major role in extremely productive comets (such as Comet Hale-Bopp), but the degree of quenching in more ``typical'' comets is not well understood. Coma-resolved mapping observations offer a means to characterize the degree of quenching.
The post-upgrade Arecibo telescope is well-suited to such coma-resolved mapping observations, with a high sensitivity. We observed two comets with perihelion production rates near 1029~mol/s to investigate the role of quenching in the inner coma. In June 2000, we observed the long-period comet C/1999 S4 LINEAR, and found that the quenching radius is very low, consistent with its lower production rate compared to Hale-Bopp. The production rates for this comet derived from our observations are consistent with those reported from other sources. We observed Comet C/1999 T1 McNaught-Hartley in January 2001, and detected the OH lines at 12~mJy, very close to the predicted value. The detection limit is about +/-2~mJy. Results of these observations and model interpretations will be presented.
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