Multi-frequency variations of the Wolf-Rayet system HD 193793 (WC7pd+O4-5)

III. IUE observations

¹ Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands e-mail: diah@astro.rug.nl
² Space Research Organization Netherlands, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
³ present address: Australia Telescope National Facility, PO Box 76, Epping, NSW 1710, Australia
⁴ Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK e-mail: p.m.williams@roe.ac.uk
⁵ Astronomical Institute , University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands e-mail: huib;lexk@astro.uva.nl
⁶ Space Science Department, Rutherford Appleton Observatory, Chilton, Didcot, Oxon OX11 OQX, UK e-mail: ds@astro1.bnsc.rl.ac.uk
⁷ ESA-IUE Observatory, VILSPA, PO Box 50727, 28080 Madrid, Spain e-mail: wwamstek@notes.vilspa.esa.es

Corresponding author: K. A. van der Hucht, k.a.van.der.hucht@sron.nl

Received: 20 February 2001
Accepted: 15 June 2001

Abstract

The colliding-wind binary system WR 140 (HD 193793, WC7pd+O4-5, P = 7.94 yr) was monitored in the ultraviolet by IUE from 1979 to 1994 in 35 short-wavelength high-resolution spectra. An absorption-line radial-velocity solution is obtained from the photospheric lines of the O component, by comparison with a single O star. The resulting orbital parameters, e = 0.87 ± 0.05, ω = 31° ± 9° and = 25 ± 15 km s^-1, confirm the large eccentricity of the orbit, within the uncertainties of previous optical studies. This brings the weighted mean UV-optical eccentricity to e = 0.85 ± 0.04. Occultation of the O-star light by the WC wind and the WC+O colliding-wind region results into orbital modulation of the P-Cygni profiles of the C ii, C iv and Si iv resonance lines. Near periastron passage, the absorption troughs of those resonance-line profiles increase abruptly in strength and width, followed by a gradual decrease. In particular, near periastron the blue black-edges of the P-Cygni absorption troughs shift to larger outflow velocities. We discuss that the apparently larger wind velocity and velocity dispersion observed at periastron could be explained by four phenomena: (i) geometrical resonance-line eclipse effects being the main cause of the observed UV spectral variability, enhanced by sightline crossing of the turbulent wind-wind collision zone; (ii) the possibility of an orbital-plane enhanced WC7 stellar wind; (iii) possible common-envelope acceleration by the combined WC and O stellar radiation fields; and (iv) possible enhanced radiatively driven mass loss due to tidal stresses, focused along the orbiting line of centers.