G. M. Muñoz Caro

UV-photoprocessing of interstellar ice analogs: New infrared spectroscopic results

We simulate experimentally the physical conditions present in dense clouds by means of a high vacuum experimental setup at low temperature T ≈ 12 K. The accretion and photoprocessing of ices on grain surfaces is simulated in the following way: an ice layer with composition analogous to that of interstellar ices is deposited on a substrate window, while being irradi- ated by ultraviolet (UV) photons. Subsequently the sample is slowly warmed up to room temperature; a residue remains con- taining the most refractory products of photo- and thermal processing. In this paper we report on the Fourier transform-infrared (FT-IR) spectroscopy of the refractory organic material formed under a wide variety of initial conditions (ice composition, UV spectrum, UV dose and sample temperature). The refractory products obtained in these experiments are identified and the corresponding efficiencies of formation are given. The first evidence for carboxylic acid salts as part of the refractory products is shown. The features in the IR spectrum of the refractory material are attributed to hexamethylenetetramine (HMT, ((CH2)6N4)), ammonium salts of carboxylic acids ((R-COO − )(NH + )), amides (H2NC(=O)-R), esters (R-C(=O)-O-R � ) and species related to polyoxymethylene (POM, ((-CH2O-)n)). Furthermore, evidence is presented for the formation of HMT at room temperature, and the important role of H2O ice as a catalyst for the formation of complex organic molecules. These species might also be present in the interstellar medium (ISM) and form part of comets. Ongoing and future cometary missions, such as Stardust and Rosetta, will allow a comparison with the laboratory results, providing new insight into the physico-chemical conditions present during the formation of our solar system.

Discovery of the Methoxy Radical, CH3O, toward B1: Dust Grain and Gas-phase Chemistry in Cold Dark Clouds

We report on the discovery of the methoxy radical (CH3O) toward the cold and dense core B1-b based on the observation, with the IRAM 30 m radio telescope, of several lines at 3 and 2 mm wavelengths. Besides this new molecular species we also report on the detection of many lines arising from methyl mercaptan (CH3SH), formic acid (HCOOH), propynal (HCCCHO), acetaldehyde (CH3CHO), dimethyl ether (CH3OCH3), methyl formate (CH3OCOH), and the formyl radical (HCO). The column density of all these species is 1012 cm–2, corresponding to abundances of 10–11. The similarity in abundances for all these species strongly suggest that they are formed on the surface of dust grains and ejected to the gas phase through non-thermal desorption processes, most likely cosmic rays or secondary photons. Nevertheless, laboratory experiments indicate that the CH3O isomer released to the gas phase is CH2OH rather than the methoxy one. Possible gas-phase formation routes to CH3O from OH and methanol are discussed.

New results on thermal and photodesorption of CO ice using the novel InterStellar Astrochemistry Chamber (ISAC)

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 x 10 -2 , 5.4 ± 0.5 × 10 -2 , and 3.5 ± 0.5 x 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 x 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.

Ultraviolet photoproduction of ISM dust Laboratory characterisation and astrophysical relevance

The production of a hydrogenated amorphous carbon polymer (a-C:H) via the photolysis of a series of organic molecule precursors at low temperature is described. Such amorphous material is synthesised under interstellar conditions (10 K and Lyman-α photons) and represents the best candidate to explain the Diffuse Interstellar Medium absorption observed in our Galaxy and in other galaxies. We perform a series of laboratory analyses (Infrared spectroscopy, µspectroscopy, Raman, Photoluminescence and UV-visible spectroscopy) which allow a full characterisation of such polymers. This allows us to assess the importance of the polymer and possible scenarios for its role in crucial aspects of the lifecycle of dust. Such material has implications for the carbon budget at galactic scales, hydrogen formation, extended red emission, as a PAH precursor, and in explaining the 2175 A extinction bump.

