Before-and-after images taken by Hubble reveal a bright blue system that may be the origins of a weak and rare kind of stellar explosion, known as a type Iax supernova. The explosion may have left behind a “zombie star.” The findings are published in Nature this week.
Type Iax supernovae are the less luminous relatives of the more common type Ia supernovae. These explosions will typically destroy an old, dying star, but where astronomers expected to see nothing, images of this supernova reveal a survivor -- a zombie star, if you will.
While looking through Hubble Space Telescope images of the spiral galaxy NGC 1309 (pictured above), located 108 million light-years away, a team led by Saurabh Jha from Rutgers discovered a blue companion star feeding energy to a white dwarf star -- as seen in the “before” picture (2005 to 2006). The process must have ignited a nuclear reaction that released a weak supernova blast, because in the “after” photos (2013), the team found a type Iax supernova at the same location. They named the mini-supernova, SN 2012Z.
To understand this progenitor system (the source of the explosion), scientists compared it to a similar, less energetic supernova in our own galaxy. That nova, called V445 Puppis, is thought to have occurred when a white dwarf star accumulated gas from a companion helium star. The similarities suggest that this new mini-supernova also originated from a binary system, and that the white dwarf exploded as a result of the transfer of gas from its companion.
After studying the object’s colors and comparing it 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, leaving just its helium core behind.
A game of cosmic tug-of-war might explain the unusual nature of SN 2012Z. First, the more massive of the two stars would have evolved and expanded more rapidly, dumping gases onto its companion. This rapidly evolving star would have become the white dwarf. But eventually, the companion outpaced it, growing to the point that it engulfed the white dwarf. The two began to form a combined star that ejected its outer layers; afterwards, all that remained were the white dwarf and the companion star’s helium core. Then, at the end of the white dwarf’s life, it exploded as a Type Iax supernova, leaving behind only the companion’s helium core as a “zombie star.”
“Astronomers have been searching for decades for the star systems that produce Type Ia supernova explosions,” Jha says in a news release. “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.”