Astron. Astrophys. 334, L61-L64 (1998)

4. Sodium production rate

On 14 March, 1997, we derived the sodium production rate from the spectrum taken with the slit placed across the tail at 2 arcmin offset to the antisolar direction (Fig. 4). We integrate the column density, N, across the slit and multiply by the average velocity, , of Na atoms at the observed distance from the nucleus: . The derived column densities are inversely proportional to the excitation rate, which also depends on the velocity of sodium atoms and the resulting Doppler shift relative to the solar spectrum. From high-resolution Doppler-shift mesurements of the Na velocity in the coma (Arpigny et al. 1998) we estimate the mean velocity along the tail to km s^-1, resulting in a production rate 3 10²⁴ s^-1 up to 5 10²⁵ s^-1. Because part of the Na tail is not covered by our 5.5 arcmin long slit (see Fig. 4), total sodium production rates will be slightly higher than the derived value. The water production rate was about 0.5-1 10³¹ s^-1 during mid-March (Crovisier, personal communication). If we assume a normal cosmic abundance of Na/O = 2.4 10^-3 (Anders and Grevesse 1989) in comet Hale-Bopp, only a small fraction of 0.3% of sodium is released in the coma. Observations of the distant sodium tail in wide field images taken in April 1997 (Cremonese et al. 1997c) also resulted in low abundances of sodium. Our observations support that most of the sodium is confined in the cometary dust and only a small fraction is released in the coma and nucleus.

© European Southern Observatory (ESO) 1998

Online publication: May 15, 1998

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