Mark P. Collings

Evaporation of ices near massive stars: models based on laboratory temperature programmed desorption data

Hot cores and their precursors contain an integrated record of the physics of the collapse process in the chemistry of the ices deposited during that collapse. In this paper, we present results from a new model of the chemistry near high-mass stars in which the desorption of each species in the ice mixture is described as indicated by new experimental results obtained under conditions similar to those in hot cores. Our models show that provided there is a monotonic increase in the temperature of the gas and dust surrounding the protostar, the changes in the chemical evolution of each species due to differential desorption are important. The species H2S, SO, SO2, OCS, H2CS, CS, NS, CH3OH, HCOOCH3 ,C H 2CO, C2H5OH show a strong time dependence that may be a useful signature of time evolution in the warm-up phase as the star moves on to the main sequence. This preliminary study demonstrates the consequences of incorporating reliable temperature programmed desorption data into chemical models. Ke yw ords: stars: formation ‐ ISM: abundances ‐ ISM: clouds ‐ ISM: molecules.

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.

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.

Laboratory surface astrophysics experiment

In this article we describe the design and construction of a laboratory astrophysics experiment that recreates the harsh conditions of the Interstellar Medium (ISM) and is used to study the heterogeneous chemistry that occurs there. The Nottingham Surface Astrophysics Experiment is used to determine, empirically, accurately, and usually for the first time, key physical and chemical constants that are vital for modeling and understanding the ISM. It has been designed specifically to investigate gas–solid interactions under interstellar conditions. The pressure regime is ideally matched to molecular densities in dusty disks in protostellar or protoplanetary regions. The ultrahigh vacuum system is routinely capable of obtaining pressures that are only three orders of magnitude above those in the ISM, with similar relative concentrations of the two most abundant gases in such regions, H2 and CO, and an absence of any other major gas components. A short introduction describes the astronomical motivation behind this experiment. In Sec. II we then give details of the design, construction, and calibration of each component of the experiment. The cryostat system has far exceeded design expectations, and reaches temperatures between 7 and 500 K. This is comparable with the ISM, where dust temperatures from 10 K have been observed. Line-of-sight mass spectrometry, reflection absorption infrared spectroscopy, and quartz crystal microbalance mass measurements were combined into a single instrument for the first time. The instrument was carefully calibrated, and its control and data acquisition system was developed to ensure that experimental parameters are recorded as accurately as possible. In Sec. III we present some of the experimental results from this system that have not been published elsewhere. The results presented here demonstrate that the system can be used to determine desorption enthalpies, ΔdesH, bonding systems, and sticking probabilities between a variety of gases and ices common to the ISM. This instrument will greatly facilitate our understanding of surface processes that occur in the ISM, and allow us to investigate “mimic” ISM systems in a controlled environment. In this article we illustrate that laboratory surface astrophysics is an exciting and emerging area of research, and this instrument in particular will have a major impact through its contributions to both surface science and astronomy.

DESORPTION OF HOT MOLECULES FROM PHOTON IRRADIATED INTERSTELLAR ICES

We present experimental measurements of photodesorption from ices of astrophysical relevance. Layers of benzene and water ice were irradiated with a laser tuned to an electronic transition in the benzene molecule. The translational energy of desorbed molecules was measured by time-of-flight (ToF) mass spectrometry. Three distinct photodesorption processes were identified: a direct adsorbate-mediated desorption producing benzene molecules with a translational temperature of around 1200 K, an indirect adsorbate-mediated desorption resulting in water molecules with a translational temperature of around 450 K, and a substrate-mediated desorption of both benzene and water producing molecules with translational temperatures of around 530 and 450 K, respectively. The translational temperature of each population of desorbed molecules is well above the temperature of the ice matrix. The implications for gas-phase chemistry in the interstellar medium are discussed.

The profile of the 2140 cm−1 solid CO band on different substrates

We have studied the profile of the 2140 cm(-1) fundamental band of solid carbon monoxide (CO) at low temperature (10-15 K) by infrared transmission spectroscopy and by reflection absorption infrared (RAIR) spectroscopy. In particular, transmission spectra have been taken after CO had been adsorbed on a bare crystalline silicon substrate and on pre-adsorbed solid N(2) layers of different thickness. RAIR spectra have been taken after CO had been adsorbed on a bare gold substrate and on pre-adsorbed solid N(2) layers of different thickness. Laboratory spectra show that the profile of the fundamental CO band at about 2140 cm(-1) is different in the different instances considered. In particular, we have found that the relative intensity of the LO and TO modes of the CO band depends on the thickness of the N(2) layer. Here we present the experimental results and show that these can be predicted by the elementary electromagnetic theory.

Thermal desorption of C6H6 from surfaces of astrophysical relevance

The thermal desorption of C(6)H(6) from two astrophysically relevant surfaces has been studied using temperature programmed desorption. Desorption from an amorphous SiO(2) substrate was used as a mimic for bare interstellar grains, while multilayer films of amorphous solid water (ASW) were used to study the adsorption of C(6)H(6) on grains surrounded by H(2)O dominated icy mantles. Kinetic parameters were obtained through a combination of kinetic modeling, leading edge analysis, and by considering a distribution of binding sites on the substrate. The latter is shown to have a significant impact on the desorption of small exposures of C(6)H(6) from the amorphous SiO(2) substrate. In the case of adsorption on ASW, dewetting behavior and fractional order desorption at low coverage strongly suggest the formation of islands of C(6)H(6) on the H(2)O surface. The astrophysical implications of these observations are briefly outlined.

Surface science investigations of photoprocesses in model interstellar ices

The kinetic energy of benzene and water molecules photodesorbed from astrophysically relevant ices on a sapphire substrate under irradiation by a UV laser tuned to the S-1 pi* transition of benzene has been measured using time-of-flight mass spectrometry. Three distinct photodesorption mechanisms have been identified - a direct adsorbate-mediated desorption of benzene, an indirect adsorbate-mediated desorption of water, and a substrate-mediated desorption of both benzene and water. The translational temperature of each desorbing population was well in excess of the ambient temperature of the ice matrix

Warm cores around regions of low-mass star formation

Warm cores (or hot corinos) around low-mass protostellar objects show a rich chemistry with strong spatial variations. This chemistry is generally attributed to the sublimation of icy mantles on dust grains initiated by the warming effect of the stellar radiation. We have used a model of the chemistry in warm cores in which the sublimation process is based on extensive laboratory data; these data indicate that sublimation from mixed ices occurs in several well-defined temperature bands. We have determined the position of these bands for the slow warming by a solar-mass star. The resulting chemistry is dominated by the sublimation process and by subsequent gas-phase reactions; strong spatial and temporal variations in certain molecular species are found to occur, and our results are, in general, consistent with observational results for the well-studied source IRAS 16293-2422. The model used is similar to the one that describes the chemistry of hot cores. We infer that the chemistry of both hot cores and warm cores may be described by the same model (suitably adjusted for different physical parameters).