Detailed laboratory experiments on the formation of HD from atom recombination on amorphous solid water films show that this process is extremely efficient in a temperature range of 8 to 20 kelvin, temperatures relevant for H2 formation on dust grain surfaces in the interstellar medium (ISM). The fate of the 4.5 electron volt recombination energy is highly dependent on film morphology. These results suggest that grain morphology, rather than the detailed chemical nature of the grain surface, is most important in determining the energy content of the H2 as it is released from the grain into the ISM.

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Abstract.A stellar hydrodynamic pulsation model has been combined with a SiO maser model in an attempt to calculate the temporal variability of SiO maser emission in the circumstellar envelope (CE) of a model AGB star. This study investigates whether the variations in local physical conditions brought about by shocks are the predominant contributing factor to SiO maser variability because, in this work, the radiative part of the pump is constant. We find that some aspects of the variability are not consistent with a pump provided by shock-enhanced collisions alone. In these simulations, gas parcels of relatively enhanced SiO abundance are distributed in a model CE by a Monte Carlo method, at a single epoch of the stellar cycle. From this epoch on, Lagrangian motions of individual parcels are calculated according to the velocity fields encountered in the model CE during the stellar pulsation cycle. The potentially masing gas parcels therefore experience different densities and temperatures, and have varying line-of-sight velocity gradients throughout the stellar cycle, which may or may not be suitable to produce maser emission. At each epoch (separated by 16.6 days), emission lines from the parcels are combined to produce synthetic spectra and VLBI-type images. We report here the results for v = 1, J = 1-0 (43-GHz) and J = 2-1 (86-GHz) masers and compare synthetic lineshapes and images with those observed. Strong SiO maser emission is calculated to form in an unfilled ring within a few stellar radii of the photosphere, indicating a tangential amplification process. The diameter of the synthetic maser ring is dependent upon stellar phase, as clearly observed for TX Cam, and upon maser transition. Proper motions of brightly masing parcels are comparable to measurements for some maser components in R Aqr and TX Cam, although we are unable to reproduce all of the observed motions. Synthetic lineshapes peak at the stellar velocity, have typical Mira linewidths and vary in intensity with stellar phase. However, the model fails quantitatively in several respects. We attribute these failings to (i) lack of an accurate, time-varying stellar IR field (ii) post-shock kinetic temperatures which are too high, due to the cooling function included in our model and (iii) the lack of a detailed treatment of the chemistry of the inner CE. We expect the use of oxygen-rich hydrodynamical stellar models which are currently under development to alleviate these problems.

A B S T R A C TSputtering yields are reported for the release of Mg, Fe, Si and O under impact of He, C, O, Si and Fe on grain material composed of Mg-and Fe-bearing silicates. The yields were derived using the trim code, which simulates the results of the transport of ions in matter by means of classical Monte Carlo techniques. The energetics of the sputtering process are a key factor in the sputtering calculations, and so detailed determinations have been made of the energy with which atoms are bound to the lattice, using solid-state simulation programs. The probability of ejection of an atom is computed at a given energy, for a number of angles of incidence, and integrated to obtain the mean yield at that energy. These numerical results are then fitted with a simple function of energy for convenience in subsequent applications.A grid of C-type shock models has been computed, using our new sputtering yields, for pre-shock densities in the range 10 4 < n H nH 1 2nH 2 < 10 6 cm 23 and shock speeds 20 < v s < 45 km s 21 X Sputtered fractions can be high, exceeding 50 per cent for shock speeds in excess of approximately 40 km s 21 . The column densities of Si and SiO were also computed, for comparison with observations. Key words: shock waves ± ISM: jets and outflows. I N T R O D U C T I O NIn an earlier work (Field et al. 1997; hereafter FMFF), sputtering yields were reported for amorphous carbon and amorphous SiO 2 targets, for sputtering of C, Si and O from these targets by impact of He, C, O, Si and Fe. The consequences for the chemistry in C-type shocks were very briefly explored with special reference to the production of gas-phase SiO. Our purpose in the present work is twofold. In the first place, we seek to make available values of sputtering yields for targets that mimic more closely the composition of interstellar grains. To this end, the sputtering of grains composed of Mg-and Fe-bearing silicates is studied here. The results are relevant to the oxygen-rich environments in which outflows, associated with star formation, are typically found. In the second place, the new sputtering data are used in a sophisticated C-type shock model to yield a grid of results for a range of shock parameters, notably the shock velocity, pre-shock gas density and magnetic field. These models determine, for example, the budget of refractory material in the gas phase, in particular of SiO. These results may be used in conjunction with observational data (e.g. Dutrey, Guilloteau & Bachiller 1997; Lefloch et al. 1998) as a guide to the shock conditions in any particular region.In Section 2 the technical means for calculating sputtering yields are described, with particular reference to the energetics of the processes involved in sputtering. Sputtering yields are presented in Section 3 for magnesium silicate (Mg 2 SiO 4 ), or forsterite', iron silicate (Fe 2 SiO 4 ), or`fayalite' and an`olivine', MgFeSiO 4 , for a variety of projectile species. In Section 4, the grid of shock models is presented and brief reference is made to...

