HCO+ Imaging of Comet C/1995 O1 Hale-Bopp

A.J. Lovell, F.P. Schloerb, C.H. De Vries, J.E. Dickens, W.M Irvine, M.C. Senay (FCRAO U. Massachusetts-Amherst)

We present maps of the HCO+ J=1-0 emission in the coma of comet C/1995 O1 Hale-Bopp. The maps were assembled over the time period from March to June of 1997 using the QUARRY 15-element focal plane array receiver on the Five-College Radio Astronomy Observatory (FCRAO) 14-m antenna. The HCO+ emission is detectable over an extended region corresponding to 300,000 km at the comet, compared to the 50,000 - 100,000 km scale that characterizes our observations of neutral species. Maps made throughout the apparition show significant day-to-day variability in the structure of the HCO+ coma. The strongest HCO+ features are not typically located at the nucleus position. A common morphology is a peak in the emission located 50,000 - 100,000 km anti-sunward from the nucleus. However, on some days the HCO+ emission forms a ring around the position of the nucleus. Individual spectra within the maps display broad (approximately 4 km/s) lines, redshifted by 1-2 km/s or more from the nominal nuclear velocity. Simple models of the production and outflow of HCO+ can account for the tailward offset of the peak emission, the enhanced line width, and the redshift of the lines with a tailward acceleration of the ions, presumably due to interaction with the solar wind. However, the explanation of the complex spatial distribution of intensities may require a more complex, MHD model.