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Optical Spectroscopy of Plaskett's Star

N. Linder
Université de Liège (Belgium)

G. Rauw
Université de Liège (Belgium)

F. Martins
Université de Montpellier (France)

H. Sana
ESO (Chile)

M. De Becker
Université de Liège (Belgium)

E. Gosset
Université de Liège (Belgium)

Plaskett's star (HD 47 129) is a very massive O + O binary that belongs to the Mon OB2 association. Previous work suggested that this system displays the Struve-Sahade effect although the results were quite controversial. We present here the analysis of an extensive set of high resolution optical spectra of HD 47 129. A disentangling method enables us to determine the individual spectra of each star and to derive a new orbital solution, with Mp sin³i = 45.4 ± 2.4 Msun and Ms sin³i = 47.3 ± 0.3 Msun. A Doppler tomography technique applied to the emission lines H alpha and He II 4686 yields a Doppler map that illustrates the wind interactions in the system, confirming our results from X-ray spectroscopy. An atmosphere code is used to determine the different chemical abundances of the system components and the wind parameters. HD 47 129 appears to be an O8 III/I + O7.5 III binary system in a post RLOF evolutionary stage and the chemical peculiarities indicate that the primary component is an evolved massive star, as already suggested by our X-ray data. Furthermore, the secondary star has a large rotational velocity (v sin i ~ 300 km/s) that deforms its surface, leading to a non-uniform distribution in effective temperature. This could explain the phase-locked variations in the equivalent widths of the secondary lines previously thought to be the signature of the Struve-Sahade effect. We suggest that the wind of the secondary star is confined near the equatorial plane because of its high rotational velocity, affecting the ram pressure equilibrium in the wind interaction zone.