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 massive star-forming region Monoceros R2 (Mon R2) hosts the closest ultra-compact Hii region, where the photondominated region (PDR) between the ionized and molecular gas can be spatially resolved with current single-dish telescopes. Aims. We aim at studying the chemistry of deuterated molecules toward Mon R2 to determine the deuterium fractions around a high-UV irradiated PDR and investigate the chemistry of these species. Methods. We used the IRAM-30 m telescope to carry out an unbiased spectral survey toward two important positions (namely IF and MP2) in Mon R2 at 1, 2, and 3 mm. This spectral survey is the observational basis of our study of the deuteration in this massive starforming region. Our high spectral resolution observations (∼0.25-0.65 km s −1 ) allowed us to resolve the line profiles of the different species detected. Results. We found a rich chemistry of deuterated species at both positions of Mon R2, with detections of C 2 D, DCN, DNC, DCO + , D 2 CO, HDCO, NH 2 D, and N 2 D + and their corresponding hydrogenated species and rarer isotopologs. The high spectral resolution of our observations allowed us to resolve three velocity components: the component at 10 km s −1 is detected at both positions and seems associated with the layer most exposed to the UV radiation from IRS 1; the component at 12 km s −1 is found toward the IF position and seems related to the foreground molecular gas; finally, a component at 8.5 km s −1 is only detected toward the MP2 position, most likely related to a low-UV irradiated PDR. We derived the column density of the deuterated species (together with their hydrogenated counterparts), and determined the deuterium fractions as D frac = [XD]/ [XH]. The values of D frac are around 0.01 for all the observed species, except for HCO + and N 2 H + , which have values 10 times lower. The values found in Mon R2 are similar to those measured in the Orion Bar, and are well explained with a pseudo-time-dependent gas-phase model in which deuteration occurs mainly via ion-molecule reactions with H 2 D + , CH 2 D + and C 2 HD + . Finally, the [H 13 CN]/[HN 13 C] ratio is very high (∼11) for the 10 km s −1 component, which also agree with our model predictions for an age of ∼0.01 to a few 0.1 Myr. Conclusions. The deuterium chemistry is a good tool for studying the low-mass and high-mass star-forming regions. However, while low-mass star-forming regions seem well characterized with D frac (N 2 H + ) or D frac (HCO + ), a more complete chemical modeling is required to date massive star-forming regions. This is due to the higher gas temperature together with the rapid evolution of massive protostars.

Context. M 82 is one of the nearest and brightest starburst galaxies. It has been extensively studied in the past decade and by now is considered the prototypical extragalactic photon-dominated region (PDR) and a reference for studying star formation feedback. Aims. Our aim is to characterize the molecular chemistry in M 82 at spatial scales of giant molecular clouds (GMCs), ∼100 pc, to investigate the feedback effects of the star formation activity. Methods. We present interferometric observations of the CN 1 → 0 (113.491 GHz), N 2 H + 1 → 0 (93.173 GHz), H (41) + ] ratio can be explained as the consequence of differences in the local intestellar UV field and in the average cloud sizes within the nucleus of the galaxy. Conclusions. Our high spatial resolution imaging of the starburst galaxy M 82 shows that the star formation activity has a strong impact on the chemistry of the molecular gas. In particular, the entire nucleus behaves as a giant PDR whose chemistry is determined by the local UV flux. The detection of N 2 H + shows the existence of a population of clouds with A v > 20 mag all across the galaxy plane. These clouds constitute the molecular gas reservoir for the formation of new stars and, although it is distributed throughout the nucleus, the highest concentration occurs in the outer x1 bar orbits (R ∼ 280 pc).

Context. Herbig Ae stars (HAe) are the precursors of Vega-type systems, hence crucial objects in planet formation studies. Thus far, only a few disks associated with HAe stars have been studied using millimetre interferometers. Aims. Our aim is to determine the dust evolution and the lifetime of the disks associated with Herbig Ae stars. Methods. We imaged the continuum emission at ∼3 mm and ∼1.3 mm of the Herbig Ae/Be stars BD+61154, RR Tau, VY Mon, and LkHα 198 using the Plateau de Bure Interferometer (PdBI). These stars are in the upper end of the stellar mass range of the Herbig Ae stars (M * > 3 M ). Our measurements were used to complete the spectral energy distribution (SED). The modelling of the SED, in particular the FIR-mm part, allows us to determine the masses and dust properties of these disks. Results. We detected the disks associated with BD+61154, RR Tau, and VY Mon with disk masses of 0.35 M , 0.05 M , and 0.40 M , respectively. The disk around LkHα 198 was not detected with an upper limit to the disk mass of 0.004 M . We detected, however, the disks associated with the younger stellar objects LkHα 198-IR and LkHα 198-mm that are located in the vicinity of LkHα 198. The fitting of the mm part of the SED reveal that the grains in the mid-plane of the disks around BD+61154, RR Tau, and VY Mon have sizes of ∼1-1000 μm. Therefore, grains have not grown to centimetre sizes in these disks yet. Conclusions. These massive (M * > 3 M ) and young (∼1 Myr) HAe stars are surrounded by massive ( 0.04 M ) disks with grains of micron-millimetre sizes. Although grain growth is proceeding in these disks, their evolutionary stage is prior to the formation of planetesimals. These disks are less evolved than those detected around T Tauri and Herbig Be stars.