On the chemistry and distribution of HOC⁺in M 82

Context. The molecular gas composition in the inner 1 kpc disk of the starburst galaxy M 82 resembles that of Galactic Photon Dominated Regions (PDRs). In particular, large abundances of the reactive ions HOC+ and CO+ have been measured in the nucleus of this galaxy. Two explanations have been proposed for such high abundances: the influence of intense UV fields from massive stars, or a significant role of X-Rays. Aims. Our aim is to investigate the origin of the high abundances of reactive ions in M 82. Methods. We have completed our previous 30 m HOC+ J = 1 -> 0 observations with the higher excitation HCO+ and HOC+ J = 4 -> 3 and 3 -> 2 rotational lines. In addition, we have obtained with the IRAM Plateau de Bure Interferometer (PdBI) a 4 " resolution map of the HOC+ emission in M 82, the first ever obtained in a Galactic or extragalactic source. Results. Our HOC+ interferometric image shows that the emission of the HOC+ 1 -> 0 line is mainly restricted to the nuclear disk, with the maxima towards the E and W molecular peaks. In addition, line excitation calculations imply that the HOC+ emission arises in dense gas (n >= 10(4) cm(-3)). Therefore, the HOC+ emission is arising in the dense PDRs embedded in the M 82 nuclear disk, rather than in the intercloud phase and/or wind. Conclusions. We have improved our previous chemical model of M 82 by (i) using the new version of the Meudon PDR code; (ii) updating the chemical network; and (iii) considering two different types of clouds (with different thickness) irradiated by the intense interstellar UV field (G(0) = 10(4) in units of the Habing field) prevailing in the nucleus of M 82. Most molecular observations (HCO+, HOC+, CO+, CN, HCN, H3O+) are well explained assuming that similar to 87% of the mass of the molecular gas is forming small clouds (A(v) = 5 mag) while only similar to 13% of the mass is in large molecular clouds (Av = 50 mag). Such a small number of large molecular clouds suggests that M 82 is an old starburst, where star formation has almost exhausted the molecular gas reservoir.