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.
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