H. J. Fraser

Thermal desorption of water ice in the interstellar medium

Water (H2O) ice is an important solid constituent of many astrophysical environments. To comprehend the role of such ices in the chemistry and evolution of dense molecular clouds and comets, it is necessary to understand the freeze-out, potential surface reactivity, and desorption mechanisms of such molecular systems. Consequently, there is a real need from within the astronomical modelling community for accurate empirical molecular data pertaining to these processes. Here we give the first results of a laboratory programme to provide such data. Measurements of the thermal desorption of H2O ice, under interstellar conditions, are presented. For ice deposited under conditions that realistically mimic those in a dense molecular cloud, the thermal desorption of thin films (�50 molecular layers) is found to occur with zero order kinetics characterised by a surface binding energy, Edes, of 5773 ±60 K, and a pre-exponential factor, A, of 10 30±2 molecules cm 2 s 1 . These results imply that, in the dense interstellar medium, thermal desorption of H2O ice will occur at significantly higher temperatures than has previously been assumed.

Carbon Monoxide Entrapment in Interstellar Ice Analogs

The adsorption and desorption of CO on and from amorphous H2O ice at astrophysically relevant temperatures has been studied using temperature programmed desorption (TPD) and reflection-absorption infrared spectroscopy (RAIRS). Solid CO is able to diffuse into the porous structure of H2O at temperatures as low as 15 K. When heated, a phase transition between two forms of amorphous H2O ice occurs over the 30-70 K temperature range, causing the partial collapse of pores and the entrapment of CO. Trapped CO is released during crystallization and desorption of the H2O film. This behavior may have a significant impact on both gas-phase and solid-phase chemistry in a variety of interstellar environments.

Photodesorption of CO Ice

At the high densities and low temperatures found in star forming regions, all molecules other than H2 should stick on dust grains on timescales shorter than the cloud lifetimes. Yet these clouds are detected in the millimeter lines of gaseous CO. At these temperatures, thermal desorption is negligible and hence a non-thermal desorption mechanism is necessary to maintain molecules in the gas phase. Here, the first laboratory study of the photodesorption of pure CO ice under ultra high vacuum is presented, which gives a desorption rate of 3 × 10 3 CO molecules per UV (7–10.5 eV) photon at 15 K. This rate is factors of 10 2 -10 5 larger than previously estimated and is comparable to estimates of other non-thermal desorption rates. The experiments constrains the mechanism to a single photon desorption process of ice surface molecules. The measured efficiency of this process shows that the role of CO photodesorption in preventing total removal of molecules in the gas has been underestimated. Subject headings: Molecular data — Molecular processes — ISM: abundances — Physical Data and Processes: astrochemistry — ISM: molecules

Competition between CO and N2 Desorption from Interstellar Ices

Millimeter observations of pre- and protostellar cores show that the abundances of the gas-phase tracer molecules, C 18 O and N2H, anticorrelate with each other and often exhibit “holes” where the density is greatest. These results are reasonably reproduced by astrochemical models, provided that the ratio between the binding energies of N2 and CO, , is taken to be between 0.5 and 0.75. This Letter is the first experimental report of RBE the desorption of CO and N2 from layered and mixed ices at temperatures relevant to dense cores, studied under ultrahigh vacuum laboratory conditions using temperature programmed desorption. From control experiments with pure ices, , given K and K. In mixed R p 0.923 0.003 E (N -N ) p 790 25 E (CO-CO) p 855 25 BE b 22 b

Laboratory studies of the interaction of carbon monoxide with water ice

The interaction of carbon monoxide (CO) with vapour-deposited water(H2O) ices has been studied using temperature programmed desorption (TPD) and Fourier transform reflection-absorption infrared spectroscopy (FT-RAIRS) over a range of astrophysically relevant temperatures. Such measurements have shown that CO desorption from amorphous H2Oices is a much more complex process than current astrochemical models suggest. Re-visiting previously reported laboratory experiments (Collings et al., 2003), a rate model has been constructed to explain, in a phenomenological manner, the desorption of CO over astronomically relevant time scales. The model presented here can be widely applied to a range of astronomical environments where depletion of CO from the gas phase is relevant. The model accounts for the two competing processes of CO desorption and migration, and also enables the entrapment of some of the CO in the ice matrix and its subsequent release as the water ice crystallises and then desorbs. The astronomical implications of this model are discussed.

Desorption of CO and O2 interstellar ice analogs

Context. Aims. Solid O2 has been proposed as a possible reservoir for oxygen in dense clouds through freeze-out processes. The aim of this work is to characterize quantitatively the physical processes that are involved in the desorption kinetics of CO-O2 ices by interpreting laboratory temperature programmed desorption (TPD) data. This information is used to simulate the behavior of CO-O2 ices under astrophysical conditions. Methods. The TPD spectra have been recorded under ultra high vacuum conditions for pure, layered and mixed morphologies for different thicknesses, temperatures and mixing ratios. An empirical kinetic model is used to interpret the results and to provide input parameters for astrophysical models. Results. Binding energies are determined for different ice morphologies. Independent of the ice morphology, the desorption of O2 is found to follow 0 th -order kinetics. Binding energies and temperature-dependent sticking probabilities for CO–CO, O2–O2 and CO–O2 are determined. O2 is slightly less volatile than CO, with binding energies of 912±15 versus 858±15 K for pure ices. In mixed and layered ices, CO does not co-desorb with O2 but its binding energies are slightly increased compared with pure ice whereas those for O2 are slightly decreased. Lower limits to the sticking probabilities of CO and O2 are 0.9 and 0.85, respectively, at temperatures below 20 K. The balance between accretion and desorption is studied for O2 and CO in astrophysically relevant scenarios. Only minor differences are found between the two species, i.e., both desorb between 16 and 18 K in typical environments around young stars. Thus, clouds with significant abundances of gaseous CO are unlikely to have large amounts of solid O2. Conclusions.

