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Sequential Star Formation in RCW 34: A Spectroscopic Census of the Stellar Content of High-Mass Star-Forming Regions
In this paper, we present VLT/SINFONI integral field spectroscopy of RCW34 along with Spitzer/IRAC photometry of the surroundings. RCW 34consists of three different regions. A large bubble has been detected inthe IRAC images in which a cluster of intermediate- and low-mass classII objects is found. At the northern edge of this bubble, an H II regionis located, ionized by 3 OB stars, of which the most massive star hasspectral type O8.5V. Intermediate-mass stars (2-3 M sun) aredetected of G- and K-spectral type. These stars are still in thepre-main-sequence (PMS) phase. North of the H II region, aphoton-dominated region is present, marking the edge of a densemolecular cloud traced by H2 emission. Several class 0/Iobjects are associated with this cloud, indicating that star formationis still taking place. The distance to RCW 34 is revised to 2.5 ±0.2 kpc and an age estimate of 2 ± 1 Myr is derived from theproperties of the PMS stars inside the H II region. Between the class IIsources in the bubble and the PMS stars in the H II region, no agedifference could be detected with the present data. The presence of theclass 0/I sources in the molecular cloud, however, suggests that theobjects inside the molecular cloud are significantly younger. The mostlikely scenario for the formation of the three regions is that starformation propagated from south to north. First the bubble is formed,produced by intermediate- and low-mass stars only, after that, the H IIregion is formed from a dense core at the edge of the molecular cloud,resulting in the expansion similar to a champagne flow. More recently,star formation occurred in the rest of the molecular cloud. Twodifferent formation scenarios are possible. (1) The bubble with thecluster of low- and intermediate-mass stars triggered the formation ofthe O star at the edge of the molecular cloud, which in its turn inducesthe current star formation in the molecular cloud. (2) An externaltriggering is responsible for the star formation propagating from southto north.Based on observations collected at the European Southern Observatory atParanal, Chile (ESO program 078.C-0780).
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