  The molecular gas composition in the inner 1 kpc disk of the starburst galaxy
M82 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. To investigate the origin of the large abundances of
reactive ions in M82, we have completed our previous 30m 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+ 1-0 emission, the first
ever obtained in a Galactic or extragalactic source. 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. Therefore, the HOC+ emission is arising in the dense PDRs embedded
in the M82 nuclear disk, rather than in the intercloud phase and/or wind. We
have improved our previous chemical model of M82 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). Most
molecular observations (HCO+, HOC+, CO+, CN, HCN, H3O+) are well explained
assuming 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=50
mag). Such small number of large molecular clouds suggests that M82 is an old
starburst, where star formation has almost exhausted the molecular gas
reservoir.
