The shortest-period Wolf-Rayet binary in the Small Magellanic Cloud: Part of a high-order multiple system

Spectral and orbital analysis of SMC AB 6

¹ Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Str. 24/25, 14476 Potsdam, Germany
e-mail: shtomer@astro.physik.uni-potsdam.de
² Département de physique and Centre de Recherche en Astrophysique du Québec (CRAQ), Université de Montréal, C.P. 6128, Succ. Centre-Ville, Montréal, Québec H3C 3J7, Canada
³ Queen’s University 99 University Ave, Kingston, Ontario, Canada
⁴ American Association of Variable Star Observers, 49 Bay State Road, Cambridge, MA 02138, USA
⁵ Institute of Astrophysics, KU Leuven, Celestijnenlaan 200 D, 3001 Leuven, Belgium

Received: 12 March 2018
Accepted: 30 April 2018

Abstract

Context. SMC AB 6 is the shortest-period (P = 6.5 d) Wolf-Rayet (WR) binary in the Small Magellanic Cloud. This binary is therefore a key system in the study of binary interaction and formation of WR stars at low metallicity. The WR component in AB 6 was previously found to be very luminous (log L = 6.3 [L_⊙]) compared to its reported orbital mass (≈8 M_⊙), placing it significantly above the Eddington limit.

Aims. Through spectroscopy and orbital analysis of newly acquired optical data taken with the Ultraviolet and Visual Echelle Spectrograph (UVES), we aim to understand the peculiar results reported for this system and explore its evolutionary history.

Methods. We measured radial velocities via cross-correlation and performed a spectral analysis using the Potsdam Wolf-Rayet model atmosphere code. The evolution of the system was analyzed using the Binary Population and Spectral Synthesis evolution code.

Results. AB 6 contains at least four stars. The 6.5 d period WR binary comprises the WR primary (WN3:h, star A) and a rather rapidly rotating (v_eq = 265 km s⁻¹) early O-type companion (O5.5 V, star B). Static N III and N IV emission lines and absorption signatures in He lines suggest the presence of an early-type emission line star (O5.5 I(f), star C). Finally, narrow absorption lines portraying a long-term radial velocity variation show the existence of a fourth star (O7.5 V, star D). Star D appears to form a second 140 d period binary together with a fifth stellar member, which is a B-type dwarf or a black hole. It is not clear that these additional components are bound to the WR binary. We derive a mass ratio of M_O∕M_WR = 2.2 ± 0.1. The WR star is found to be less luminous than previously thought (log L = 5.9 [L_⊙]) and, adopting M_O = 41 M_⊙ for star B, more massive (M_WR = 18 M_⊙). Correspondingly, the WR star does not exceed the Eddington limit. We derive the initial masses of M_i,WR = 60 M_⊙ and M_i,O = 40 M_⊙ and an age of 3.9 Myr for the system. The WR binary likely experienced nonconservative mass transfer in the past supported by the relatively rapid rotation of star B.

Conclusions. Our study shows that AB 6 is a multiple – probably quintuple – system. This finding resolves the previously reported puzzle of the WR primary exceeding the Eddington limit and suggests that the WR star exchanged mass with its companion in the past.