2023A&A...674A..88D

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_{\mathrm{max}}^{\mathrm{WNL}} $), power-law index (π^WNL), and intrinsic binary fraction ($ f_{\mathrm{int}}^{\mathrm{WNL}} $).
Results. Adopting a threshold C of 50 km s^–1, we derived $ f_{\mathrm{obs}}^{\mathrm{WNL}} $ = 0.36 ± 0.15. Our Bayesian analysis produces $ f_{\mathrm{int}}^{\mathrm{WNL}} $$ \,=\,0.42^{+0.15}_{-0.17} $, π^WNL$ \,=\,-0.70^{+0.73}_{-1.02} $ and $ \log P_{\mathrm{max}}^{\mathrm{WNL}}=4.90^{+0.09}_{-3.40} $ for the parent WNL population. The combined analysis of the Galactic WN population results in $ f_{\mathrm{int}}^{\mathrm{WN}} $$ \,=\,0.52^{+0.14}_{-0.12} $, π^WN$ \,=\,-0.99^{+0.57}_{-0.50} $ and $ \log P_{\mathrm{max}}^{\mathrm{WN}} $$ \,=\,4.99^{+0.00}_{-1.11} $. 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.