Author:

Keywords:

Science & Technology, Physical Sciences, Astronomy & Astrophysics, stars: massive, binaries: spectroscopic, stars: Wolf-Rayet, Magellanic Clouds, stars: individual: SMC AB 6, stars: atmospheres, BLANKETED MODEL ATMOSPHERES, RADIATION-DRIVEN WINDS, UPPER MAIN-SEQUENCE, VLT-FLAMES SURVEY, MASS-LOSS RATES, LINED O STARS, COLLIDING WINDS, WN STARS, STELLAR WINDS, PHYSICAL-PROPERTIES, 0201 Astronomical and Space Sciences, 5101 Astronomical sciences, 5107 Particle and high energy physics, 5109 Space sciences

Abstract:

© ESO 2018 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.