Spectacular ultraviolet flash may finally explain how white dwarfs
explode
Event also could give insight into dark energy and the creation of iron


Date:
July 23, 2020
Source:
Northwestern University
Summary:
For just the second time ever, astrophysicists have spotted a
spectacular flash of ultraviolet (UV) light accompanying a white
dwarf explosion. An extremely rare type of supernova, the event
is poised to offer insights into several long-standing mysteries,
including what causes white dwarfs to explode, how dark energy
accelerates the cosmos and how the universe creates heavy metals,
such as iron.



FULL STORY ==========================================================================
For just the second time ever, astrophysicists have spotted a spectacular
flash of ultraviolet (UV) light accompanying a white dwarf explosion.


==========================================================================
An extremely rare type of supernova, the event is poised to offer insights
into several long-standing mysteries, including what causes white dwarfs
to explode, how dark energy accelerates the cosmos and how the universe
creates heavy metals, such as iron.

"The UV flash is telling us something very specific about how this
white dwarf exploded," said Northwestern University astrophysicist Adam
Miller, who led the research. "As time passes, the exploded material
moves farther away from the source. As that material thins, we can see
deeper and deeper. After a year, the material will be so thin that we
will see all the way into the center of the explosion." At that point,
Miller said, his team will know more about how this white dwarf --
and all white dwarfs, which are dense remnants of dead stars -- explode.

The paper will be published on July 23 in the Astrophysical Journal.

Miller is a fellow in Northwestern's Center for Interdisciplinary
Exploration and Research in Astrophysics (CIERA) and director of the
Legacy Survey of Space and Time Corporation Data Science Fellowship
Program.



========================================================================== Common event with a rare twist Using the Zwicky Transient Facility in California, researchers first spotted the peculiar supernova in December
2019 -- just a day after it exploded. The event, dubbed SN2019yvq,
occurred in a relatively nearby galaxy located 140 million light-years
from Earth, very close to tail of the dragon-shaped Draco constellation.

Within hours, astrophysicists used NASA's Neil Gehrels Swift Observatory
to study the phenomenon in ultraviolet and X-ray wavelengths. They
immediately classified SN2019yvq as a type Ia (pronounced "one-A")
supernova, a fairly frequent event that occurs when a white dwarf
explodes.

"These are some of the most common explosions in the universe," Miller
said.

"But what's special is this UV flash. Astronomers have searched for this
for years and never found it. To our knowledge, this is actually only the second time a UV flash has been seen with a type Ia supernova." Heated
mystery The rare flash, which lasted for a couple days, indicates that something inside or nearby the white dwarf was incredibly hot. Because
white dwarfs become cooler and cooler as they age, the influx of heat
puzzled astronomers.



==========================================================================
"The simplest way to create UV light is to have something that's very,
very hot," Miller said. "We need something that is much hotter than
our sun -- a factor of three or four times hotter. Most supernovae are
not that hot, so you don't get the very intense UV radiation. Something
unusual happened with this supernova to create a very hot phenomenon."
Miller and his team believe this is an important clue to understanding why white dwarfs explode, which has been a long-standing mystery in the field.

Currently, there are multiple competing hypotheses. Miller is particularly interested in exploring four different hypotheses, which match his team's
data analysis from SN2019yvq.

Potential scenarios that could cause a white dwarf to explode with a UV
flash are:
* 1. A white dwarf consumes its companion star and becomes so
large and
unstable that it explodes. The white dwarf's and companion star's
materials collide, causing a flash of UV emission; 2. Extremely hot
radioactive material in the white dwarf's core mixes with its outer
layers, causing the outer shell to reach higher temperatures than
usual; 3. An outer layer of helium ignites carbon within the white
dwarf, causing an extremely hot double explosion and a UV flash;
4. Two white dwarfs merge, triggering an explosion with colliding
ejecta that emit UV radiation.

"Within a year," Miller said, "we'll be able to figure out which one of
these four is the most likely explanation." Earth-shattering insights
Once the researchers know what caused the explosion, they will apply
those findings to learn more about planet formation and dark energy.

Because most of the iron in the universe is created by type Ia supernovae, better understanding this phenomenon could tell us more about our
own planet.

Iron from exploded stars, for example, formed the core of all rocky
planets, including Earth.

"If you want to understand how the Earth formed, you need to
understand where iron came from and how much iron was needed," Miller
said. "Understanding the ways in which a white dwarf explodes gives us
a more precise understanding of how iron is created and distributed
throughout the universe." Illuminating dark energy White dwarfs
already play an enormous role in physicists' current understanding of
dark energy as well. Physicists predict that white dwarfs all have the
same brightness when they explode. So type Ia supernovae are considered "standard candles," allowing astronomers to calculate exactly how far
the explosions lie from Earth. Using supernovae to measure distances
led to the discovery of dark energy, a finding recognized with the 2011
Nobel Prize in Physics.

"We don't have a direct way to measure the distance to other galaxies,"
Miller explained. "Most galaxies are actually moving away from us. If
there is a type Ia supernova in a distant galaxy, we can use it to measure
a combination of distance and velocity that allows us to determine the acceleration of the universe. Dark energy remains a mystery. But these supernovae are the best way to probe dark energy and understand what
it is." And by better understanding white dwarfs, Miller believes we potentially could better understand dark energy and how fast it causes
the universe to accelerate.

"At the moment, when measuring distances, we treat all of these explosions
as the same, yet we have good reason to believe that there are multiple explosion mechanisms," he said. "If we can determine the exact explosion mechanism, we think we can better separate the supernovae and make more
precise distance measurements."

========================================================================== Story Source: Materials provided by Northwestern_University. Original
written by Amanda Morris. Note: Content may be edited for style and
length.


========================================================================== Journal Reference:
1. A. A. Miller, M. R. Magee, A. Polin, K. Maguire, E. Zimmerman,
Y. Yao, J.

Sollerman, S. Schulze, D. A. Perley, M. Kromer, S. Dhawan,
M. Bulla, I.

Andreoni, E. C. Bellm, K. De, R. Dekany, A. Delacroix, C. Fremling,
A.

Gal-Yam, D. A. Goldstein, V. Z. Golkhou, A. Goobar, M. J. Graham, I.

Irani, M. M. Kasliwal, S. Kaye, Y.-L. Kim, R. R. Laher,
A. A. Mahabal, F.

J. Masci, P. E. Nugent, E. Ofek, E. S. Phinney, S. J. Prentice, R.

Riddle, M. Rigault, B. Rusholme, T. Schweyer, D. L. Shupe, M. T.

Soumagnac, G. Terreran, R. Walters, L. Yan, J. Zolkower,
S. R. Kulkarni.

The Spectacular Ultraviolet Flash from the Peculiar Type Ia
Supernova 2019yvq. The Astrophysical Journal, 2020; 898 (1):
56 DOI: 10.3847/1538- 4357/ab9e05 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200723115907.htm

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