Thermal desorption characteristics of CO, O2 and CO2 on non-porous water, crystalline water and silicate surfaces at submonolayer and multilayer coverages

The desorption characteristics of molecules on interstellar dust grains are important for modelling the behaviour of molecules in icy mantles and, critically, in describing the solid–gas interface. In this study, a series of laboratory experiments exploring the desorption of three small molecules from three astrophysically relevant surfaces is presented. The desorption of CO, O2 and CO2 at both submonolayer and multilayer coverages was investigated from non-porous water, crystalline water and silicate surfaces. Experimental data were modelled using the Polanyi–Wigner equation to produce a mathematical description of the desorption of each molecular species from each type of surface, uniquely describing both the monolayer and multilayer desorption in a single combined model. The implications of desorption behaviour over astrophysically relevant time-scales are discussed.

CO2 FORMATION IN QUIESCENT CLOUDS: AN EXPERIMENTAL STUDY OF THE CO + OH PATHWAY

The formation of CO2 in quiescent regions of molecular clouds is not yet fully understood, despite CO2 having an abundance of around 10%-34% H2O. We present a study of the formation of CO2 via the nonenergetic route CO + OH on nonporous H2O and amorphous silicate surfaces. Our results are in the form of temperature-programmed desorption spectra of CO2 produced via two experimental routes: O-2 + CO + H and O-3 + CO + H. The maximum yield of CO2 is around 8% with respect to the starting quantity of CO, suggesting a barrier to CO + OH. The rate of reaction, based on modeling results, is 24 times slower than O-2 + H. Our model suggests that competition between CO2 formation via CO + OH and other surface reactions of OH is a key factor in the low yields of CO2 obtained experimentally, with relative reaction rates of k(CO+H) << k(CO+OH) < k(H2O2+H) < k(OH+H), k(O2+H). Astrophysically, the presence of CO2 in low AV regions of molecular clouds could be explained by the reaction CO + OH occurring concurrently with the formation of H2O via the route OH + H.

Effects of CO2 on H2O band profiles and band strengths in mixed H2O:CO2 ices

Context. H2O is the most abundant component of astrophysical ices. In most lines of sight it is not possible to fit both the H 2O 3 µm stretching, the 6 µm bending and the 13 µm libration band intensities with a single pure H2O spectrum. Recent Spitzer observations have revealed CO2 ice in high abundances and it has been suggested that CO2 mixed into H2O ice can affect the positions, shapes and relative strengths of the 3 µm and 6 µm bands. Aims. We investigate whether the discrepancy in intensity between H2O bands in interstellar clouds and star forming regions can be explained by CO2 mixed into the observed H2O ice affecting the bands differently. Methods. Laboratory infrared transmission spectroscopy is used to record spectra of H2O:CO2 ice mixtures at astrophysically relevant temperatures and composition ratios. Results. The H2O peak profiles and band strengths are significantly di fferent in H2O:CO2 ice mixtures compared to pure H2O ice. The ratio between the strengths of the 3 µm and 6 µm bands drops linearly with CO2 concentration such that it is 50% lower in a 1:1 mixture compared to pure H2O ice. In all H2O:CO2 mixtures, a strong free-OH stretching band appears around 2.73 µm, which can be used to put an upper limit on the CO2 concentration in the H2O ice. The H2O bending mode profile also changes drastically with CO 2 concentration; the broad pure H2O band gives way to two narrow bands as the CO2 concentration is increased. This makes it crucial to constr ain the environment of H2O ice to enable correct assignments of other species contributing t o the interstellar 6 µm absorption band. The amount of CO2 present in the H2O ice of B5:IRS1 is estimated by simultaneously comparing the H2O stretching and bending regions and the CO2 bending mode to laboratory spectra of H2O, CO2, H2O:CO2 and HCOOH.

Physics and chemistry of icy particles in the universe: answers from microgravity

Abstract During the last century, the presence of icy particles throughout the universe has been confirmed by numerous ground and space based observations. Ultrathin icy layers are known to cover dust particles within the cold regions of the interstellar medium, and drive a rich chemistry in energetic star-forming regions. The polar caps of terrestrial planets, as well as most of the outer-solar-system satellites, are covered with an icy surface. Smaller solar system bodies, such as comets and Kuiper Belt Objects (KBOs), contain a significant fraction of icy materials. Icy particles are also present in planetary atmospheres and play an important role in determining the climate and the environmental conditions on our host planet, Earth. Water ice seems universal in space and is by far the most abundant condensed-phase species in our universe. Many research groups have focused their efforts on understanding the physical and chemical nature of water ice. However, open questions remain as to whether ices produced in Earths laboratories are indeed good analogs for ices observed in space environments. Although temperature and pressure conditions can be very well controlled in the laboratory, it is very difficult to simulate the time-scales and gravity conditions of space environments. The bulk structure of ice, and the catalytic properties of the surface, could be rather different when formed in zero gravity in space. The author list comprises the members of the ESA Topical Team: Physico-chemistry of ices in space. In this paper we present recent results including ground-based experiments on ice and dust, models as well as related space experiments performed under microgravity conditions. We also investigate the possibilities of designing a new infrastructure, and /or making improvements to the existing hardware in order to study ices on the International Space Station (ISS). The type of multidisciplinary facility that we describe will support research in crystal growth of ices and other solid refractory materials, aerosol microphysics, light scattering properties of solid particles, the physics of icy particle aggregates, and radiation processing of molecular ices. Studying ices in microgravity conditions will provide us with fundamental data on the nature of extraterrestrial ices and allow us to enhance our knowledge on the physical and chemical processes prevailing in different space environments.