An X‐Ray Census of Young Stars in the Massive Southern Star‐forming Complex NGC 6357

Wang, Junfeng; Townsley, Leisa K.; Feigelson, E. D.; Getman, Konstantin V.; Broos, P. S.; Garmire, G. P.; Tsujimoto, Masahiro

doi:10.1086/509147

Cited by 67 publications

(156 citation statements)

References 105 publications

(212 reference statements)

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“…Considering that we have found 28 (for 3.2 < A V < 7.8) to 85 (for 3.2 < A V < 15) stars down to ∼6.5 M (assuming an age of 1 Myr), a Scalo IMF yields ∼3600 to ∼11 000 members in the Sofi field. This is consistent with ∼10 000 (in fact ∼5000 if the distance is shortened to 1.7 kpc) found by Wang et al (2007) by scaling their derived X-ray luminosity function (obtained from sources on a larger area than our SofI field) to that of the ONC. Thus, the stellar mass of Pis24 core amounts to ∼2−6 × 10 3 M for a Scalo IMF.…”

Section: Star Formation On the Smaller Scale: Stellar Population Of Psupporting

confidence: 90%

“…Assuming a distance of 1.7 kpc and adopting the evolutionary tracks of Palla & Stahler (1999) for 1 Myr old PMS stars, this corresponds to a mass completeness limit of ∼2 M for A V between 5-10 mag, which is consistent with what was found by Wang et al (2007).…”

Section: X-ray Source Population Of Pismis 24supporting

confidence: 85%

“…Recent simulations of a non-rotating 60 M star have shown that this would display a supergiant appearance (i.e., luminosity class I) already on the zero age main sequence (ZAMS; Groh et al 2014). Wang et al (2007) obtained deep X-ray Chandra/ACIS observations of Pismis 24, finding ∼800 X-ray sources that cluster around Pismis 24 1 and Pismis 24 17, mostly intermediate-and low-mass PMS cluster members. The radial distribution of X-ray source surface density is characterised by a core ∼2 in radius superimposed on a halo with a radius of at least 8 .…”

Section: Introductionmentioning

confidence: 99%

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Young open clusters in the Galactic star forming region NGC 6357

Massi

Giannetti

Carlo

et al. 2014

A&A

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Context. NGC 6357 is an active star forming region with very young massive open clusters. These clusters contain some of the most massive stars in the Galaxy and strongly interact with nearby giant molecular clouds. Aims. We study the young stellar populations of the region and of the open cluster Pismis 24, focusing on their relationship with the nearby giant molecular clouds. We seek evidence of triggered star formation "propagating" from the clusters. Methods. We used new deep JHK s photometry, along with unpublished deep Spitzer/IRAC mid-infrared photometry, complemented with optical HST/WFPC2 high spatial resolution photometry and X-ray Chandra observations, to constrain age, initial mass function, and star formation modes in progress. We carefully examine and discuss all sources of bias (saturation, confusion, different sensitivities, extinction). Results. NGC 6357 hosts three large young stellar clusters, of which Pismis 24 is the most prominent. We found that Pismis 24 is a very young (∼1-3 Myr) open cluster with a Salpeter-like initial mass function and a few thousand members. A comparison between optical and infrared photometry indicates that the fraction of members with a near-infrared excess (i.e., with a circumstellar disk) is in the range 0.3-0.6, consistent with its photometrically derived age. We also find that Pismis 24 is likely subdivided into a few different subclusters, one of which contains almost all the massive members. There are indications of current star formation triggered by these massive stars, but clear age trends could not be derived (although the fraction of stars with a near-infrared excess does increase towards the Hii region associated with the cluster). The gas out of which Pismis 24 formed must have been distributed in dense clumps within a cloud of less dense gas ∼1 pc in radius. Conclusions. Our findings provide some new insight into how young stellar populations and massive stars emerge, and evolve in the first few Myr after birth, from a giant molecular cloud complex.

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Section: Star Formation On the Smaller Scale: Stellar Population Of Psupporting

confidence: 90%

Section: X-ray Source Population Of Pismis 24supporting

confidence: 85%

Section: Introductionmentioning

confidence: 99%

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Young open clusters in the Galactic star forming region NGC 6357

Massi

Giannetti

Carlo

et al. 2014

A&A

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“…6) were previously identified, the well-known Pismis 24 and AH03J1726-34.4 (Dias et al 2002). For Pismis 24, Wang et al (2007) identified 34 O-B3 stars from X-ray studies. The information on the cluster AH03J1726-34.4 is very sparse.…”

