Revealing the "missing" low-mass stars in the S254-S258 star forming region by deep X-ray imaging
Abstract
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.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- September 2011
- DOI:
- 10.1051/0004-6361/201117074
- arXiv:
- arXiv:1106.2003
- Bibcode:
- 2011A&A...533A.121M
- Keywords:
-
- stars: formation;
- stars: pre-main sequence;
- open clusters and;
- associations: individual: S254-S258;
- X-rays: stars;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- Accepted for publication in Astronomy &