AnXMM-Newtonlook at the Wolf-Rayet star WR 40

Abstract

Abstract. We present the results of an XMM-Newton observation of the field of the Wolf-Rayet star WR 40. Despite a nominal exposure of 20 ks and the high sensitivity of the satellite, the star itself is not detected: we thus derive an upper limit on its X-ray flux and luminosity. Joining this result to recent reports of a non-detection of some WC stars, we suggest that the X-ray emission from single normal Wolf-Rayet stars could often be insignificant despite remarkable instabilities in the wind. On the basis of a simple modelling of the opacity of the Wolf-Rayet wind of WR 40, we show that any X-ray emission generated in the particular zone where the shocks are supposed to be numerous will indeed have little chance to emerge from the dense wind of the Wolf-Rayet star. We also report the non-detection of the ejecta nebula RCW 58 surrounding WR 40. Concerning the field around these objects, we detected 33 X-ray sources, most of them previously unknown: we establish a catalog of these sources and cross-correlate it with catalogs of optical/infrared sources.

Absence of hot gas within the Wolf-Rayet bubble around WR 16

“…(b) For NGC 6888 this is the semi-major axis. (c) Column density taken from Toalá et al (2012) for S 308, Gosset et al (2005) for RCW 58, and Zhekov & Park (2011) for NGC 6888. The observed X-ray surface brightness and luminosity of these sources have been scaled from these references to the distances used here and extended to the 0.3-1.5 keV energy range in all cases for consistency.…”

“…To put this non-detection in context with previous observations of WR nebulae, we show the stellar and nebular parameters (distance, spectral type of the WR star, stellar wind velocity, nebular radius, observed X-ray surface brightness, and luminosity) for the four WR nebulae observed to date in Table 1 by the present generations of X-ray satellites, namely S 308, NGC 6888, RCW 58, and that around WR 16. The surface brightness and luminosity of RCW 58 in the 0.3-1.5 keV energy range have been computed from the count rate reported by Gosset et al (2005) for a distance of 2.26 kpc (van der Hucht 2001) after adopting the same plasma temperature as for the WR nebula around WR 16. The observed surface brightness presented in Table 1, which is obtained by dividing the observed X-ray flux by the area sustained by the nebula, provides a distance independent measurement of the relative X-ray brightness of each nebula.…”

“…The temperature and abundances of the dominant X-ray-emitting plasma can be explained by means of mixing between the outer cold material and the shocked fast stellar wind. On the other hand, RCW 58 has not been detected by XMM-Newton (Gosset et al 2005). The cause of this non-detection of hot gas may be twofold.…”

“…X-ray emission from these objects is expected to arise from shock-heated plasma that is produced when the WR stellar wind rams into material previously ejected during the red supergiant (RSG) or luminous blue variable (LBV) phase (García-Segura & Mac Low 1995). To date, there are only three WR nebulae reported in the literature that had been observed with the latest generation of X-ray satellites (Chandra, Suzaku, and XMM-Newton) : S 308, RCW 58, andNGC 6888 around WR 6, WR 40, andWR 136, respectively (Chu et al 2003, 2006;Gosset et al 2005;Zhekov & Park 2011;Toalá et al 2012).…”

The spectrum of the very massive binary system WR 20a (WN6ha + WN6ha): Fundamental parameters and wind interactions

“…This feature is not unusual for Wolf-Rayet stars: extra X-ray absorption is produced by the material in the stellar wind and is observed for instance in the case of the WN6ha star WR 25 (Raassen et al 2003). Even worse, in the case of the WN8 star WR 40, the wind absorption is probably so large that no X-ray emission is actually escaping (Gosset et al 2005). The best-fit plasma temperature found by Tsujimoto et al (2004) is rather high for early-type stars, which could be a signature of the colliding wind interaction.…”