Welcome to the new version of CaltechAUTHORS. Login is currently restricted to library staff. If you notice any issues, please email coda@library.caltech.edu
Published August 1993 | public
Journal Article

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.

Additional Information

© 1993 Academic Press. Received January 25; revised May 10, 1993. We have benefited from valuable discussions with Drs. Michael Combi and William Smyth. Dr. Robert Wu kindly supplied his data in advance of publication. We thank Tony Misch and Lick Observatory for obtaining the Comet Levy spectrum. This paper has been greatly improved through the critical readings and discussions of Elisabeth Moyer and Toby Falk and through the insightful comments of the reviewers. This work was supported by NASA Grant NAGW-2883, and M.E.B. acknowledges the support of a NASA Graduate Fellowship and the hospitality of the Lunar and Planetary Institute at which the manuscript was completed.

Additional details

August 22, 2023
October 19, 2023