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