Spectroscopic searches for evolutionary orbital period changes in WR+OB binaries: The case of WR 127 (Hen 3-1772)

Abstract

Aims. We aim to determine the secular evolution of the orbital period of the short-period binary system WR 127 (WN3b+O9.5V, P ≈ 9.555d). Methods. We performed new low-resolution spectroscopic observations of WR 127 with the 2.5 m CMO SAI telescope to construct the radial velocity curves of the components. Our results suggest component masses of MWRsin3(i) = 11.8 ± 1.4 M⊙ and MOsin3(i) = 17.2 ± 1.4 M⊙. By comparing these values with archival radial velocity curves we were able to create an (O − C) plot with an accuracy sufficient to search for the orbital period change in WR 127. Results. We report the reliable detection of a secular increase in the orbital period of WR 127 at a rate of Ṗ = 0.83 ± 0.14 s yr−1, which corresponds to a dynamical mass-loss rate from the Wolf-Rayet (WR) star of ĖWR = (2.6 ± 0.5) × 10−5 M⊙ yr−1. Conclusions. The mass-loss rate from WR stars in three Wolf-Rayet+OB binaries (WR 127, CX Cep, and V444 Cyg) as inferred from spectroscopic and photometric measurements suggests a preliminary empirical correlation between a WR star’s mass and its dynamical mass-loss rate of ṀWR ∼ MWR1.8. This relation is important for the understanding of the evolution of massive close binaries that include WR stars as such an evolution is a precursor of gravitational-wave binary merging events with neutron stars and black holes.