A star in a distant galaxy blew up in a powerful explosion, solving an
astronomical mystery
Dr. Iair Arcavi, a Tel Aviv University researcher at the Raymond and Beverly
Sackler Faculty of Exact Sciences, participated in a study that discovered a
new type of stellar explosion - an electron-capture supernova. While they
have been theorized for 40 years, real-world examples have been elusive.
Such supernovas arise from the explosions of stars 8-9 times the mass of the
sun. The discovery also sheds new light on the thousand-year mystery of the
supernova from A.D. 1054 that was seen by ancient astronomers, before
eventually becoming the Crab Nebula, that we know today.
A supernova is the explosion of a star following a sudden imbalance between
two opposing forces that shaped the star throughout its life. Gravity tries
to contract every star. Our sun, for example, counter balances this force
through nuclear fusion in its core, which produces pressure that opposes the
gravitational pull. As long as there is enough nuclear fusion, gravity will
not be able to collapse the star. However, eventually, nuclear fusion will
stop, just like gas runs out in a car, and the star will collapse. For stars
like the sun, the collapsed core is called a white dwarf. This material in
white dwarfs is so dense that quantum forces between electrons prevent
further collapse.
For stars 10 times more massive than our sun, however, electron quantum
forces are not enough to stop the gravitational pull, and the core continues
to collapse until it becomes a neutron star or a black hole, accompanied by
a giant explosion. In the intermediate mass range, the electrons are
squeezed (or more accurately, captured) onto atomic nuclei. This removes the
electron quantum forces, and causes the star to collapse and then explode.
Historically, there have been two main supernova types. One is a
thermonuclear supernova -- 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. Another main supernova type is a
core-collapse supernova where a massive star -- one more than about 10 times
the mass of the sun -- runs out of nuclear fuel and has its core collapsed,
creating a black hole or a neutron star. Theoretical work suggested that
electron-capture supernovae would occur on the borderline between these two
types of supernovae.
That's the theory that was developed in the 1980's 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. The stars
should lose a lot of mass of particular composition before exploding, and
the supernova itself should be relatively weak, have little radioactive
fallout, and produce neutron-rich elements.
The new study, published in Nature Astronomy, focuses on the supernova
SN2018zd, discovered in 2018 by Japanese amateur astronomer Koihchi Itagaki.
Dr. Iair Arcavi, of the astrophysics department at Tel Aviv University, also
took part in the study. This supernova, located in the galaxy NGC 2146, has
all of the properties expected from an electron-capture supernova, which
were not seen in any other supernova. In addition, because the supernova is
relatively nearby - only 31 million light years away - the researchers were
able to identify the star in pre-explosion archival images taken by the
Hubble Space Telescope. Indeed, the star itself also fits the predictions of
the type of star that should explode as an electron-capture supernovae, and
is unlike stars that were seen to explode as the other types of supernovae.
While some supernovae discovered in the past had a few of the indicators
predicted for electron-capture supernovae, only SN2018zd had all six - a
progenitor star that fits within the expected mass range, strong
pre-supernova mass loss, an unusual chemical composition, a weak explosion,
little radioactivity, and neutron-rich material. "We started by asking
'what's this weirdo?'" said Daichi Hiramatsu of the University of California
Santa Barbara and Las Cumbres Observatory, who led the study. "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 one of the most famous
supernovae of the past. In A.D. 1054 a supernova happened in our own Milky
Way Galaxy, and according to Chinese and Japanese records, it was so bright
that it could be seen in the daytime and cast shadows at night. The
resulting remnant, the Crab Nebula, has been studied in great detail, and
was found to have an unusual composition. It was previously the best
candidate for an electron-capture supernova, but this was uncertain partly
because the explosion happened nearly a thousand years ago. The new result
increases the confidence that the historic 1054 supernova was an
electron-capture supernova.
"It's amazing that we can shed light on historical events in the Universe
with modern instruments," says Dr. Arcavi. "Today, with robotic telescopes
that scan the sky in unprecedented efficiency, we can discover more and more
rare events which are critical for understanding the laws of nature, without
having to wait 1000 years between one event and the next."
Reference:
“The electron-capture origin of supernova 2018zd” by Daichi Hiramatsu, D.
Andrew Howell, Schuyler D. Van Dyk, Jared A. Goldberg, Keiichi Maeda,
Takashi J. Moriya, Nozomu Tominaga, Ken’ichi Nomoto, Griffin Hosseinzadeh,
Iair Arcavi, Curtis McCully, Jamison Burke, K. Azalee Bostroem, Stefano
Valenti, Yize Dong, Peter J. Brown, Jennifer E. Andrews, Christopher
Bilinski, G. Grant Williams, Paul S. Smith, Nathan Smith, David J. Sand,
Gagandeep S. Anand, Chengyuan Xu, Alexei V. Filippenko, Melina C. Bersten,
Gastón Folatelli, Patrick L. Kelly, Toshihide Noguchi and Koichi Itagaki, 28
June 2021, Nature Astronomy. DOI:
10.1038/s41550-021-01384-2
Tags:
Space & Astrophysics