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

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

Research output: Contribution to journalArticlepeer-review

16 Scopus citations


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 (MBe=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 YHe ≈ 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), percent 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.

Original languageEnglish (US)
Pages (from-to)3602-3630
Number of pages29
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
StatePublished - Nov 1 2022

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • binaries: spectroscopic
  • stars: abundances
  • stars: emission-line, Be


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