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.

Context High-mass stars and star clusters commonly form within hub-filament systems. Monoceros R2 (hereafter Mon R2), at a distance of 830 pc, harbors one of the closest such systems, making it an excellent target for case studies. Aims We investigate the morphology, stability and dynamical properties of the Mon R2 hub-filament system. Methods We employ observations of the 13CO and C18O 1→0 and 2→1 lines obtained with the IRAM-30m telescope. We also use H2 column density maps derived from Herschel dust emission observations. Results We identified the filamentary network in Mon R2 with the DisPerSE algorithm and characterized the individual filaments as either main (converging into the hub) or secondary (converging to a main filament) filaments. The main filaments have line masses of 30–100 M⊙ pc−1 and show signs of fragmentation, while the secondary filaments have line masses of 12–60 M⊙ pc−1 and show fragmentation only sporadically. In the context of Ostriker’s hydrostatic filament model, the main filaments are thermally supercritical. If non-thermal motions are included, most of them are trans-critical. Most of the secondary filaments are roughly transcritical regardless of whether non-thermal motions are included or not. From the morphology and kinematics of the main filaments, we estimate a mass accretion rate of 10−4–10−3 M⊙ yr−1 into the central hub. The secondary filaments accrete into the main filaments with a rate of 0.1–0.4×10−4 M⊙ yr−1. The main filaments extend into the central hub. Their velocity gradients increase towards the hub, suggesting acceleration of the gas.We estimate that with the observed infall velocity, the mass-doubling time of the hub is ~ 2:5 Myr, ten times larger than the free-fall time, suggesting a dynamically old region. These timescales are comparable with the chemical age of the Hii region. Inside the hub, the main filaments show a ring- or a spiral-like morphology that exhibits rotation and infall motions. One possible explanation for the morphology is that gas is falling into the central cluster following a spiral-like pattern.

Context. We study the chemistry of small hydrocarbons in the photon-dominated regions (PDRs) associated with the ultra-compact H ii region (UCH ii) Mon R2.Aims. Our goal is to determine the variations in the abundance of small hydrocarbons in a high-UV irradiated PDR and investigate the chemistry of these species. Methods. We present an observational study of the small hydrocarbons CH, CCH, and c-C 3 H 2 in Mon R2 that combines spectral mapping data obtained with the IRAM-30 m telescope and the Herschel space observatory. We determine the column densities of these species, and compare their spatial distributions with that of polycyclic aromatic hydrocarbon (PAH), which trace the PDR. We compare the observational results with different chemical models to explore the relative importance of gas-phase, grain-surface, and time-dependent chemistry in these environments. Results. The emission of the small hydrocarbons show different spatial patterns. The CCH emission is extended, while CH and c-C 3 H 2 are concentrated towards the more illuminated layers of the PDR. The ratio of the column densities of c-C 3 H 2 and CCH shows spatial variations up to a factor of a few, increasing from N(c-C 3 H 2 )/N(CCH) ≈ 0.004 in the envelope to a maximum of ≈0.015−0.029 towards the 8 μm emission peak. Comparing these results with other galactic PDRs, we find that the abundance of CCH is quite constant over a wide range of G 0 , whereas the abundance of c-C 3 H 2 is higher in low-UV PDRs, with the N(c-C 3 H 2 )/N(CCH) ratio ranging ≈0.008-0.08 from high to low UV PDRs. In Mon R2, the gas-phase steady-state chemistry can account relatively well for the abundances of CH and CCH in the most exposed layers of the PDR, but falls short by a factor of 10 of reproducing c-C 3 H 2 . In the low-density molecular envelope, time-dependent effects and grain surface chemistry play dominant roles in determining the hydrocarbon abundances. Conclusions. Our study shows that the small hydrocarbons CCH and c-C 3 H 2 present a complex chemistry in which UV photons, grain-surface chemistry, and time dependent effects contribute to determining their abundances. Each of these effects may be dominant depending on the local physical conditions, and the superposition of different regions along the line of sight leads to the variety of measured abundances.

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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.

