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Aims. In this paper we study the Spitzer and TIMMI2 infrared spectra of post-AGB disc sources, both in the Galaxy and the LMC. Using the observed infrared spectra we determine the mineralogy and dust parameters of the discs, and look for possible differences between the Galactic and extragalactic sources. Methods. Modelling the full spectral range observed allows us to determine the dust species present in the disc and different physical parameters such as grain sizes, dust abundance ratios, and the dust and continuum temperatures. Results. We find that all the discs are dominated by emission features of crystalline and amorphous silicate dust. Only a few sample sources show features due to CO 2 gas or carbonaceous molecules such as PAHs and C 60 fullerenes. Our analysis shows that dust grain processing in these discs is strong, resulting in large average grain sizes and a very high crystallinity fraction. However, we do not find any correlations between the derived dust parameters and properties of the central source. There also does not seem to be a noticeable difference between the mineralogy of the Galactic and LMC sources. Even though the observed spectra are very similar to those of protoplanetary discs around young stars, showing similar mineralogy and strong grain processing, we do find evidence for differences in the physical and chemical processes of the dust processing.

We present spectroscopic observations of a sample of 15 embedded young stellar objects (YSOs) in the Large Magellanic Cloud (LMC). These observations were obtained with the Spitzer Infrared Spectrograph (IRS) as part of the SAGE-Spec Legacy program. We analyze the two prominent ice bands in the IRS spectral range: the bending mode of CO 2 ice at 15.2 µm and the ice band between 5 and 7 µm that includes contributions from the bending mode of water ice at 6 µm amongst other ice species. The 5−7 µm band is difficult to identify in our LMC sample due to the conspicuous presence of PAH emission superimposed onto the ice spectra. We identify water ice in the spectra of two sources; the spectrum of one of those sources also exhibits the 6.8-µm ice feature attributed in the literature to ammonium and methanol. We model the CO 2 band in detail, using the combination of laboratory ice profiles available in the literature.We find that a significant fraction ( 50%) of CO 2 ice is locked in a water-rich component, consistent with what is observed for Galactic sources. The majority of the sources in the LMC also require a pure-CO 2 contribution to the ice profile, evidence of thermal processing. There is a suggestion that CO 2 production might be enhanced in the LMC, but the size of the available sample precludes firmer conclusions. We place our results in the context of the star formation environment in the LMC. Subject headings: astrochemistry -circumstellar matter -stars: formationgalaxies: individual (LMC) -Magellanic Clouds (ISM) of the Large and Small Magellanic Cloud (LMC and SMC) have significantly lower metallicities (Z LMC ∼ 0.4 Z ⊙ and Z SMC ∼ 0.2 Z ⊙ ), than the solar neighborhood ISM (Z Gal ∼ Z ⊙ ). Only relatively recently are we able to study the details of the star formation process in the MCs (for a review of recent results see Oliveira 2009). The star-formation process is a complex interplay of various chemo-physical processes. During the onset of gravitational collapse of a molecular cloud, sufficiently dense cores can only develop if the heat produced during the contraction can be dissipated. The most efficient cooling mechanisms are via radiation through fine-structure lines of carbon and oxygen (e.g., via the emission lines of [C ii] at 158 µm and [O i] at 63 and 146 µm), and rotational transitions in abundant molecules such as CO, O 2 and water (Goldsmith & Langer 1978). These cooling agents all contain at least one heavy atom -H 2 has no permanent dipole moment, and emission through its rotational transitions is very unlikely, making it an inefficient coolant. Besides also contributing to the thermal balance in the molecular cloud, dust grains are crucial in driving cloud chemistry, as dust opacity shields molecules from radiation and grain surfaces enable chemical reactions to occur that would not happen in the gas phase. Surface chemistry is also thought to play an important role in the formation of H 2 (e.g., Williams 2003) and H 2 O (e.g., Hollenbach et al. 2009). Simple 15.2 µm. Oliveira et al. (20...

We investigate the gas-phase and grain-surface chemistry in the inner 30 AU of a typical protoplanetary disk using a new model which calculates the gas temperature by solving the gas heating and cooling balance and which has an improved treatment of the UV radiation field. We discuss inner-disk chemistry in general, obtaining excellent agreement with recent observations which have probed the material in the inner regions of protoplanetary disks. We also apply our model to study the isotopic fractionation of carbon. Results show that the fractionation ratio, 12 C/ 13 C, of the system varies with radius and height in the disk. Different behaviour is seen in the fractionation of different species. We compare our results with 12 C/ 13 C ratios in the Solar System comets, and find a stark contrast, indicative of reprocessing.Subject headings: astrochemistry -solar system: formation -planetary systems: protoplanetary disks 1 Present address: Jodrell Bank Centre for Astrophysics, Alan Turing Building, The University of Manchester, Oxford Road, Manchester M13 9PL, UK 1 The Atacama Large Millimeter Array, due for completion in 2012 (www.alma.info) 2 When there is more than one 13 C per molecule, the fractionation ratio is taken to be [ 12 C]/[ 13 C]

