Context. Hydrogenation reactions of CO in inter-and circumstellar ices are regarded as an important starting point in the formation of more complex species. Previous laboratory measurements by two groups of the hydrogenation of CO ices provided controversial results about the formation rate of methanol. Aims. Our aim is to resolve this controversy by an independent investigation of the reaction scheme for a range of H-atom fluxes and different ice temperatures and thicknesses. To fully understand the laboratory data, the results are interpreted theoretically by means of continuous-time, random-walk Monte Carlo simulations. Methods. Reaction rates are determined by using a state-of-the-art ultra high vacuum experimental setup to bombard an interstellar CO ice analog with H atoms at room temperature. The reaction of CO + H into H 2 CO and subsequently CH 3 OH is monitored by a Fourier transform infrared spectrometer in a reflection absorption mode. In addition, after each completed measurement, a temperature programmed desorption experiment is performed to identify the produced species according to their mass spectra and to determine their abundance. Different H-atom fluxes, morphologies, and ice thicknesses are tested. The experimental results are interpreted using Monte Carlo simulations. This technique takes into account the layered structure of CO ice. Results. The formation of both formaldehyde and methanol via CO hydrogenation is confirmed at low temperature (T = 12−20 K). We confirm that the discrepancy between the two Japanese studies is caused mainly by a difference in the applied hydrogen atom flux, as proposed by Hidaka and coworkers. The production rate of formaldehyde is found to decrease and the penetration column to increase with temperature. Temperature-dependent reaction barriers and diffusion rates are inferred using a Monte Carlo physical chemical model. The model is extended to interstellar conditions to compare with observational H 2 CO/CH 3 OH data.

Context. Gas-phase complex organic molecules are commonly detected in the warm inner regions of protostellar envelopes, so-called hot cores. Recent models show that photochemistry in ices followed by desorption may explain the observed abundances. There is, however, a general lack of quantitative data on UV-induced complex chemistry in ices. Aims. This study aims to experimentally quantify the UV-induced production rates of complex organics in CH 3 OH-rich ices under a variety of astrophysically relevant conditions. Methods. The ices are irradiated with a broad-band UV hydrogen microwave-discharge lamp under ultra-high vacuum conditions, at 20-70 K, and then heated to 200 K. The reaction products are identified by reflection-absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD), through comparison with RAIRS and TPD curves of pure complex species, and through the observed effects of isotopic substitution and enhancement of specific functional groups, such as CH 3 , in the ice. Results. Complex organics are readily formed in all experiments, both during irradiation and during the slow warm-up of the ices after the UV lamp is turned off. The relative abundances of photoproducts depend on the UV fluence, the ice temperature, and whether pure CH 3 OH ice or CH 3 OH:CH 4 /CO ice mixtures are used. C 2 H 6 , CH 3 CHO, CH 3 CH 2 OH, CH 3 OCH 3 , HCOOCH 3 , HOCH 2 CHO and (CH 2 OH) 2 are all detected in at least one experiment. Varying the ice thickness and the UV flux does not affect the chemistry. The derived product-formation yields and their dependences on different experimental parameters, such as the initial ice composition, are used to estimate the CH 3 OH photodissociation branching ratios in ice and the relative diffusion barriers of the formed radicals. At 20 K, the pure CH 3 OH photodesorption yield is 2.1(±1.0) × 10 −3 per incident UV photon, the photo-destruction cross section 2.6(±0.9) × 10 −18 cm 2 . Conclusions. Photochemistry in CH 3 OH ices is efficient enough to explain the observed abundances of complex organics around protostars. Some complex molecules, such as CH 3 CH 2 OH and CH 3 OCH 3 , form with a constant ratio in our ices and this can can be used to test whether complex gas-phase molecules in astrophysical settings have an ice-photochemistry origin. Other molecular ratios, e.g. HCO-bearing molecules versus (CH 2 OH) 2 , depend on the initial ice composition and temperature and can thus be used to investigate when and where complex ice molecules form.

Even though water is the main constituent in interstellar icy mantles, its chemical origin is not well understood. Three different formation routes have been proposed following hydrogenation of O, O 2 , or O 3 on icy grains, but experimental evidence is largely lacking. We present a solid state astrochemical laboratory study in which one of these routes is tested. For this purpose O 2 ice is bombarded by H or D atoms under ultrahigh vacuum conditions at astronomically relevant temperatures ranging from 12 to 28 K. The use of reflection absorption infrared spectroscopy (RAIRS) permits derivation of reaction rates and shows efficient formation of H 2 O (D 2 O) with a rate that is surprisingly independent of temperature. This formation route converts O 2 into H 2 O via H 2 O 2 and is found to be orders of magnitude more efficient than previously assumed. It should therefore be considered as an important channel for interstellar water ice formation as illustrated by astrochemical model calculations.

