2010A&A...510A..32M

We study three Galactic HII regions - RCW 79, RCW 82, and RCW 120 - where triggered star formation is taking place. Two stellar populations are observed: the ionizing stars of each HII region and young stellar objects on their borders. Our goal is to show that they represent two distinct populations, as expected from successive star-forming events. We use near-infrared integral field spectroscopy obtained with SINFONI on the VLT to make a spectral classification. We derived the stellar and wind properties of the ionizing stars using atmosphere models computed with the code CMFGEN. The young stellar objects were classified according to their K-band spectra. In combination with published near and mid infrared photometry, we constrained their nature. Linemaps were constructed to study the geometry of their close environment. We identify the ionizing stars of each region. RCW 79 is dominated by a cluster of a dozen O stars, identified for the first time by our observations. RCW 82 and RCW 120 are ionized by two and one O star, respectively. All ionizing stars are early-to-late O stars, close to the main sequence. The cluster ionizing RCW 79 formed 2.3±0.5Myr ago. Similar ages are estimated, albeit with a larger uncertainty, for the ionizing stars of the other two regions. The total mass-loss rate and ionizing flux is derived for each region. In RCW 79, where the richest cluster of ionizing stars is found, the mechanical wind luminosity represents only 0.1% of the ionizing luminosity, questioning the influence of stellar winds on the dynamics of these three HII regions. The young stellar objects show four main types of spectral features: H₂ emission, Brγ emission, CO bandheads emission, and CO bandheads absorption. These features are typical of young stellar objects surrounded by disks and/or envelopes, confirming that star formation is taking place on the borders of the three HII regions. The radial velocities of most YSOs are consistent with that of the ionized gas, firmly establishing their association with the HII regions. Exceptions are found in RCW 120 where differences up to 50km/s are observed. Outflows are detected in a few YSOs. All YSOs have moderate-to-strong near-IR excess. In the [24] versus K-[24] diagram, the majority of the sources dominated by H₂ emission lines stand out as redder and brighter than the rest of the YSOs. The quantitative analysis of their spectra indicates that, for most of them, the H₂ emission is essentially thermal and likely produced by shocks. We tentatively propose that they represent an earlier phase of evolution compared to sources dominated by Brγ and CO bandheads. We suggest that they still possess a dense envelope in which jets or winds create shocks. The other YSOs have partly lost their envelopes and show signatures of accretion disks. Overall, the YSOs show distinct spectroscopic signatures compared to the ionizing sources, confirming the presence of two stellar populations.