We present far-infrared observations of Monoceros R2 (a giant molecular cloud at approximately 830 pc distance, containing several sites of active star formation), as observed at 70 µm, 160 µm, 250 µm, 350 µm, and 500 µm by the Photodetector Array Camera and Spectrometer (PACS) and Spectral and Photometric Imaging Receiver (SPIRE) instruments on the Herschel Space Observatory as part of the Herschel imaging survey of OB young stellar objects (HOBYS) Key programme. The Herschel data are complemented by SCUBA-2 data in the submillimetre range, and WISE and Spitzer data in the mid-infrared. In addition, C 18 O data from the IRAM 30-m Telescope are presented, and used for kinematic information. Sources were extracted from the maps with getsources, and from the fluxes measured, spectral energy distributions were constructed, allowing measurements of source mass and dust temperature. Of 177 Herschel sources robustly detected in the region (a detection with high signal-to-noise and low axis ratio at multiple wavelengths), including protostars and starless cores, 29 are found in a filamentary hub at the centre of the region (a little over 1% of the observed area). These objects are on average smaller, more massive, and more luminous than those in the surrounding regions (which together suggest that they are at a later stage of evolution), a result that cannot be explained entirely by selection effects. These results suggest a picture in which the hub may have begun star formation at a point significantly earlier than the outer regions, possibly forming as a result of feedback from earlier star formation. Furthermore, the hub may be sustaining its star formation by accreting material from the surrounding filaments.

Context. Carbon monosulphide (CS) is among the most abundant gas-phase S-bearing molecules in cold dark molecular clouds. It is easily observable with several transitions in the millimeter wavelength range, and has been widely used as a tracer of the gas density in the interstellar medium in our Galaxy and external galaxies. However, chemical models fail to account for the observed CS abundances when assuming the cosmic value for the elemental abundance of sulfur. Aims. The CS+O → CO + S reaction has been proposed as a relevant CS destruction mechanism at low temperatures, and could explain the discrepancy between models and observations. Its reaction rate has been experimentally measured at temperatures of 150−400 K, but the extrapolation to lower temperatures is doubtful. Our goal is to calculate the CS+O reaction rate at temperatures <150 K which are prevailing in the interstellar medium. Methods. We performed ab initio calculations to obtain the three lowest potential energy surfaces (PES) of the CS+O system. These PESs are used to study the reaction dynamics, using several methods (classical, quantum, and semiclassical) to eventually calculate the CS + O thermal reaction rates. In order to check the accuracy of our calculations, we compare the results of our theoretical calculations for T ~ 150−400 K with those obtained in the laboratory. Results. Our detailed theoretical study on the CS+O reaction, which is in agreement with the experimental data obtained at 150–400 K, demonstrates the reliability of our approach. After a careful analysis at lower temperatures, we find that the rate constant at 10 K is negligible, below 10−15 cm3 s−1, which is consistent with the extrapolation of experimental data using the Arrhenius expression. Conclusions. We use the updated chemical network to model the sulfur chemistry in Taurus Molecular Cloud 1 (TMC 1) based on molecular abundances determined from Gas phase Elemental abundances in Molecular CloudS (GEMS) project observations. In our model, we take into account the expected decrease of the cosmic ray ionization rate, ζH2, along the cloud. The abundance of CS is still overestimated when assuming the cosmic value for the sulfur abundance.

Context. AB Aur is a Herbig Ae star that hosts a prototypical transition disk. The disk shows a plethora of features connected with planet formation mechanisms, such as spiral arms, dust cavities, and dust traps. Understanding the physical and chemical characteristics of these features is crucial to advancing our knowledge of the planet formation processes. Aims. We aim to characterize the gaseous disk around the Herbig Ae star AB Aur. A complete spectroscopic study was performed using NOEMA to determine the physical and chemical conditions with high spatial resolution. Methods. We present new NOrthern Extended Millimeter Array (NOEMA) interferometric observations of the continuum and 12CO, 13CO, C18O, H2CO, and SO lines obtained at high resolution. We used the integrated intensity maps and stacked spectra to derive reliable estimates of the disk temperature. By combining our 13CO and C18O observations, we computed the gas-to-dust ratio along the disk. We also derived column density maps for the different species and used them to compute abundance maps. The results of our observations were compared with a set of Nautilus astrochemical models to obtain insight into the disk properties. Results. We detected continuum emission in a ring that extends from 0.6′′ to ~2.0′′, peaking at 0.97′′ and with a strong azimuthal asymmetry. The molecules observed show different spatial distributions, and the peaks of the distributions are not correlated with the binding energy. Using H2CO and SO lines, we derived a mean disk temperature of 39 K. We derived a gas-to-dust ratio that ranges from 10 to 40 along the disk. Abundance with respect to 13CO for SO (~2 × 10−4) is almost one order of magnitude greater than the value derived for H2CO (1.6 × 10−5). The comparison with Nautilus models favors a disk with a low gas-to-dust ratio (40) and prominent sulfur depletion. Conclusions. From a very complete spectroscopic study of the prototypical disk around AB Aur, we derived, for the first time, the gas temperature and the gas-to-dust ratio along the disk, providing information that is essential to constraining hydrodynamical simulations. Moreover, we explored the gas chemistry and, in particular, the sulfur depletion. The derived sulfur depletion is dependent on the assumed C/O ratio. Our data are better explained with C/O ~ 0.7 and S/H = 8 × 10−8.