A spectroscopic multiplicity survey of Galactic Wolf-Rayet stars

Abstract

Context. Massive stars are powerful cosmic engines that have a huge impact on their surroundings and host galaxies. The majority of massive stars will interact with a companion star during their evolution. The effects of this interaction on their end-of-life products are currently poorly constrained. In the phases immediately preceding core-collapse, massive stars in the Galaxy with Mi ≳ 20 M⊙ may appear as classical Wolf-Rayet (WR) stars. The multiplicity properties of the WR population are thus required to further our understanding of stellar evolution at the upper-mass end. Aims. As the final contribution of a homogeneous radial velocity (RV) survey, this work aims to constrain the multiplicity properties of northern Galactic late-type nitrogen-rich Wolf-Rayet (WNL) stars. We compare their intrinsic binary fraction and orbital period distribution to the carbon-rich (WC) and early-type nitrogen-rich (WNE) populations from previous works. Methods. We obtained high-resolution spectra of the complete magnitude-limited sample of 11 Galactic WNL stars with the Mercator telescope on the island of La Palma. We used cross-correlation with a log-likelihood framework to measure relative RVs and flagged binary candidates based on the peak-to-peak RV dispersion. By using Monte Carlo sampling and a Bayesian framework, we computed the three-dimensional likelihood and one-dimensional posteriors for the upper period cut-off (log PmaxWNL), power-law index (πWNL), and intrinsic binary fraction (fintWNL). Results. Adopting a threshold C of 50 km s−1, we derived fobsWNL = 0.36 ± 0.15. Our Bayesian analysis produces fintWNL = 0.42−0.17+0.15, πWNL = −0.70−1.02+0.73 and log PmaxWNL = 4.90−3.40+0.09 for the parent WNL population. The combined analysis of the Galactic WN population results in fintWN = 0.52−0.12+0.14, πWN = −0.99−0.50+0.57 and log PmaxWN = 4.99−1.11+0.00. The observed period distribution of Galactic WN and WC binaries from the literature is in agreement with what is found. Conclusions. The period distribution of Galactic WN binaries peaks at P ∼ 1–10 d and that of the WC population at P ∼ 5000 d. This shift cannot be reconciled by orbital evolution due to mass loss or mass transfer. At long periods, the evolutionary sequence O (→LBV) → WN → WC seems feasible. The high frequency of short-period WN binaries compared to WC binaries suggests that they either tend to merge, or that the WN components in these binaries rarely evolve into WC stars in the Galaxy.