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

The nucleus of M82 has been mapped in several 3mm and 1mm lines of CN, HCN,C2H, c-C3H2, CH3C2H, HC3N and HOC+ using the IRAM 30m telescope. These specieshave been purposely selected as good tracers of photon-dominated chemistry. Wehave measured [CN]/[HCN] ~ 5 in the inner 650 pc galaxy disk. Furthermore, wehave detected the HOC+ 1--0 line with an intensity similar to that of theH13CO+ 1--0 line. This implies a [HCO+]/[HOC+] ratio of ~40. These resultscorroborate the existence of a giant photo-dissociation region (PDR) in thenucleus of M82. In fact, the low [HCO+]/[HOC+] ratio can only be explained ifthe nucleus of M82 is formed by small (r<0.02-0.2 pc) and dense (n ~ a few10^4--10^5 cm^{-3}) clouds immersed in an intense UV field (G_0 ~ 10^4 in unitsof the Habing field). The detection of the hydrocarbons c-C3H2 and CH3C2H inthe nucleus of M82 suggests that a complex carbon chemistry is developing inthis giant PDR.Comment: 4 pages, 2 fig