A stripped-companion origin for Be stars: clues from the putative black
  holes HR 6819 and LB-1
  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 $(\log g \approx 2.75)$. Both
stars show periodic radial velocity (RV) variability, but the RV semi-amplitude
of the B star's orbit is $K_{\rm B}= (62.7 \pm 1)\,\rm km\,s^{-1}$, while that
of the Be star is only $K_{\rm Be} = (4.5\pm 2)\,\rm km\,s^{-1}$. 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 $\approx 0.5\,M_{\odot}$
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 models to investigate their evolutionary history, finding
that they likely formed from intermediate-mass ($3-7\,M_{\odot}$) primaries
stripped by slightly lower-mass secondaries and are progenitors to Be + sdOB
binaries such as $\phi$ Persei. The lifetime of their current evolutionary
phase is on average $2\times 10^5$ years, of order half a percent 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 ($20-100\%$) of field
Be stars form through accretion of material from a binary companion.