UV-induced photodesorption of ice is a non-thermal evaporation process that can explain the presence of cold molecular gas in a range of interstellar regions. Information on the average UV photodesorption yield of astrophysically important ices exists for broadband UV lamp experiments. UV fields around low-mass pre-main sequence stars, around shocks and in many other astrophysical environments are however often dominated by discrete atomic and molecular emission lines. It is therefore crucial to consider the wavelength dependence of photodesorption yields and mechanisms. In this work, for the first time, the wavelength-dependent photodesorption of pure CO ice is explored between 90 and 170 nm. The experiments are performed under ultra high vacuum conditions using tunable synchrotron radiation. Ice photodesorption is simultaneously probed by infrared absorption spectroscopy in reflection mode of the ice and by quadrupole mass spectrometry of the gas phase. The experimental results for CO reveal a strong wavelength dependence directly linked to the vibronic transition strengths of CO ice, implying that photodesorption is induced by electronic transition (DIET). The observed dependence on the ice absorption spectra implies relatively low photodesorption yields at 121.6 nm (Lyman α), where CO barely absorbs, compared to the high yields found at wavelengths coinciding with transitions into the first electronic state of CO (A 1 Π at 150 nm); the CO photodesorption rates depend strongly on the UV profiles encountered in different star formation environments.

Complex organic molecules (COMs) have been observed not only in the hot cores surrounding low-and high-mass protostars, but also in cold dark clouds. Therefore, it is interesting to understand how such species can be formed without the presence of embedded energy sources. We present new laboratory experiments on the lowtemperature solid state formation of three complex moleculesmethyl formate (HC(O)OCH 3 ), glycolaldehyde (HC(O)CH 2 OH) and ethylene glycol (H 2 C(OH)CH 2 OH)through recombination of free radicals formed via H-atom addition and abstraction reactions at different stages in the CO→H 2 CO→CH 3 OH hydrogenation network at 15 K. The experiments extend previous CO hydrogenation studies and aim at resembling the physical-chemical conditions typical of the CO freeze-out stage in dark molecular clouds, when H 2 CO and CH 3 OH form by recombination of accreting CO molecules and Hatoms on ice grains. We confirm that H 2 CO, once formed through CO hydrogenation, not only yields CH 3 OH through ongoing H-atom addition reactions, but is also subject to H-atom-induced abstraction reactions, yielding CO again. In a similar way, H 2 CO is also formed in abstraction reactions involving CH 3 OH. The dominant methanol H-atom abstraction product is expected to be CH 2 OH, while H-atom additions to H 2 CO should at least partially proceed through CH 3 O intermediate radicals. The occurrence of H-atom abstraction reactions in ice mantles leads to more reactive intermediates (HCO, CH 3 O and CH 2 OH) than previously thought, when assuming sequential H-atom addition reactions only. This enhances the probability to form COMs through radical-radical recombination without the need of UV photolysis or cosmic rays as external triggers.

Wavelength dependent photodesorption rates have been determined using synchrotron radiation, for condensed pure and mixed methanol ice in the 7 -14 eV range. The VUV photodesorption of intact methanol molecules from pure methanol ices is found to be of the order of 10 −5 molecules/photon, that is two orders of magnitude below what is generally used in astrochemical models. This rate gets even lower (< 10 −6 molecules/photon) when the methanol is mixed with CO molecules in the ices. This is consistent with a picture in which photodissociation and recombination processes are at the origin of intact methanol desorption from pure CH 3 OH ices. Such low rates are explained by the fact that the overall photodesorption process is dominated by the desorption of the photofragments CO, CH 3 , OH, H 2 CO and CH 3 O/CH 2 OH, whose photodesorption rates are given in this study. Our results suggest that the role of the photodesorption as a mechanism to explain the observed gas phase abundances of methanol in cold media is probably overestimated. Nevertheless, the photodesorption of radicals from methanol-rich ices may stand at the origin of the gas phase presence of radicals such as CH 3 O, therefore opening new gas phase chemical routes for the formation of complex molecules.

NH 3 and CH 3 OH are key molecules in astrochemical networks leading to the formation of more complex N-and O-bearing molecules, such as CH 3 CN and CH 3 OCH 3 . Despite a number of recent studies, little is known about their abundances in the solid state. This is particularly the case for low-mass protostars, for which only the launch of the Spitzer Space Telescope has permitted high-sensitivity observations of the ices around these objects. In this work, we investigate the ∼8-10 μm region in the Spitzer IRS (InfraRed Spectrograph) spectra of 41 low-mass young stellar objects (YSOs). These data are part of a survey of interstellar ices in a sample of low-mass YSOs studied in earlier papers in this series. We used both an empirical and a local continuum method to correct for the contribution from the 10 μm silicate absorption in the recorded spectra. In addition, we conducted a systematic laboratory study of NH 3 -and CH 3 OH-containing ices to help interpret the astronomical spectra. We clearly detect a feature at ∼9 μm in 24 low-mass YSOs. Within the uncertainty in continuum determination, we identify this feature with the NH 3 ν 2 umbrella mode and derive abundances with respect to water between ∼2% and 15%. Simultaneously, we also revisited the case of CH 3 OH ice by studying the ν 4 C-O stretch mode of this molecule at ∼9.7 μm in 16 objects, yielding abundances consistent with those derived by Boogert et al. based on a simultaneous 9.75 and 3.53 μm data analysis. Our study indicates that NH 3 is present primarily in H 2 O-rich ices, but that in some cases, such ices are insufficient to explain the observed narrow FWHM. The laboratory data point to CH 3 OH being in an almost pure methanol ice, or mixed mainly with CO or CO 2 , consistent with its formation through hydrogenation on grains. Finally, we use our derived NH 3 abundances in combination with previously published abundances of other solid N-bearing species to find that up to 10%-20% of nitrogen is locked up in known ices.

No abstract