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

Abstract: (abbreviated) In the central part of the S254-S258 star forming complex, a dense embedded cluster of very young stellar objects (S255-IR) is sandwiched between the two HII 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 HII regions. The presence or absence of low-mass stars associated with these B-stars can discriminate between the possible scenarios. We performed a deep Chandra X-ray observation of the S254-S258 region in order to efficiently discriminate young stars from the numerous older field stars in the area. We detected 364 X-ray point sources, providing a complete sample of all young stars in the observed region down to ~0.5 Msun. A clustering analysis identifies three significant clusters, containing 64 X-ray sources in total. After accounting for X-ray background contaminants, this implies that about 250 X-ray sources constitute a widely scattered population of young stars. This number 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 HII regions have swept up the central cloud and trigger star formation therein.