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(Abridged) We explore the chemistry of the most abundant C, O, S, and N bearing species in molecular clouds, in the context of the IRAM 30 m Large Programme Gas phase Elemental abundances in Molecular Clouds (GEMS). In this work, we aim to assess the limitations introduced in the observational works when a uniform density is assumed along the line of sight for fitting the observations, developing a very simple numerical model of a turbulent box. We perform a MHD simulation in order to reproduce the turbulent steady-state of a turbulent box with properties typical of a molecular filament before collapse. We post-process the results of the MHD simulation with a chemical code to predict molecular abundances, and then post-process this cube with a radiative transfer code to create synthetic emission maps for a series of rotational transitions observed during the GEMS project. From the chemical point of view, we find that turbulence produces variations on the predicted abundances, but they are more or less critical depending on the chosen transition and the chemical age. When compared to real observations, the results from the turbulent simulation provides a better fit than when assuming a uniform gas distribution along the line of sight. In the view of our results, we conclude that taking into account turbulence when fitting observations might significantly improve the agreement with model predictions. This is especially important for sulfur bearing species that are very sensitive to the variations of density produced by turbulence at early times (0.1 Myr). The abundance of CO is also quite sensitive to turbulence when considering the evolution beyond a few 0.1 Myr., Comment: Published in Astronomy and Astrophysics

The reservoir of sulfur accounting for sulfur depletion in the gas of dense clouds and circumstellar regions is still unclear. One possibility is the formation of sulfur chains, which would be difficult to detect by spectroscopic techniques. This work explores the formation of sulfur chains experimentally, both in pure H$_2$S ice samples and in H$_2$O:H$_2$S ice mixtures. An ultra-high vacuum chamber, ISAC, eqquipped with FTIR and QMS, was used for the experiments. Our results show that the formation of H$_2$S$_x$ species is efficient, not only in pure H$_2$S ice samples, but also in water-rich ice samples. Large sulfur chains are formed more efficiently at low temperatures ($\approx$10 K), while high temperatures ($\approx$50 K) favour the formation of short sulfur chains. Mass spectra of H$_2$S$_x$, x~=~2-6, species are presented for the first time. Their analysis suggests that H$_2$S$_x$ species are favoured in comparison with S$_x$ chains. Nevertheless, the detection of several S$_x^+$ fragments at high temperatures in H$_2$S:H$_2$O ice mixtures suggests the presence of S$_8$ in the irradiated ice samples, which could sublimate from 260~K. ROSINA instrument data from the cometary Rosetta mission detected mass-to-charge ratios 96 and 128. Comparing these detections with our experiments, we propose two alternatives: 1) H$_2$S$_4$ and H$_2$S$_5$ to be responsible of those S$_3^+$ and S$_4^+$ cations, respectively, or 2) S$_8$ species, sublimating and being fragmented in the mass spectrometer. If S$_8$ is the parent molecule, then S$_5^+$ and S$_6^+$ cations could be also detected in future missions by broadening the mass spectrometer range.

Acetic acid (CH3COOH) was detected in the gas toward interstellar clouds, hot cores, protostars and comets. Its formation in ice mantles was proposed and acetic acid awaits detection in the infrared spectra of the ice as most other COMs except methanol. The thermal annealing and UV-irradiation of acetic acid in the ice was simulated experimentally in this work under astrophysically relevant conditions. The experiments were performed under ultra-high vacuum conditions. An ice layer was formed by vapor deposition onto a cold substrate, and was warmed up or exposed to UV photons. The ice was monitored by infrared spectroscopy while the molecules desorbing to the gas phase were measured using a quadrupole mass spectrometer. The transformation of the CH3COOH monomers to cyclic dimers occurs at 120 K and the crystal form composed of chain polymers was observed above 160 K during warm-up of the ice. Ice sublimation proceeds at 189 K in our experiments. Upon UV-irradiation simpler species and radicals are formed, which lead to a residue made of complex molecules after warm-up to room temperature. The possible formation of oxalic acid needs to be confirmed. The photodestruction of acetic acid molecules is reduced when mixed with water in the ice. This work may serve to search for the acetic acid photoproducts in lines of sight where this species is detected. A comparison of the reported laboratory infrared spectra with current JWST observations allows to detect or set upper imits on the CH3COOH abundances in interstellar and circumstellar ice mantles., Comment: Accepted to MNRAS

