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

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 He I 5875 Å and He II 5412 Å line profiles, and the continuum light curves of three Stokes parameters (I, Q, U) 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 ṀWR = 0.6 (±0.2) × 10-5 M⊙ 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 RWR = 5.0 R⊙) to 0.075 (for RWR = 2.5 R⊙). We also found that the blue-side of He I 5876 Å and 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.