A spectroscopic multiplicity survey of Galactic Wolf-Rayet stars III. The northern late-type nitrogen-rich sample

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.