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- Heikkilä, Arto, 1965
(författare)
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Studies of Interstellar Molecular Gas in the Magellanic Clouds
- 1998
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Results of observational studies of interstellar molecular gas clouds in the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC), two nearby metal-poor and dust-poor dwarf galaxies, are presented. Spectral line radiation emitted by interstellar molecules has been used to probe the physical state and the chemical composition of individual molecular clouds. In addition, observations of the C18O/C17O ratio in a few Galactic and extragalactic sources are presented. Two molecular species, deuterated formyl ion (DCO+) and hydrogen sulphide (H2S), were detected for the first time in an extragalactic source (N159W in the LMC). Moreover, a definite detection of C17O outside the Local Group has been obtained. The density and the temperature of the molecular gas in the Magellanic Clouds are estimated to be typically 104-106cm-3 and 15-30,K, respectively. The abundances of trace molecules (relative to H2) in clouds in the LMC are typically five to twenty times lower than in Galactic disc clouds. These underabundances are most likely caused by a combination of the lower metallicity and a more rapid photodissociation in the LMC. The effects of the metallicity and far-ultraviolet radiation are further emphasized in two of the clouds, where trace molecules are even less abundant: the cloud (N27) studied in the SMC and a cloud located near the centre of the 30 Doradus nebula in the LMC. Several features, most notably the abundance of C2H as well as the correlations of the HCO+/CO and the C2H/CO ratios with the star-formation activity, suggest that the chemistry in molecular clouds in the LMC and the SMC largely resembles that in Galactic photon-dominated regions. The gas-phase 12C/13C and 32S/34S abundance ratios are found to be similar to those measured in Galactic clouds. In particular, no trend of a higher 12C/13C ratio due to the low metallicity is discernible. On the other hand, the 18O/17O abundance ratio, the average in the LMC is estimated to be 1.6±0.3, is significantly lower than in Galactic clouds (by a factor of two) and centres of starburst galaxies (by a factor of five). Provided that the current understanding of stellar nucleosynthesis applies, a low 18O/17O abundance ratio indicates that massive stars have contributed only little to the enrichment of the interstellar medium. This suggests that the star-formation history in the LMC has been characterised by a steep initial mass function together with a low average star-formation rate.
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- Suutarinen, A., et al.
(författare)
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CH abundance gradient in TMC-1
- 2011
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 531, s. A121-
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Tidskriftsartikel (refereegranskat)abstract
- Aims. The aim of this study is to examine if the well-known chemical gradient in TMC-1 is reflected in the amount of rudimentaryforms of carbon available in the gas-phase. As a tracer we use the CH radical which is supposed to be well correlated with carbonatoms and simple hydrocarbon ions.Methods. We observed the 9-cm Λ-doubling lines of CH along the dense filament of TMC-1. The CH column densities were comparedwith the total H2 column densities derived using the 2MASS NIR data and previously published SCUBA maps and with OH columndensities derived using previous observations with Effelsberg. We also modelled the chemical evolution of TMC-1 adopting physicalconditions typical of dark clouds using the UMIST Database for Astrochemistry gas-phase reaction network to aid the interpretationof the observed OH/CH abundance ratios.Results. The CH column density has a clear peak in the vicinity of the cyanopolyyne maximum of TMC-1. The fractional CHabundance relative to H2 increases steadily from the northwestern end of the filament where it lies around 1.0 × 10−8, to the southeastwhere it reaches a value of 2.0 × 10−8. The OH and CH column densities are well correlated, and we obtained OH/CH abundanceratios of ∼16–20. These values are clearly larger than what has been measured recently in diffuse interstellar gas and is likely to berelated to C to CO conversion at higher densities. The good correlation between CH and OH can be explained by similar productionand destruction pathways. We suggest that the observed CH and OH abundance gradients are mainly due to enhanced abundances ina low-density envelope which becomes more prominent in the southeastern part and seems to continue beyond the dense filament.Conclusions. An extensive envelope probably signifies an early stage of dynamical evolution, and conforms with the detection of alarge CH abundance in the southeastern part of the cloud. The implied presence of other simple forms of carbon in the gas phase provides a natural explanation for the observation of "early-type" molecules in this region.
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