The presence of methanol among the common ice components in interstellar clouds and protostellar envelopes has been confirmed by the James Webb Space Telescope. Methanol is often detected in the gas phase toward lines of sight shielded from UV radiation. We measured the volumetric density of methanol ice, grown under simulated interstellar conditions, and the infrared spectroscopy at different deposition temperatures and during the warm-up. The IR band strengths are provided and the experimental spectra are compared to those computed with a model. The transition from amorphous to crystalline methanol ice was also explored. Finally, we propose new observations of methanol ice at high resolution to probe the methanol ice structure., Comment: 11 pages, 11 figures

Gas phase Elemental abundances in molecular CloudS (GEMS) is an IRAM 30m large program aimed at determining the elemental abundances of carbon (C), oxygen (O), nitrogen (N), and sulfur (S) in a selected set of prototypical star-forming filaments. In particular, the elemental abundance of S remains uncertain by several orders of magnitude and its determination is one of the most challenging goals of this program. We have carried out an extensive chemical modeling of the fractional abundances of CO, HCO$^+$, HCN, HNC, CS, SO, H$_2$S, OCS, and HCS$^+$ to determine the sulfur depletion toward the 244 positions in the GEMS database. These positions sample visual extinctions from A$_V$ $\sim$ 3 mag to $>$50 mag, molecular hydrogen densities ranging from a few 10$^3$~cm$^{-3}$ to 3$\times$10$^6$~cm$^{-3}$, and T$_k$ $\sim$ 10$-$35 K. Most of the positions in Taurus and Perseus are best fitted assuming early-time chemistry, t=0.1 Myr, $\zeta_{H_2}$$\sim$ (0.5$-$1)$\times$10$^{-16}$ s$^{-1}$, and [S/H]$\sim$1.5$\times$10$^{-6}$. On the contrary, most of the positions in Orion are fitted with t=1~Myr and $\zeta_{H_2}$$\sim$ 10$^{-17}$ s$^{-1}$. Moreover, $\sim$40% of the positions in Orion are best fitted assuming the undepleted sulfur abundance, [S/H]$\sim$1.5$\times$10$^{-5}$. Our results suggest that sulfur depletion depends on the environment. While the abundances of sulfur-bearing species are consistent with undepleted sulfur in Orion, a depletion factor of $\sim$20 is required to explain those observed in Taurus and Perseus. We propose that differences in the grain charge distribution in the envelopes of the studied clouds might explain these variations. The shocks associated with past and ongoing star formation could also contribute to enhance [S/H] in Orion., Comment: 22 pages, 15 figures, Astronomy and Astrophysics, in press

In the context of the IRAM 30m Large Program GEMS, we present a study of thioformaldehyde in several starless cores located in star-forming filaments of Taurus, Perseus, and Orion. We investigate the influence of the environmental conditions on the abundances of these molecules in the cores, and the effect of time evolution. We have modelled the observed lines of H2CS, HDCS, and D2CS using the radiative transfer code RADEX. We have also used the chemical code Nautilus to model the evolution of these species depending on the characteristics of the starless cores. We derive column densities and abundances for all the cores. We also derive deuterium fractionation ratios, Dfrac, to determine and compare the evolutionary stage between different parts of each star-forming region. Our results indicate that the north region of the B213 filament in Taurus is more evolved than the south, while the north-eastern part of Perseus presents an earlier evolutionary stage than the south-western zone. Model results also show that Dfrac decreases with the cosmic-ray ionisation rate, while it increases with density and with the degree of sulphur depletion. In particular, we only reproduce the observations when the initial sulphur abundance in the starless cores is at least one order of magnitude lower than the solar elemental sulphur abundance. The progressive increase in HDCS/H2CS and D2CS/H2CS with time makes these ratios powerful tools for deriving the chemical evolutionary stage of starless cores. However, they cannot be used to derive the temperature of these regions, since both ratios present a similar evolution at two different temperature ranges (7-11 K and 15-19 K). Regarding chemistry, (deuterated) thioformaldehyde is mainly formed through gas-phase reactions (double-replacement and neutral-neutral displacement reactions), while surface chemistry plays an important role as a destruction mechanism., Comment: 31 pages, 26 figures

