Astronomers have detected a white dwarf that miraculously survived its own thermonuclear detonation, and raised questions about how and why these stars create supernovae.
A white dwarf is the evolutionary endpoint of a ground-like a star. After such a star swells to become one red giantthen runs out of fuel needed for nuclear fusion reactions, the star ejects its outer layers to form a planetary nebula, or a diffuse gas shell. As the nebula expands and dissipates, it leaves behind the inert core of the star, which is what we call a white dwarf.
White dwarfs are only the size of Earth, but contain a mass equivalent to that of a star. As such, they are dense objects with a gravity strong enough to remove material from any companion star in close orbit. This material flows into the white dwarf and, once enough material has accumulated, explodes in a thermonuclear detonation that normally destroys the star in what scientists call type Ia. supernovae.
Or so we thought.
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In 2012, when astronomers saw a supernova called 2012Z explode in the spiral galaxy NGC 1309 face, which is about 120 million. light years far from Earth in the constellation Eridanus, the Hubble Space Telescope he quickly entered the scene. Hubble had made images of NGC 1309 many times during the years before the supernova, and astronomers were able to identify the parent stellar system that exploded, finding that it contained a white dwarf snatching material from an aging star rich in heli, perhaps one red giant.
These observations marked the first time scientists had ever imagined the progenitor of a type Ia supernova. Still, something seemed wrong with the Hubble data.i Not only did the images show that the parent had somehow survived the explosion, but the white dwarf was also somehow even brighter than before. .
“No one expected to see a brighter surviving star,” said Curtis McCully, an astrophysicist at the California-based Las Cumbres Observatory, in a statement. “That was a real puzzle.”
Astronomers have identified what they suspect to be failed Ia-type supernovae that leave behind remaining “zombie” stars, a phenomenon called Iax-type supernovae. One of the most notable supernovae was SN 2008ha, which exploded in the galaxy UGC 12682 about 69 million light-years away from Earth in the constellation of Pegasus. However, SN 2012Z is the first time astronomers have confirmed that the exploding star survived, based on images taken both before and after the supernova.
McCully and his colleagues, after studying SN 2012Z, have a partial theory about what happened. The researchers suggest that the thermonuclear explosion was not powerful enough to blow up the white dwarf completely, and that much of the debris fell back on the star, inflating the white dwarf as it became what the astronomers call it a “bound remnant.” Over time, scientists expect the star to return to its original quince state.
Artist print of a white dwarf stealing stuff from a nearby star. (Image Credit: NASA Goddard Space Flight Center Concept Image Lab)
The surviving white dwarf could shine brighter than before due to light from a number of sources, including the brightly tied rest itself, the companion warmed by the shock. star which received the weight of the explosion and the radioactive decay of the material that escaped into space during the explosion.
Normally, the light from a type Ia supernova is fed by the radioactive decay of the isotopes of cobalt-56 and -57 in the material ejected into space. (Isotopes are varieties of an atom that have the same number of protons and electrons but a different number of neutrons.) These isotopes have half-lives of 77 and 271 days, respectively, so that after a few years, most of cobalt has disintegrated and the light from the supernova fades drastically.
However, SN 2012Z has faded much more slowly, so if radioactive decay is the main source of light, it must be an isotope with a much longer half-life. McCully’s team suggests iron-55, which has a half-life of 2.7 years.
Why SN 2012Z suffered from this failed supernova, instead of being destroyed in a typical Type Ia, remains a mystery.
“The implications for type Ia supernovae are profound,” McCully said. “We’ve found that supernovae can at least grow to the limit and explode. However, explosions are weak, at least sometimes. We now need to understand what causes a supernova to fail and become an Iax type, and what makes it a successful.
Type Ia supernovae have a standardizable change in their brightness over time, which astronomers call a light curve, which makes determining their distance much easier. As such, they have become crucial in the studies of cosmic expansion i dark energy. By better understanding how they explode, astronomers will be able to measure the expansion of the planet universe and the force of dark energy with even greater accuracy than before.
The findings were published on February 1 a Astrophysics Journal.
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