Supernova 2012Z in Spiral Galaxy NGC 130
Credit: NASA, ESA, C. McCully and S. Jha (Rutgers University), R. Foley (University of Illinois), and Z. Levay (STScI).
Astronomers using NASA's Hubble Space Telescope for the first time have spotted a star system that later produced an unusual supernova explosion of a white dwarf, the stripped-down core of an ordinary star at the end of its life.
Examining archived Hubble images taken before the supernova,
astronomers say they have detected the blue companion star of the white
dwarf. The white dwarf slowly siphoned fuel from its companion,
eventually igniting a runaway nuclear reaction in the dead star, and
producing a weak supernova blast.
This particular supernova is classified as a Type Iax, a recently
identified class of stellar explosion. These exploding stars are less
energetic and fainter than Type Ia supernovae, which also originate
from exploding white dwarfs in binary systems. Astronomers originally
thought these weaker stellar blasts were unique Type Ia supernovae. So
far, they have identified more than 30 of these mini-explosions, which
occur at one-fifth the rate of Type Ia supernovae.
"Astronomers have been searching for decades for the progenitors of
Type Ia's," said 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 tomorrow in the journal Nature.
The weak supernova, dubbed SN 2012Z, was found in the Lick
Observatory Supernova Search in January 2012. Fortuitously, Hubble's
Advanced Camera for Surveys also observed the supernova's host galaxy,
NGC 1309, in 2005, 2006, and 2010, before the supernova outburst. NGC
1309 resides 110 million light-years away. Curtis McCully, a graduate
student at Rutgers and lead author of the team's paper, reprocessed the
pre-explosion images to make them sharper and noticed an object at the
supernova's position. "I was very surprised to see anything at the
supernova's location. We expected that 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.
The likelihood that the object detected was just a chance alignment
unrelated to the supernova is less than one percent. After studying the
object's colors and computer simulations showing possible type Iax
progenitor systems, the team concluded that what they were seeing was
most likely the light of a star that had lost its outer hydrogen
envelope, revealing its helium core.
"Back in 2009, when we were just starting to understand this class,
we predicted that 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 with
this Hubble study, but it is essentially validation of our claim."
According to the team, one possible scenario for the oddball star
system predicts that a seesaw game ensues between the stars, with each
star donating mass to the other. The stars originally weighed about
seven and four times that of our Sun, respectively. The more massive
seven-solar-mass star evolves quickly, dumping its hydrogen and helium
onto its smaller companion. Now slimmed down to just one solar mass, the
once-more- massive star is left with a carbon and oxygen core, becoming
a white dwarf. The companion star, which began with just four solar
masses, is now bulked up and begins to evolve quickly, growing larger
and engulfing the white dwarf. The outer layers of this "combined" star
are ejected, leaving behind the white dwarf and the two-solar-mass
helium core of the companion star. The white dwarf is still siphoning
matter from its partner until it becomes unstable and explodes as a
mini-supernova, ejecting about half a solar mass of material.
Unlike a normal Type Ia supernova, which destroys its white dwarf,
the explosion of a Type Iax is thought to leave behind a battered and
bruised white dwarf. Since this dead star comes back to life as it
explodes, astronomers have nicknamed it a "zombie star."
The team acknowledges that they can't totally rule out other
possibilities for the object's identity, including the possibility that
it was simply a single, massive star that exploded as a supernova. To
settle those uncertainties and confirm their hypothesis, the team plans
to use Hubble again in 2015 to observe the area when the supernova's
light has dimmed enough to show any possible zombie star and helium
companion.
The astronomers already have seen the aftermath of one Type Iax
supernova blast. Hubble images taken of supernova 2008ha in January
2013, more than four years after it exploded, show an object at the
supernova's location. The supernova resides in the galaxy UGC 12682,
located 69 million light-years away. The object could be either the
zombie remnant star or the companion. Based on the object's colors, the
team suggests in a separate paper that the star is the companion,
weighing more than three solar masses. It is significantly less
luminous and redder than the SN 2012Z progenitor system. The findings
will be published in the Aug. 11 issue of 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, the SN 2008ha paper's lead
author. "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 progenitors.
CONTACT
Donna Weaver / Ray VillardSpace Telescope Science Institute, Baltimore, Md.
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu
Source: HubbleSite
