Comet Hale-Bopp's Diffuse Sodium Tail

J.K. Wilson, M. Mendillo, J. Baumgardner (Boston University/CSP)

We have imaged a diffuse sodium tail of comet Hale-Bopp, using large field-of-view observations from the McDonald Observatory in March 1997. We subtracted off-band images from sodium-filter images to obtain the atomic sodium tail. The sodium tail was between the dust and ion tails, as seen from the earth. It was broader than the dust or ion tails, and its radially integrated brightness increased with distance from the nucleus for at least 8 million km. It did not resemble the narrow sodium tail observed by Cremonese et al. (1997) in April, implying a new source mechanism began operating in the intervening time. Our preliminary analysis indicates that the sodium in the diffuse tail could not have traveled from the nucleus in atomic form, nor been liberated from molecular ions in the plasma tail. At this point, we find dust to be the most likely source for the sodium gas. However, since the brightest part of the sodium tail does not coincide with the visible dust tail, any sodium-producing dust must be either dark, or too small to backscatter visible light.

Cremonese, G., H. Rauer, A. Fitzsimmons, I.A.U. Circular no. 6643, Central Bureau for Astronomical Telegrams, Smithsonian Astrophysical Observatory, Cambridge, 1997.