On the chemistry and distribution of HOC$\mathsf{^+}$ in M 82

摘要: Context. The molecular gas composition in the inner 1 kpc disk of starburst galaxy M 82 resembles that Galactic Photon Dominated Regions (PDRs). In particular, large abundances reactive ions HOC + and CO have been measured nucleus this galaxy. Two explanations proposed for such high abundances: influence intense UV fields from massive stars, or a significant role X-Rays. Aims. Our aim is to investigate origin 82. Methods. We completed our previous 30 m J = → 0 observations with higher excitation HCO 4 3a nd 3→ 2 rotational lines. addition, we obtained IRAM Plateau de Bure Interferometer (PdBI) �� resolution map emission 82, first ever extragalactic source. Results. interferometric image shows line mainly restricted nuclear disk, maxima towards E W peaks. calculations imply arises dense (n ≥ 10 cm −3 ). Therefore, arising PDRs embedded rather than intercloud phase and/or wind. Conclusions. improved chemical model by (i) using new version Meudon PDR code; (ii) updating network; (iii) considering two different types clouds (with thickness) irradiated interstellar field (G0 units Habing field) prevailing Most (HCO , ,C O CN, HCN, H3O ) are well explained assuming ∼87% mass forming small (Av 5 mag) while only ∼13% 50 mag). Such number suggests an old starburst, where star formation has almost exhausted reservoir.