The nucleus of M82 has been mapped in several 3 and 1 mm lines of CN, HCN, C 2 H, c-C 3 H 2 , CH 3 C 2 H, HC 3 N, and HOC ϩ using the IRAM 30 m telescope. These species have been purposely selected as good tracers of photon-dominated chemistry. We have measured in the inner 650 pc galaxy disk. Furthermore, [CN]/[HCN] ∼ 5 we have detected the HOC ϩ line with an intensity similar to that of the H 13 CO ϩ line. This implies 1 r 0 1 r 0 an [HCO ϩ ]/[HOC ϩ ] ratio of ∼40. These results corroborate the existence of a giant photodissociation region (PDR) in the nucleus of M82. In fact, the low [HCO ϩ ]/[HOC ϩ ] ratio can only be explained if the nucleus of M82 is formed by small ( pc) and dense (n ∼ a few times 10 4 -10 5 cm Ϫ3 ) clouds immersed in an r ! 0.02-0.2 intense UV field ( in units of the Habing field). The detection of the hydrocarbons c-C 3 H 2 and CH 3 C 2 H 4 G ∼ 10 0 in the nucleus of M82 suggests that a complex carbon chemistry is developing in this giant PDR.

Context. Ultracompact (UC) Hii regions constitute one of the earliest phases in the formation of a massive star and are characterized by extreme physical conditions (G 0 > 10 5 Habing field and n > 10 6 cm −3 ). The UC Hii Mon R2 is the closest example and an excellent target to study the chemistry in these complex regions. Aims. Our goal is to investigate the chemistry of the molecular gas around UC Hii Mon R2 and the variations caused by the different local physical conditions. Methods. We carried out 3 mm and 1 mm spectral surveys using the IRAM 30-m telescope towards three positions that represent different physical environments in Mon R2: (i) the ionization front (IF) at (0 , 0 ), and two peaks in the molecular cloud; (ii) molecular Peak 1 (hereafter MP1) at the offset (+15 , −15 ); and (iii) molecular Peak 2 (hereafter MP2) at the farther offset (0 , 40 ). In addition, we carried out extensive modeling to explain the chemical differences between the three observed regions. Results. We detected more than 30 different species (including isotopologues and deuterated compounds). In particular, we detected SO + and C 4 H confirming that ultraviolet (UV) radiation plays an important role in the molecular chemistry of this region. In agreement with this interpretation, we detected the typical photo-dissociation region (PDR) molecules CN, HCN, HCO, C 2 H, and c-C 3 H 2 . There are chemical differences between the observed positions. While the IF and the MP1 have a chemistry similar to that found in high UV field and dense PDRs such as the Orion Bar, the MP2 is similar to lower UV/density PDRs such as the Horsehead nebula. Our chemical modeling supports this interpretation. In addition to the PDR-like species, we detected complex molecules such as CH 3 CN, H 2 CO, HC 3 N, CH 3 OH, and CH 3 C 2 H that are not usually found in PDRs. The sulfur compounds CS, HCS + , C 2 S, H 2 CS, SO, and SO 2 and the deuterated species DCN and C 2 D were also identified. The origin of these complex species requires further study. In Mon R2, we have the two classes of PDRs, a high UV PDR towards the IF and the adjacent molecular bar, and a low-UV PDR, which extends towards the north-west following the border of the cloud.

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, H 3 O + ) are well explained assuming that ∼87% of the mass of the molecular gas is forming small clouds (A v = 5 mag) while only ∼13% of the mass is in large molecular clouds (A v = 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.

Context. Mon R2, at a distance of 830 pc, is the only ultracompact H ii region (UCH ii) where the associated photon-dominated region (PDR) can be resolved with Herschel. Owing to its brightness and proximity, it is one of the best-suited sources for investigating the chemistry and physics of highly UV-irradiated PDRs. O abundance, however, is larger (∼1 × 10 −7 ) in the high-velocity wings detected toward the H ii region. Possible explanations for this larger abundance include an expanding hot PDR and/or an outflow. Ammonia seems to be present only in the envelope of the core with an average abundance of ∼2 × 10 −9 relative to H 2 . Conclusions. The Meudon PDR code, which includes only gas-phase chemical networks, can account for the measured water abundance in the high velocity gas as long as we assume that it originates from a 1 mag hot expanding layer of the PDR, i.e. that the outflow has only a minor contribution to this emission. To explain the water and ammonia abundances in the rest of the cloud, the molecular freeze out and grain surface chemistry would need to be included.