Analysis of POSS Images of Comet–Asteroid Transition Object 107P/1949 W1 (Wilson–Harrington)

Abstract

We have analyzed the only two known images taken of Comet 107P = Asteroid (4015) Wilson–Harrington while it was a distinctly cometary object. The images reside on two Palomar Observatory Sky Survey (POSS) photographic plates taken on 19 November 1949 UT and provide a unique way of studying the fading gasps of a dying comet. The comet appears as a streak, a tail is evident, but the coma is indistinguishable. A comparison of the profiles of the streaks and the stellar PSF yielded no coma, implying the coma's scale height is small (upper limit of a few hundred kilometers). Finson–Probstein modeling of the tail demonstrates that it is not a dust tail. If it were, the size of the particles would have to be tens to hundreds of micrometers in size (which contradicts the tail's blue color [vs. the Sun]) and they would have to have been released several weeks before the observations (which contradicts observers' reports that the tail dissipated in a few days). Instead, we are seeing CO+and H2O+fluorescence in a plasma tail. With this composition, the tail's blue hue and short lifetime are explained. The lag angle of the tail on the image is about 15°, larger than the “typical” for Type I tails, but the value is not implausible. We show that the deviation of the solar wind from radial need not have been atypical to explain it. We have calibrated the relevant portions of the photographic plates and, from the surface brightness of the tail and an estimate of its age, we have calculated a plausible maximum to the production rate of H2O and CO during Wilson–Harrington's outburst:QH2O≈QCO≈ 5 × 1027molecules sec−1. The measurements indicate that CO is roughly as abundant as H2O on the dormant comet's nucleus.