Gas phase Elemental abundances in Molecular CloudS (GEMS) is an IRAM 30m Large Program designed to estimate the S, C, N, and O depletions and gas ionization degree, X(e-), in a set of star-forming filaments of Taurus, Perseus and Orion. Our immediate goal is to build up a complete database of molecular abundances that can serve as an observational basis for estimating X(e-) and the C, O, N, and S depletions through chemical modeling. We observed and derived the abundances of 14 species (13CO, C18O, HCO+, H13CO+, HC18O+, HCN, H13CN, HNC, HCS+, CS, SO, 34SO, H2S, and OCS) in 244 positions, covering the AV 3 to 100 mag, n(H2) a few 10$^{3}$ to 10$^6$ cm$^{-3}$, and Tk 10 to 30 K ranges in these clouds, avoiding protostars, HII regions, and outflows. A statistical analysis is carried out to identify general trends between different species and with physical parameters. Relations between molecules reveal strong linear correlations which define three different families: (1) 13CO and C18O; (2) H13CO+, HC18O+, H13CN, and HNC; and (3) the S-bearing molecules. The abundances of the CO isotopologs increase with the gas kinetic temperature until TK 15 K. For higher temperatures, the abundance remains constant with a scatter of a factor of 3. The abundances of H13CO+, HC18O+, H13CN, and HNC are well correlated with each other, and all of them decrease with molecular hydrogen density, following the law n(H2)$^{-0.8\pm0.2}$. The abundances of S-bearing species also decrease with n(H2) at a rate of (S-bearing/H)gas n(H2)$^{-0.6\pm0.1}$. The abundances of molecules belonging to groups 2 and 3 do not present any clear trend with gas temperature. At scales of molecular clouds, the C18O abundance is the quantity that better correlates with the cloud mass. We discuss the utility of the 13CO/C18O, HCO+/H13CO+, and H13CO+/H13CN abundance ratios as chemical diagnostics of star formation in external galaxies., Comment: 28 pages, 22 figures. Accepted for publication in A&A

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. Chemical models fail to account for the observed CS abundances when assuming the cosmic value for the elemental abundance of sulfur. 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. Here we calculate the CS+O reaction rate at temperatures <150 K which are prevailing in the interstellar medium. We performed ab initio calculations to obtain the three lowest PES of the CS+O system. These PESs are used to study the reaction dynamics, using several methods to eventually calculate the CS+O thermal reaction rates. We compare the results of our theoretical calculations for 150-400 K with those obtained in the laboratory. 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, which is consistent with the extrapolation of experimental data using the Arrhenius expression. We use the updated chemical network to model the sulfur chemistry in TMC1 based on molecular abundances determined from GEMS project observations. In our model, we take into account the expected decrease of the cosmic ray ionization rate along the cloud. The abundance of CS is still overestimated when assuming the cosmic value for the sulfur abundance., Comment: 11 pages, 10 figures, accepted in Astronomy & Astrophysics

