Aims. Massive stars form in clusters, and they are often found in different evolutionary stages located close to each other. To understand evolutionary and environmental effects during the formation of high-mass stars, we observed three regions of massive star formation at different evolutionary stages, and all are found that in the same natal molecular cloud. Methods. The three regions, S255IR, S255N, and S255S, were observed at 1.3 mm with the submillimeter array (SMA), and followup short spacing information was obtained with the IRAM 30 m telescope. Near infrared (NIR) H + K-band spectra and continuum observations were taken for S255IR with VLT-SINFONI to study the different stellar populations in this region.Results. This combination of millimeter (mm) and near infrared data allow us to characterize different stellar populations within the young forming cluster in detail. While we find multiple mm continuum sources toward all regions, their outflow, disk, and chemical properties vary considerably. The most evolved source S255IR exhibits a collimated bipolar outflow visible in CO and H 2 emission, and the outflows from the youngest region S255S are still small and fairly confined in the regions of the mm continuum peaks. Also the chemistry toward S255IR is the most evolved, exhibiting strong emission from complex molecules, while much fewer molecular lines are detected in S255N, and in S255S we detect only CO isotopologues and SO lines. Also, rotational structures are found toward S255N and S255IR. Furthermore, a comparison of the NIR SINFONI and mm data from S255IR clearly reveal two different (proto) stellar populations with an estimated age difference of approximately 1 Myr. Conclusions. A multiwavelength spectroscopy and mapping study reveals different evolutionary phases of the star formation regions. We propose the triggered outside-in collapse star formation scenario for the bigger picture and the fragmentation scenario for S255IR.

We present high-resolution, near-infrared images of the ultracompact H ii region G5.89Ϫ0.39 that were taken during a commissioning run of the NACO adaptive optics/near-infrared camera system at the Very Large Telescope. For the first time, these data reveal the exact location of a very promising candidate for the primary, ionizing star of this region and provide a good estimate of its spectral type. We very briefly discuss the morphology of the region and the implications of finding the star at a location where it was quite unexpected, namely, off-center of a shell. Subject headings: H ii regions -stars: early-type -stars: formation On-line material: color figure L92 FELDT ET AL.Vol. 599

In this paper, we present VLT/SINFONI integral field spectroscopy of RCW 34 along with Spitzer/IRAC photometry of the surroundings. RCW 34 consists of three different regions. A large bubble has been detected in the IRAC images in which a cluster of intermediate-and low-mass class II objects is found. At the northern edge of this bubble, an H ii region is located, ionized by 3 OB stars, of which the most massive star has spectral type O8.5V. Intermediate-mass stars (2-3 M ) are detected of G-and K-spectral type. These stars are still in the premain-sequence (PMS) phase. North of the H ii region, a photon-dominated region is present, marking the edge of a dense molecular cloud traced by H 2 emission. Several class 0/I objects are associated with this cloud, indicating that star formation is 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 the properties of the PMS stars inside the H ii region. Between the class II sources in the bubble and the PMS stars in the H ii region, no age difference could be detected with the present data. The presence of the class 0/I sources in the molecular cloud, however, suggests that the objects inside the molecular cloud are significantly younger. The most likely scenario for the formation of the three regions is that star formation propagated from south to north. First the bubble is formed, produced by intermediate-and low-mass stars only, after that, the H ii region 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. Two different formation scenarios are possible. (1) The bubble with the cluster of low-and intermediate-mass stars triggered the formation of the O star at the edge of the molecular cloud, which in its turn induces the current star formation in the molecular cloud. (2) An external triggering is responsible for the star formation propagating from south to north.