Young massive stars or clusters are often observed at the peripheries of HII regions. What triggers star formation at such locations? Among the scenarios that have been proposed, the `collect and collapse' process is particularly attractive because it permits the formation of massive objects via the fragmentation of the dense shocked layer of neutral gas surrounding the expanding ionized zone. However, until our recent article on Sh 104, it had not been convincingly demonstrated that this process actually takes place. In the present paper we present our selection of seventeen candidate regions for this process; all show high-luminosity near-IR clusters and/or mid-IR point sources at their peripheries. The reality of a `collect and collapse' origin of these presumably second-generation stars and clusters will be discussed in forthcoming papers, using new near-IR and millimetre observations.

/pdf/triggered-massive-star-formation-on-the-borders-of-galactic-4lozr06uum.pdf

Triggered massive-star formation on the borders of Galactic HII regions. I. A search for `collect and collapse' candidates

Young massive stars or clusters are often observed at the peripheries of H  regions. What triggers star formation at such locations? Among the scenarios that have been proposed, the "collect and collapse" process is particularly attractive because it permits the formation of massive objects via the fragmentation of the dense shocked layer of neutral gas surrounding the expanding ionized zone. However, until our recent article on Sh 104, it had not been convincingly demonstrated that this process actually takes place. In the present paper we present our selection of seventeen candidate regions for this process; all show high-luminosity near-IR clusters and/or mid-IR point sources at their peripheries. The reality of a "collect and collapse" origin of these presumably second-generation stars and clusters will be discussed in forthcoming papers, using new near-IR and millimetre observations.

/pdf/triggered-massive-star-formation-on-the-borders-of-galactic-49xzjmlqdq.pdf

Triggered massive-star formation on the borders of Galactic H II regions I. A search for "collect and collapse" candidates

We present SEST-SIMBA 1.2-mm continuum maps and ESO-NTT SOFI JHK images of the Galactic HII region RCW 79. The millimetre continuum data reveal the presence of massive fragments located in a dust emission ring surrounding the ionized gas. The two most massive fragments are diametrically opposite each other in the ring. The near-IR data, centred on the compact HII region located at the south-eastern border of RCW 79, show the presence of an IR-bright cluster containing massive stars along with young stellar objects with near-IR excesses. A bright near- and mid-IR source is detected towards maser emissions, 1.2 pc north-east of the compact HII region centre. Additional information, extracted from the Spitzer GLIMPSE survey, are used to discuss the nature of the bright IR sources observed towards RCW 79. Twelve luminous Class I sources are identified towards the most massive millimetre fragments. All these facts strongly indicate that the massive-star formation observed at the border of the HII region RCW 79 has been triggered by its expansion, most probably by the collect and collapse process.

/pdf/triggered-massive-star-formation-on-the-borders-of-galactic-2ryazcqp0c.pdf

Triggered massive-star formation on the borders of Galactic HII regions. II. Evidence for the collect and collapse process around RCW 79

We present SEST-SIMBA 1.2-mm continuum maps and ESO-NTT SOFIJHKS images of the Galactic H ii region RCW 79. The millimetre continuum data reveal the presence of massive fragments located in a dust emission ring surrounding the ionized gas. The two most massive fragments are diametrically opposite each other in the ring. The near-IR data, centred on the compact H ii region located at the south-eastern border of RCW 79, show the presence of an IR-bright cluster containing massive stars along with young stellar objects with near-IR excesses. A bright near- and mid-IR source is detected towards maser emissions, 1.2 pc north-east of the compact H ii region centre. Additional information, extracted from the Spitzer GLIMPSE survey, are used to discuss the nature of the bright IR sources observed towards RCW 79. Twelve luminous Class I sources are identified towards the most massive millimetre fragments. All these facts strongly indicate that the massive-star formation observed at the border of the H ii region RCW 79 has been triggered by its expansion, most probably by the collect and collapse process.

/pdf/triggered-massive-star-formation-on-the-borders-of-galactic-xgguteo3pm.pdf

Triggered massive-star formation on the borders of Galactic H ii regions ? II. Evidence for the collect and collapse process around RCW 79

Attention is given to low- and medium-resolution spectra in the 3-micron region of 24 Herbig Ae/Be stars obtained in a search for organic features from the dust around young stars. The 3.29-micron emission feature from aromatic hydrocarbons was detected in three objects: Lk H-alpha 25, XY Per, and AS 310. Two other stars, HD 245185 and HK Ori, may have weak features. About 20 percent of the Herbig Ae/Be surveyed to date have firmly detected 3.29-micron features. The available data indicate that the 3.29-micron feature is more extended around Herbig Ae/Be stars of earlier spectral type, possibly due to dehydrogenization or destruction of the aromatics near these stars. It is suggested that the total number of aromatics excited by the stars is also greater around the earlier-type objects.

Dust emission features in 3-micron spectra of Herbig Ae/Be stars

We present 3-12 μ m ISOCAM observations of the five Galactic compact H ii regions Sh 61, Sh 138, Sh 152, Sh 156 and Sh 186. The unidentified infrared bands (UIBs) centred at 3.3, 6.2, 7.7, 8.6 and 11.2 μ m -and underlying continuum -are imaged using the SW1, SW2, LW4, LW6 and LW8 filters. Images are also obtained at 5.985, 6.911, 8.222, 10.520 and 12.000 μ m using the circular variable filter (CVF). We show that the 5.985 μ m emission represents a true continuum reference for the 6.2 μ m band, allowing a derivation of this band's properties. Due to uncertainties in the continuum estimates, only lower limits can be given for the 3.3, 7.7 and 11.3 μ m band fluxes. These limits agree with previous results found in the literature. The distribution of the bands coincide. The 3.3 μ m emission is not observed in high extinction zones, suggesting a lower temperature of the carriers and/or a higher abundance of larger molecules in those zones. The 6.2 μ m band emission peaks outside the ionized zone, in the photodissociation region. The 6.2 μ m band luminosity correlates with the far UV field intensity, suggesting a UV excitation. We also find a correlation between the spatial distribution of the 6.2 μ m band emission and zones of strong 2.122 μ m H 2 emission due to ultraviolet fluorescence. This suggests that both emissions are due to UV excitation. The 6.2 μ m emission is slightly closer to the exciting star. This suggests that the band carriers survive in the H i zone. The 12 μ m emission traces the continuum emission from very small grains, when present, and follows well the distribution of UIB emission. This suggests a link between the two emission carriers. The emission peak observed on the star in Sh 61 and Sh 156 indicates that the continuum from very small grains dominates the emission in highly excited regions.

/pdf/isocam-3-12-mm-imaging-of-five-galactic-compact-hii-regions-4g3kl2mtvq.pdf

ISOCAM 3-12 μm imaging of five galactic compact Hii regions

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ISOCAM 3-12 μm imaging of five galactic compact Hii regions