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 36″ and 23″, respectively. These data all show a slight velocity shift (∼1.2 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 × 10¹² 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 ∼10 km s^-1 for both HCO⁺ and HNC, and the weak-to-strong line intensity ratios (∼5%) 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 ∼10⁵-10 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 H₂ + CO⁺ is likely to be an important route to this ion in the outer coma beyond the collisionopause, where it has its peak abundance.