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 M i ≳ 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 P max WNL ), power-law index ( π WNL ), and intrinsic binary fraction ( f int WNL ). Results. Adopting a threshold C of 50 km s −1 , we derived f obs WNL = 0.36 ± 0.15. Our Bayesian analysis produces f int WNL = 0.42 −0.17 +0.15 , π WNL = −0.70 −1.02 +0.73 and log P max WNL = 4.90 −3.40 +0.09 for the parent WNL population. The combined analysis of the Galactic WN population results in f int WN = 0.52 −0.12 +0.14 , π WN = −0.99 −0.50 +0.57 and log P max WN = 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.

Topics & Concepts