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Aims. This work aims to study the unexplained sulfur depletion observed toward dense clouds and protostars. Methods. We made simulation experiments of the UV-photoprocessing and sublimation of H 2 S and H 2 S:H 2 O ice in dense clouds and circumstellar regions, using the InterStellar Astrochemistry Chamber (ISAC), a state-of-the-art ultra-high-vacuum setup. The ice was monitored in situ by mid-infrared spectroscopy in transmittance. Temperature-programmed desorption (TPD) of the ice was performed using a quadrupole mass spectrometer (QMS) to detect the volatiles desorbing from the ice.Results. Comparing our laboratory data to infrared observations of protostars we obtained a more accurate upper limit of the abundance of H 2 S ice toward these objects. We determined the desorption temperature of H 2 S ice, which depends on the initial H 2 S:H 2 O ratio. UV-photoprocessing of H 2 S:H 2 O ice led to the formation of several species. Among them, H 2 S 2 was found to photodissociate forming S 2 and, by elongation, other species up to S 8 , which are refractory at room temperature. A large fraction of the missing sulfur in dense clouds and circumstellar regions could thus be polymeric sulfur residing in dust grains.

Pure methanol ices have been irradiated with monochromatic soft X-rays of 300 and 550 eV close to the 1s resonance edges of C and O, respectively, and with a broadband spectrum (250-1200 eV). The infrared (IR) spectra of the irradiated ices show several new products of astrophysical interest such as CH 2 OH, H 2 CO, CH 4 , HCOOH, HCOCH 2 OH, CH 3 COOH, CH 3 OCH 3 , HCOOCH 3 , and (CH 2 OH) 2 , as well as HCO, CO, and CO 2 . The effect of X-rays is the result of the combined interactions of photons and electrons with the ice. A significant contribution to the formation and growth of new species in the CH 3 OH ice irradiated with X-rays is given by secondary electrons, whose energy distribution depends on the energy of X-ray photons. Within a single experiment, the abundances of the new products increase with the absorbed energy. Monochromatic experiments show that product abundances also increase with the photon energy. However, the abundances per unit energy of newly formed species show a marked decrease in the broadband experiment as compared to irradiations with monochromatic photons, suggesting a possible regulatory role of the energy deposition rate. The number of new molecules produced per absorbed eV in the X-ray experiments has been compared to those obtained with electron and ultraviolet (UV) irradiation experiments.

Aims. We present the novel InterStellar Astrochemistry Chamber (ISAC), designed for studying solids (ice mantles, organics, and silicates) in interstellar and circumstellar environments: characterizing their physico-chemical properties and monitoring their evolution as caused by (i) vacuum-UV irradiation; (ii) cosmic ray irradiation; and (iii) thermal processing. Experimental study of thermal and photodesorption of the CO ice reported here simulates the freeze-out and desorption of CO on grains, providing new information on these processes. Methods. ISAC is an UHV set-up, with base pressure down to P = 2.5 × 10 −11 mbar, where an ice layer is deposited at 7 K and can be UV-irradiated. The evolution of the solid sample was monitored by in situ transmittance FTIR spectroscopy, while the volatile species were monitored by QMS. Results. The UHV conditions of ISAC allow experiments under extremely clean conditions. Transmittance FTIR spectroscopy coupled to QMS proved to be ideal for in situ monitoring of ice processes that include radiation and thermal annealing. Thermal desorption of CO starting at 15 K, induced by the release of H 2 from the CO ice, was observed. We measured the photodesorption yield of CO ice per incident photon at 7, 8, and 15 K, respectively yielding 6.4 ± 0.5 × 10 −2 , 5.4 ± 0.5 × 10 −2 , and 3.5 ± 0.5 × 10 −2 CO molecules photon (7.3-10.5 eV) −1 . Our value of the photodesorption yield of CO ice at 15 K is about one order of magnitude higher than the previous estimate. We confirmed that the photodesorption yield is constant during irradiation and independent of the ice thickness. Only below ∼5 monolayers ice thickness the photodesorption rate decreases, which suggests that only the UV photons absorbed in the top 5 monolayers led to photodesorption. The measured CO photodesorption quantum yield at 7 K per absorbed photon in the top 5 monolayers is 3.4 molecules photon −1 . Conclusions. Experimental values were used as input for a simple model of a quiescent cloud interior. Photodesorption seems to explain the observations of CO in the gas phase for densities below 3-7 ×10 4 cm −3 . For the same density of a cloud, 3 × 10 4 cm −3 , thermal desorption of CO is not triggered until T = 14.5 K. This has important implications for CO ice mantle build up in dark clouds.

We study the chemical evolution of H 2 O:CO:NH 3 ice mixtures irradiated with soft X-rays, in the range 250 − 1250 eV. We identify many nitrogen-bearing molecules such as e.g., OCN − , NH + 4 , HNCO, CH 3 CN, HCONH 2 , and NH 2 COCONH 2 . Several infrared features are compatible with glycine or its isomers.During the irradiation, we detected through mass spectroscopy many species desorbing the ice. Such findings support either the infrared identifications and reveal less abundant species with not clear infrared features. Among them, m/z = 57 has been ascribed to methyl isocyanate (CH 3 NCO), a molecule of prebiotic relevance, recently detected in protostellar environments.During the warm up after the irradiation, several infrared features including 2168 cm −1 band of OCN − , 1690 cm −1 band of formamide, and the 1590 cm −1 band associated to three different species, HCOO − , CH 3 NH 2 and NH + 3 CH 2 COO survive up to room temperature. Interestingly, many high masses have been also detected. Possible candidates are methyl-formate, (m/z = 60, HCOOCH 3 ), ethanediamide (m/z = 88, NH 2 COCONH 2 ), and N-acetyl-L-aspartic acid (m/z = 175). This latter species is compatible with the presence of the m/z = 43, 70 and 80 fragments.Photo-desorption of organics is relevant for the detection of such species in the gas-phase of cold environments, where organic synthesis in ice mantles should dominate. We estimate the gas-phase enrichment of some selected species in the light of a protoplanetary disc model around young solartype stars.