Spectroscopic Searches for Evolutionary Orbital Period Changes in WR+OB Binaries: the Case of WR 127 (Hen 3-1772)

Abstract

We aim at searching for secular evolution of the orbital period in the short-period binary system WR 127 (WN3b+O9.5V, $P = 9.555^d$). We performed new low-resolution spectroscopic observations of WR 127 on 2.5-m CMO SAI telescope to construct the radial velocity curves of the components suggesting the component masses $M_\mathrm{WR}\sin^3(i) = 11.8\pm1.4$ $M_{\odot}$, $M_\mathrm{O}\sin^3(i)=17.2\pm1.4$ $M_{\odot}$. The comparison with archival radial velocity curves enabled us to calculate the $(O-C)$ plot with accuracy sufficient to search for the orbital period change in WR 127. We report on the reliable detection of a secular increase in the orbital period of WR 127 at a rate of $\dot{P} = 0.83\pm0.14~\mbox{s~yr}^{-1}$ corresponding to the dynamical mass-loss rate from the WR star $\dot{M}_\mathrm{WR} = (2.6\pm0.5)\times 10^{-5}$ $M_{\odot}$ yr$^{-1}$. The mass-loss rate from WR stars in three WR+OB binaries (WR 127, CX Cep and V444 Cyg) as inferred from spectroscopic and photometric measurements suggests a preliminary empirical correlation between the WR star mass and the dynamical mass-loss rate $\dot M_\mathrm{WR}\sim M_\mathrm{WR}^{1.8}$. This relation is important for the understanding of the evolution of massive close binaries with WR stars -- precursors of gravitational-wave binary merging events with neutron stars and black holes.