A stripped-companion origin for Be stars: Clues from the putative black holes HR 6819 and LB-1

Kareem El-Badry, Eliot Quataert

Research output: Contribution to journalArticlepeer-review

38 Scopus citations


HR 6819 is a bright (V = 5.36), blue star recently proposed to be a triple containing a detached black hole (BH). We show that the system is a binary and does not contain a BH. Using spectral decomposition, we disentangle the observed composite spectra into two components: a rapidly rotating Be star and a slowly rotating B star with low surface gravity (log g ≈ 2.75). Both stars show periodic radial velocity (RV) variability, but the RV semi-amplitude of the B star's orbit is K B= (62.7 pm 1)\, km, s-1, while that of the Be star is only K Be = (4.5\pm 2)\, km, s-1. This implies that the B star is less massive by at least a factor of 10. The surface abundances of the B star bear imprints of CNO burning. We argue that the B star is a bloated, recently stripped helium star with mass ≈0.5, M⊙ that is currently contracting to become a hot subdwarf. The orbital motion of the Be star obviates the need for a BH to explain the B star's motion. A stripped-star model reproduces the observed luminosity of the system, while a normal star with the B star's temperature and gravity would be more than 10 times too luminous. HR 6819 and the binary LB-1 probably formed through similar channels. We use MESA (Modules for Experiments in Stellar Astrophysics) models to investigate their evolutionary history, finding that they likely formed from intermediate-mass (3-7, M⊙) primaries stripped by slightly lower-mass secondaries and are progenitors to Be + sdOB binaries such as φ Persei. The lifetime of their current evolutionary phase is on average 2 × 105 yr, of the order of half a per cent of the total lifetime of the Be phase. This implies that many Be stars have hot subdwarf and white dwarf companions, and that a substantial fraction (20\\100 per cent) of field Be stars form through accretion of material from a binary companion.

Original languageEnglish (US)
Pages (from-to)3436-3455
Number of pages20
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
StatePublished - Apr 1 2021
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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


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