Revealing the "missing" low-mass stars in the S254-S258 star forming region by deep X-ray imaging ?
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.