An exceptional X-ray view of the young open cluster NGC 6231: what XMM-Newton has taught us

Considered as the core of the Sco OB1 association, the young open cluster NGC 6231 harbours a rich O-type star population. In 2001, the XMM-Newton satellite targeted the cluster for a nominal duration of about 180 ks. Thanks to the detector sensitivity, the EPIC cameras provided an unprecedented X-ray view of NGC 6231, revealing about 600 point-like sources. In this contribution, we review the main results that have been obtained thanks to this unprecedented data set. Concerning the O-type stars, we present the latest developments related to the so-called 'canonical' Lx-Lbol relation. The dispersion around this relation might actually be much smaller than previously thought. In our data set, the sole mechanism that yields a significant deviation from this scheme is wind interaction. It is also the sole mechanism that induces a significant variation of the early-type star X-ray flux. In a second part of this contribution, we probe the properties of the optically faint X-ray sources. Most of them are believed to be low mass pre-main sequence stars. Their analysis provides direct insight into the star formation history of the cluster.Comment: 6 pages, 5 figures, to appear in "The X-Ray Universe 2005", ESA Symposium held at El Escorial, Madrid (Spain), 26-30 Sep 200

The massive star binary fraction in young open clusters I. NGC 6231 revisited

We present the results of a long-term high-resolution spectroscopy campaign on the O-type stars in NGC 6231. We revise the spectral classification and multiplicity of these objects and we constrain the fundamental properties of the O-star population. Almost three quarters of the O-type stars in the cluster are members of a binary system. The minimum binary fraction is 0.63, with half the O-type binaries having an orbital period of the order of a few days. The eccentricities of all the short-period binaries are revised downward, and henceforth match a normal period-eccentricity distribution. The mass-ratio distribution shows a large preference for O+OB binaries, ruling out the possibility that, in NGC 6231, the companion of an O-type star is randomly drawn from a standard IMF. Obtained from a complete and homogeneous population of O-type stars, our conclusions provide interesting observational constraints to be confronted with the formation and early-evolution theories of O stars.Comment: 16 pages, 14 figures. Accepted by MNRA

The Struve-Sahade effect in the optical spectra of O-type binaries I. Main-sequence systems

We present a spectroscopic analysis of four massive binary systems that are known or are good candidates to display the Struve-Sahade effect (defined as the apparent strengthening of the secondary spectrum of the binary when the star is approaching, and the corresponding weakening of the lines when it is receding). We use high resolution optical spectra to determine new orbital solutions and spectral types of HD 165052, HD 100213, HD 159176 and DH Cep. As good knowledge of the fundamental parameters of the considered systems is necessary to examine the Struve-Sahade effect. We then study equivalent width variations in the lines of both components of these binaries during their orbital cycle. In the case of these four systems, variations appear in the equivalent widths of some lines during the orbital cycle, but the definition given above can any longer be valid, since it is now clear that the effect modifies the primary spectrum as much as the secondary spectrum. Furthermore, the lines affected, and the way in which they are affected, depend on the considered system. For at least two of them (HD 100213 and HD 159176) these variations probably reflect the ellipsoidal variable nature of the system.Comment: 12 pages, 20 figures, in press A&

An XMM-Newton view of the young open cluster NGC 6231 -- II. The OB star population

In this second paper, 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% 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 $\log L_\mathrm{X} - \log L_\mathrm{bol}=-6.912\pm0.153$. 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 our detection threshold and within our 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. Our analysis points towards the detected emission being associated with a physical PMS companion >... [see paper for the complete abstract]Comment: 21 pages, 14 figures, Table 2 and Figs 2 to 5 will be available through the CDS only, accepted for publication by MNRAS, Fig 1 not included in the present preprint because of size limitation

The rotation rates of massive stars: the role of binary interaction through tides, mass transfer and mergers

Rotation is thought to be a major factor in the evolution of massive stars, especially at low metallicity, with consequences for their chemical yields, ionizing flux and final fate. Determining the natal rotation-rate distribution of stars is of high priority given its importance as a constraint on theories of massive star formation and as input for models of stellar populations in the local Universe and at high redshift. Recently, it has become clear that the majority of massive stars interact with a binary companion before they die. We investigate how this affects the distribution of rotation rates. For this purpose, we simulate a massive binary-star population typical for our Galaxy assuming continuous star formation. We find that, because of binary interaction, 20^+5_-10% of all massive main-sequence stars have projected rotational velocities in excess of 200km/s. We evaluate the effect of uncertain input distributions and physical processes and conclude that the main uncertainties are the mass transfer efficiency and the possible effect of magnetic braking, especially if magnetic fields are generated or amplified during mass accretion and stellar mergers. The fraction of rapid rotators we derive is similar to that observed. If indeed mass transfer and mergers are the main cause for rapid rotation in massive stars, little room remains for rapidly rotating stars that are born single. This implies that spin down during star formation is even more efficient than previously thought. In addition, this raises questions about the interpretation of the surface abundances of rapidly rotating stars as evidence for rotational mixing. Furthermore, our results allow for the possibility that all early-type Be stars result from binary interactions and suggest that evidence for rotation in explosions, such as long gamma-ray bursts, points to a binary origin.Comment: 14 pages, 5 figures, accepted for publication in ApJ., no changes with v1 apart from fixed typos/ref

