An Interferometric Study of HCN in Comet Hale-Bopp (C/1995 O1)

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Abstract

We observed comet Hale-Bopp in HCN emission at 88.63 GHz using the BIMA array. We present images of the HCN emission with 9'' angular and 0.33 km s-1 spectral resolution for 12 days between 1997 March and April. These images show changes in the HCN distribution on short-term, daily, and weekly scales. There is a general trend of increasing brightness as the comet approaches the Sun. We investigate the evidence for jets through the characteristics of distinct emission spurs that appear in images created from 2 hr segments of the data. Within many 2 hr time spans, there is more than one active spot on the surface of the nucleus, although it may be active only for a short period. We explore deviations from spherical outflow by subtracting a Haser model parent distribution from the observed images. The excess emission in these "difference maps" is attributed to gas jets, or dust jets that produce an extended source of gas such as icy grains. The excess emission contains up to 15% of the total flux density, but these deviations are not enough to undermine the utility of spherical models of this comet. We calculated the HCN gas production rate for each of the 12 days and find 1.2 × 1028 ≤ Q ≤ 1.8 × 1028 s-1. Quantitative investigation of the extent to which production rate calculations are affected by deviations from sphericity—via a nonspherically symmetric Monte Carlo model—shows that gas productions rates derived from a spherical model of comet Hale-Bopp should be accurate as a first approximation for determining global parameters. Finally, through comparison of continuum observations with our HCN observations, we determined a dust-to-gas ratio of 2.3 in the inner coma.

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