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Session 72 - Comet Hale-Bopp.
Display session, Friday, January 09
Exhibit Hall,
We present a comprehensive set of images of comet C/1995 O1 Hale-Bopp in the millimeter-wave emission of the gases HCN, CS, and HCO^+. The HCN J=1-0 rotational transition was monitored from October, 1996 until June, 1997, using the QUARRY 15-element focal plane array receiver on the Five-College Radio Astronomy Observatory (FCRAO) 14-m antenna. In addition, maps of neutral CS J=2-1 and the ion HCO^+ J=1-0 were assembled over the time period from March to June of 1997. The HCO^+ emission is detectable over an extended region corresponding to 300,000 km at the comet, compared to the 100,000 km scale that characterizes HCN and CS. Maps show significant day-to-day variability in the structure of the HCO^+ coma with a common feature that the strongest emission does not typically coincide with the nucleus position. The peak neutral emission is typically symmetric about the nucleus, while the peak in HCO^+ is shifted anti-sunward by 100,000 km. Individual spectra within the HCO^+ maps display broad (approximately 4 km/s) lines, redshifted by 1-2 km/s or more from the nominal nuclear velocity, whereas neutral spectra are symmetric about 0 km/s. The observed features of the HCO^+ emission require not only that ions accelerate away from the sun, but that their formation is inhibited in a critical region surrounding the nucleus. We offer a simple formation path for HCO^+, involving abundant constituents of the coma.
