NEUTRAL SODIUM FROM COMET HALE-BOPP - A 3RD TYPE OF TAIL

We report on the discovery and analysis of a striking neutral sodium g as tail associated with comet C/1995 O1 Hale-Bopp. Sodium D line emiss ion has been observed at heliocentric distance r less than or equal to 1.4 AU in some long-period comets, and the presence of neutral sodium in the tailward direction of a few bright comets has been noted, but the extent, and in particular the source, has never been clear. Here w e describe the first observations and analysis of a neutral sodium gas tail in comet Hale-Bopp, which is entirely different from the previou sly known ion and dust tails. We show that the observed characteristic s of this third type of tail are consistent with it being produced by radiation pressure due to resonance fluorescence of sodium atoms and t hat the lifetime for photoionization is consistent with recent theoret ical calculations.