UV photodestruction of CH bonds and the evolution of the 3.4 mu m feature carrier. II. The case of hydrogenated carbon grains

We present the results of a laboratory program aimed at studying the effects induced by energetic UV photons on hydrogenated carbon particles. Experiments have been performed under simulated diffuse and dense interstellar medium conditions. To monitor the effects of UV irradiation on grains IR spectroscopy has been used. In both circumstances UV photons lead to a reduction of the aliphatic 3.4 μm band. An estimation of the destruction cross section by UV photons for the hydrogenated carbon particles has been derived from the reduction of the 3.4 μm intensity band as a function of the UV fluence. The results of the present work, together with previous laboratory data, can shed light on the enigmatic difference observed for the 3.4 μm band between dense and diffuse interstellar medium clouds. This difference is compatible with the transformation of hydrogenated carbon particles produced by UV photons and hydrogen atoms and with the changes of the grain properties in the two environments.

VACUUM ULTRAVIOLET EMISSION SPECTRUM MEASUREMENT OF A MICROWAVE-DISCHARGE HYDROGEN-FLOW LAMP IN SEVERAL CONFIGURATIONS: APPLICATION TO PHOTODESORPTION OF CO ICE

We report measurements of the vacuum ultraviolet (VUV) emission spectra of a microwave-discharge hydrogen-flow lamp (MDHL), a common tool in astrochemistry laboratories working on ice VUV photoprocessing. The MDHL provides hydrogen Ly-α (121.6 nm) and H2 molecular emission in the 110-180 nm range. We show that the spectral characteristics of the VUV light emitted in this range, in particular the relative proportion of Ly-α to molecular emission bands, strongly depend on the pressure of H2 inside the lamp, the lamp geometry (F type versus T type), the gas used (pure H2 versus H2 seeded in He), and the optical properties of the window used (MgF2 versus CaF2). These different configurations are used to study the VUV irradiation of CO ice at 14 K. In contrast to the majority of studies dedicated to the VUV irradiation of astrophysical ice analogs, which have not taken into consideration the emission spectrum of the MDHL, our results show that the processes induced by photons in CO ice from a broad energy range are different and more complex than the sum of individual processes induced by monochromatic sources spanning the same energy range, as a result of the existence of multistate electronic transitions and discrepancy in absorption cross sections between parent molecules and products in the Ly-α and H2 molecular emission ranges.

The effects of circularly polarized light on amino acid enantiomers produced by the UV irradiation of interstellar ice analogs

Two irradiation experiments on interstellar ice analogs at 80 K under interstellar-like conditions were performed with the LURE SU5 synchrotron beamline to assess, for the first time, the photochemical effect of circularly polarized ultraviolet light (UV CPL) at 167 nm (7.45 eV) with right and left polarizations on such ice mixtures. Methods. This effect was measured by determining the enantiomeric excesses (e.e.s) for two amino acids formed in the solid organic residues produced during the subsequent warm-up of the irradiated samples to room temperature: alanine, the most abundant chiral proteinaceous amino acid produced (both polarizations) and 2,3-diaminopropanoic acid (DAP), a non-proteinaceous amino acid (rightpolarization experiment). These excesses were compared to those measured for the same amino acids produced after unpolarized UV irradiation of the same ice mixtures (expected to be zero), in order to determine the contribution of CPL only. A careful estimate of all the associated uncertainties (statistical and systematic errors) was also developed. Results. It appears that the enantiomeric photochemical effect at this wavelength is weak, since both alanine and DAP e.e.s were found to be small, at most of the order of 1% in absolute values, and tends to be inconclusive since the effects obtained for both amino acids and both polarizations are not those expected. In light of these results, the hypothesis that CPL may be one source responsible for the e.e.s measured for such amino acids in some meteorites and, more generally, that CPL may be directly related to the origin of biomolecular homochirality on Earth is discussed.

