HCO+ IMAGING OF COMET HALE-BOPP (C 1995 O1)

The HCO+ J = 1-0 rotational transition at 89.189 GHz has been mapped i n comet Hale-Bopp (C/1995 O1) over a total of 38 individual days spann ing the period 1997 March 10-June 20 with the Five College Radio Astro nomy Observatory 14 m antenna. HCO+ is detectable over an extended reg ion of the comet, with the peak emission commonly located 50,000-100,0 00 km in the antisolar direction. Maps made throughout the apparition show significant variability in the structure of the HCO+ coma, someti mes on timescales of several hours. The HCO+ brightness is usually dep ressed at the nucleus position, and on some occasions, the emission is spread into a ring around the position of the nucleus. Individual spe ctra within the maps display broad (approximately 4 km s(-1)) Lines re dshifted by 1-2 km s(-1) or more from the nominal velocity of the nucl eus, with the redshift typically increasing in the antisolar direction . The spectra and maps may be generally explained by models in which t he ions are accelerated tailward at a rate on the order of 10 cm s(-2) , provided that HCO+ is destroyed within 50,000-100,000 km of the nucl eus.