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

Abstract

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 $\log L_{\mathrm{X}}-\log L_{\mathrm{bol}}=-6.912\pm 0.153$. 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 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]

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 limitations

References in corpus (1)

• An XMM-Newton view of the young open cluster NGC 6231 -- I. The catalogue

Cited by in corpus (53)

• Mass loss from hot massive stars
• X-Ray Spectroscopy of Stars
• The young stellar population in M17 revealed by Chandra
• A Chandra Study of the Rosette Star-Forming Complex. I. The Stellar Population and Structure of the Young Open Cluster NGC 2244
• The R136 star cluster dissected with Hubble Space Telescope/STIS. III. The most massive stars and their clumped winds
• Non-thermal emission processes in massive binaries
• The Tarantula Massive Binary Monitoring VI: Characterisation of hidden companions in 51 single-lined O-type binaries, a flat mass-ratio distribution, and black-hole binary candidates
• No breakdown of the radiatively-driven wind theory in low-metallicity environments
• XMM-Newton X-ray study of early type stars in the Carina OB1 association
• A spectroscopic investigation of the O-type star population in four Cygnus OB associations. II. Determination of the fundamental parameters
• NLTE models of line-driven stellar winds III. Influence of X-ray radiation on wind structure of O stars
• HD148937: a multiwavelength study of the third Galactic member of the Of?p class
• Chandra monitoring of the very massive binary WR20a and the young massive cluster Westerlund2
• A multiwavelength investigation of the massive eclipsing binary Cyg OB2 #5
• Three discoveries of gamma Cas analogs from dedicated XMM-Newton observations of Be stars
• An XMM-Newton view of the young open cluster NGC 6231 III. Optically faint X-ray sources
• Binarity at LOw Metallicity (BLOeM): a spectroscopic VLT monitoring survey of massive stars in the SMC
• X-ray irradiation of the winds in binaries with massive components
• Massive stars in the Small Magellanic Cloud : Evolution, rotation and surface abundances
• Atmospheric NLTE models for the spectroscopic analysis of blue stars with winds. V. Complete comoving frame transfer, and updated modeling of X-ray emission
• Discovery of X-ray Emission from the Wolf-Rayet star WR142 of oxygen subtype
• Towards an understanding of the Of?p star HD191612: Phase-resolved multiwavelength observations
• The X-ray emission of Be+stripped star binaries
• Clumping and X-Rays in cooler B supergiant stars
• A Chandra X-ray study of the young star cluster NGC 6231: low-mass population and initial mass function
• X-ray properties of early-type stars in the Tarantula Nebula from T-ReX
• The Correlation between X-Ray Line Ionization and Optical Spectral Types of the OB Stars
• The IACOB project VIII. Searching for empirical signatures of binarity in fast-rotating O-type stars
• Sejong Open Cluster Survey (SOS). III. The Young Open Cluster NGC 1893 in the H II Region W8
• A new study on a type Iax stellar remnant and its probable association with SN 1181
• Phase-resolved XMM-Newton and swift observations of WR 25
• Wind properties of Milky Way and SMC massive stars: empirical Z dependence from CMFGEN models
• On the origin of close massive binaries in the M17 star-forming region
• X-ray diagnostics of massive star winds
• X-ray and optical spectroscopy of the massive young open cluster IC1805
• The Structure of the Young Star Cluster NGC 6231. I. Stellar Population
• Chasing the Identification of ASCA Galactic Objects (ChIcAGO) - An X-ray Survey of Unidentified Sources in the Galactic Plane I: Source Sample and Initial Results
• Constraints on the multiplicity of the most massive stars known: R136 a1, a2, a3, and c
• Multi-frequency study of DEM L299 in the Large Magellanic Cloud
• Resolving the dynamical mass tension of the massive binary 9 Sagittarii
• The multiplicity of massive stars in the Scorpius OB1 association through high-contrast imaging
• A VLT/FLAMES survey for massive binaries in Westerlund 1: VI. Properties of X-ray bright massive cluster members
• Chandra observations of the planetary nebula IC 4593
• X-ray emission of massive stars and their winds
• An interferometric study of B star multiplicity
• The First Candidate Colliding-Wind Binary in M33
• X-Shooting ULLYSES: massive stars at low metallicity: XII. The clumped winds of O-type (super)giants in the Large Magellanic Cloud
• LIFELINE: The program for the simulation of the X-ray line profiles in massive colliding wind binaries
• Searching for compact objects in the single-lined spectroscopic binaries of the young Galactic cluster NGC 6231
• Fundamental parameters of the massive eclipsing binary HM1 8
• A Deep Chandra Observation of the Giant HII Region N11 I. X-ray Sources in the Field
• The observed multiplicity properties of B-type stars in the Galactic young open cluster NGC 6231
• Tentative insight into the multiplicity of the persistent dust maker WR106 from X-ray observations