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

1. Introduction

For a long time it has been a commonly accepted idea that the SO/CS abundance ratio may probe evolutionary effects in molecular clouds. The present project started in 1996 and is based upon this idea, together with our hope that these investigations may be used to guide searches for interstellar O₂ by the SWAS (Melnick et al. 1997) and Odin (Hjalmarson 1997; Nordh 1997) satellites and also may lead to a better understanding of the presence, or absence, of O₂ at detectable levels. The reason for this is the similarity between the dominant formation and destruction mechanisms for SO and O₂ in dense interstellar clouds, viz,

and

Therefore we expect that the abundances of SO and O₂ will remain low, because of the efficient loss reactions, Eq. (2), as long as the C abundance remains high. Only when C has been "locked up" by the formation of CO, will the SO and O₂ abundances become high, i.e. at "late times". CS, on the other hand, reaches a high abundance level already at "early times". This behaviour has been clearly demonstrated by time dependent chemical models, e.g. by Bergin et al. (1995), Bergin & Langer (1997), El-Nawawy et al. (1997), and Taylor et al. (1998). The same behaviour is also found in models which use enhanced ion-dipole rate coefficients at low temperatures (cf. Lee et al. 1996).

Already Langer et al. (1984) demonstrated by means of chemical modelling the dramatic influence of the initial C/O elemental abundance ratio on the O₂ concentration. The many failures to detect interstellar O₂ [e.g. the recent low gas phase limits by Combes et al. (1997); Maréchal et al. (1997a); Olofsson et al. (1998); Melnick et al. (1999) and references therein] then may be explained by an elevated C/O ratio, somewhat above the commonly used ratio of about 0.4. Recently Maréchal et al. (1997b) have reported an extensive study on the chemistry and emissivity of O₂ in interstellar clouds. They predict O₂ abundances and emissivities in several O₂ transitions for varying cloud conditions and confirm the conclusion by Langer et al. (1984) that the initial C/O abundance ratio very strongly influences the O₂ abundance. Until recently dedicated observational studies determining the eventual variations of the SO/CS abundance ratio within and between molecular clouds have been rather rare (e.g. Swade 1989a, 1989b; Heithausen et al. 1998; Bergin et al. 1997; Pratap et al. 1997). Gerin et al. (1997) have observed chemical inhomogeneities in high latitude and dark clouds and also have performed chemical modelling. These authors advocate that in some clouds chemical bistability caused by a high ionization phase, in addition to the usually adopted low ionization phase, may play a role (cf. also Le Bourlot et al. 1995).

In this paper we analyse SO() and CS() observational data resulting from extensive mapping of 19 molecular clouds performed with the Onsala 20 m telescope (Nilsson et al. 2000; hereafter Paper I). In Sect. 2 we present integrated intensity ratio maps, and tabulate relevant ratios between SO, ³⁴SO, CS and C³⁴S, together with the estimated SO/CS abundance ratios. In the subsequent Sect. 3 we present results from astrochemistry simulations. In Sect. 4 we discuss our observational findings in terms of the astrochemistry modelling results, and in Sect. 5 we summarize our findings and draw some main conlusions.

In a subsequent Paper III (Olofsson et al., in prep.) we will combine the present data (available in Paper I) with newly acquired C¹⁸O(1-0) maps. In this way we can discuss the actual SO and CS abundance variations (rather than the SO/CS abundance ratio) across and between the sources treated in this paper in the light of chemical modelling.

© European Southern Observatory (ESO) 2000

Online publication: June 26, 2000
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