An absence of binary companions to Wolf-Rayet stars in the Small Magellanic Cloud. Implications for mass loss and black hole masses at low metallicities

Abstract

To predict black hole mass distributions at high redshifts, we need to understand whether very massive single stars ($M 40$\,M$_ with low metallicities ($Z$) lose their hydrogen-rich envelopes, like their metal-rich counterparts, or whether a binary companion is required to achieve this. To test this, we undertook a deep spectroscopic search for binary companions of the seven known apparently single Wolf-Rayet (WR) stars in the Small Magellanic Cloud (SMC; where $Z 1/5\,Z_ v lines in these spectra, we monitored radial velocity (RV) variations to search for binary motion. We find low RV variations of between 6 and 23\,km/s for the seven WR stars, with a median standard deviation of $5$\,km/s. Our Monte Carlo simulations imply probabilities below sim 5<!PCT!> that any of our target WR stars have a binary companion more massive than sim 5\,M$_ with orbital periods of less than a year. We estimate that the probability that all our target WR stars have companions with orbital periods shorter than 10\,yr is below sim $ and argue that the observed modest RV variations may originate from intrinsic atmosphere or wind variability. Our findings imply that metal-poor massive stars born with $M 40$\,M$_ can lose most of their hydrogen-rich envelopes via stellar winds or eruptive mass loss, which strongly constrains their initial mass--black hole mass relation. We also identify two of our seven target stars (SMC\,AB1 and SMC\,AB11) as runaway stars with a peculiar RV of sim 80\,km/s. Moreover, with all five previously detected WR binaries in the SMC exhibiting orbital periods of less than 20\,d, a puzzling absence of intermediate-to-long-period WR binaries has emerged, with strong implications for the outcome of massive binary interactions at low metallicities.