An interferometric study of HCN in comet Hale-Bopp (C/1995 O1)

We observed comet Hale-Bopp in HCN emission at 88.63 GHz using the BIMA arr ay. 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 image s show changes in the HCN distribution on short-term, daily, and weekly sca les. There is a general trend of increasing brightness as the comet approac hes the Sun. We investigate the evidence for jets through the characteristi cs of distinct emission spurs that appear in images created from 2 hr segme nts 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 s hort period. We explore deviations from spherical outflow by subtracting a Haser model parent distribution from the observed images. The excess emissi on 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 c ontains up to 15% of the total flux density, but these deviations are not e nough to undermine the utility of spherical models of this comet. We calcul ated the HCN gas production rate for each of the 12 days and find 1.2 x 10( 28) less than or equal to Q less than or equal to 1.8 X 10(28) s(-1). Quant itative investigation of the extent to which production rate calculations a re affected by deviations from sphericity-via a nonspherically symmetric Mo nte Carlo model-shows that gas productions rates derived from a spherical m odel of comet Hale-Bopp should be accurate as a first approximation for det ermining global parameters. Finally, through comparison of continuum observ ations with our HCN observations, we determined a dust-to-gas ratio of 2.3 in the inner coma.