Section: The Ob Stars Censusmentioning

confidence: 99%

Statistical study of OB stars in NGC 6334 and NGC 6357

Russeil

Zavagno

Adami

et al. 2012

A&A

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Context. Star-forming complexes are large structures exhibiting massive star-formation at different stages of evolution, from dense cores to well-developed H ii regions. They are very interesting for the study of the formation and evolution of stars. NGC 6334 and NGC 6357 are two active and relatively nearby star-forming complexes. From the extinction map and the sub-mm cold dust emission, and because they have similar velocities, these regions are most likely connected. However, located in the direction of the Galactic center their radial velocity is not representative of their distance. An alternative is then to determine the distance of NGC 6334 and NGC 6357 from their stellar content. Aims. Our aim is to perform a census of O-B3 ionising stars in NGC 6334 and NGC 6357, to determine the extinction coefficient, and the distance of both regions. A census of O-B3 stars is an essential basis for estimating the statistical lifetime of the earliest massive star-forming phases. Methods. We performed a U, B, V, and R photometric survey of a large area covering NGC 6334 and NGC 6357 with the VIMOS (ESO-VLT) and the MOSAIC (CTIO) instruments. This allows us to have a complete census of O to B3 stars up to V = 22.6 mag. The OB stars are selected based on their U − B and B − V colors. The most robust extinction coefficient is determined from color−color plots before computing the distance of the OB stars. Results. We find a higher value than typical of the diffuse interstellar medium for R V of 3.53 ± 0.08 and 3.56 ± 0.15 for NGC 6357 and NGC 6334, respectively. Adopting these R V values, the distances of NGC 6357 and NGC 6334 are 1.9 ± 0.4 kpc and 1.7 ± 0.3 kpc. We conclude that, within the error bars, both regions are thus at the same distance of 1.75 kpc (weighted mean). We confirm that the value of R V is linked to the large dust grain content. In particular, we found that there are more very small grains in NGC 6357 than in NGC 6334, suggesting that NGC 6357 could be more evolved than NGC 6334. Placed in the Galactic context, the NGC 6334-NGC 6357 complex appears to be located at the inner edge of the Sagittarius-Carina arm. Our census of O to B3 stars leads to a count of ∼230, which allows us to determine the statistical lifetime of the earliest phases of the massive stars. The starless and the protostellar phases have a mean statistical lifetime of ∼1.5 × 10 4 yr and ∼2.2 × 10 5 yr, respectively.

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“…the initial mass function (IMF). Although the correlation between stellar mass and X-ray luminosity shows a considerable scatter (see, e.g., , X-ray studies of a large number of young stellar clusters have shown that the XLF appears to be rather universal and constant in different environments (see Feigelson & Getman 2005;Getman et al 2006;Wang et al 2007).…”

Section: The X-ray Luminosity Functionmentioning

confidence: 99%

Revealing the “missing” low-mass stars in the S254-S258 star forming region by deep X-ray imaging

Mucciarelli

Preibisch

Zinnecker

2011

A&A

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Context. X-ray observations provide a very good way to reveal the population of young stars in star forming regions avoiding the biases introduced when selecting samples based on infrared excess. Aims. The aim of this study was to find an explanation for the remarkable morphology of the central part of the S254-S258 star forming complex, where a dense embedded cluster of very young stellar objects (S255-IR) is sandwiched between the two H ii regions S255 and S257. This interesting configuration had led to different speculations such as dynamical ejection of the B-stars from the central cluster or triggered star formation in a cloud that was swept up in the collision zone between the two expanding H ii regions. The presence or absence, and the spatial distribution of low-mass stars associated with these B-stars can discriminate between the possible scenarios. Methods. We performed a deep Chandra X-ray observation of the S254-S258 region in order to efficiently discriminate young stars (with and without circumstellar matter) from the numerous older field stars in the area. Results. We detected 364 X-ray point sources in a 17 × 17 field (≈8× 8 pc). This X-ray catalog provides, for the first time, a complete sample of all young stars in the region down to ∼0.5 M . A clustering analysis identifies three significant clusters: the central embedded cluster S255-IR and two smaller clusterings in S256 and S258. Sixty-four X-ray sources can be classified as members in one of these clusters. After accounting for X-ray background contaminants, this implies that about 250 X-ray sources constitute a widely scattered population of young stars, distributed over the full field-of-view of our X-ray image. This distributed young stellar population is considerably larger than the previously known number of non-clustered young stars selected by infrared excesses. Comparison of the X-ray luminosity function with that of the Orion Nebula Cluster suggests a total population of ∼2000 young stars in the observed part of the S254-S258 region. Conclusions. The observed number of ∼250 X-ray detected distributed young stars agrees well with the expectation for the low-mass population associated to the B-stars in S255 and S257 as predicted by an IMF extrapolation. These results are consistent with the scenario that these two B-stars represent an earlier stellar population and that their expanding H ii regions have swept up the central cloud and trigger star formation (i.e. the central embedded cluster S255-IR) therein.

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An X‐Ray Census of Young Stars in the Massive Southern Star‐forming Complex NGC 6357

Cited by 67 publications

References 105 publications

Young open clusters in the Galactic star forming region NGC 6357

Young open clusters in the Galactic star forming region NGC 6357

Statistical study of OB stars in NGC 6334 and NGC 6357

Revealing the “missing” low-mass stars in the S254-S258 star forming region by deep X-ray imaging

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