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Molecular oxygen has been the subject of many observational searches as chemical models predicted it to be a reservoir of oxygen. Although it has been detected in two regions of the interstellar medium, its rarity is a challenge for astrochemical models. In this paper, we have combined the physical conditions computed with smoothed particle hydrodynamics simulations with our full gas–grain chemical model Nautilus, to study the predicted O2 abundance in interstellar material forming cold cores. We thus follow the chemical evolution of gas and ices in parcels of material from the diffuse interstellar conditions to the cold dense cores. Most of our predicted O2 abundances are below 10−8 (with respect to the total proton density) and the predicted column densities in simulated cold cores are at maximum a few 10−14 cm−2, in agreement with the non-detection limits. This low O2 abundance can be explained by the fact that, in a large fraction of the interstellar material, the atomic oxygen is depleted on to the grain surface (and hydrogenated to form H2O) before O2 can be formed in the gas-phase and protected from ultraviolet photodissociations. We could achieve this result only because we took into account the full history of the evolution of the physical conditions from the diffuse medium to the cold cores. [ABSTRACT FROM AUTHOR]

GEMS is an IRAM 30 m 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 Taurus molecular cloud (TMC) 1. Extensive millimeter observations have been carried out with the IRAM 30 m telescope (3 and 2 mm) and the 40 m 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 an 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 $\zeta_{\textrm{H}_2}$ ζ H 2 to ~(0.5–1.8) × 10−16 s−1 in the translucent cloud. [ABSTRACT FROM AUTHOR]

The protonated form of CO2, HOCO+, is assumed to be an indirect tracer of CO2 in the millimeter/submillimeter regime since CO2 lacks a permanent dipole moment. Here, we report the detection of two rotational emission lines (40,4-30,3 and 50,5-40,4) of HOCO+ in IRAS 16293-2422. For our observations, we have used EMIR heterodyne 3mmreceiver of the IRAM 30m telescope. The observed abundance of HOCO+ is compared with the simulations using the 3-phase NAUTILUS chemical model. Implications of the measured abundances of HOCO+ to study the chemistry of CO2 ices using JWST-MIRI and NIRSpec are discussed as well. [ABSTRACT FROM AUTHOR]

A wide variety of molecules have recently been detected in the Horsehead nebula photodissociation region (PDR) suggesting that: (i) gas-phase and grain chemistries should both contribute to the formation of organic molecules; and (ii) far-ultraviolet (FUV) photodesorption may explain the release into the gas phase of grain surface species. In order to tackle these specific problems and more generally in order to better constrain the chemical structure of these types of environments we present a study of the Horsehead nebula gas-grain chemistry. To do so we used the 1D astrochemical gas-grain code Nautilus with an appropriate physical structure computed with the Meudon PDR code and compared our modeled outcomes with published observations and with previously modeled results when available. The use of a large set of chemical reactions coupled with the time-dependent code Nautilus allows us to reproduce most of the observations well, including those of the first detections in a PDR of the organic molecules HCOOH, CH2CO, CH3CHO and CH3CCH, which are mostly associated with hot cores. We also provide some abundance predictions for other molecules of interest. Understanding the chemistry behind the detection of these organic molecules is crucial to better constrain the environments these molecules can probe. [ABSTRACT FROM AUTHOR]

Cyclic-C3HD (c-C3HD) is a singly deuterated isotopologue of c-C3H2, which is one of the most abundant and widespread molecules in our Galaxy. We observed IRAS 16293-2422 in the 3-mm band with a single frequency set-up using the EMIR heterodyne 3 mm receiver of the IRAM 30-m telescope. We observed seven lines of c-C3HD and three lines of c-C3H2. Observed abundances are compared with astrochemical simulations using the NAUTILUS gas-grain chemical model. Assuming that the size of the protostellar envelope is 3000 au and assuming the same excitation temperatures for both c-C3H2 and c-C3HD, we obtain a deuterium fraction of 14-3+4 per cent. [ABSTRACT FROM AUTHOR]

