Phase-resolvedXMM-Newtonobservations of the massive WR+O binary WR 22

Abstract

Aims. To better understand the phenomenon of colliding winds in massive binary stars, we study the X-ray lightcurve of a WR+O system of the Carina region, a system well known for the high mass of its primary. Methods. Phase-resolved X-ray observations of the massive WR+O binary system WR 22 were performed with the XMM-Newton facility. We observed the object at seven different phases from near apastron to near periastron. Results. The X-ray spectrum can be represented by a two-component, optically thin, thermal plasma model with a first one at a typical temperature of 0.6 keV and a second hotter one in the range 2.0-4.5 keV. The hot component is indicative of a colliding wind phenomenon, but its flux is remarkably constant with time despite the high eccentricity of the orbit. Although surprising at first, this actually does not contradict the results of the hydrodynamical simulations of the wind collision that we performed. When the system goes from apastron to periastron, the soft part of the X-ray flux is progressively lowered by an increasing intervening absorbing column. This behaviour can be interpreted in terms of an X-ray emitting plasma located near the O star, but not fully intrinsic to it, and accompanying the star when it dives into the wind of the WR component. A model is presented that interprets most of the observational constraints. This model suggests that the mass-loss rate ofṀ WR ∼ 1.6 × 10 −5 M yr −1 assumed for the WR could still be slightly too high, whereas it is already lower than other published values. From the comparison of the observed and the expected absorptions at phases near periastron, we deduce that the hard X-ray emitting collision zone should at least have a typical size of 50-60 R , but that the size for the soft X-ray emitting region could reach 244 R if the assumed mass-loss rate is correct. We also present an upper limit to the X-ray luminosity of the WR component that further questions the existence of intrinsic X-ray emission from single WN stars.

X-ray observations of central stars of planetary nebulae and their winds

“…The X-ray emission from the CSPNe of NGC 2392, NGC 6543, and NGC 6826 have plasma temperatures that are typical of single OB stars and whose X-ray to bolometric luminosity ratios are consistent with the canonical ∼10 −7 value for OB stars (Berghöfer et al 1997;Nazé 2009). Furthermore, the stellar winds of these CSPNe show evidence of variability of P Cygni profiles of UV high-excitation lines (Patriarchi & Perinotto 1995, 1997Guerrero & de Marco 2012, in prep.…”

The 2.35 year itch of Cygnus OB2 #9

“…Adiabatic systems should be those with long periods, like Cyg OB2 #9, but not many longperiod binaries have been monitored in X-rays, and results were mixed. γ 2 Vel and WR22 did not show any 1/D variation, despite the changing separation of their components (Rauw et al 2000b;Gosset et al 2009). Cyg OB2 #8A and WR140 deviate from expectations at periastron, since the collisions then become radiative (De Becker et al 2006;Pollock et al 2002;De Becker et al 2012).…”

“…2 in Antokhin et al 2003), though the results need confirmation, in view of the huge noise, and in HD 93403, though the intrinsic emission of the stars blurred the results, yielding a luminosity change lower than that expected from a simple 1/D relation (Rauw et al 2002). Among evolved systems, WR25 seems to display a rather "clean" 1/D variation in its luminosity (Gosset 2007;Güdel & Nazé 2009). For most of its orbit, it is also the case of η Car (Okazaki et al 2008), though the system shows large cycle-to-cycle variations in flux, and there are strong indications that the wind collision region collapse onto the companion star at periastron (Parkin et al 2011b, and references therein).…”

“…This is especially relevant for systems with two very different winds, e.g. WR+O: two spectacular cases in this respect are γ 2 Vel (Willis et al 1995) and WR22 (Gosset et al 2009). The similarity of the two components in Cyg OB2 #9 makes such variations less prominent; indeed, no significant change in absorption is detected in the data.…”

Colliding winds in low-mass binary star systems: wind interactions and implications for habitable planets

“…Since then, many detailed observational and theoretical studies of X-ray emission from WWC regions in CWBs have been carried out (e.g. Stevens et al 1996;Gosset et al 2009). …”