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

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

Nov 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004apj...615.1054m&link_type=abstract

The Astrophysical Journal, Volume 615, Issue 2, pp. 1054-1062.

Astronomy and Astrophysics

Astronomy

3

Astrochemistry, Comets: Individual: Name: Hale-Bopp (C/1995 O1), Line: Profiles, Molecular Data, Radio Lines: Solar System, Sun: Solar Wind

Scientific paper

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 ~105-106 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.

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