The nature of the main sulphur reservoir in star-forming regions is a long standing mystery. The observed abundance of sulphur-bearing species in dense clouds is only about 0.1 per cent of the same quantity in diffuse clouds. Therefore, the main sulphur species in star-forming regions of the interstellar medium are still unknown. IRAS 16293-2422 is one of the regions where production of S-bearing species is favourable due to its conditions which allows the evaporation of ice mantles.We carried out observations in the 3mmband towards the solar type protostar IRAS 16293-2422 with the IRAM 30 m telescope. We observed a single frequency setup with the EMIR heterodyne 3 mm receiver with an Lower Inner tuning frequency of 89.98 GHz. Several lines of the complex sulphur species CH3SH were detected. Observed abundances are compared with simulations using the NAUTILUS gas-grain chemical model. Modelling results suggest that CH3SH has the constant abundance of 4 × 10-9 (compared to H2) for radii lower than 200 au and is mostly formed on the surfaces. Detection of CH3SH indicates that there may be several new families of S-bearing molecules (which could form starting from CH3SH) which have not been detected or looked for yet. [ABSTRACT FROM AUTHOR]

Recent observations have revealed the existence of complex organic molecules (COMs) in cold dense cores and pre-stellar cores. The presence of these molecules in such cold conditions is not well understood and remains a matter of debate since the previously proposed ‘warm-up’ scenario cannot explain these observations. In this paper, we study the effect of Eley–Rideal and complex induced reaction mechanisms of gas-phase carbon atoms with the main ice components of dust grains on the formation of COMs in cold and dense regions. Based on recent experiments, we use a low value for the chemical desorption efficiency (which was previously invoked to explain the observed COM abundances). We show that our introduced mechanisms are efficient enough to produce a large amount of COMs in the gas phase at temperatures as low as 10 K. [ABSTRACT FROM AUTHOR]

The aim of this paper is to provide a newset of branching ratios (BRs) for interstellar and planetary chemical networks based on a semiempirical model. We applied, instead of zero-order theory (i.e., only the most exoergic decaying channel is considered), a statistical microcanonical model based on the construction of breakdown curves and using experimental high velocity collision BRs for their parameterization. We applied the model to ion-molecule, neutral-neutral, and ion-pair reactions implemented in the few popular databases for astrochemistry, such as KIDA, OSU, and UMIST. We studied the reactions of carbon and hydrocarbon species with electrons, He+, H+, CH+, CH, C, and C+ leading to intermediate complexes of the type C+n=2,10, Cn=2,4H, C3H2, C+ n=2,10, Cn=2,4H+, or C3H+2 . Comparison of predictions with measurements supports the validity of the model. Huge deviations with respect to database values are often obtained. Effects of the new BRs in time-dependent chemistry for dark clouds and for photodissociation region chemistry with conditions similar to those found in the Horsehead Nebula are discussed. [ABSTRACT FROM AUTHOR]

We present results from the Herschel and IRAM projects to map M33 in the dust continuum and main emission lines, particularly C[II] and CO. The temperature of the cool dust decreases with distance from the center of M33 from ~25K to ~13K. The CO emission generally follows the dust temperature and the overall dust emission. However, about 1/6 of the molecular clouds are not associated with massive stars, such that about 1/6th the lifetime of an entity identifiable as a molecular cloud is in a pre-star formation state. These clouds are less CO-bright than those with massive stars. The largest sample of molecular clouds currently available for an external galaxy shows that the cloud CO luminosity function, usually viewed as the cloud H2 mass, steepens with radius such that smaller clouds are more numerous in the outer parts. The observations of the C[II] line with Herschel indicate that the C[II] emission traces on-going star formation rather than the neutral gas. This identification will be tested via velocity-resolved Herschel/HIFI C[II] spectra in the near future. [ABSTRACT FROM PUBLISHER]