Mass-loss rate determination for the massive binary V444 Cygni using 3-D Monte-Carlo simulations of line and polarization variability

Free access article

Issue		A&A Volume 388, Number 3, June IV 2002


Page(s)		957 - 977
Section		Stellar atmospheres
DOI		http://dx.doi.org/10.1051/0004-6361:20020443

A&A 388, 957-977 (2002)
DOI: 10.1051/0004-6361:20020443

Mass-loss rate determination for the massive binary V444 Cygni using 3-D Monte-Carlo simulations of line and polarization variability

R. Kurosawa¹, D. J. Hillier¹ and J. M. Pittard²

¹ Department of Physics and Astronomy, University of Pittsburgh, 3941 O'Hara Street, Pittsburgh, PA 15260, USA
² Department of Physics & Astronomy, The University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK

(Received 24 December 2001 / Accepted 22 March 2002)

Abstract
A newly developed 3-D Monte Carlo model is used, in conjunction with a multi-line non-LTE radiative transfer model, to determine the mass-loss rate of the Wolf-Rayet (W-R) star in the massive binary V444 Cyg (WN5+O6). This independent estimate of mass-loss rate is attained by fitting the observed $\ion{He}{i}$ 5876 Å and $\ion{He}{ii}$ 5412 Å line profiles, and the continuum light curves of three Stokes parameters $\left( I,\, Q,\, U\right)$ in the V band simultaneously. The high accuracy of our determination arises from the use of many observational constraints, and the sensitivity of the continuum polarization to the mass-loss rate. Our best fit model suggests that the mass-loss rate of the system is $\dot{M}_{{\rm WR}}=0.6\left( \pm 0.2\right) \times 10^{-5}\, M_{\odot }\, {\rm yr}^{-1}$ , and is independent of the assumed distance to V444 Cyg . The fits did not allow a unique value for the radius of the W-R star to be derived. The range of the volume filling factor for the W-R star atmosphere is estimated to be in the range of 0.050 (for $R_{{\rm WR}}=5.0\, R_{\odot}$ ) to 0.075 (for $R_{{\rm WR}}=2.5\, R_{\odot}$ ). We also found that the blue-side of $\ion{He}{i}$ 5876 Å and $\ion{He}{ii}$ 5412 Å lines at phase 0.8 is relatively unaffected by the emission from the wind-wind interaction zone and the absorption by the O-star atmosphere; hence, the profiles at this phase are suitable for spectral line fittings using a spherical radiative transfer model.

Key words: stars: mass-loss -- stars: individual: V444 Cygni -- polarization -- stars: binaries: eclipsing -- stars: Wolf-Rayet

Offprint request: R. Kurosawa, kurosawa@phyast.pitt.edu

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