XMM-Newton observations of the young open cluster around λ Orionis^⋆,⋆⋆

¹ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Florence, Italy
e-mail: francio@arcetri.astro.it
² Chester F. Carlson Center for Imaging Science, Rochester Institute of Technology, 54 Lomb Memorial Drive, 14623 Rochester, USA

Received: 21 June 2010
Accepted: 30 March 2011

Abstract

Aims. We studied the X-ray properties of the young (~1−8 Myr) open cluster around the hot (O8 III) star λ Ori and compared them with those of the similarly-aged σ Ori cluster to investigate possible effects of the different ambient environment.

Methods. We analysed an XMM-Newton observation of the cluster using EPIC imaging and low-resolution spectral data. We studied the variability of detected sources, and performed a spectral analysis of the brightest sources in the field using multi-temperature models.

Results. We detected 167 X-ray sources, of which 58 are identified with known cluster members and candidates, from massive stars down to low-mass stars with spectral types  ~M5.5. Another 23 sources were identified with new possible photometric candidates. Late-type stars have a median log L_X/L_bol ~  −3.3, close to the saturation limit. Variability was observed in  ~35% of late-type members or candidates, including six flaring sources. The emission from the central hot star λ Ori is dominated by plasma at 0.2 − 0.3 keV, with a weaker component at 0.7 keV, consistently with a wind origin. The coronae of late-type stars can be described by two plasma components with temperatures T₁ ~ 0.3−0.8 keV and T₂ ~ 0.8−3 keV, and subsolar abundances Z ~ 0.1−0.3 Z_⊙, similar to what is found in other star-forming regions and associations. No significant difference was observed between stars with and without circumstellar discs, although the smallness of the sample of stars with discs and accretion does not allow us to draw definitive conclusions.

Conclusions. The X-ray properties of λ Ori late-type stars are comparable to those of the coeval σ Ori cluster, suggesting that stellar activity in λ Ori has not been significantly affected by the different ambient environment.