The investigation of particle acceleration in colliding-wind massive binaries with SIMBOL-X

An increasing number of early-type (O and Wolf-Rayet) colliding wind binaries (CWBs) is known to accelerate particles up to relativistic energies. In this context, non-thermal emission processes such as inverse Compton (IC) scattering are expected to produce a high energy spectrum, in addition to the strong thermal emission from the shock-heated plasma. SIMBOL-X will be the ideal observatory to investigate the hard X-ray spectrum (above 10 keV) of these systems, i.e. where it is no longer dominated by the thermal emission. Such observations are strongly needed to constrain the models aimed at understanding the physics of particle acceleration in CWB. Such systems are important laboratories for investigating the underlying physics of particle acceleration at high Mach number shocks, and probe a different region of parameter space than studies of supernova remnants.Comment: 2 pages, 2 figures, to appear in the proceedings of the workshop "Simbol-X: the hard X-ray universe in focus", held in Bologna, Italy (14-16 May 2007

Evidence for a physically bound third component in HD 150136

Context. HD150136 is one of the nearest systems harbouring an O3 star. Although this system was for a long time considered as binary, more recent investigations have suggested the possible existence of a third component. Aims. We present a detailed analysis of HD 150136 to confirm the triple nature of this system. In addition, we investigate the physical properties of the individual components of this system. Methods. We analysed high-resolution, high signal-to-noise data collected through multi-epoch runs spread over ten years. We applied a disentangling program to refine the radial velocities and to obtain the individual spectra of each star. With the radial velocities, we computed the orbital solution of the inner system, and we describe the main properties of the orbit of the outer star such as the preliminary mass ratio, the eccentricity, and the orbital-period range. With the individual spectra, we determined the stellar parameters of each star by means of the CMFGEN atmosphere code. Results. We offer clear evidence that HD 150136 is a triple system composed of an O3V((f\ast))-3.5V((f+)), an O5.5-6V((f)), and an O6.5-7V((f)) star. The three stars are between 0-3 Myr old. We derive dynamical masses of about 64, 40, and 35 Msun for the primary, the secondary and the third components by assuming an inclination of 49{\deg}. It currently corresponds to one of the most massive systems in our galaxy. The third star moves with a period in the range of 2950 to 5500 d on an outer orbit with an eccentricity of at least 0.3. This discovery makes HD 150136 the first confirmed triple system with an O3 primary star. However, because of the long orbital period, our dataset is not sufficient to constrain the orbital solution of the tertiary component with high accuracy.Comment: 13 pages, 11 figures, accepted at A&

The Struve-Sahade effect in the optical spectra of O-type binaries I. Main-sequence systems

We present a spectroscopic analysis of four massive binary systems that are known or are good candidates to display the Struve-Sahade effect (defined as the apparent strengthening of the secondary spectrum of the binary when the star is approaching, and the corresponding weakening of the lines when it is receding). We use high resolution optical spectra to determine new orbital solutions and spectral types of HD 165052, HD 100213, HD 159176 and DH Cep. As good knowledge of the fundamental parameters of the considered systems is necessary to examine the Struve-Sahade effect. We then study equivalent width variations in the lines of both components of these binaries during their orbital cycle. In the case of these four systems, variations appear in the equivalent widths of some lines during the orbital cycle, but the definition given above can any longer be valid, since it is now clear that the effect modifies the primary spectrum as much as the secondary spectrum. Furthermore, the lines affected, and the way in which they are affected, depend on the considered system. For at least two of them (HD 100213 and HD 159176) these variations probably reflect the ellipsoidal variable nature of the system.Comment: 12 pages, 20 figures, in press A&

A new investigation of the binary HD 48099

With an orbital period of about 3.078 days, the double-lined spectroscopic binary HD 48099 is, still now, the only short-period O+O system known in the Mon OB2 association. Even though an orbital solution has already been derived for this system, few information are available about the individual stars. We present, in this paper, the results of a long-term spectroscopic campaign. We derive a new orbital solution and apply a disentangling method to recover the mean spectrum of each star. To improve our knowledge concerning both components, we determine their spectral classifications and their projected rotational velocities. We also constrain the main stellar parameters of both stars by using the CMFGEN atmosphere code and provide the wind properties for the primary star through the study of IUE spectra. This investigation reveals that HD 48099 is an O5.5 V((f))+O9 V binary with M_1 sin^3 i = 0.70 M_{\sun} and M_2 sin^3 i = 0.39 M_{\sun}, implying a rather low orbital inclination. This result, combined with both a large effective temperature and log g, suggests that the primary star (v sini ~ 91 km s^-1) is actually a fast rotator with a strongly clumped wind and a nitrogen abundance of about 8 times the solar value.Comment: 12 pages, 7 figures, accepted by Ap