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Closest Black Hole or no Black Hole at all…? (ESOcast 252)
Artist’s animation of HR 6819
In 2020 a team led by European Southern
Observatory (ESO) astronomers reported the closest black hole to Earth,
located just 1000 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 scrutinised,” 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-metre 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 PhD 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.
More information
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” (DOI: 10.1051/0004-6361/202143004) to appear in Astronomy & Astrophysics.
It has received funding from the European Research Council
(ERC) under the European Union’s Horizon 2020 research and innovation
programme (grant agreement number 772225: MULTIPLES; PI: Hugues Sana).
The team is composed of A. J. Frost (Institute of Astronomy, KU
Leuven, Belgium [KU Leuven]), J. Bodensteiner (European Southern
Observatory, Garching, Germany [ESO]), Th. Rivinius (European Southern
Observatory, Santiago, Chile [ESO Chile]), D. Baade (ESO), A. Mérand
(ESO), F. Selman (ESO Chile), M. Abdul-Masih (ESO Chile), G. Banyard (KU
Leuven), E. Bordier (KU Leuven, ESO Chile), K. Dsilva (KU Leuven), C.
Hawcroft (KU Leuven), L. Mahy (Royal Observatory of Belgium, Brussels,
Belgium), M. Reggiani (KU Leuven), T. Shenar (Anton Pannekoek Institute
for Astronomy, University of Amsterdam, The Netherlands), M. Cabezas
(Astronomical Institute, Academy of Sciences of the Czech Republic,
Prague, Czech Republic [ASCR]), P. Hadrava (ASCR), M. Heida (ESO), R.
Klement (The CHARA Array of Georgia State University, Mount Wilson
Observatory, Mount Wilson, USA) and H. Sana (KU Leuven).
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Links
Contacts:
Abigail Frost
KU Leuven
Leuven, Belgium
Tel: +56-2-2463-3280
Cell: +56-9-3548-9255
Email: abi.frost@kuleuven.be
Thomas Rivinius
European Southern Observatory
Santiago, Chile
Tel: +56-9-8288-4950
Email: triviniu@eso.org
Julia Bodensteiner
European Southern Observatory
Garching bei München, Germany
Tel: +49-89-3200-6409
Email: julia.bodensteiner@eso.org
Dietrich Baade
European Southern Observatory
Garching bei München, Germany
Tel: +49-89-6096-295
Email: dbaade@eso.org
Hugues Sana
KU Leuven
Leuven, Belgium
Tel: +32-16-3743-61
Email: hugues.sana@kuleuven.be
Bárbara Ferreira
ESO Media Manager
Garching bei München, Germany
Tel: +49-89-3200-6670
Cell: +49-151-2416-6400
Email: press@eso.org
Source: ESO/News