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The molecular gas composition in the inner 1 kpc disk of the starburst galaxyM82 resembles that of Galactic Photon Dominated Regions (PDRs). In particular,large abundances of the reactive ions HOC+ and CO+ have been measured in thenucleus of this galaxy. To investigate the origin of the large abundances ofreactive ions in M82, we have completed our previous 30m HOC+ J=1-0observations with the higher excitation HCO+ and HOC+ J=4-3 and 3-2 rotationallines. In addition, we have obtained with the IRAM Plateau de BureInterferometer (PdBI) a 4" resolution map of the HOC+ 1-0 emission, the firstever obtained in a Galactic or extragalactic source. Our HOC+ interferometricimage shows that the emission of the HOC+ 1-0 line is mainly restricted to thenuclear disk, with the maxima towards the E. and W. molecular peaks. Inaddition, line excitation calculations imply that the HOC+ emission arises indense gas. Therefore, the HOC+ emission is arising in the dense PDRs embeddedin the M82 nuclear disk, rather than in the intercloud phase and/or wind. Wehave improved our previous chemical model of M82 by (i) using the new versionof the Meudon PDR code, (ii) updating the chemical network, and (iii)considering two different types of clouds (with different thickness). Mostmolecular observations (HCO+, HOC+, CO+, CN, HCN, H3O+) are well explainedassuming that ~ 87 % of the mass of the molecular gas is forming small clouds(Av=5 mag) while only ~ 13 % of the mass is in large molecular clouds (Av=50mag). Such small number of large molecular clouds suggests that M82 is an oldstarburst, where star formation has almost exhausted the molecular gasreservoir.

On the chemistry and distribution of HOC+ in M82: More evidence for extensive PDRs