The Velocity Distribution of Cometary Hydrogen: Evidence for High Velocities?
We present high-velocity and spatial resolution two-dimensional spectra of Hα 6563 Å emission in comets Austin (1989c_1) and Levy (1990c) obtained using the Hamilton Echelle spectrograph on the 3-m Shane telescope at Lick Observatory. Because of the high signal-to-noise of these spectra, achieved through long integrations with long-slit spectroscopy on these relatively bright comets, we can directly observe the hydrogen velocity distribution. The hydrogen velocity distributions in both comets confirm the presence of the components expected from water dissociation and collisional thermalization in the inner coma. In addition, the velocity distribution in comet Austin apparently has a spatially symmetric high-velocity component not predicted by current theory, although the ubiquitous emissions from a unidentified cometary molecule could coincidentally mimic the signature of this fast hydrogen. Comet Levy also shows high-velocity hydrogen, but, unlike the spatially symmetric emission in Austin, the fast hydrogen in Levy is located exclusively on the sunward side of the nucleus. The cause of the Levy fast hydrogen is unknown, but the source could be charge exchange with the solar wind related to the variability and sunward dust ejection observed in that comet by IUE and HST.