Modelling the asymmetric wind of the luminous blue variable binary MWC 314^⋆

¹ Royal Observatory of Belgium, Ringlaan 3, 1180 Brussels, Belgium
e-mail: alobel@sdf.org; Alex.Lobel@oma.be
² Geneva Observatory, Geneva University, Chemin des Maillettes 51, 1290 Sauverny, Switzerland
³ European Southern Observatory, 3107 Alonso de Cordova, Vitacura, Santiago, Chile
⁴ University of São João Del Rei, CAP, 36420-000 Ouro Branco, MG, Brazil
⁵ University of Leuven, Instituut voor Sterrenkunde, Celestijnenlaan 200 D, 3001 Heverlee, Belgium
⁶ Université Libre de Bruxelles, Boulevard du Triomphe, 1050 Brussels, Belgium
⁷ Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany

Received: 20 September 2012
Accepted: 17 July 2013

Abstract

Aims. We present a spectroscopic analysis of MWC 314, a luminous blue variable (LBV) candidate with an extended bipolar nebula. The detailed spectroscopic variability is investigated to determine if MWC 314 is a massive binary system with a supersonically accelerating wind or a low-mass B[e] star. We compare the spectrum and spectral energy distribution to other LBVs (such as P Cyg) and find very similar physical wind properties, indicating strong kinship.

Methods. We combined long-term high-resolution optical spectroscopic monitoring and V-band photometric observations to determine the orbital elements and stellar parameters and to investigate the spectral variability with the orbital phases. We developed an advanced model of the large-scale wind-velocity and wind-density structure with 3-D radiative transfer calculations that fit the orbitally modulated P Cyg profile of He i λ5876, showing outflow velocities above 1000 km   s^-1.

Results. We find that MWC 314 is a massive semi-detached binary system of ≃1.22 AU, observed at an inclination angle of i = 72.8° with an orbital period of 60.8 d and e = 0.23. The primary star is a low-vsini LBV candidate of m₁ = 39.6  M_⊙ and R₁ = 86.8  R_⊙. The detailed radiative transfer fits show that the geometry of wind density is asymmetric around the primary star with increased wind density by a factor of 3.3, which leads the orbit of the primary. The variable orientation causes the orbital modulation that is observed in absorption portions of P Cyg wind lines. Wind accretion in the system produces a circumbinary disc.

Conclusions. MWC 314 is in a crucial evolutionary phase of close binary systems, when the massive primary star has its H envelope being stripped and is losing mass to a circumbinary disc. MWC 314 is a key system for studying the evolutionary consequences of these effects.