'Closest black hole' system found to contain no black hole

Date:
March 2, 2022
Source:
ESO
Summary:
In 2020, 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. In a
new paper, 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.



FULL STORY ==========================================================================
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" 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. Me'rand (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).

========================================================================== Story Source: Materials provided by ESO. Note: Content may be edited
for style and length.


========================================================================== Related Multimedia:
* Images,_video_and_animation_related_to_HR_6819 ========================================================================== Journal Reference:
1. A. J. Frost, J. Bodensteiner, Th. Rivinius, D. Baade, A. Merand, F.

Selman, M. Abdul-Masih, G. Banyard, E. Bordier, K. Dsilva,
C. Hawcroft, L. Mahy, M. Reggiani, T. Shenar, M. Cabezas,
P. Hadrava, M. Heida, R.

Klement, H. Sana. HR 6819 is a binary system with no black hole.

Astronomy & Astrophysics, 2022; 659: L3 DOI:
10.1051/0004-6361/202143004 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/03/220302092707.htm

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