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Astron. Astrophys. 336, 309-314 (1998)

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1. Introduction

Radio intensity maps of molecules in transitions with critical densities [FORMULA] cm-3 show emission mostly localised to [FORMULA] 0.1pc `cores' that are known to be sites of star-formation. It is therefore reasonable to assume that this dense material is the consequence of the partial collapse of the more diffuse gas that surrounds it, traced, for example, in CO (J=[FORMULA]) emission. The molecules most commonly used to detect these cores are ammonia and carbon monosulphide (e.g. Benson & Myers 1989, Snell et al. 1984), and a puzzling discrepancy became apparent very early in these studies. Although CS should trace higher densities than NH3 the surveys of Zhou et al. (1989), Myers et al. (1991) and Pastor et al. (1991) showed the CS (J=[FORMULA], and [FORMULA]) emission to cover systematically wider areas than the NH3 (1,1) inversion transition. The same conclusions for further sources were drawn by Juan et al. (1993), López et al. (1994) and Zinchenko et al. (1994). In addition, the CS linewidths were larger, and it was noted that there was a significant difference in the positions of the peaks of emission of CS and NH3. However, the denser gas (traced in CS) should be more confined then the less dense gas (traced in NH3).

The differences are not caused by the use of different instruments, and continue to appear whenever studies are made at similar angular resolution. Possible explanations for the anomaly may lie either in the effects of optical depth or of chemical differentiation. In the former case, Fuller (1988) proposed that a low density envelope would scatter photons from the core within, giving the impression of a wider emitting area, the effect occurring preferentially in CS since the NH3 (1,1) population is distributed over all the hyperfine components. In the latter case, Taylor, Morata & Williams (1996; Paper 1) proposed a solution based on the differences in the chemistry of the two molecules, and furthermore using chemical models and an analysis of the excitation of CS in a particular source (L1524) came to the conclusion that the extended CS emission was due to unresolved clumps rather than homogeneous gas. In this model, the difference is due to the speed at which the molecules form, NH3 only being present in observable amounts in longer-lasting clumps, perhaps only those that become star-forming cores. This model has the further advantages that it can continue to explain the observations when the CS lines are optically thin, and also the lack of coincidence of the molecular emission peaks and the larger CS linewidth.

In this paper we extend the work of Paper 1 to examine whether there are other potentially observable molecules that should show extended emission like CS, or more compact emission like ammonia, if this `unresolved clump' model is correct. Although sulphur-containing molecules are some of the most useful in observing these regions, quantitative chemical modelling is hampered by the lack of information on the elemental depletion of sulphur; hence, we are especially interested in other signature molecules. The reliability of the chemistry that leads to these predictions is also discussed.

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© European Southern Observatory (ESO) 1998

Online publication: July 7, 1998