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Astron. Astrophys. 358, 257-275 (2000)

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SO and CS observations of molecular clouds *

II. Analysis and modelling of the abundance ratios - probing O2/CO with SO/CS?

A. Nilsson 1, Å. Hjalmarson 1, P. Bergman 1 and T.J. Millar 2

1 Onsala Space Observatory, 439 92 Onsala, Sweden
2 Department of Physics, UMIST, P.O. Box 88, Manchester M60 1QD, UK

Received 24 August 1999 / Accepted 14 March 2000


We here analyse the observational SO and CS data presented in Nilsson et al. (2000). The SO/CS integrated intensity ratio maps are presented for 19 molecular clouds, together with tables of relevant ratios at strategic positions, where we have also observed 34SO and/or C34S. The SO/CS abundance ratio as calculated from an LTE analysis is highly varying within and between the sources. Our isotopomer observations and Monte Carlo simulations verify that this is not an artifact due to optical depth problems. The variation of the maximum SO/CS abundance ratio between the clouds is 0.2-7. The largest variations within a cloud are found for the most nearby objects, possibly indicating resolution effects. We have also performed time dependent chemical simulations. We compare the simulations with our observed SO/CS abundance ratios and suggest a varying oxygen to carbon initial abundance, differing temporal evolution, density differences and X-ray sources associated with young stellar objects as possible explanations to the variations. In particular, the observed variation of the maximum SO/CS abundance ratio between the clouds can be explained by using initial O/C+ abundance ratios in the range 1.3-2.5. We finally derive a relationship between the SO/CS and O2/CO abundance ratios, which may be used as a guide to find the most promising interstellar O2 search targets.

Key words: ISM: clouds – ISM: molecules – ISM: abundances – ISM: structure

* Table 1 and Figs. 4 to 21 are only available in electronic form at the CDS via anonymous ftp to ftp://cdsarc.u-strasbg.fr/J/A+A/358/257 ( or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/358/257

Send offprint requests to: Å. Hjalmarson

Correspondence to: hjalmar@oso.chalmers.se

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© European Southern Observatory (ESO) 2000

Online publication: June 26, 2000