Stellar modelling of Spica, a high-mass spectroscopic binary with a β Cep variable primary component

Abstract

Binary stars provide a valuable test of stellar structure and evolution, because the masses of the individual stellar components can be derived with high accuracy and in a model-independent way. In this work, we study Spica, an eccentric double-lined spectroscopic binary system with a β Cep type variable primary component. We use state-of-the-art modelling tools to determine accurate orbital elements of the binary system and atmospheric parameters of both stellar components. We interpret the short-period variability intrinsic to the primary component, detected on top of the orbital motion both in the photometric and spectroscopic data. The non-LTE based spectrum analysis reveals two stars of similar atmospheric chemical composition consistent with the present day cosmic abundance standard defined by Nieva & Przybilla (2012). The masses and radii of the stars are found to be 11.43±1.15 M ⊙ and 7.21±0.75 M ⊙ , and 7.47±0.54 R ⊙ and 3.74±0.53 R ⊙ for the primary and secondary, respectively. We find the primary component to pulsate in three independent modes, of which one is identified as a radial mode, while the two others are found to be non-radial, low degree l modes. The frequency of one of these modes is an exact multiple of the orbital frequency, and the l = m = 2 mode identification suggests a tidal nature for this particular mode. We find a very good agreement between the derived dynamical and evolutionary masses for the Spica system to within the observational errors of the measured masses. The age of the system is estimated to be 12.5±1 Myr.

Discovery of β Cep pulsations in the eclipsing binary V453 Cygni

“…However, there is often significant disagreement between the masses of O and B stars inferred from spectroscopy and stellar evolution theory, versus dynamical masses measurements in binary systems. This E-mail: astro.js@keele.ac.uk mass discrepancy was identified by Herrero et al (1992) and has been explained by invoking varying amounts of near-core mixing (Guinan et al 2000;Tkachenko et al 2014Tkachenko et al , 2016Tkachenko et al , 2020.…”

C60+– looking for the bucky-ball in interstellar space

“…As a divisor we used the UVES spectra of Spica (HD 116658). Recently, Tkachenko et al (2016) published an extensive study of this close binary system, which has an orbital period of ∼4 d , effective temperatures of 25,300±500 K and 20,900±500 K, logarithms of gravity 3.71±0.10 and 4.15±0.15, rotational velocities v sin i 165.3±4.5 km/s and 58.8±1.5 km/s, for the primary and secondary components respectively. When used for the removal of telluric lines, the complex and variable spectrum 1 http://gazinur.com/DECH-software.html of Spica may introduce unwanted distortion into relatively broad profiles, such as that of DIB9633, owing to the presence of stellar line(s).…”

A high-precision polarimeter for small telescopes

“…In figure 7 we show observations of Spica obtained with Mini-HIPPI that show clear periodic variability in polarization and position angle. The observations are plotted against phase on the oprbital cycle according to the ephemeris of Tkachenko et al (2016). Zero phase corresponds to the time of periastron passage.…”