In 2020, a team of astronomers from the European Southern Observatory (ESO) identified the
closest black hole to Earth in the HR 6819 system, merely 1,000 light-years away.
But the findings of the study were disputed by other researchers, including an international
team located at KU Leuven in Belgium. They believe that they might have been deceived
by a cosmic illusion. These two teams joined forces to declare that
there is no black hole in the HR 6819 system which is a “vampire” two-star system in
a rare and short-lived stage of its evolution. Thomas Rivinius, a Chile-based ESO astronomer
and lead author on that publication, was not surprised by the astronomical community’s
reaction to their discovery of the black hole. Using MPG/ESO 2.2-meter telescope Rivinius
and his colleagues determined that HR 6819 was a triple system with one star orbiting
a black hole every 40 days and a second star orbiting in a wider orbit.
However, a 2020 study by Julia Bodensteiner, then a Ph.D. candidate at KU Leuven and now
an ESO fellow, posited a different explanation for the same data.
She proposed that HR 6819 might potentially be made up of only two stars in a 40-day orbit,
with no black hole at all. This would require one of the stars stripping away and absorbing
a large portion of the mass of the other star. To solve this enigma, the two teams joined
forces to investigate HR 6819 more closely and determine which theory was more likely.
The researchers searched for relatively simple, distinct, and easily recognizable scenarios
with the correct instrument. They came to an agreement that the system
had two light sources; thus, the question was whether they were far apart, as in the
black hole scenario, or orbited each other closely, as in the stripped-star scenario.
Rivinius’ original research used observations from a small telescope, while the new team
used ESO’s Very Large Telescope (VLT) and Very Large Telescope Interferometer (VLTI)
for their study. The team discovered that the two bright stars
in HR 6819 orbit one another at barely one-third the distance between Earth and the Sun, implying
that one of them is significantly larger and spins faster than the other.
There was no such thing as a black hole. This finding proved to be the final piece of the
puzzle, and they concluded that HR 6819 is a binary system with no black hole.
So far, the best interpretation is that they captured this binary system just after one
of the stars had sucked the atmosphere from its companion star. This is a regular occurrence
in close binary systems and is known as ‘stellar vampirism’.
As the donor star was stripped of some of its material, the receiving star began to
spin faster. Catching such a post-interaction phase is very difficult because it is so brief.
This made the findings for HR 6819 quite exciting as it presents an ideal candidate for studying
how vampirism affects the evolution of giant stars, and thus the formation of their associated
phenomena such as gravitational waves and violent supernova explosions.
The newly established joint team intends to use the VLTI’s GRAVITY instrument to keep
a closer eye on HR 6819. The researchers will collaborate to investigate
the system over time in order to better understand its evolution, constrain its properties, and
apply that knowledge to learn more about other binary stars. The team remains optimistic
in its quest for black holes. Stellar-mass black holes remain elusive due
to their nature. However, the order-of-magnitude estimations suggest that the Milky Way alone
contains tens to hundreds of millions of black holes. It’s only a matter of time until
astronomers discover another one in our cosmic neighborhood.