Near-IR integral field spectroscopy of ionizing stars and young stellar objects on the borders of H II regions

Abstract

Aims. We study three Galactic H ii 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 H ii 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. Methods. 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. Results. 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 ± 0.5 Myr 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 H ii regions. The young stellar objects show four main types of spectral features: H 2 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 H ii regions. The radial velocities of most YSOs are consistent with that of the ionized gas, firmly establishing their association with the H ii regions. Exceptions are found in RCW 120 where differences up to 50 km s −1 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 H 2 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 H 2 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.

HerschelSPIRE-FTS observations of RCW 120

“…We obtain a value of ∼1925 in Habing units. This value is in good agreement with the ∼1000 derived from our measurements using the PDR code, especially if we consider that dust present in the ionized region, as shown by the 24 μm emission (Martins et al 2010), can absorb part of the radiation and diminishes that reaching the PDR surface.…”

“…5 Anderson et al (2015) results in a physical size of ∼3 pc. RCW 120 is ionized by the single star CD − 38 • 11636, with a spectral type O6-8V/III according to the latest measurements by Martins et al (2010). Labeled "the perfect bubble" throughout the literature, recent single-dish observations of the lowest CO transitions (Anderson et al 2015;Torii et al 2015) fail to detect an expanding shell of fore-and background material, which would indicate a 3D structure.…”

A gallery of bubbles

“…For example, it is observed in: i) RCW 79, which is excited by a cluster of early O-stars (Martins et al 2009), and in N49, which is excited by an O5V star (Watson et al 2008); ii) RCW 120, which is excited by an O8V star (Zavagno et al 2006) and in N4, which is excited by an O8V star (an estimation based on a radio flux of 2.02 Jy at 11-cm, Reich et al 1984, and a distance of 3.14 kpc); iii) RCW 82, which is excited by two late O-stars (Martins et al 2009). It is also observed in bubbles enclosing regions of very faint/or absent radio emission (as N8).…”

Herschelfar-infrared observations of the Carina Nebula complex

“…However, the validity of this interpretation remains unclear, since recent studies suggest that massive star feedback is usually dominated by ionizing radiation (e.g. Martins et al 2010) and the effects of stellar winds are of secondary importance (e.g. Martins et al 2012).…”