An Extreme Case of a Misaligned Highly Flattened Wind in the Wolf-Rayet Binary CX Cephei

A. Villar-Sbaffi(1), N. St-Louis(1), Anthony F. J. Moffat(1) and Vilppu Piirola(2,3)

1- Département de Physique, Université de Montréal, C.P. 6128, Succursale Centre-Ville, Montréal, QC H3C 3J7, Canada; and Observatoire du Mont Mégantic.
2- Tuorla Observatory, University of Turku, FIN-21500, Piikkio, Finland.
3- Specola Vaticana, V-00120 Citta del Vaticano.

CX Cep (WR 151) is the WR+O binary (WN5+O5V) with the second shortest period known in our Galaxy. To examine the circumstellar matter distribution and to better constraint the orbital parameters and mass-loss rate of the WR star, we obtained broadband and multi-band (i.e. UBVRI) linear polarization observations of the system. Our analysis of the phase-locked polarimetric modulation confirms the high orbital inclination of the system (i.e. $i=65^o$). Using the orbital solution of Lewis et al. (1993) we obtain masses of $33.9 M_{\odot}$ and $23.9 M_{\odot}$ for the O and WR stars respectively, which agree with their spectral types. A simple polarimetric model accounting for finite stellar size effects allowed us to derive a mass-loss rate for the WR star of $0.3-0.5\times10^{-5} M_{\odot}/yr$. This result was remarkably independent of the model's input parameters and favors an earlier spectral type for the WR component (i.e. WN4). Finally, using our multi-band observations, we fitted and subtracted from our data the interstellar polarization. The resulting constant intrinsic polarization of $3-4\%$ is misaligned in relation to the orbital plane (i.e. $\theta_{CIP}=26^o$ vs. $\Omega=75^o$) and is the highest intrinsic polarization ever observed for a WR star. This misalignment points towards a rotational (or magnetic) origin for the asymmetry and contradicts the most recent evolutionary models for massive stars (Meynet \& Maeder 2003) which predict spherically symmetric winds during the WR phase (i.e. $CIP=0\%$).

Reference: Astrophysical Journal
Status: Manuscript has been submitted