Abstract. The dependence of stellar molecular bands on the metallicity is studied using infrared L-band spectra of AGB stars (both carbon-rich and oxygen-rich) and M-type supergiants in the Large and Small Magellanic Clouds (LMC and SMC) and in the Sagittarius Dwarf Spheroidal Galaxy. The spectra cover SiO bands for oxygen-rich stars, and acetylene (C 2 H 2 ), CH and HCN bands for carbon-rich AGB stars. The equivalent width of acetylene is found to be high even at low metallicity. The high C 2 H 2 abundance can be explained with a high carbon-to-oxygen (C/O) ratio for lower metallicity carbon stars. In contrast, the HCN equivalent width is low: fewer than half of the extra-galactic carbon stars show the 3.5 µm HCN band, and only a few LMC stars show high HCN equivalent width. HCN abundances are limited by both nitrogen and carbon elemental abundances. The amount of synthesized nitrogen depends on the initial mass, and stars with high luminosity (i.e. high initial mass) could have a high HCN abundance. CH bands are found in both the extra-galactic and Galactic carbon stars. One SMC post-AGB star, SMC-S2, shows the 3.3 µm PAH band. This first detection of a PAH band from an SMC post-AGB star confirms PAHs can form in these low-metallicity stars. None of the oxygen-rich LMC stars show SiO bands, except one possible detection in a low quality spectrum. The limits on the equivalent widths of the SiO bands are below the expectation of up to 30 Å for LMC metallicity. Several possible explanations are discussed, mostly based on the effect of pulsation and circumstellar dust. The observations imply that LMC and SMC carbon stars could reach mass-loss rates as high as their Galactic counterparts, because there are more carbon atoms available and more carbonaceous dust can be formed. On the other hand, the lack of SiO suggests less dust and lower mass-loss rates in low-metallicity oxygen-rich stars. The effect on the ISM dust enrichment is discussed.

Abstract.A millimetre molecular line survey of seven high mass-loss rate carbon stars in both the northern and southern skies is presented. A total of 196 emission lines (47 transitions) from 24 molecular species were detected. The observed CO emission is used to determine mass-loss rates and the physical structure of the circumstellar envelope, such as the density and temperature structure, using a detailed radiative transfer analysis. This enables abundances for the remaining molecular species to be determined. The derived abundances generally vary between the sources by no more than a factor of five indicating that circumstellar envelopes around carbon stars with high mass-loss rates have similar chemical compositions. However, there are some notable exceptions. The most striking difference between the abundances are reflecting the spread in the 12 C/ 13 C-ratio of about an order of magnitude between the sample stars, which mainly shows the results of nucleosynthesis. The abundance of SiO also shows a variation of more than an order of magnitude between the sources and is on average more than an order of magnitude more abundant than predicted from photospheric chemistry in thermal equilibrium. The over-abundance of SiO is consistent with dynamical modelling of the stellar atmosphere and the inner parts of the wind where a pulsation-driven shock has passed. This scenario is possibly further substantiated by the relatively low amount of CS present in the envelopes. The chemistry occurring in the outer envelope is consistent with current photochemical models.

We show that the physical conditions in CRL 618 are such that efficient formation of benzene, C 6 H 6 , occurs. A combination of high temperatures, high densities, and high ionization rates drives an efficient ion-molecule chemistry involving condensation reactions of acetylene and its derivatives, rather than reactions involving atomic hydrogen, as was suggested for the interstellar synthesis of benzene. We find a column density of benzene within a factor of 2 of that observed providing that the material is trapped in a long-lived reservoir of gas in the disk around CRL 618. We note that the chemistry can give rise to other carbon chain molecules as well as a large abundance of benzonitrile, C 6 H 5 CN.

We present the results of models of the chemistry, including deuterium, in the inner regions of protostellar disks. We find good agreement with recent gas phase observations of several (non-deuterated) species. We also compare our results with observations of comets and find that in the absence of other processing e.g. in the accretion shock at the surface of the disk, or by mixing in the disk, the calculated D/H ratios in ices are higher than measured and reflect the D/H ratio set in the molecular cloud phase. Our models give quite different abundances and molecular distributions to other inner disk models because of the differences in physical conditions in the model disk. This emphasizes how changes in the assumptions about the density and temperature distribution can radically affect the results of chemical models.