EDP Sciences
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Volume 421, Number 2, July II 2004
Page(s) 715 - 727
Section Stellar atmospheres
DOI http://dx.doi.org/10.1051/0004-6361:20047159

A&A 421, 715-727 (2004)
DOI: 10.1051/0004-6361:20047159

XMM-Newton observations of the $\sigma$ Ori cluster

I. The complex RGS spectrum of the hot star $\sigma$ Ori AB
J. Sanz-Forcada, E. Franciosini and R. Pallavicini

INAF - Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy

(Received 28 January 2004 / Accepted 30 March 2004)

We present XMM-Newton observations of the young ( ${\sim} 2{-}5$ Myr) cluster around the hot (O9.5V) star $\sigma$ 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 $\sigma$ 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 $\sigma$ 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 $\sigma$ 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 $\sigma$ Ori AB; no detectable line broadenings and shifts ( ${\la}
800$  km s -1) in the spectrum of $\sigma$ Ori AB together with a remarkable low value of the $\ion{O}{vii}$ forbidden to intercombination line ratio and unusually high coronal abundances of CNO elements.

Key words: stars: coronae -- stars: winds, outflows -- stars: individual: $\sigma$ Ori -- stars: early-type -- stars: late-type

Offprint request: J. Sanz-Forcada, jsanz@rssd.esa.int

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