Observations are reported of H 2 IR emission in the S(1) v = 1−0 line at 2.121 µm in the Orion Molecular Cloud, OMC1, using the GriF instrument on the Canada-France-Hawaii Telescope. GriF uses a combination of adaptive optics and Fabry-Perot interferometry, yielding a spatial resolution of 0.15 to 0.18 and velocity discrimination as high as 1 km s −1 . 193 bright H 2 emission regions can be identified in OMC1. The general characteristics of these features are described in terms of radial velocities, brightness and spatial displacement of maxima of velocity and brightness, the latter to yield the orientation of flows in the plane of the sky. Strong spatial correlation between velocity and bright H 2 emission is found and serves to identify many features as shocks. Important results are: (i) velocities of the excited gas illustrate the presence of a zone to the south of BN-IRc2 and Peak 1, and the west of Peak 2, where there is a powerful blue-shifted outflow with an average velocity of −18 km s −1 . This is shown to be the NIR counterpart of an outflow previously identified in the radio, originating from either source I or source n. (ii) There is a band of weak radial velocity features (<5 km s −1 ) in Peak 1. (iii) A small proportion of the flows may represent sites of low mass star formation and one region shows evidence of multiple flows which may indicate multiple low mass star formation within OMC1.

We report here the discovery of a new form of spontaneously polarized material. Examples of this material, in the form of films, demonstrate the property that they spontaneously harbour electric fields which may exceed 10(8) Vm(-1), achieving potentials of tens of volts on the film surface. The molecules presently identified form a diverse group, thus far of six species, with gas phase dipoles lying between 0.08 D and 0.5 D: propane (0.08 D), isopentane (0.13 D), nitrous oxide (0.167 D), isoprene (0.25 D), toluene (0.385 D) and CF(3)Cl (Freon-13; 0.5 D). Here we concentrate on an understanding of the nature of the interactions which give rise to the spontaneously polarized state, using the measured temperature dependence of the electric field in N(2)O as a diagnostic. We show that the polarized state can arise through a mechanism of non-linear dipole alignment in a single domain in which dipole alignment generates the electric field within the film and the field generates dipole alignment. Non-local interactions take place over the dimension of the thickness of the film and permeate the entire medium through the agency of the electric field. This new type of material may have wide ranging applications in devices and in nanotechnology.

We have surveyed a sample of massive star-forming regions located over a range of distances from the Galactic centre for methyl formate, HCOOCH 3 , and its isotopologues H 13 COOCH 3 and HCOO 13 CH 3 . The observations were carried out with the APEX telescope in the frequency range 283.4-287.4 GHz. Based on the APEX observations, we report tentative detections of the 13 C-methyl formate isotopologue HCOO 13 CH 3 towards the following four massive star-forming regions: Sgr B2(N-LMH), NGC 6334 IRS 1, W51 e2 and G19.61-0.23. In addition, we have used the 1 mm ALMA science verification observations of Orion-KL and confirm the detection of the 13 C-methyl formate species in Orion-KL and image its spatial distribution. Our analysis shows that the 12 C/ 13 C isotope ratio in methyl formate toward Orion-KL Compact Ridge and Hot Core-SW components (68.4±10.1 and 71.4±7.8, respectively) are, for both the 13 C-methyl formate isotopologues, commensurate with the average 12 C/ 13 C ratio of CO derived toward Orion-KL. Likewise, regarding the other sources, our results are consistent with the 12 C/ 13 C in CO. We also report the spectroscopic characterization, which includes a complete partition function, of the complex H 13 COOCH 3 and HCOO 13 CH 3 species. New spectroscopic data for both isotopomers H 13 COOCH 3 and HCOO 13 CH 3 , presented in this study, has made it possible to measure this fundamentally important isotope ratio in a large organic molecule for the first time.

Abstract.Observations are reported of IR emission of H2 from a region of the Orion molecular cloud (OMC1) between the Becklin-Neugebauer object and IRc2 to the north and the Trapezium stars to the south. Data were obtained using the ESO 3.6 m telescope in the K-band around 2 µm with the ADONIS adaptive optics system. Images of H2 v = 1−0 S(1) show a spatial resolution of ∼0.15 . Detailed investigations of the distribution of sizes of structures in our images have been performed by area-perimeter analysis, Fourier analysis and brightness distribution studies. These demonstrate that structure is not fractal but shows a preferred scales of between 3 10 −3 and 4 10 −3 pc. In an attempt to estimate the density in observed structures, predictions of both shock models and photodissociation region models have been compared with measured emission brightness in the H2 v = 1−0 S(1) line. Magnetic (C-type) shocks with velocities of 30 km s −1 and pre-shock densities of 10 6 cm −3 yield the best representation of our data, notwithstanding significant discrepancies for the brightness ratio between v = 2−1 S(1) and v = 1−0 S(1) lines. Our results show that post-shock densities are several times 10 7 cm −3 . This is sufficiently high that the passage of C-type shocks in Orion yields gravitational instability which may in turn trigger star formation in the post-shock gas.