Wind clumping and the wind-wind collision zone in the Wolf-Rayet binary γ² Velorum ^*

XMM-Newton observations at high and low state

¹ Institut für Astronomie, ETH-Zentrum, 8092 Zürich, Switzerland e-mail: hschild@astro.phys.ethz.ch
² Paul Scherrer Institut, Würenlingen & Villigen, 5232 Villigen PSI, Switzerland
³ SRON National Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands e-mail: [a.j.j.raassen;k.a.van.der.hucht;r.mewe]@sron.nl
⁴ Physikalisch-Meteorologisches Observatorium Davos, Dorfstrasse 33, 7260 Davos Dorf, Switzerland e-mail: w.schmutz@pmodwrc.ch
⁵ Astronomical Institute “Anton Pannekoek”, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands e-mail: raassen@science.uva.nl
⁶ Columbia Astrophysics Laboratory, Columbia University, 550 West 120th Street, New York, NY 10027, USA e-mail: [audard;maurice]@astro.columbia.edu
⁷ Center for Astrophysics and Space Astronomy, University of Colorado, Campus Box 389, Boulder, CO 80309-0389, USA e-mail: skinner@origins.colorado.edu

Corresponding author: M. Güdel, guedel@astro.phys.ethz.ch

Received: 8 January 2004
Accepted: 13 April 2004

Abstract

We present XMM-Newton observations of γ² Velorum (WR 11, WC8+O7.5III, P = 78.53 d), a nearby Wolf-Rayet binary system, at its X-ray high and low states. At high state, emission from a hot collisional plasma dominates from about 1 to 8 keV. At low state, photons between 1 and 4 keV are absorbed. The hot plasma is identified with the shock zone between the winds of the primary Wolf-Rayet star and the secondary O giant. The absorption at low state is interpreted as photoelectric absorption in the Wolf-Rayet wind. This absorption allows us to measure the absorbing column density and to derive a mass loss rate  = 8  10^-6  yr^-1 for the WC8 star. This mass loss rate, in conjunction with a previous Wolf-Rayet wind model, provides evidence for a clumped WR wind. A clumping factor of 16 is required. The X-ray spectra below 1 keV (12 Å) show no absorption and are essentially similar in both states. There is a rather clear separation in that emission from a plasma hotter than 5 MK is heavily absorbed in low state while the cooler plasma is not. This cool plasma must come from a much more extended region than the hot material. The Neon abundance in the X-ray emitting material is 2.5 times the solar value. The unexpected detection of C v (25.3 Å) and C vi (31.6 Å) radiative recombination continua at both phases indicates the presence of a cool (~40 000 K) recombination region located far out in the binary system.