Never-before-seen way to annihilate a star

Date:
June 22, 2023
Source:
Association of Universities for Research in Astronomy (AURA)
Summary:
Astronomers studying a powerful gamma-ray burst, may have detected
a never-before-seen way to destroy a star. Unlike most GRBs,
which are caused by exploding massive stars or the chance mergers
of neutron stars, astronomers have concluded that this GRB came
instead from the collision of stars or stellar remnants in the
jam-packed environment surrounding a supermassive black hole at
the core of an ancient galaxy.


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FULL STORY ========================================================================== International Gemini Observatory traces gamma-ray burst to nucleus
of ancient galaxy, suggesting stars can undergo demolition-derby-like collisions.

Astronomers studying a powerful gamma-ray burst (GRB) with the
Gemini South telescope, operated by NSF's NOIRLab, may have detected
a never-before-seen way to destroy a star. Unlike most GRBs, which are
caused by exploding massive stars or the chance mergers of neutron stars, astronomers have concluded that this GRB came instead from the collision
of stars or stellar remnants in the jam-packed environment surrounding
a supermassive black hole at the core of an ancient galaxy.

Most stars in the Universe die in predictable ways, depending on their
mass.

Relatively low-mass stars like our Sun slough off their outer layers in
old age and eventually fade to become white dwarf stars. More massive
stars burn brighter and die sooner in cataclysmic supernova explosions, creating ultradense objects like neutron stars and black holes. If two
such stellar remnants form a binary system, they also can eventually
collide. New research, however, points to a long-hypothesized, but never-before-seen, fourth option.

While searching for the origins of a long-duration gamma-ray burst
(GRB), astronomers using the Gemini South telescope in Chile, part of
the International Gemini Observatory operated by NSF's NOIRLab, and
other telescopes [1], have uncovered evidence of a demolition-derby-like collision of stars or stellar remnants in the chaotic and densely packed
region near an ancient galaxy's supermassive black hole.

"These new results show that stars can meet their demise in some of the
densest regions of the Universe where they can be driven to collide," said Andrew Levan, an astronomer with Radboud University in The Netherlands and
lead author of a paper appearing in the journal Nature Astronomy. "This
is exciting for understanding how stars die and for answering other
questions, such as what unexpected sources might create gravitational
waves that we could detect on Earth." Ancient galaxies are long past
their star-forming prime and would have few, if any, remaining giant
stars, the principal source of long GRBs. Their cores, however, are
teeming with stars and a menagerie of ultra-dense stellar remnants,
such as white dwarf stars, neutron stars, and black holes.

Astronomers have long suspected that in the turbulent beehive of activity surrounding a supermassive black hole, it would only be a matter of time
until two stellar objects collide to produce a GRB. Evidence for that
type of merger, however, has been elusive.

The first hints that such an event had occurred were seen on 19 October
2019 when NASA's Neil Gehrels Swift Observatory detected a bright flash of gamma rays that lasted for a little more than one minute. Any GRB lasting
more than two seconds is considered "long." Such bursts typically come
from the supernova death of stars at least 10 times the mass of our Sun --
but not always.

The researchers then used Gemini South to make long-term observations
of the GRB's fading afterglow to learn more about its origins. The
observations allowed the astronomers to pinpoint the location of
the GRB to a region less than 100 light-years from the nucleus of an
ancient galaxy, which placed it very near the galaxy's supermassive black
hole. The researchers also found no evidence of a corresponding supernova, which would leave its imprint on the light studied by Gemini South.

"Our follow-up observation told us that rather than being a massive
star collapsing, the burst was most likely caused by the merger of two
compact objects," said Levan. "By pinpointing its location to the center
of a previously identified ancient galaxy, we had the first tantalizing evidence of a new pathway for stars to meet their demise." In normal
galactic environments, the production of long GRBs from colliding
stellar remnants such as neutron stars and black holes is thought to be vanishingly rare. The cores of ancient galaxies, however, are anything
but normal and there may be a million or more stars crammed into a region
just a few light-years across. Such extreme population density may be
great enough that occasional stellar collisions can occur, especially
under the titanic gravitational influence of a supermassive black hole,
which would perturb the motions of stars and send them careening in
random directions. Eventually, these wayward stars would intersect and
merge, triggering a titanic explosion that could be observed from vast
cosmic distances.

It is possible that such events occur routinely in similarly crowded
regions across the Universe but have gone unnoticed until this point. A possible reason for their obscurity is that galactic centers are brimming
with dust and gas, which could obscure both the initial flash of the
GRB and the resulting afterglow. This particular GRB, identified as GRB 191019A, may be a rare exception, allowing astronomers to detect the
burst and study its after effects.

The researchers would like to discover more of these events. Their hope
is to match a GRB detection with a corresponding gravitational-wave
detection, which would reveal more about their true nature and confirm
their origins, even in the murkiest of environments. The Vera C. Rubin Observatory, when it comes online in 2025, will be invaluable in this
kind of research.

"Studying gamma-ray bursts like these is a great example of how the
field is really advanced by many facilities working together, from the detection of the GRB, to the discoveries of afterglows and distances
with telescopes like Gemini, through to detailed dissection of events
with observations across the electromagnetic spectrum," said Levan.

"These observations add to Gemini's rich heritage developing our
understanding of stellar evolution," says Martin Still, NSF's program
director for the International Gemini Observatory. "The time sensitive observations are a testament to Gemini's nimble operations and sensitivity
to distant, dynamic events across the Universe."
* RELATED_TOPICS
o Space_&_Time
# Black_Holes # Stars # Galaxies # Astrophysics #
Nebulae # Astronomy # Cosmic_Rays # Space_Telescopes
* RELATED_TERMS
o Supernova o Chandra_X-ray_Observatory o Blue_supergiant_star o
Interstellar_medium o Spitzer_space_telescope o Globular_cluster
o Gravitational_wave o Astronomy

========================================================================== Story Source: Materials provided by Association_of_Universities_for_Research_in_Astronomy_ (AURA). Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Levan, A. J., Malesani, D. B., Gompertz, B. P., et al. A
long-duration
gamma-ray burst of dynamical origin from the nucleus of an ancient
galaxy. Nature Astronomy, 2023 DOI: 10.1038/s41550-023-01998-8 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/06/230622120817.htm

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