Spectroscopic searches for evolutionary orbital period changes in WR+OB binaries: the case of V444 Cyg

Abstract

ABSTRACT We present the results of new photometric and spectroscopic observations of the WN5+O6 binary V444 Cyg and a detailed analysis of extant spectroscopy and photometry. Using elements of the spectroscopic orbit and assuming e ≈ 0, i ≈ 78° we determined the masses and orbit sizes of the components of V444 Cyg as MO6 ≈ 26.4 M⊙, MWN5 ≈ 10.7 M⊙, aO6 ≈ 10.6 R⊙, aWN5 ≈ 26.1 R⊙. Based on new and archival light curves and using the Hertzsprung method, we improved the photometric estimate of the secular increase rate of the orbital period in V444 Cyg, obtaining $\dot{P}_{\mathrm{ph}} = 0.119\pm 0.003$ s yr−1. From a comparison of the new and archival radial velocity curves of V444 Cyg, we independently derived the secular orbital period change rate as $\dot{P}_{\mathrm{sp}} = 0.147\pm 0.032$ s yr−1, in agreement with the photometric $\dot{P}_{\mathrm{ph}}$. The obtained secular increase rate of the binary orbital period $\dot{P}$ and the mean radii of the components enabled us to estimate the stellar wind mass-loss rate from the WR star as $\dot{M}_{\mathrm{WN5}} = -(6.0\pm 0.4)\times 10^{-6}~{\rm M}_{\odot }\mbox{ yr}^{-1}$.