An absence of binary companions to Wolf-Rayet stars in the Small Magellanic Cloud: implications for mass loss and black hole masses at low metallicity
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2024
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| _version_ | 1866929493730394112 |
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| author | Schootemeijer, A. Shenar, T. Langer, N. Grin, N. Sana, H. Schürmann, G. Gräfener C. Wang, C. Xu, X. -T. |
| author_facet | Schootemeijer, A. Shenar, T. Langer, N. Grin, N. Sana, H. Schürmann, G. Gräfener C. Wang, C. Xu, X. -T. |
| contents | In order to predict the black hole mass distributions at high redshift, we need to understand whether very massive single stars ($M>40$ M$_\odot$) at low metallicity $Z$ lose their hydrogen-rich envelopes, like their metal-rich counterparts, or whether a binary companion is required to achieve this. To test this, we undertake a deep spectroscopic search for binary companions of the seven apparently single Wolf-Rayet (WR) stars in the Small Magellanic Cloud (SMC; $Z \simeq 1/5 Z_\odot$). For each of them, we acquired six high-quality VLT-UVES spectra spread over 1.5 years. By using the narrow N V lines in these spectra, we monitor radial velocity (RV) variations to search for binary motion. We find low RV variations 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 ~5% for any of our target WR stars to have a binary companion more massive than ~5 M$_\odot$ at 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 ~10$^{-5}$, 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 $M \gtrsim 40$ M$_\odot$ 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 radial velocity of ~80 km/s. Moreover, with all five previously detected WR binaries in the SMC exhibiting orbital periods of below 20 d, a puzzling absence of intermediate-to-long-period WR binaries has emerged, with strong implications for the outcome of massive binary interaction at low metallicity. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2406_01420 |
| institution | arXiv |
| publishDate | 2024 |
| record_format | arxiv |
| spellingShingle | An absence of binary companions to Wolf-Rayet stars in the Small Magellanic Cloud: implications for mass loss and black hole masses at low metallicity Schootemeijer, A. Shenar, T. Langer, N. Grin, N. Sana, H. Schürmann, G. Gräfener C. Wang, C. Xu, X. -T. Solar and Stellar Astrophysics Astrophysics of Galaxies In order to predict the black hole mass distributions at high redshift, we need to understand whether very massive single stars ($M>40$ M$_\odot$) at low metallicity $Z$ lose their hydrogen-rich envelopes, like their metal-rich counterparts, or whether a binary companion is required to achieve this. To test this, we undertake a deep spectroscopic search for binary companions of the seven apparently single Wolf-Rayet (WR) stars in the Small Magellanic Cloud (SMC; $Z \simeq 1/5 Z_\odot$). For each of them, we acquired six high-quality VLT-UVES spectra spread over 1.5 years. By using the narrow N V lines in these spectra, we monitor radial velocity (RV) variations to search for binary motion. We find low RV variations 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 ~5% for any of our target WR stars to have a binary companion more massive than ~5 M$_\odot$ at 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 ~10$^{-5}$, 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 $M \gtrsim 40$ M$_\odot$ 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 radial velocity of ~80 km/s. Moreover, with all five previously detected WR binaries in the SMC exhibiting orbital periods of below 20 d, a puzzling absence of intermediate-to-long-period WR binaries has emerged, with strong implications for the outcome of massive binary interaction at low metallicity. |
| title | An absence of binary companions to Wolf-Rayet stars in the Small Magellanic Cloud: implications for mass loss and black hole masses at low metallicity |
| topic | Solar and Stellar Astrophysics Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2406.01420 |