Sulphur is one of the most abundant elements in the Universe. Surprisingly, sulphuretted molecules are not as abundant as expected in the interstellar medium, and the identity of the main sulphur reservoir is still an open question. Our goal is to investigate the H$_{2}$S chemistry in dark clouds, as this stable molecule is a potential sulphur reservoir. Using millimeter observations of CS, SO, H$_{2}$S, and their isotopologues, we determine the physical conditions and H$_{2}$S abundances along the cores TMC 1-C, TMC 1-CP, and Barnard 1b. The gas-grain model Nautilus is then used to model the sulphur chemistry and explore the impact of photo-desorption and chemical desorption on the H$_2$S abundance. Our model shows that chemical desorption is the main source of gas-phase H$_2$S in dark cores. The measured H$_{2}$S abundance can only be fitted if we assume that the chemical desorption rate decreases by more than a factor of 10 when $n_{\rm H}>2\times10^{4}$. This change in the desorption rate is consistent with the formation of thick H$_2$O and CO ice mantles on grain surfaces. The observed SO and H$_2$S abundances are in good agreement with our predictions adopting an undepleted value of the sulphur abundance. However, the CS abundance is overestimated by a factor of $5-10$. Along the three cores, atomic S is predicted to be the main sulphur reservoir. We conclude that the gaseous H$_2$S abundance is well reproduced, assuming undepleted sulphur abundance and chemical desorption as the main source of H$_2$S. The behavior of the observed H$_{2}$S abundance suggests a changing desorption efficiency, which would probe the snowline in these cores. Our model, however, overestimates the observed gas-phase CS abundance. Given the uncertainty in the sulphur chemistry, our data are consistent with a cosmic elemental S abundance with an uncertainty of a factor of 10., Comment: 23 pages, 16 figures

CO$_2$ ice has a phase transition at 35 K when its structure changes from amorphous to crystalline. Using Reflection absorption Infrared Spectroscopy (RAIRS), \"Oberg et al. observed that the photodesorption yield of CO$_2$ ice deposited at 60 K and irradiated at 18 K is 40% lower than that of CO$_2$ ice deposited and irradiated at 18 K. In this work, CO$_2$ ices were deposited at 16-60 K and UV-irradiated at 16 K to rule out the temperature effect and figure out the relationship between photodesorption yield and ice structure. IR spectroscopy is a common method used for measurement of the photodesorption yield in ices. We found that undetectable C atoms produced in irradiated CO$_2$ ice can account for 33% of the amount of depleted CO$_2$ molecules in the ice. A quantitative calibration of QMS was therefore performed to convert the measured ion current into photodesorption yield. During various irradiation periods, the dominant photodesorbing species were CO, O$_2$, and CO$_2$, and their photodesorption yields in CO$_2$ ices deposited at different temperature configurations were almost the same, indicating that ice morphology has no effect on the photodesorption yield of CO$_2$ ice. In addition, we found that the lower desorption yield reported by Mart\'in-Dom\'enech et al. is due to a linear relationship between the photodesorption yield and the combination of energy distribution of Microwave-Discharge Hydrogen-flow Lamp (MDHL) and UV absorption cross section of ices., Comment: 11 pages, 11 figures

We have observed 29 transitions corresponding to 12 distinct species and 7 additional isotopologs toward the deeply embedded Class 0 young stellar object Ser-emb 1 with the Atacama Large Millimeter Array at 1 mm. The detected species include CH3OH and two complex organic molecules, CH3OCH3 and CH3OCHO. The emission of CH3OH and the two COMs is compact, and the CH3OH rotational temperature is 261 (46) K, implying that Ser-emb 1 hosts a hot corino. The derived CH3OH, CH3OCH3 and CH3OCHO column densities are at least 1.2 (0.4)E17 cm-2, 9.2 (3.8)E16 cm-2, and 9.1 (3.6)E16 cm-2, respectively, comparable to the values found for other Class 0 hot corinos. In addition, we observe evidence of rotation at compact scales: two of the more strongly detected lines, corresponding to C18O and H2CO present spatially resolved red- and blue-shifted compact emission orthogonal to the direction of a jet and outflow traced by CO, SiO, and several other molecules. The spatial coincidence of the hot corino emission and a possible disk in a compact region around the central protostar suggests that these structures may be physically and/or chemically related.

