Collisional Quenching of OH Radio Emission from Comet Hale-Bopp

C.H. De Vries, A.J. Lovell, F.P. Schloerb, J.E. Dickens, W.M. Irvine, M. Senay (FCRAO, University of Massachusetts), H.A. Wootten (NRAO)

OH radio emission arises from the molecule's ground state lambda doublet. In comets, the excitation of these energy levels is usually dominated by a process involving absorption of solar UV photons followed by a radiative cascade back to the ground state which leads to a population inversion or anti-inversion and results in strong emission. However, when the coma density is high enough, as in a high production rate comet like C/1995 O1 (Hale-Bopp), collisions between the OH molecule and other species can ``quench'' the population inversion and shut down the strong radio emission from the molecule. We present observations of this quenching effect in Comet Hale-Bopp. The comet has been monitored in the 18-cm OH emission during the time period from August, 1996, through June, 1997, using the NRAO 140-foot antenna in Green Bank, West Virginia. The observed lines were frequently strong enough to permit ``mapping'' of the coma with the 18-arcmin antenna beam, and maps were made using a hexagonal ring pattern 9 and 18 arcminutes from the nucleus position. The radio OH brightness distribution and line intensity indicate that considerable quenching of the OH radio emission occurs within the inner coma. Comparison of the observations to quenching models suggests that, near perihelion, radio emission from all OH molecules within a few hundred thousand kilometers of the nucleus was effectively shut off by collisional quenching of the OH excitation and that only a few percent of the total number of OH molecules in the coma were sensed at radio wavelengths. This derived quenching scale is generally consistent with previous observations of the effect in lower production rate comets when the high production rate of Hale-Bopp is considered.