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Astron. Astrophys. 320, 972-992 (1997)

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1. Introduction

The HII region/molecular cloud complexes NGC 6334 and NGC 6357, in the Sagittarius spiral arm, are very active regions of star formation in the Galaxy. Of the two complexes, NGC 6334 seems to be the most active in forming massive stars, but as demonstrated by Felli et al. (1990; hereafter Fea90) massive star formation is presently taking place in NGC 6357 as well.

Optical images show that NGC 6357 (a.k.a. RCW 131, W 22, Sh-2 11) comprises several distinct HII regions (see e.g. Fig. 2 of Fea90) in different stages of evolution. Numerous O and B stars are located in the region, which have been studied by Neckel (1978, 1984) and Lortet et al. (1984), and there are a number of thermal radio-continuum sources. The complex has been (rather crudely) mapped in the far IR (FIR) and CO by McBreen et al. (1983), who found widespread CO emission at [FORMULA] [FORMULA] to -1  [FORMULA], often peaking in intensity near the position of the FIR sources.

The regions around three of the radio sources were studied in detail by Fea90 with the VLA at 6 cm. One of the three regions (G353.2+0.7) showed no compact HII regions, nor early type stars, and will not be considered further. The other two, viz. G353.1+0.6 and G353.2+0.9, were found to contain bright structures on scale sizes from [FORMULA] to several arcminutes, and are the subject of the present study. G353.1+0.6 is a more evolved HII region, ionized by visible early-type stars located outside the radio nebulosity but inside a faint optical diffuse nebulosity. Here most of the radio-continuum emission arises from the obscured region north of the nebula; the properties of several of the radio structures found with the VLA are consistent with their being ionized skins of extended molecular clouds. No signs of recent star formation were found associated with this HII region. G353.2+0.9, on the other hand, is the brightest component in NGC 6357 at all wavelengths studied, and was found by Fea90 to be younger. The H [FORMULA] and radio-continuum emission both show a sharp boundary in brightness towards the south, indicating an ionization front seen almost edge-on, and suggesting the presence of obscuring material to the south of the HII region. Inside the nebulosity several indicators of recently formed bright stars, such as compact HII regions and sources with IR excess emission were found. These are probably responsible for the excitation of the nebula (see Fig. 7a in Fea90).

In view of the morphology of the HII regions, it is of interest to study the molecular clouds associated with G353.1+0.6 and G353.2+0.9 in greater detail. In particular, a study of the HII region/molecular cloud interfaces should allow one to directly compare the conditions immediately adjacent to the ionization fronts of two HII regions with different evolutionary status and star formation activity. In this paper we analyze observations of several molecular species and transitions. We adopt a distance [FORMULA]  kpc to both regions, from early type stars (Neckel 1978). The kinematic near-distance ([FORMULA] 1 kpc), based on the radial velocity of CO (McBreen et al. 1983; this work) and radio recombination lines (Wilson et al. 1970), is rather uncertain since the longitude of these sources is within [FORMULA] of the direction of the galactic center. In Sect. 2 we describe the observations and the data reduction. The results for G 353.1+0.6 and G 353.2+0.9 are presented and discussed in Sects. 3 and 4, and the main conclusions are summarized in Sect. 5.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998