Modelling of Sigma Scorpii, a high-mass binary with a Beta Cep variable primary component

A. Tkachenko (1), C. Aerts (1,2), K. Pavlovski (3), P. Degroote (1), P. I. Papics (1), E. Moravveji (1), H. Lehmann (4), V. Kolbas (3), and K. Clemer (1)

(1) Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium; (2) Department of Astrophysics, IMAPP, Radboud University Nijmegen, 6500 GL Nijmegen, The Netherlands; (3) Department of Physics, University of Zagreb, Bijenicka cesta 32, 10000 Zagreb, Croatia; (4) Thueringer Landessternwarte Tautenburg, 07778 Tautenburg, Germany

High-mass binary stars are known to show an unexplained discrepancy between the dynamical masses of the individual components and those predicted by models. In this work, we study Sigma Scorpii, a double-lined spectroscopic binary system consisting of two B-type stars residing in an eccentric orbit. The more massive primary component is a Beta Cep-type pulsating variable star. Our analysis is based on a time-series of some 1000 high resolution spectra collected with the CORALIE spectrograph in 2006, 2007, and 2008. We use two different approaches to determine the orbital parameters of the star; the spectral disentangling technique is used to separate the spectral contributions of the individual components in the composite spectra. The non-LTE based spectrum analysis of the disentangled spectra reveals two stars of similar spectral type and atmospheric chemical composition. Combined with the orbital inclination angle estimate found in the literature, our orbital elements allow a mass estimate of 14.7 +/- 4.5 and 9.5 +/- 2.9 solar masses for the primary and secondary component, respectively. The primary component is found to pulsate in three independent modes, of which two are identified as fundamental and second overtone radial modes, while the third is an l = 1 non-radial mode. Seismic modelling of the pulsating component refines stellar parameters to 13.5 +0.5/-1.4 and 8.7 +0.6/-1.2 solar masses , and delivers radii of 8.95 +0.43/-0.66 and 3.90 +0.58/-0.36 solar radii for the primary and secondary, respectively. The age of the system is estimated to be ~12 Myr.

Reference: Monthly Notices of the Royal Astronomical Society (MNRAS)
Status: Manuscript has been accepted