The two inset images show before-and-after images
captured by NASA’s Hubble Space Telescope of Supernova 2012Z in the spiral
galaxy NGC 1309. The white X at the top of the main image marks the location of
the supernova in the galaxy.
Image Credit:
NASA, ESA
Using NASA’s Hubble Space Telescope, a team of astronomers has spotted a star
system that could have left behind a “zombie star” after an unusually weak
supernova explosion.
A supernova typically obliterates the exploding white dwarf, or dying star.
On this occasion, scientists believe this faint supernova may have left behind a
surviving portion of the dwarf star -- a sort of zombie star.
While examining Hubble images taken years before the stellar explosion,
astronomers identified a blue companion star feeding energy to a white dwarf, a
process that ignited a nuclear reaction and released this weak supernova blast.
This supernova, Type Iax, is less common than its brighter cousin, Type Ia.
Astronomers have identified more than 30 of these mini-supernovas that may leave
behind a surviving white dwarf.
“Astronomers have been searching for decades for the star systems that
produce Type Ia supernova explosions,” said scientist Saurabh Jha of Rutgers
University in Piscataway, New Jersey. “Type Ia’s are important because they’re
used to measure vast cosmic distances and the expansion of the universe. But we
have very few constraints on how any white dwarf explodes. The similarities
between Type Iax’s and normal Type Ia’s make understanding Type Iax progenitors
important, especially because no Type Ia progenitor has been conclusively
identified. This discovery shows us one way that you can get a white dwarf
explosion.”
The team’s results will appear in the Thursday, Aug. 7 edition of the journal
Nature.
The weak supernova, dubbed SN 2012Z, resides in the host galaxy NGC 1309
which is 110 million light-years away. It was discovered in the Lick Observatory
Supernova Search in January 2012. Luckily, Hubble’s Advanced Camera for Surveys
also observed NGC 1309 for several years prior the supernova outburst, which
allowed scientists to compare before-and-after images.
Curtis McCully, a graduate student at Rutgers and lead author of the team’s
paper, sharpened the Hubble pre-explosion images and noticed a peculiar object
near the location of the supernova.
“I was very surprised to see anything at the location of the supernova. We
expected the progenitor system would be too faint to see, like in previous
searches for normal Type Ia supernova progenitors. It is exciting when nature
surprises us,” McCully said.
After studying the object’s colors and comparing with computer simulations of
possible Type Iax progenitor systems, the team concluded they were seeing the
light of a star that had lost its outer hydrogen envelope, revealing its helium
core.
The team plans to use Hubble again in 2015 to observe the area, giving time
for the supernova’s light to dim enough to reveal any possible zombie star and
helium companion to confirm their hypothesis.
“Back in 2009, when we were just starting to understand this class, we
predicted these supernovae were produced by a white dwarf and helium star binary
system,” said team member Ryan Foley of the University of Illinois at
Urbana-Champaign, who helped identify Type Iax supernovae as a new class.
“There’s still a little uncertainty in this study, but it is essentially
validation of our claim.”
One possible explanation for the unusual nature of SN 2012Z is that a game of
seesaw ensued between the bigger and smaller of the star pair. The more massive
star evolved more quickly to expand and dump its hydrogen and helium onto the
smaller star. The rapidly evolving star became a white dwarf. The smaller star
bulked up, grew larger and engulfed the white dwarf. The outer layers of this
combined star were ejected, leaving behind the white dwarf and the helium core
of the companion star. The white dwarf siphoned matter from the companion star
until it became unstable and exploded as a mini-supernova, leaving behind a
surviving zombie star.
Astronomers already have located the aftermath of another Type Iax supernova
blast. Images were taken with Hubble in January 2013 of supernova 2008ha,
located 69 million light-years away in the galaxy UGC 12682, in more than four
years after it exploded. The images show an object in the area of the supernova
that could be the zombie star or the companion. The findings will be published
in The Astrophysical Journal.
“SN 2012Z is one of the more powerful Type Iax supernovae and SN 2008ha is
one of the weakest of the class, showing that Type Iax systems are very
diverse,” explained Foley, lead author of the paper on SN 2008ha. “And perhaps
that diversity is related to how each of these stars explodes. Because these
supernovae don’t destroy the white dwarf completely, we surmise that some of
these explosions eject a little bit and some eject a whole lot.”
The astronomers hope their new findings will spur the development of improved
models for these white dwarf explosions and a more complete understanding of the
relationship between Type Iax and normal Type Ia supernovae and their
corresponding star systems.
The Hubble Space Telescope is a project of international cooperation between
NASA and the European Space Agency. NASA's Goddard Space Flight Center in
Greenbelt, Maryland, manages the telescope. The Space Telescope Science
Institute (STScI) in Baltimore conducts Hubble science operations. STScI is
operated for NASA by the Association of Universities for Research in Astronomy,
Inc., in Washington.
For images and more information about Hubble, visit:
and
NASA
Guillermo Gonzalo Sánchez Achutegui
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