Ion Chemistry in Photon-Dominated Regions: Examining the HCO+/HOC+/CO+ Chemical Network

Details Ion Chemistry in Photon-Dominated Regions: Examining the HCO+/HOC+/CO+ Chemical Network Ion Chemistry in Photon-Dominated Regions: Examining the HCO+/HOC+/CO+ Chemical Network

May 2000

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000aas...196.2614s&link_type=abstract

American Astronomical Society, 196th AAS Meeting, #26.14; Bulletin of the American Astronomical Society, Vol. 32, p.712

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Photon-dominated regions (PDRs), the interfaces between molecular clouds and hot OB stars, present chemical environments distinctly different from cold molecular clouds. Theoretical models of these regions predict enhanced abundances of open-shell molecular ions and other exotic species, but to date the only molecule to show a definite enhancement is CO+ (e.g. Störzer et. al. 1995, A&A 296, L9; Fuente and Martín-Pintado 1997, Ap.J. 477, L107). HOC+, the metastable isomer of HCO+, is also predicted to be enhanced in PDRs resulting from high abundances of C+ and CO+. In order to test this theory, we have begun to observe millimeter-wave transitions of HCO+, HOC+, and CO+ towards photon-dominated regions, using the NRAO 12m telescope. Observations towards the Orion Bar suggest a strong enhancement of HOC+, with [HCO+]/[HOC+] of order 270, in comparision with the normal molecular cloud ratio of 2000-9000 (Apponi et. al. 1999, Ap.J. 519, L89; Apponi and Ziurys 1997, Ap.J. 481, 800). We have subsequently detected HOC+ and CO+ towards NGC2023 and S140. A preliminary analysis suggests that HOC+ is not particulary enhanced in these objects, although geometric effects may be responsible for these results. The abundances of HOC+ and CO+, however, do appear to be correlated.

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