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M. Mendillo, J. K. Wilson, J. Baumgardner (Center for Space Physics, B.U.), G. Cremonese (Osservatorio Astronomico, Padova, Italy), C. Barbieri (Dipartimento di Astronomia, Padova)
While the presence of sodium in comet tails has been known for decades, the recent application of imaging techniques to portray the Na component of comet Hale-Bopp added a new dimension to the study of sources and dynamics of cometary gases. In this paper, we report on preliminary results of a search for additional image sets showing sodium tails in comets. New observations made at the McDonald Observatory in January 1998 searched for Na components of two comets: Tempel-Tuttle (magnitude ~9.5) at a heliocentric distance of 1.1 AU (pre-perihelion) and geocentric distance of 0.4 AU, and Hartley-2 (magnitude ~8.5) at a heliocentric distance of 1.1 AU (post-perihelion) and a geocentric distance of 0.8 AU. In both cases, using the same techniques that detected Na in Hale-Bopp, we found no evidence for sodium above the detection threshold of 10 Rayleighs. Earlier observations of comet Hyakutake (magnitude ~1.5) at 0.9 AU from the sun (pre-perihelion), made from the site of the National Telescope Galileo (TNG) on La Palma in April 1996, revealed a well-formed sodium tail. The Hyakutake sodium tail had a peak brightness of ~100 Rayleighs near the coma, and a brightness of 20 Rayleighs at 2 million kilometers in the anti-sunward direction. Preliminary model studies suggest that the brightness pattern cannot be explained with a nucleus source alone; an extended sodium source in the tail region is indicated.