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EI2GYB > ASTRO    29.08.21 09:56l 154 Lines 8138 Bytes #999 (0) @ WW
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Subj: The Goldilocks Supernova
Path: IZ3LSV<IR1UAW<IW2OHX<UA6ADV<I0OJJ<GB7CIP<EI2GYB
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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