Birth of a Be star: an APOGEE search for Be stars forming through binary mass transfer
| Autor: | Kareem El-Badry, Charlie Conroy, Eliot Quataert, Hans-Walter Rix, Jonathan Labadie-Bartz, Tharindu Jayasinghe, Todd Thompson, Phillip Cargile, Keivan G Stassun, Ilya Ilyin |
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| Rok vydání: | 2022 |
| Předmět: | |
| Zdroj: | Monthly Notices of the Royal Astronomical Society. 516:3602-3630 |
| ISSN: | 1365-2966 0035-8711 |
| DOI: | 10.1093/mnras/stac2422 |
| Popis: | Motivated by recent suggestions that many Be stars form through binary mass transfer, we searched the APOGEE survey for Be stars with bloated, stripped companions. From a well-defined parent sample of 297 Be stars, we identified one mass-transfer binary, HD 15124. The object consists of a main-sequence Be star ($M_{\rm Be}=5.3\pm 0.6 \,M_{\odot}$) with a low-mass ($M_{\rm donor}=0.92\pm 0.22\,M_{\odot}$), subgiant companion on a 5.47-day orbit. The emission lines originate in an accretion disk caused by ongoing mass transfer, not from a decretion disk as in classical Be stars. Both stars have surface abundances bearing imprint of CNO processing in the donor's core: the surface helium fraction is $Y_{\rm He}\approx 0.6$, and the nitrogen-to-carbon ratio is 1000 times the solar value. The system's properties are well-matched by binary evolution models in which mass transfer begins while a $3-5\,M_{\odot}$ donor leaves the main sequence, with the secondary becoming the Be star. These models predict that the system will soon become a detached Be + stripped star binary like HR 6819 and LB-1, with the stripped donor eventually contracting to become a core helium-burning sdOB star. Discovery of one object in this short-lived ($\sim$1 Myr) evolutionary phase implies the existence of many more that have already passed through it and are now Be + sdOB binaries. We infer that $(28_{-16}^{+27})\,\%$ of Be stars have stripped companions, most of which are faint. Together with the dearth of main-sequence companions to Be stars and recent discovery of numerous Be + sdOB binaries in the UV, our results imply that binarity plays an important role in the formation of Be stars. 25 pages, 19 figures, accepted to MNRAS |
| Databáze: | OpenAIRE |
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