Theories of a pre-RNA world suggest that glycolonitrile (HOCH$_2$CN) is a key species in the process of ribonucleotide assembly, which is considered as a molecular precursor of nucleic acids. In this Letter, we report the first detection of this pre-biotic molecule in the interstellar medium (ISM) by using ALMA data obtained at frequencies between 86.5$\,$GHz and 266.5$\,$GHz toward the Solar-type protostar IRAS16293-2422 B. A total of 15 unblended transitions of HOCH$_2$CN were identified. Our analysis indicates the presence of a cold (T$\rm _{ex}$=24$\pm$8$\,$K) and a warm (T$\rm _{ex}$=158$\pm$38$\,$K) component meaning that this molecule is present in both the inner hot corino and the outer cold envelope of IRAS16293 B. The relative abundance with respect to H$_2$ is (6.5$\pm$0.6)$\times$10$^{-11}$ and $\geq$(6$\pm$2)$\times$10$^{-10}$ for the warm and cold components respectively. Our chemical modelling seems to underproduce the observed abundance for both the warm and cold component under various values of the cosmic-ray ionisation rate ($\zeta$). Key gas phase routes for the formation of this molecule might be missing in our chemical network., Comment: 5 pages, 4 figures, 2 tables, accepted in Monthly Notices of the Royal Astronomical Society Letters

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 (3mm and 2mm) and the 40m Yebes telescope (1.3cm and 7mm) 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 ~20mag. Two phases with differentiated chemistry can be distinguished: i) the translucent envelope with molecular hydrogen densities of (1-5)x10$^3$ cm$^{-3}$; and ii) the dense phase, located at Av>10mag, with molecular hydrogen densities >10$^4$ cm$^{-3}$. Observations and modeling show that the gas phase abundances of C and O progressively decrease along the C+/C/CO transition zone where C/H~8x10$^{-5}$ 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) x10$^{-6}$, 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~8x10$^{-8}$). Based on our chemical modeling, we constrain the value of $\zeta_{\rm H_2}$ to ~(0.5 - 1.8) x10$^{-16}$ s$^{-1}$ in the translucent cloud., Comment: 26 pages, 20 figures, A&A in press

Ice mantles detected on the surface of dust grains toward the coldest regions of the interstellar medium can be photoprocessed by the secondary ultraviolet (UV) field present in dense cloud interiors. In this work, we present UV-irradiation experiments under astrophysically relevant conditions of pure NH3 ice samples in an ultra-high vacuum chamber where solid samples were deposited onto a substrate at 8 K. The ice analogs were subsequently photoprocessed with a microwave-discharged hydrogen- flow lamp. The induced radiation and photochemistry led to the production of H2, N2 and N2H4. In addition, photodesorption to the gas phase of the original ice component, NH3, and two of the three detected photoproducts, H2 and N2, was observed thanks to a quadrupole mass spectrometer (QMS). Calibration of the QMS allowed quantifica- tion of the photodesorption yields, leading to Ypd(NH3)=2.1+2.1-1.0 x 10^-3 molecules per incident photon, which remained constant during the whole experiments, while photodesorption of H2 and N2 increased with fluence, pointing toward an indirect photodesorption mecha- nism involving energy transfer for these species. Photodesorption yield of N2 molecules after a fluence equivalent to that experienced by ice mantles in space was similar to that of the NH3 molecules (Ypd (N2) = 1.7+1.7-0.9 x 10^-3 molecules incident photon ).

