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

El-Badry, Kareem and Conroy, Charlie and Quataert, Eliot and Rix, Hans-Walter and Labadie-Bartz, Jonathan and Jayasinghe, Tharindu and Thompson, Todd and Cargile, Phillip and Stassun, Keivan G. and Ilyin, Ilya (2022) Birth of a Be star: an APOGEE search for Be stars forming through binary mass transfer. Monthly Notices of the Royal Astronomical Society, 516 (3). pp. 3602-3630. ISSN 0035-8711. doi:10.1093/mnras/stac2422.

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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_⊙⁠) with a low-mass (⁠M_(donor) = 0.92 ± 0.22 M_⊙⁠), 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) ≈ 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_⊙ 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 (∼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 ∼100× 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.

Item Type:Article
Related URLs:
URLURL TypeDescription
El-Badry, Kareem0000-0002-6871-1752
Conroy, Charlie0000-0002-1590-8551
Quataert, Eliot0000-0001-9185-5044
Rix, Hans-Walter0000-0003-4996-9069
Labadie-Bartz, Jonathan0000-0002-2919-6786
Jayasinghe, Tharindu0000-0002-6244-477X
Thompson, Todd0000-0003-2377-9574
Cargile, Phillip0000-0002-1617-8917
Stassun, Keivan G.0000-0002-3481-9052
Ilyin, Ilya0000-0002-0551-046X
Additional Information:© 2022 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model ( We thank the referee for helpful comments. We are also grateful to David Whelan and Julia Bodensteiner for feedback on an early version of the manuscript, to Nathan Sandford and Yuan-Sen Ting for help generating model spectra, and to Robert Kurucz for making his codes publicly available. HWR acknowledges support from the GIF grant 1498. Funding for the Sloan Digital Sky Survey IV has been provided by the Alfred P. Sloan Foundation, the U.S. Department of Energy Office of Science, and the Participating Institutions. JLB acknowledges support from FAPESP (grant 2017/23731-1). DATA AVAILABILITY. Data associated with this paper are available upon request from the corresponding author.
Funding AgencyGrant Number
German-Israeli Foundation for Research and Development1498
Alfred P. Sloan FoundationUNSPECIFIED
Department of Energy (DOE)UNSPECIFIED
Participating InstitutionsUNSPECIFIED
Fundação de Amparo à Pesquisa do Estado de Sao Paulo (FAPESP)2017/23731-1
Issue or Number:3
Record Number:CaltechAUTHORS:20230105-896050000.59
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:118733
Deposited By: George Porter
Deposited On:06 Jan 2023 16:01
Last Modified:06 Jan 2023 16:01

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