The thermal desorption characteristics of 16 astrophysically relevant species from laboratory analogues of the icy mantles on interstellar dust grains have been surveyed in an extensive set of preliminary temperature programmed desorption experiments. The species can be separated into three categories based on behaviour. Water‐like species have a single relevant desorption coincident with water. CO‐like species show the volcano desorption and co‐desorption of trapped molecules, monolayer desorption from the surface of water ice, and multilayer desorption if initially present in sufficient abundance in an outer layer separated from the water ice. Intermediate species show the two desorptions of trapped molecules, and may show a small monolayer desorption for molecules small enough to have a limited ability to diffuse through the structure of porous amorphous water ice. Methods by which the results obtained under laboratory conditions can be adapted for astrophysical situations are discussed.

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, CH2CO, 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.

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 condi- tions 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 charac- terised by a surface binding energy, Edes, of 5773 ±60 K, and a pre-exponential factor, A, of 1030±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

Publisher's description: The book leads the advanced undergraduate through the wide range of disciplines related to an understanding of the interstellar medium and is suitable for any student studying either physics or astrophysics. The first edition was published in 1980 by Manchester University Press.Chapter headings are: 1) Introduction; 2) How we obtain information about the interstellar medium; 3) Microscopic processes in the interstellar medium; 4) Interstellar grains; 5) Radiatively excited regions; 6) Introduction to gas dynamics; 7) Gas dynamical effects on massive stars on the interstellar medium; 8) star formation and star forming regions. Answers to problems; Index. Some useful tables are given on pp. xiii-xiv.

The adsorption and desorption of CO on and from amorphous H 2 O 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 H 2 O at temperatures as low as 15 K. When heated, a phase transition between two forms of amorphous H 2 O 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 H 2 O film. This behavior may have a significant impact on both gas-phase and solid-phase chemistry in a variety of interstellar environments.

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A reliable estimate of the molecular gas content in galaxies plays a crucial role in determining their dynamical and star‐forming properties. However, H2, the dominant molecular species, is difficult to observe directly, particularly in the regions where most molecular gas is thought to reside. Its mass is therefore commonly inferred by assuming a direct proportionality with the integrated intensity of the 12CO(J= 1 → 0) emission line, using a CO‐to‐H2 conversion factor, X. Although a canonical value for X is used extensively in such estimates, there is increasing evidence, both theoretical and observational, that the conversion factor may vary by over an order of magnitude under conditions different from those of the local neighbourhood. In an effort to understand the influence of changing environmental conditions on the conversion factor, we derive theoretical estimates of X for a wide range of physical parameters using a photon‐dominated region (PDR) time‐dependent chemical model, benchmarking key results against those of an independent PDR code to ensure reliability. Based on these results, the sensitivity of the X factor to change in each physical parameter is interpreted in terms of the chemistry and physical processes within the cloud. In addition to confirming previous observationally derived trends, we find that the time‐dependence of the chemistry, often neglected in such models, has a considerable influence on the value of the conversion factor.

Molecular line observations may serve as diagnostics of the degree to which the number density of cosmic ray protons, having energies of 10s to 100s of MeV each, is enhanced in starburst galaxies and galaxies with active nuclei. Results, obtained with the ucl_pdr code, for the fractional abundances of molecules as functions of the cosmic ray induced ionization rate, ζ, are presented. The aim is not to model any particular external galaxies. Rather, it is to identify characteristics of the dependencies of molecular abundances on ζ, in part to enable the development of suitable observational programmes for cosmic ray dominated regions (CRDRs) which will then stimulate detailed modelling. For a number density of hydrogen nuclei of of 104 cm−3, and high visual extinction, the fractional abundances of some species increase as ζ increases to 10−14 s−1, but for much higher values of ζ the fractional abundances of all molecular species are significantly below their peak values. We show in particular that OH, H2O, H+3, H3O+ and OH+ attain large fractional abundances (≥10−8) for ζ as large as 10−12 s−1. HCO+ is a poor tracer of CRDRs when ζ > 10−13 s−1. Sulphur‐bearing species may be useful tracers of CRDR gas in which ζ∼ 10−16 s−1. Ammonia has a large fractional abundance for ζ≤ 10−16 s−1 and nitrogen appears in CN‐bearing species at significant levels as ζ increases, even up to ∼10−14 s−1. In this paper, we also discuss our model predictions, comparing them to recent detections in both Galactic and extragalactic sources. We show that they agree well, to a first approximation, with the observational constraints.