OH radio observations of comet P/Halley

Abstract

We present the first results of an extensive program of OH observations of P/Halley obtained with the NRAO 43 m antenna between July 1985 and May 1986. Using Radio Model 1986 a, we find that the OH production rate derived from these data follows an r h −2 law for heliocentric distances in the range r h < 2AU. Beyond 2 AU, the production rate decreases in a manner which is qualitatively consistent with nominal models of the sublimation of water ice, although quantitative agreement would require some modification of the nominal physical properties of the nucleus. Comparison of radio OH production rates to those estimated by ultraviolet techniques continues to show that the radio technique finds systematically lower production rates. Observations of the OH line shape show that the line width increases as the comet approaches perihelion, and that the broader line widths persist after perihelion to at least r h =1 AU. Detailed modeling of the profiles suggests that this may be accounted for through an increase in the velocity of the OH parent. However, the OH parent velocities which are required to produce the observed line widths are not quantitatively consistent with other estimates of the parent velocity. Finally, the mean velocity of the OH emission (after correction for the OH Greenstein effect) is slightly blue shifted compared to the nucleus velocity and suggests that the outgassing from the nucleus may occur preferentially on its sunward side.