Photon-dominated Chemistry in the Nucleus of M82: Widespread HOC + Emission in the Inner 650 Parsec Disk

Abstract

The nucleus of M82 has been mapped in several 3 and 1 mm lines of CN, HCN, C2H, c-C3H2, CH3C2H, HC3N, and HOC+ using the IRAM 30 m telescope. These species have been purposely selected as good tracers of photon-dominated chemistry. We have measured [CN]/[HCN] ?5 in the inner 650 pc galaxy disk. Furthermore, we have detected the HOC+ 1 ? 0 line with an intensity similar to that of the H13CO+ 1 ? 0 line. This implies an [HCO+]/[HOC+] ratio of ?40. These results corroborate the existence of a giant photodissociation region (PDR) in the nucleus of M82. In fact, the low [HCO+]/[HOC+] ratio can only be explained if the nucleus of M82 is formed by small (r < 0.02-0.2 pc) and dense (n ? a few times 104-105 cm -3) clouds immersed in an intense UV field (G0 9? 104 in units of the Habing field). The detection of the hydrocarbons c-C3H2 and CH3C2H in the nucleus of M82 suggests that a complex carbon chemistry is developing in this giant PDR. ? 2005. The American Astronomical Society. All rights reserved.