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EI2GYB > ASTRO 29.08.21 10:56l 154 Lines 8138 Bytes #999 (0) @ WW
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Subj: The Goldilocks Supernova
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Sent: 210829/0854Z @:EI2GYB.DGL.IRL.EURO #:13839 BPQ6.0.22
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The Goldilocks Supernova
The discovery of a new type of supernova illuminates a medieval mystery
Scientists have discovered the first convincing evidence for a new type of
stellar explosion -- an electron-capture supernova.
While they have been theorized for 40 years, real-world examples have been
elusive.
They are thought to arise from the explosions of massive super-asymptotic
giant branch (SAGB) stars, for which there has also been scant evidence.
A worldwide team led by UC Santa Barbara scientists at Las Cumbres
Observatory has discovered the first convincing evidence for a new type
of stellar explosion -- an electron-capture supernova.
While they have been theorized for 40 years, real-world examples have
been elusive.
They are thought to arise from the explosions of massive super-asymptotic
giant branch (SAGB) stars, for which there has also been scant evidence.
The discovery, published in Nature Astronomy, also sheds new light on the
thousand-year mystery of the supernova from A.D. 1054 that was visible
all over the world in the daytime, before eventually becoming the Crab Nebula.
Historically, supernovae have fallen into two main types: thermonuclear
and iron-core collapse.
A thermonuclear supernova is the explosion of a white dwarf star after it
gains matter in a binary star system.
These white dwarfs are the dense cores of ash that remain after a
low-mass star (one up to about 8 times the mass of the sun) reaches the
end of its life.
An iron core-collapse supernova occurs when a massive star -- one more
than about 10 times the mass of the sun -- runs out of nuclear fuel and
its iron core collapses, creating a black hole or neutron star.
Between these two main types of supernovae are electron-capture supernovae.
These stars stop fusion when their cores are made of oxygen, neon and
magnesium; they aren't massive enough to create iron.
While gravity is always trying to crush a star, what keeps most stars
from collapsing is either ongoing fusion or, in cores where fusion has
stopped, the fact that you can't pack the atoms any tighter.
In an electron capture supernova, some of the electrons in the
oxygen-neon-magnesium core get smashed into their atomic nuclei in a
process called electron capture. This removal of electrons causes the
core of the star to buckle under its own weight and collapse, resulting
in an electron-capture supernova.
If the star had been slightly heavier, the core elements could have fused
to create heavier elements, prolonging its life. So it is a kind of
reverse Goldilocks situation: The star isn't light enough to escape its
core collapsing, nor is it heavy enough to prolong its life and die later
via different means.
That's the theory that was formulated beginning in 1980 by Ken'ichi
Nomoto of the University of Tokyo and others.
Over the decades, theorists have formulated predictions of what to look
for in an electron-capture supernova and their SAGB star progenitors.
The stars should have a lot of mass, lose much of it before exploding,
and this mass near the dying star should be of an unusual chemical
composition.
Then the electron-capture supernova should be weak, have little
radioactive fallout, and have neutron-rich elements in the core.
The new study is led by Daichi Hiramatsu, a graduate student at UC Santa
Barbara and Las Cumbres Observatory (LCO).
Hiramatsu is a core member of the Global Supernova Project, a worldwide
team of scientists using dozens of telescopes around and above the globe.
The team found that the supernova SN 2018zd had many unusual
characteristics, some of which were seen for the first time in a supernova.
It helped that the supernova was relatively nearby -- only 31 million
light-years away -- in the galaxy NGC 2146.
This allowed the team to examine archival images taken by the Hubble
Space Telescope prior to the explosion and to detect the likely progenitor
star before it exploded.
The observations were consistent with another recently identified
SAGB star in the Milky Way, but inconsistent with models of red
supergiants, the progenitors of normal iron core-collapse supernovae.
The authors looked through all published data on supernovae, and found
that while some had a few of the indicators predicted for electron-capture
supernovae, only SN 2018zd had all six: an apparent SAGB progenitor,
strong pre-supernova mass loss, an unusual stellar chemical composition,
a weak explosion, little radioactivity and a neutron-rich core.
"We started by asking 'what's this weirdo?'" Hiramatsu said. "Then we
examined every aspect of SN 2018zd and realized that all of them can
be explained in the electron-capture scenario."
The new discoveries also illuminate some mysteries of the most famous
supernova of the past.
In A.D. 1054 a supernova happened in the Milky Way Galaxy that,
according to Chinese and Japanese records, was so bright that it
could be seen in the daytime for 23 days, and at night for nearly two years.
The resulting remnant, the Crab Nebula, has been studied in great detail.
The Crab Nebula was previously the best candidate for an electron-capture
supernova, but its status was uncertain partly because the explosion
happened nearly a thousand years ago.
The new result increases the confidence that the historic SN 1054 was
an electron-capture supernova.
It also explains why that supernova was relatively bright compared to
the models: Its luminosity was probably artificially enhanced by the
supernova ejecta colliding with material cast off by the progenitor
star as was seen in SN 2018zd.
Ken Nomoto at the Kavli IPMU of the University of Tokyo expressed
excitement that his theory had been confirmed. "I am very pleased that
the electron-capture supernova was finally discovered, which my
colleagues and I predicted to exist and have a connection to the
Crab Nebula 40 years ago," he said.
"I very much appreciate the great efforts involved in obtaining
these observations.
This is a wonderful case of the combination of observations and theory."
Hiramatsu added, "It was such a 'Eureka moment' for all of us that we
can contribute to closing the 40-year-old theoretical loop, and for me
personally because my career in astronomy started when I looked at
the stunning pictures of the Universe in the high school library, one
of which was the iconic Crab Nebula taken by the Hubble Space Telescope."
"The term Rosetta Stone is used too often as an analogy when we find a
new astrophysical object," said Andrew Howell, a staff scientist at
Las Cumbres Observatory and adjunct faculty at UCSB, "but in this case
I think it is fitting.
This supernova is literally helping us decode thousand-year-old records
from cultures all over the world. And it is helping us associate one
thing we don't fully understand, the Crab Nebula, with another thing we
have incredible modern records of, this supernova. In the process it is
teaching us about fundamental physics: how some neutron stars get made,
how extreme stars live and die, and about how the elements we're made of
get created and scattered around the universe."
Howell also is the leader of the Global Supernova Project, and lead
author Hiramatsu 's Ph.D. advisor.
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