Birth of a Be star: an APOGEE search for Be stars forming through binary mass transfer

Author(s)

El-Badry, Kareem ; Conroy, Charlie ; Quataert, Eliot ; Rix, Hans-Walter ; Labadie-Bartz, Jonathan ; Jayasinghe, Tharindu ; Thompson, Todd ; Cargile, Phillip ; Stassun, Keivan G. ; Ilyin, Ilya

• Document type: Journal article
• Source: Monthly Notices of the Royal Astronomical Society; v. 516, n. 3, p. 29-pg., 2022-09-19.

Abstract

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-Be = 5.3 +/- 0.6 M-circle dot) with a low-mass (M-donor = 0.92 +/- 0.22 M-circle dot), subgiant companion on a 5.47-d orbit. The emission lines originate in an accretion disc caused by ongoing mass transfer, not from a decretion disc 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-He approximate to 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-circle dot donor leaves the main sequence, with the originally less massive component 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 sdO/B star. Discovery of one object in this short-lived (similar to 1 Myr) evolutionary phase implies the existence of many more that have already passed through it and are now Be + sdO/B binaries. We infer that (10-60) per cent of Be stars have stripped companions, most of which are similar to 100 x fainter than the Be stars in the optical. Together with the dearth of main-sequence companions to Be stars and recent discovery of numerous Be + sdO/B binaries in the UV, our results imply that binarity plays an important role in the formation of Be stars. (AU)

• FAPESP's process: 17/23731-1 - Analysis and Modeling of a Large Sample of Galactic Be Stars
• Grantee: Jonathan Michael Labadie-Bartz
• Support Opportunities: Scholarships in Brazil - Post-Doctoral