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 $M_{\mathrm{WR}}{\sin }^3(i)=11.8\pm 1.4$ $M_{\odot }$, $M_{\mathrm{O}}{\sin }^3(i)=17.2\pm 1.4$ $M_{\odot }$. 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 ${\dot{M}}_{\mathrm{WR}}=(2.6\pm 0.5)\times 10^{-5}$ $M_{\odot }$ 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.

5 pages, 5 figures. Accepted for publication in Astronomy & Astrophysics Letters on March 4, 2024