The conspicuous absence of X-ray emission from carbon-enriched Wolf-Rayet stars

¹ Astrophysik, Univerität Potsdam, Am Neuen Palais 10, 14469 Potsdam, Germany
² Department of Astronomy, University of Wisconsin, Madison, WI 53706, USA
³ ESA/Vilspa, Apartado 50727, 28080 Madrid, Spain
⁴ Department of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, UK

Corresponding author: L. M. Oskinova, lida@astro.physik.uni-potsdam.de

Received: 9 December 2003
Accepted: 21 February 2003

Abstract

The carbon-rich WC5 star WR 114 was not detected during a 15.9 ksec XMM-Newton  observation, implying an upper limit to the X-ray luminosity of L_X   erg s^-1 and to the X-ray to bolometric luminosity ratio of L_X /L_bol  . This confirms indications from earlier less sensitive measurements that there has been no convincing X-ray detection of any single WC star. This lack of detections is reinforced by XMM-Newton  and Chandra observations of WC stars. Thus the conclusion has to be drawn that the stars with radiatively-driven stellar winds of this particular class are insignificant X-ray sources. We attribute this to photoelectronic absorption by the stellar wind. The high opacity of the metal-rich and dense winds from WC stars puts the radius of optical depth unity at hundreds or thousands of stellar radii for much of the X-ray band. We believe that the essential absence of hot plasma so far out in the wind exacerbated by the large distances and correspondingly high ISM column densities makes the WC stars too faint to be detectable with current technology. The result also applies to many WC stars in binary systems, of which only about 20% are identified X-ray sources, presumably due to colliding winds.