In 2020 a team led by European Southern Observatory (ESO) astronomers
reported the closest black hole to Earth, located just 1,000 light-years
away in the HR 6819 system. But the results of their study were contested by
other researchers, including by an international team based at KU Leuven,
Belgium. In a paper published today, these two teams have united to report
that there is in fact no black hole in HR 6819, which is instead a "vampire"
two-star system in a rare and short-lived stage of its evolution.
The original study on HR 6819 received significant attention from both the
press and scientists. Thomas Rivinius, a Chile-based ESO astronomer and lead
author on that paper, was not surprised by the astronomy community's
reception to their discovery of the black hole. "Not only is it normal, but
it should be that results are scrutinized," he says, "and a result that
makes the headlines even more so."
Rivinius and his colleagues were convinced that the best explanation for the
data they had, obtained with the MPG/ESO 2.2-meter telescope, was that HR
6819 was a triple system, with one star orbiting a black hole every 40 days
and a second star in a much wider orbit. But a study led by Julia
Bodensteiner, then a Ph.D. student at KU Leuven, Belgium, proposed a
different explanation for the same data: HR 6819 could also be a system with
only two stars on a 40-day orbit and no black hole at all. This alternative
scenario would require one of the stars to be "stripped," meaning that, at
an earlier time, it had lost a large fraction of its mass to the other star.
"We had reached the limit of the existing data, so we had to turn to a
different observational strategy to decide between the two scenarios
proposed by the two teams," says KU Leuven researcher Abigail Frost, who led
the new study published today in Astronomy & Astrophysics.
To solve the mystery, the two teams worked together to obtain new, sharper
data of HR 6819 using ESO's Very Large Telescope (VLT) and Very Large
Telescope Interferometer (VLTI). "The VLTI was the only facility that would
give us the decisive data we needed to distinguish between the two
explanations," says Dietrich Baade, author on both the original HR 6819
study and the new Astronomy & Astrophysics paper. Since it made no sense
to ask for the same observation twice, the two teams joined forces, which
allowed them to pool their resources and knowledge to find the true nature
of this system.
"The scenarios we were looking for were rather clear, very different and
easily distinguishable with the right instrument," says Rivinius. "We agreed
that there were two sources of light in the system, so the question was
whether they orbit each other closely, as in the stripped-star scenario, or
are far apart from each other, as in the black hole scenario."
To distinguish between the two proposals, the astronomers used both the
VLTI's GRAVITY instrument and the Multi Unit Spectroscopic Explorer (MUSE)
instrument on ESO's VLT.
"MUSE confirmed that there was no bright companion in a wider orbit, while
GRAVITY's high spatial resolution was able to resolve two bright sources
separated by only one-third of the distance between the Earth and the Sun,"
says Frost. "These data proved to be the final piece of the puzzle, and
allowed us to conclude that HR 6819 is a binary system with no black hole."
"Our best interpretation so far is that we caught this binary system in a
moment shortly after one of the stars had sucked the atmosphere off its
companion star. This is a common phenomenon in close binary systems,
sometimes referred to as 'stellar vampirism' in the press," explains
Bodensteiner, now a fellow at ESO in Germany and an author on the new study.
"While the donor star was stripped of some of its material, the recipient
star began to spin more rapidly."
"Catching such a post-interaction phase is extremely difficult as it is so
short," adds Frost. "This makes our findings for HR 6819 very exciting, as
it presents a perfect candidate to study how this vampirism affects the
evolution of massive stars, and in turn the formation of their associated
phenomena including gravitational waves and violent supernova explosions."
The newly formed Leuven-ESO joint team now plans to monitor HR 6819 more
closely using the VLTI's GRAVITY instrument. The researchers will conduct a
joint study of the system over time, to better understand its evolution,
constrain its properties, and use that knowledge to learn more about other
binary systems.
As for the search for black holes, the team remains optimistic.
"Stellar-mass black holes remain very elusive owing to their nature," says
Rivinius. "But order-of-magnitude estimates suggest there are tens to
hundreds of millions of black holes in the Milky Way alone," Baade adds. It
is just a matter of time until astronomers discover them.
This research was presented in the paper "HR 6819 is a binary system with no
black hole: Revisiting the source with infrared interferometry and optical
integral field spectroscopy" to appear in Astronomy & Astrophysics.
Reference:
"HR 6819 is a binary system with no black hole: Revisiting the source with
infrared interferometry and optical integral field spectroscopy" Astronomy
& Astrophysics (2022).
DOI: 10.1051/0004-6361/202143004
Tags:
Space & Astrophysics