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

Abstract

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 ± 1) km s⁻¹⁠, while that of the Be star is only K_(Be) = (4.5 ± 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.

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