XMM-Newton observations of the σ Ori cluster. I. The complex RGS spectrum of the hot star σ Ori AB

We present XMM-Newton observations of the young ({∼} 2{-}5 Myr) cluster around the hot (O9.5V) star σ Orionis AB, aimed at obtaining a high resolution RGS spectrum of the hot star as well as EPIC imaging data for the whole field. We show that the RGS spectrum of σ Ori AB may be contaminated by weaker nearby sources which required the development of a suitable procedure to extract a clean RGS spectrum and to determine the thermal structure and wind properties of the hot star. We also report on the detection of a flare from the B2Vp star σ Ori E and we discuss whether the flare originated from the hot star itself or rather from an unseen late-type companion. Other results of this observation include: the detection of 174 X-ray sources in the field of σ Ori of which 76 are identified as cluster members, including very low-mass stars down to the substellar limit; the discovery of rotational modulation in a late-type star near σ Ori AB; no detectable line broadenings and shifts (⪉ 800 km s^-1) in the spectrum of σ Ori AB together with a remarkable low value of the O VII forbidden to intercombination line ratio and unusually high coronal abundances of CNO elements.