AnXMM-Newtonview of the young open cluster NGC 6231 - II. The OB star population

Abstract

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).

Analysis of Galactic late-type O dwarfs: more constraints on the weak wind problem

“…In this paper, we use a fixed shock temperature of 3 × 10 6 K and the filling factor is chosen to keep log L x /L Bol close to −7.0 (within ±0.1). We recall that a log L x /L Bol of about −7.0 is the typical value observed (see e.g., Sana et al 2006). For HD 326329 however, we used models with a higher ratio, in accord with the observations (log L X /L Bol ∼ −6.5; Sana et al 2006).…”

“…For HD 216898, HD 66788, and HD 216532, we have fixed log L X /L Bol at the canonical value of ∼ − 7.0. For HD 326329 and ζ Oph, we chose L X /L Bol ratios close to those recently observed (Oskinova 2005;Oskinova et al 2006;Sana et al 2006). Overall, the stellar properties of our sample are quite homogeneous.…”

X-ray emission from hydrodynamical simulations in non-LTE wind models

“…However, this is not supported by observations. The L X − L relation of low luminosity stars is either the same as that of high luminosity stars, or, for B stars, the slope is even less steep (Sana et al 2006). The reason for this discrepancy between observation and theory is unclear.…”

“…By doing this, our list may be considered as representative of a diverse population of O stars found in real star clusters. This allows us to compare the trend in Xray luminosity of our synthetic O star population with the real clusters recently observed by Sana et al (2006) and Antokhin et al (2008).…”

“…The fact that a substantial fraction of the wind is too hot to give a significant signature in the ultraviolet spectrum of the star could be an explanation for the weak-wind problem (Krtička & Kubát 2009). Sana et al (2006) and Antokhin et al (2008) divide the X-rays into three energy bands, soft (0.5−1.0 keV), medium (1.0−2.5 keV), and hard (2.5−10.0 keV). The observations show that the slope of the L X − L relation is roughly the same in the soft and medium band as in the total X-ray emission, whereas the observations in the hard band show a large dispersion.…”