UV photodestruction of CH bonds and the evolution of the 3.4 mu m feature carrier. I. The case of aliphatic and aromatic molecular species

Experiments simulating the processing of various hydrocarbon species under diffuse and dense cloud conditions by UV irradiation were performed. The results indicate that such molecules will be efficiently dehydrogenated in interstellar space. It is argued that the presence of hydrogen in the aliphatic grain material in diffuse clouds results from an equilibrium between dehydrogenation by UV processing and re-hydrogenation by the impinging atomic gas. In dense clouds, the presence of the ice layer will prevent the re-hydrogenation process, causing the carbonaceous grain material to be gradually de-hydrogenated if UV photons are able to penetrate into the dense medium. The implications of this study for the evolution of the carbonaceous component of dust in the interstellar medium are discussed.

Thermal desorption of circumstellar and cometary ice analogs

Context. Thermal annealing of interstellar ices takes place in sever al stages of star formation. Knowledge of this process comes from a combination of astronomical observations and laboratory simulations under astrophysically relevant conditions. Aims. For the first time we present the results of temperature progr ammed desorption (TPD) experiments with pre-cometary ice analogs composed of up to five molecular components: H 2O, CO, CO2, CH3OH, and NH3. Methods. The experiments were performed with an ultra-high vacuum chamber. A gas line with a novel design allows the controlled preparation of mixtures with up to five molecular components . Volatiles desorbing to the gas phase were monitored using a quadrupole mass spectrometer, while changes in the ice structure and composition were studied by means of infrared spectroscopy. Results. The TPD curves of water ice containing CO, CO2, CH3OH, and NH3 present desorption peaks at temperatures near those observed in pure ice experiments, volcano desorption peaks after water ice crystallization, and co-desorption peaks w ith water. Desorption peaks of CH3OH and NH3 at temperatures similar to the pure ices takes place when their abundance relative to water is above ∼ 3 % in the ice matrix. We found that CO, CO2, and NH3 also present co-desorption peaks with CH3OH, which cannot be reproduced in experiments with binary water-rich ice mixtures. These are extensively used in the study of thermal desorption of interstellar ices. Conclusions. These results reproduce the heating of circumstellar ices i n hot cores and can be also applied to the late thermal evolution of comets. In particular, TPD curves represent a benchmark for the analysis of the measurements that mass spectrometers on board the ESA-Rosetta cometary mission will perform on the coma of comet 67P/Churyumov-Gerasimenko, which will be active before the arrival of Rosetta according to our predictions.

UV-photoprocessing of interstellar ice analogs : Detection of hexamethylenetetramine-based species

The physical conditions governing the dense cloud environment are reproduced in a high vacuum experimental setup at low temperature T 12 K. The accretion and photoprocessing of ices on grain surfaces is simulated by depositing an ice layer on a cold finger, while it is irradiated by ultraviolet (UV) photons. After irradiation the sample is slowly warmed to room temperature; a residue remains, containing the most refractory products of photo- and thermal processing. In this paper we report on the analysis of the residues performed by means of gas chromatography-mass spectrometry (GC-MS). A number of new molecules based on hexamethylenetetramine (HMT, C 6 H 1 2 N 4 ), the most abundant component of the residues reported here, were detected: methyl-HMT (C 6 H 1 1 N 4 -CH 3 ), hydroxy-HMT (C 6 H 1 1 N 4 -OH), methanyl-HMT (C 6 H 1 1 N 4 -CH 2 OH), amin-aldehyd-HMT (C 6 H 1 1 N 4 -NH-CHO) and methanyl-aldehyd-HMT (C 6 H 1 1 N 4 -CHOH-CHO). To the best of our knowledge, this is the first reported synthesis of these molecules. Currently, these are the heaviest identified components of the residue. These species might also be present in the interstellar medium, given that the ice was submitted to high temperatures, of the order of 300 K, and form part of comets. Our work serves as preparation for the ESA-Rosetta mission, which plans to do in situ analysis of the composition of a comet nucleus with the COSAC instrumentation.