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 + have been detected toward comet Hale-Bopp (C/1995 O1) using the Arizona Radio Observatory 12 m telescope. The J ¼ 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 ¼ 3 ! 2 transition at 268 GHz on 1997 March 9, with angular resolutions of 36 00 and 23 00 , respectively. These data all show a slight velocity shift (� 1.2 km s � 1 ) from the nominal comet velocity, and the J ¼ 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;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: 6k m s � 1 ,w hich is considerably weaker than the main feature and appears to have a counterpart in the HNC, J ¼ 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 highvelocity 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 � 10 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 H2 þ CO þ is likely to be an important route to this ion in the outer coma beyond the collisionopause,