Abstract. The XMM-OM instrument extends the spectral coverage of the XMM-Newton observatory into the ultraviolet and optical range. It provides imaging and time-resolved data on targets simultaneously with observations in the EPIC and RGS. It also has the ability to track stars in its field of view, thus providing an improved post-facto aspect solution for the spacecraft. An overview of the XMM-OM and its operation is given, together with current information on the performance of the instrument.

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 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 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 log LX= 0.22(±0.06) log Lbol+ 22.8(±2.4).

Abstract. We present the first high resolution X-ray spectrum of the bright O4Ief supergiant star ζ Puppis, obtained with the Reflection Grating Spectrometer on-board XMM-Newton. The spectrum exhibits bright emission lines of hydrogen-like and helium-like ions of nitrogen, oxygen, neon, magnesium, and silicon, as well as neon-like ions of iron. The lines are all significantly resolved, with characteristic velocity widths of order 1000-1500 km s −1 . The nitrogen lines are especially strong, and indicate that the shocked gas in the wind is mixed with CNO-burned material, as has been previously inferred for the atmosphere of this star from ultraviolet spectra. We find that the forbidden to intercombination line ratios within the helium-like triplets are anomalously low for N VI, O VII, and Ne IX. While this is sometimes indicative of high electron density, we show that in this case, it is instead caused by the intense ultraviolet radiation field of the star. We use this interpretation to derive constraints on the location of the X-ray emitting shocks within the wind that are consistent with current theoretical models for this system.

Abstract. We analyse the optical spectrum of the very massive binary system WR 20a (WN6ha + WN6ha). The most prominent emission lines, Hα and He  λ 4686, display strong phase-locked profile variability. From the variations of their equivalent widths and from a tomographic analysis, we find that part of the line emission probably arises in a wind interaction region between the stars. Our analysis of the optical spectrum of WR 20a indicates a reddening of A V 6.0 mag and a distance of ∼7.9 kpc, suggesting that the star actually belongs to the open cluster Westerlund 2. The location of the system at ∼1.1 pc from the cluster core could indicate that WR 20a was gently ejected from the core via dynamical interactions. Using a non-LTE model atmosphere code, we derive the fundamental parameters of each component: T eff = 43 000 ± 2000 K, log L bol /L 6.0,Ṁ = 8.5 × 10 −6 M yr −1 (assuming a clumped wind with a volume filling factor f = 0.1). Nitrogen is enhanced in the atmospheres of the components of WR 20a, while carbon is definitely depleted. Finally, the position of the binary components in the Hertzsprung-Russell diagram suggests that they are core hydrogen burning stars in a pre-LBV stage and their current atmospheric chemical composition probably results from rotational mixing that might be enhanced in a close binary compared to a single star of same age.

Abstract.We analyse spectroscopic observations of WR 20a revealing that this star is a massive early-type binary system with a most probable orbital period of ∼3.675 days. Our spectra indicate that both components are most likely of WN6ha or O3If * /WN6ha spectral type. The orbital solution for a period of 3.675 days yields extremely large minimum masses of 70.7 ± 4.0 and 68.8± 3.8 M for the two stars. These properties make WR 20a a cornerstone system for the study of massive star evolution.