GEMS is an IRAM 30m Large Program whose aim is determining the elemental depletions and the ionization fraction in a set of prototypical star-forming regions. This paper presents the first results from the prototypical dark cloud TMC 1. Extensive millimeter observations have been carried out with the IRAM 30m telescope (3 mm and 2 mm) and the 40m Yebes telescope (1.3 cm and 7 mm) to determine the fractional abundances of CO, HCO+, HCN, CS, SO, HCS+, and N2H+ in three cuts which intersect the dense filament at the well-known positions TMC 1-CP, TMC 1-NH3, and TMC 1-C, covering a visual extinction range from AV ~ 3 to ~20 mag. Two phases with differentiated chemistry can be distinguished: i) the translucent envelope with molecular hydrogen densities of 1–5×103 cm−3; and ii) the dense phase, located at AV > 10 mag, with molecular hydrogen densities >104 cm−3. Observations and modeling show that the gas phase abundances of C and O progressively decrease along the C+/C/CO transition zone (AV ~ 3 mag) where C/H ~ 8×10−5 and C/O~0.8–1, until the beginning of the dense phase at AV ~ 10 mag. This is consistent with the grain temperatures being below the CO evaporation temperature in this region. In the case of sulfur, a strong depletion should occur before the translucent phase where we estimate a S/H ~ (0.4 - 2.2) ×10−6, an abundance ~7-40 times lower than the solar value. A second strong depletion must be present during the formation of the thick icy mantles to achieve the values of S/H measured in the dense cold cores (S/H ~8×10−8). Based on our chemical modeling, we constrain the value of ζH2 to ~ (0.5 - 1.8) ×10−16 s−1 in the translucent cloud.

Aims. Our goal is to investigate the properties of the circumstellar disks around intermediate mass stars to determine their occurrence, lifetime and evolution. Methods. We completed a search for circumstellar disks around Herbig Be stars using the NRAO Very Large Array (VLA) and the IRAM Plateau de Bure (PdB) interferometers. Thus far, we have observed 6 objects with 4 successful detections. The results towards 3 of these stars (R Mon, MWC 1080, MWC 137) were presented elsewhere. We present our new VLA and PdBI data for the three objects MWC 297, Z CMa, and LKHα 215. We constructed the SED from near-IR to centimeter wavelengths by adding our millimeter and centimeter data to the available data at other wavelengths, mainly Spitzer images. The entire SED was fitted using a disk+envelope model. In addition, we compiled all the disk millimeter observations in the literature and completed a statistical analysis of all the data. Results. We show that the disk mass is usually only a small percentage (less than 10%) of the mass of the entire envelope in HBe stars. For the disks, there are large source-to-source variations. Two disks in our sample, R Mon and Z CMa, have similar sizes and masses to those found in T Tauri and Herbig Ae stars. The disks around MWC 1080 and MWC 297 are, however, smaller (r out < 100 AU). We did not detect the disks towards MWC 137 and LkHα 215 at millimeter wavelengths, which limits the mass and the size of the possible circumstellar disks. Conclusions. A comparison between our data and previous results for T Tauri and Herbig Ae stars indicates that although massive disks (∼0.1 M ) are found in young objects (∼10 4 yr), the masses of the disks around Herbig Be stars are usually 5-10 times lower than those around lower mass stars. We propose that disk photoevaporation is responsible for this behavior. In Herbig Be stars, the UV radiation disperses the gas in the outer disk on a timescale of a few 10 5 yr. Once the outer part of the disk has vanished, the entire gaseous disk is photoevaporated on a very short timescale (∼10 5 yr) and only a small, dusty disk consisting of large grains remains.

Aims. Very few molecular species have been detected in circumstellar disks surrounding young stellar objects. We are carrying out an observational study of the chemistry of circumstellar disks surrounding T Tauri and Herbig Ae stars. First results of this study are presented in this note. Methods. We used the EMIR receivers recently installed at the IRAM 30 m telescope to carry out a sensitive search for molecular lines in the disks surrounding AB Aur, DM Tau, and LkCa 15. Results. We detected lines of the molecules HCO + , CN, H 2 CO, SO, CS, and HCN toward AB Aur. In addition, we tentatively detected DCO + and H 2 S lines. The line profiles suggest that the CN, HCN, H 2 CO, CS, and SO lines arise in the disk. This makes it the first detection of SO in a circumstellar disk. We have unsuccessfully searched for SO toward DM Tau and LkCa 15, and for c-C 3 H 2 toward AB Aur, DM Tau, and LkCa 15. Our upper limits show that contrary to all molecular species observed so far, SO is not as abundant in DM Tau as it is in AB Aur.Conclusions. Our results demonstrate that the disk associated with AB Aur is rich in molecular species. Our chemical model shows that the detection of SO is consistent with that expected from a very young disk where the molecular adsorption onto grains does not yet dominate the chemistry.

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.