In regions where stars form, variations in density and temperature can cause gas to freeze-out onto dust grains forming ice mantles, which influences the chemical composition of a cloud. The aim of this paper is to understand in detail the depletion (and desorption) of CO on (from) interstellar dust grains. Experimental simulations were performed under two different (astrophysically relevant) conditions. In parallel, Kinetic Monte Carlo simulations were used to mimic the experimental conditions. In our experiments, CO molecules accrete onto water ice at temperatures below 27 K, with a deposition rate that does not depend on the substrate temperature. During the warm-up phase, the desorption processes do exhibit subtle differences indicating the presence of weakly bound CO molecules, therefore highlighting a low diffusion efficiency. IR measurements following the ice thickness during the TPD confirm that diffusion occurs at temperatures close to the desorption. Applied to astrophysical conditions, in a pre-stellar core, the binding energies of CO molecules, ranging between 300 K and 850 K, depend on the conditions at which CO has been deposited. Because of this wide range of binding energies, the depletion of CO as a function of AV is much less important than initially thought. The weakly bound molecules, easily released into the gas phase through evaporation, change the balance between accretion and desorption, which result in a larger abundance of CO at high extinctions. In addition, weakly bound CO molecules are also be more mobile, and this could increase the reactivity within interstellar ices., Comment: Accepted in ApJ

We report the detection of the prebiotic molecule CH3NCO in a solar-type protostar, IRAS16293-2422 B. A significant abundance of this species on the surface of the comet 67P/Churyumov-Gerasimenko has been proposed, and it has recently been detected in hot cores around high-mass protostars. We observed IRAS16293-2422 B with ALMA in the 90 GHz to 265 GHz range, and detected 8 unblended transitions of CH3NCO. From our Local Thermodynamic Equilibrium analysis we derived an excitation temperature of 110+-19 K and a column density of (4.0+-0.3)x10^15 cm^-2 , which results in an abundance of <=(1.4+-0.1)x10^-10 with respect to molecular hydrogen. This implies a CH3NCO/HNCO and CH3NCO/NH2CHO column density ratios of ~0.08. Our modelling of the chemistry of CH3NCO suggests that both ice surface and gas phase formation reactions of this molecule are needed to explain the observations.

Methanol and formaldehyde are two simple organic molecules that are ubiquitously detected in the interstellar medium. An origin in the solid phase and a subsequent nonthermal desorption into the gas phase is often invoked to explain their abundances in some of the environments where they are found. Experimental simulations under astrophysically relevant conditions have been carried out to find a suitable mechanism for that process. We explore the in situ formation and subsequent photon-induced desorption of these species, studying the UV photoprocessing of pure ethanol ice, and a more realistic binary H2O:CH4 ice analog. Ice samples deposited onto an infrared transparent window at 8 K were UV-irradiated using a microwave-discharged hydrogen flow lamp. Evidence of photochemical production of these two species and subsequent UV-photon-induced desorption into the gas phase were searched for by means of a Fourier transform infrared spectrometer and a quadrupole mass spectrometer, respectively. Formation of CH3OH was only observed during photoprocessing of the H2O:CH4 ice analog, but no photon-induced desorption was detected. Photochemical production of H2CO was observed in both series of experiments. Photochemidesorption of formaldehyde, i.e., photon-induced formation on the ice surface and inmediate desorption, was observed, with a yield of 4.4 x 10-4 (molecules/incident photon) when the H2O:CH4 ice analogs were photoprocessed. While certain C-bearing species, in particular H2CO, were found to desorb upon irradiation, nonthermal desorption of CH3OH was not observed. So far, there is no experimental evidence of any efficient CH3 OH desorption induced by UV photons.

