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We present the results of an optical spectroscopic campaign on the massive binary HD 152219 located near the core of the NGC 6231 cluster. Though the primary to secondary optical brightness ratio is probably about 10, we clearly detect the secondary spectral signature and we derive the first reliable SB2 orbital solution for the system. The orbital period is close to 4.2403 d and the orbit is slightly eccentric . The system is most probably formed by an O9.5 giant and a B1-2 V-III star. We derive minimal masses of and for the primary and secondary, respectively, and we constrain the stellar radius at values about 11 and . INTEGRAL-Optical Monitoring Camera (OMC) data reveal that HD 152219 is the third O-type eclipsing binary known in NGC 6231. In the Hertzsprung–Russell (HR) diagram, the primary component lies on the blue edge of the β Cep-type instability strip and its spectral lines display clear profile variations that are reminiscent of those expected from non-radial pulsations. Finally, we report the analysis of XMM–Newton observations of the system. The X-ray spectrum is relatively soft and is well reproduced by a two-temperature mekal model with and . The X-ray flux is most probably variable on a time-scale of days. The average X-ray luminosity during our campaign is , but shows fluctuations of about 10 per cent around this value. [ABSTRACT FROM AUTHOR]

Highlights: [•] This paper reports on the very first dedicated X-ray observation of the famous yellow hypergiant IRC+10420. [•] The target is not detected and upper limits on the X-ray flux are consistently derived and discussed. [•] The non-detection is interpreted in terms of high circumstellar absorption and likely weak intrinsic emission. [•] Other X-ray point sources in the field of view are briefly discussed. [ABSTRACT FROM AUTHOR]

Context. Probing the structures of stellar winds is of prime importance for the understanding of massive stars. Based on their optical spectral morphology and variability, it has been suggested that the stars in the Oef class feature large-scale structures in their wind. Aims. High-resolution X-ray spectroscopy and time-series of X-ray observations of presumably single O-type stars can help us understand the physics of their stellar winds. Methods. We have collected XMM-Newton observations and coordinated optical spectroscopy of the O6 Ief star λ Cep to study its X-ray and optical variability and to analyse its high-resolution X-ray spectrum. We investigate the line profile variability of the He ii λ 4686 and Hα emission lines in our time series of optical spectra, including a search for periodicities. We further discuss the variability of the broadband X-ray flux and analyse the high-resolution spectrum of λ Cep using line-by-line fits as well as a code designed to fit the full high-resolution X-ray spectrum consistently. Results. During our observing campaign, the He ii λ 4686 line varies on a timescale of ∼18 h. On the contrary, the Hα line profile displays a modulation on a timescale of 4.1 days which is likely the rotation period of the star. The X-ray flux varies on timescales of days and could in fact be modulated by the same 4.1-day period as Hα, although both variations are shifted in phase. The highresolution X-ray spectrum reveals broad and skewed emission lines as expected for the X-ray emission from a distribution of windembedded shocks. Most of the X-ray emission arises within less than 2 R* above the photosphere. Conclusions. The properties of the X-ray emission of λ Cep generally agree with the expectations of the wind-embedded shock model. There is mounting evidence for the existence of large-scale structures that modulate the Hα line and about 10% of the X-ray emission of λ Cep. [ABSTRACT FROM AUTHOR]

In this second paper of the series, we pursue the analysis of the 180-ks XMM–Newton campaign towards the young open cluster NGC 6231 and we focus on its rich OB star population. We present a literature-based census of the OB stars in the field of view with more than one hundred objects, among which 30 per cent 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 . 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 per cent according to whether ‘cool’ dwarfs are included or not), much smaller than that obtained from previous studies. At our detection threshold and with our sample, the sole identified mechanism that produces significant modulations in the O star X-ray emission is related to wind interaction. We thus propose that the intrinsic X-ray emission of non-peculiar O-type stars can be considered as constant for a given star. In addition, the level of X-ray emission is accurately related to the star luminosity or, equivalently, to its wind properties. Among B-type stars, the detection rate is only about 25 per cent in the subtype range B0–B4 and remains mostly uniform throughout the different subpopulations while it drops significantly at later subtypes. The associated X-ray spectra are harder than those of O-type stars. Our analysis points towards the detected emission being associated with a physical (in a multiple system) PMS companion. However, we still observe a correlation between the bolometric luminosity of the B stars and the measured X-ray luminosity. The best-fitting power law in the 0.5–10.0 keV band yields . [ABSTRACT FROM AUTHOR]