Sodium D-emission in comets is the only tracer of a non-volatile cometary constituent accessible in ground-based observations, except in sungrazing comets, where other metallic lines are seen. It has been observed in several comets in the past (e.g.: Greenstein and Arpigny 1962; Rahe et al. 1976; Oppenheimer 1980; Hicks and Fink 1997). However, the recent intensive observing campaigns of the perihelion passage of comet Hale-Bopp (C/1995 O1) have resulted in a unique coverage of several aspects of Na emission in one comet. The most striking observation was the discovery of a several degrees long sodium tail in wide field images (Cremonese et al. 1997a). In addition, high-resolution measurements of the Doppler shift of cometary Na atoms provide information on their kinematic in the coma region (Arpigny et al. 1998) and in the tail (Cremonese et al. 1997b).
Observations of the inner coma sodium distribution in comets suggest the existence of an extended source for Na atoms, in addition to a nucleus source (Huebner 1970; Oppenheimer 1980; Arpigny et al. 1998). Release from dust particles or neutral parent molecules has been discussed. Recently, the existence of an ionic parent molecule has been suggested and the effect of collisions of sodium with heavy neutrals was evaluated (Combi et al. 1997). The spatial sodium profiles presented here provide further constraints on the nature of the sodium source.
© European Southern Observatory (ESO) 1998
Online publication: May 15, 1998