A fraction of the missing sulfur in dense clouds and circumstellar regions could be in the form of three species not yet de- tected in the interstellar medium: H2S2, HS2, and S2 according to experimental simulations performed under astrophysically relevant conditions. These S-S bonded molecules can be formed by the energetic processing of H2S-bearing ice mantles on dust grains, and subsequently desorb to the gas phase. The detection of these species could partially solve the sulfur depletion problem, and would help to improve our knowledge of the poorly known chemistry of sulfur in the interstellar medium. To this purpose we performed dedicated ground-based observations toward the low-mass warm core IRAS16293-2422. Observations in the submillimeter regime were obtained with the APEX 12 m telescope during 15 hours of observation, targeting a wide selection of the predicted rotational transitions of the three molecules. The 1{\sigma} noise rms values were extracted in the spectral regions where the targeted species should have been detected. These values were a factor of 2-7 lower than those reached by previous observations toward the same source, and allowed us to estimate a 1{\sigma} upper limit to their molecular abundances of 8.1E-9 , 1.1E-8 , and 2.9E-7 relative to H2, for H2S2, HS2, and S2, respectively. The upper limit abundances of the three molecules containing the S2 unit are up to two orders of magnitude lower than the H2S abundance in the source, and one order of magnitude lower than the expected abundances from the experimental simulations using ice analogs. Subsequent gas-phase chemistry after desorption could lower the abundances of the three species to undetectable levels in our observations.

Ice mantles that formed on top of dust grains are photoprocessed by the secondary ultraviolet (UV) field in cold and dense molecular clouds. UV photons induce photochemistry and desorption of ice molecules. Experimental simulations dedicated to ice analogs under astrophysically relevant conditions are needed to understand these processes. We present UV-irradiation experiments of a pure CO2 ice analog. Calibration of the QMS allowed us to quantify the photodesorption of molecules to the gas phase. This information was added to the data provided by the FTIR on the solid phase to obtain a complete quantitative study of the UV photoprocessing of an ice analog. Experimental simulations were performed in an ultra-high vacuum chamber. Ice samples were deposited onto an infrared transparent window at 8K and were subsequently irradiated with a microwave-discharged hydrogen flow lamp. After irradiation, ice samples were warmed up until complete sublimation was attained. Photolysis of CO2 molecules initiates a network of photon-induced chemical reactions leading to the formation of CO, CO3 ,O2 , and O3 . During irradiation, photon-induced desorption of CO and, to a lesser extent, O2 and CO2 took place through a process called indirect desorption induced by electronic transitions (DIET), with maximum photodesorption yields (Ypd) of 1.2 x 10-2 molecules/incident photon , 9.3 x 10-4 molecules/incident photon , and 1.1 x 10-4 molecules/incident photon , respectively. Calibration of mass spectrometers allows a direct quantification of photodesorption yields instead of the indirect values that were obtained from infrared spectra in most previous works. Supplementary information provided by infrared spectroscopy leads to a complete quantification, and therefore a better understanding, of the processes taking place in UV-irradiated ice mantles.

We measure the band strengths in the far-infrared region of interstellar ice analogs of astrophysically relevant species, such as H$_2$O, CO$_2$, CH$_3$OH, and NH$_3$, deposited at low temperature (8-10 $\mathrm{K}$), followed by warm-up, to induce amorphous-crystalline phase transitions when relevant. The spectra of pure H$_2$O, NH$_3$, and CH$_3$OH ices have been measured in the near-, mid- and far-infrared spectroscopic regions using the Interstellar Astrochemistry Chamber (ISAC) ultra-high-vacuum setup. In addition, far-infrared spectra of NH$_3$ and CO$_2$ were measured using a different set-up equipped with a bolometer detector. Band strengths in the far-infrared region were estimated using the corresponding near- and mid-infrared values as a reference. We also performed theoretical calculations of the amorphous and crystalline structures of these molecules using solid state computational programs at density functional theory (DFT) level. Vibrational assignment and mode intensities for these ices were predicted. Infrared band strengths in the 25-500 ${\mu}$m range have been determined for the considered ice samples by direct comparison in the near- and mid-infrared regions. Our values were compared to those we calculated from the literature complex index of refraction. We found differences of a factor of two between the two sets of values. The calculated far-infrared band strengths provide a benchmark for interpreting the observational data from future space telescope missions, allowing the estimation of the ice column densities., Comment: Key words. astrochemistry -- methods: laboratory: solid state -- ISM: molecules -- techniques: spectroscopic -- infrared: ISM