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Title: The massive binary HD 152218 revisited: A new colliding wind system in NGC 6231
Authors: H. Sana, Y. Nazé, B. O'Donnell, G. Rauw, E.Gosset

We present the results of an optical and X-ray monitoring campaign on the short-period massive SB2 binary HD 152218. Combining our HiRes spectroscopic data with previous observations, we unveil the contradictions between the published orbital solutions. In particular, we solve the aliasing on the period and derive a value close to 5.604 d. Our eccentricity e = 0.259 ± 0.006 is slightly lower than previously admitted. We show that HD 152218 is probably undergoing a relatively rapid apsidal motion of about 3deg/yr and we confirm the O9IV + O9.7V classification. We derive minimal masses of 15.82 ± 0.26 Msol operator and 12.00 ± 0.19 Msol operator and constrain the radius of the components to R1 = 10.3 ± 1.3 Rsol and R2 = 7.8 ± 1.7 Rsol. We also report the results of an XMM-Newton monitoring of the HD 152218 X-ray emission throughout its orbital motion. The averaged X-ray spectrum is relatively soft and it is well reproduced by a 2-T optically thin thermal plasma model with component temperatures about 0.3 and 0.7 keV. The system presents an increase of its X-ray flux by about 30% near apastron compared to periastron, which is interpreted as the signature of an ongoing wind-wind interaction process occurring within the wind acceleration region.

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Title: An XMM-Newton view of the young open cluster NGC 6231 -- II. The OB star population
Authors: H. Sana, G. Rauw, Y. Naze, E. Gosset, J.-M. Vreux

In this second paper, researchers pursue the analysis of the 180 ks XMM-Newton campaign towards the young open cluster NGC 6231 and they focus on its rich OB star population. The researchers present a literature-based census of the OB stars in the field of view with more than one hundred objects, among which 30% can be associated with an X-ray source. All the O-type stars are detected in the X-ray domain as soft and reasonably strong emitters. In the 0.5-10.0 keV band, their X-ray luminosities scale with their bolometric luminosities as log LX - log Lbol =-6.912 ±0.153. Such a scaling law holds in the soft (0.5-1.0 keV) and intermediate (1.0-2.5 keV) bands but breaks down in the hard band.
While the two colliding wind binaries in our sample clearly deviate from this scheme, the remaining O-type objects show a very limited dispersion (40% or 20% according to whether `cool' dwarfs are included or not), much smaller than that obtained from previous studies. At the researchers detection threshold and within their sample, the sole identified mechanism that produces significant modulations in the O star X-ray emission is related to wind interaction. The intrinsic X-ray emission of non-peculiar O-type stars seems thus constant for a given star and the level of its X-ray emission is accurately related to the its luminosity or, equivalently, to its wind properties. Among B-type stars, the detection rate is only about 25% in the sub-type range B0-B4 and remains mostly uniform throughout the different sub-populations while it drops significantly at later sub-types. The associated X-ray spectra are harder than those of O-type stars. The researchers analysis points towards the detected emission being associated with a physical PMS companion.

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