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(abbreviated) In the central part of the S254-S258 star forming complex, adense embedded cluster of very young stellar objects (S255-IR) is sandwichedbetween the two HII regions S255 and S257. This interesting configuration hadled to different speculations such as dynamical ejection of the B-stars fromthe central cluster or triggered star formation in a cloud that was swept up inthe collision zone between the two expanding HII regions. The presence orabsence of low-mass stars associated with these B-stars can discriminatebetween the possible scenarios. We performed a deep Chandra X-ray observationof the S254-S258 region in order to efficiently discriminate young stars fromthe numerous older field stars in the area. We detected 364 X-ray pointsources, providing a complete sample of all young stars in the observed regiondown to ~0.5 Msun. A clustering analysis identifies three significant clusters,containing 64 X-ray sources in total. After accounting for X-ray backgroundcontaminants, this implies that about 250 X-ray sources constitute a widelyscattered population of young stars. This number agrees well with theexpectation for the low-mass population associated to the B-stars in S255 andS257 as predicted by an IMF extrapolation. These results are consistent withthe scenario that these two B-stars represent an earlier stellar population andthat their expanding HII regions have swept up the central cloud and triggerstar formation therein.

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