Non-synchronous rotations in massive binary systems

II. The case of HD 96264A^⋆

¹ Instituto de Astrofísica de La Plata, CONICET–UNLP, Paseo del Bosque s/n, B1900FWA La Plata, Argentina
e-mail: cputkuri@fcaglp.unlp.edu.ar
² Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, La Plata, Argentina
³ Las Campanas Observatory, Carnegie Observatories, Casilla 601, La Serena, Chile
⁴ Departamento de Astronomía, Universidad de La Serena, Av. Cisternas 1200 Norte, La Serena, Chile

Received: 23 November 2020
Accepted: 6 March 2021

Abstract

Context. The OWN Survey has detected several O-type stars with composite spectra whose individual components show very different line broadening. Some of these stars have been revealed as binary systems whose components are asynchronous. This fact may be related to the processes acting in these systems (e.g., angular-momentum transfer, tidal forces, etc.) or to the origin of the binaries themselves.

Aims. We aim to determine the orbital and physical parameters of the massive star HD 96264A in order to confirm its binary nature and to constrain the evolutionary status of its stellar components.

Methods. We computed the spectroscopic orbit of the system based on the radial velocity analysis of 37 high-resolution, high-S/N, multi-epoch optical spectra. We disentangled the composite spectrum and determined the physical properties of the individual stellar components using FASTWIND models incorporated to the IACOB-GBAT tool. We also computed a set of evolutionary models to estimate the age of the system and explore its tidal evolution.

Results. HD 96264A is a binary system composed of an O9.2 IV primary and a B0 V(n) secondary, with minimum masses of 15.0 ± 0.5 M_⊙ and 9.9 ± 0.4 M_⊙, respectively, in a wide and eccentric orbit (P = 124.336 ± 0.008 d; e = 0.265 ± 0.005). The primary and secondary components have different projected rotational velocities (∼40 and ∼215 km s⁻¹ respectively), and the physical properties derived through quantitative spectroscopic analyses include masses of ∼20.5 M_⊙ and 16.8 M_⊙, respectively. The evolutionary models indicate an approximate age of 4.5 Myr for both stars in the pair, corresponding to current masses and radii of 26.0 M_⊙ and 10.8 R_⊙ for the primary, and 17.9 M_⊙ and 7.0 R_⊙ for the secondary.

Conclusions. The youth and wide orbit of the system indicate that the non-synchronous rotational nature of its components is a consequence of the stellar formation process rather than tidal evolution. This circumstance should be accounted for in theories of binary star formation.