Time-series spectropolarimetry of the short-period Wolf-Rayet+O star binary CQ Cephei

Astronomy and Astrophysics – Astrophysics

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Wolf-Rayet Stars, Time Series Analysis, O Stars, Stellar Spectra, Astronomical Polarimetry, Stellar Mass Ejection, Binary Stars, Absorption Spectra, Early Stars

Scientific paper

We present time-resolved, very precise linear spectropolarimetry of the short-period WR+O star binary CQ Cephei. We find that the N IV 4058 A line is the best spectral diagnostic of the WR orbital motion, and we derive a semiamplitude K(WR) = 290 +/- 1 km/s and a systemic velocity gamma = -72 +/- 1 km/s. We identify an antiphase spectral feature that we attribute to He I 4143 A absorption by the O star. A semiamplitude of 360 +/- 18 km/s is found for the O star, giving a mass ratio (WR:O) of 1.24 +/- 0.06. Spectropolarimetric measurements, combined with published photopolarimetry, enable a determination of the orbital inclination. The emission-line polarization vectors allow us to separate the intrinsic and interstellar polarizations, and we find that the polarization attributable to scattering of the WR radiation in the expanding wind is small but may be increasing towards the UV. We note that the inclinations derived from photometric analyses of eclipsing WR+O binaries are systematically lower than those computed from polarimetric measurements. The light curves from Hiltner are analyzed with LIGHT2 to confirm that the system is in a near-contact state. The resultant masses and astrophysical parameters show a normal main-sequence O star with a WR star that has parameters typical of the WN 6 type. The distance modulus for CQ Cep derived from our results is (V0 - MV) = 12.0.

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