HCO+ observations toward comet Hale-Bopp (C/1995 O1):: Ion-molecule chemistry and evidence for a volatile secondary source

Several millimeter-wave transitions of HCO+ have been detected toward comet Hale-Bopp (C/1995 O1) using the Arizona Radio Observatory 12 m telescope. The J = 2 --> 1 transition at 178 GHz was observed toward the comet nucleus near perihelion on 1997 March 10 and 20, as well as the J = 3 --> 2 transition at 268 GHz on 1997 March 9, with angular resolutions of 3600 and 2300, respectively. These data all show a slight velocity shift (similar to1.2 km s(-1)) from the nominal comet velocity, and the J = 3 --> 2 profile is asymmetric with a redshifted wing. These differences likely arise from ion acceleration by the solar wind. A rotational diagram analysis of the data yielded a column density of 1.1 x 10(12) cm(-2) for HCO+ in Hale-Bopp, which corresponds to an average number density of 36 cm(-3). The data taken on March 9 show a second velocity component redshifted by 7.0 +/- 0.6 km s(-1), which is considerably weaker than the main feature and appears to have a counterpart in the HNC, J = 3 --> 2 data, observed within an hour of the HCO+ measurements. The velocity difference between the main and secondary emission lines deprojected onto the extended solar radius vector is similar to10 km s(-1) for both HCO+ and HNC, and the weak-to-strong line intensity ratios (similar to5%) are identical to within observational errors, suggesting a common high-velocity volatile secondary source. A plausible model that may account for the redshifted velocity components is a comoving, localized debris field of submicron refractory grains accelerated by solar radiation pressure located similar to10(5)-10(6) km from the nucleus. The parent material of the weaker redshifted HNC and HCO+ lines may be predominately complex organic polymers. An examination of the production rates for HCO+ suggests that the reaction H-2 + CO+ is likely to be an important route to this ion in the outer coma beyond the collisionopause, where it has its peak abundance.