HCO + Observations toward Comet Hale‐Bopp (C/1995 O1): Ion‐Molecule Chemistry and Evidence for a Volatile Secondary Source

Abstract

Several millimeter-wave transitions of HCO+ were 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 obsd. toward the comet nucleus near perihelion on 1997 Mar. 10 and 20, as well as the J = 3 -> 2 transition at 268 GHz on 1997 Mar. 9, with angular resolns. of 36\" and 23\", resp. These data all show a slight velocity shift (.apprx.1.2 km s-1) from the nominal comet velocity, and the J = 3 -> 2 profile is asym. with a red shifted wing. These differences likely arise from ion acceleration by the solar wind. A rotational diagram anal. of the data yielded a column d. of 1.1 * 1012 cm-2 for HCO+ in Hale-Bopp, which corresponds to an av. no. d. of 36 cm-3. The data taken on Mar. 9 show a second velocity component red shifted 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, obsd. 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 .apprx.10 km s-1 for both HCO+ and HNC, and the weak-to-strong line intensity ratios (.apprx.5%) are identical to within observational errors, suggesting a common high-velocity volatile secondary source. A plausible model that may account for the red shifted velocity components is a comoving, localized debris field of submicron refractory grains accelerated by solar radiation pressure located .apprx.105-106 km from the nucleus. The parent material of the weaker red shifted HNC and HCO+ lines may be predominately complex org. polymers. An examn. of the prodn. 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. [on SciFinder (R)]