Astronomy News

Hubble Finds Star System Linked to Possible 'Zombie Star' Supernova

Siv Schwink
8/7/2014  8:00 am


Hubble finds star system linked to possible ‘zombie star’ supernova


After decades of searching, astronomers have found for the first time ever the progenitor of a white dwarf supernova. The stellar explosion is located 110 million light years away from Earth. The discovery solves the mystery of what creates one of these energetic celestial explosions. The exciting findings come from an international collaboration led by researchers at Rutgers University in Piscataway, New Jersey and at the University of Illinois at Urbana-Champaign. The work is published in the August 7 issue of the journal Nature. <http://www.nature.com/doifinder/10.1038/nature13615>.


The same researchers, examining a second similar supernova, may additionally have detected for the first time the remains of one of these weak supernovas—an undead "zombie star." These findings will be published in the August 11 issue of The Astrophysical Journal. <http://iopscience.iop.org/0004-637X/>


The two discoveries, taken together, outline the life span from birth to death of this variety of white dwarf supernova.


progenitor of a supernova


Using NASA’s Hubble Space Telescope, the team took images of a recently discovered white dwarf supernova and, given the extraordinary resolution and clarity of the Hubble images, were able to determine which of several candidate stars occupying nearly the same position from our vantage point on Earth had exploded, generating the supernova. Using archived Hubble images of the progenitor star system, the team was able to determine the nature of the exploding system—a white dwarf and its blue companion star.


“The most common kind of white dwarf supernova is Type Ia. We use this class of supernova to measure cosmic distances, to map out the expansion history of the Universe, and to understand dark energy,” comments co-author Ryan Foley, an assistant professor of astronomy and affiliate assistant professor of physics at the U. of I. who is a noted expert in white dwarf supernovas. “Astronomers have unsuccessfully searched for the progenitors of Type Ia supernovas for a long time. We've looked in nearby galaxies and in our own Milky Way, but we've never seen their progenitor systems."


The weak supernova, dubbed SN 2012Z, was first spotted in January 2012 during the Lick Observatory Supernova Search. It was categorized as a Type Iax, a recently discovered class of stellar explosion. These exploding stars are less energetic and fainter than Type Ia supernovas, which also originate from exploding white dwarfs in binary systems. So far, astronomers have identified more than 30 of these mini-supernovas, which occur at one-quarter the rate of Type Ia supernovas.


Rutgers astronomer Saurabh Jha is co-author of the study. He explains, “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 white dwarf slowly siphoned fuel from its companion, eventually igniting a runaway nuclear reaction in the white dwarf, and producing a weak supernova blast. This kind of stellar system was first predicted by U. of I. Distinguished Professor of Astronomy Icko Iben more than two decades ago.


Noting that Hubble’s Advanced Camera for Surveys had observed the supernova’s host galaxy, NGC 1309, in 2005, 2006, and 2010, before the supernova outburst, the team pointed Hubble at the supernova again in 2012 to precisely measure the position of SN 2012Z. Curtis McCully, a graduate student at Rutgers and lead author of the team’s Nature 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,” McCully remarks. “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.”


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, our team predicted that these supernovas were produced by a white dwarf and helium star binary system,” says Foley, who helped identify Type Iax supernovas as a new class. “There’s still a little uncertainty with this Hubble study, but it is essentially validation of our claim.”
The discovery also validates the theoretical work of Iben, who in 1991 predicted the correct progenitor system, that these explosions should be relatively weak, and even the correct occurrence rate.


“It’s terrific to confirm a theoretical prediction, but to confirm a long-standing prediction by your colleague is absolutely fantastic,” asserts Foley.


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 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 star comes back to life after it “dies” in the supernova, 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 position of the supernova when its light has dimmed enough to show any possible zombie star and helium companion.


a “zombie star” appearance


The astronomers already have seen the aftermath of one Type Iax supernova blast, in the galaxy UGC 12682, 69 million light-years from Earth. Hubble images taken of SN 2008ha in January 2013, more than four years after it exploded, show an object at the supernova’s location.


The object could be either the zombie star or the companion. In a second paper, the team shows that the object is much redder than the helium star in the SN 2012Z system, and consistent with models of zombie stars. While the star could be a more evolved companion star, it could also be the first detection of a zombie star. The findings will be published in The Astrophysical Journal on August 11.


“SN 2012Z is one of the more powerful Type Iax supernovas and SN 2008ha is one of the weakest of the class, showing that Type Iax systems are very diverse,” explains Foley, who is lead author on this study. “And perhaps that diversity is related to how each of these stars explodes. Because these supernovas 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 supernovas and their progenitors, which could eventually contribute in a substantial way to our understanding of dark energy.



The collaboration on the Nature article includes scholars from Rutgers University; University of Illinois at Urbana-Champaign; The Kavli Institute for Theoretical Physics at the University of California, Santa Barbara; The Harvard-Smithsonian Center for Astrophysics; The University of Texas at Austin; The Space Telescope Science Instititute on the campus of The Johns Hopkins University and operated by the Association of Universities for Research in Astronomy for NASA; and Aarhus University in Denmark.


The collaboration on the Astrophysical Journal article includes scholars from the University of Illinois at Urbana-Champaign; Rutgers University; The Kavli Institute for Theoretical Physics at the University of California, Santa Barbara; The Harvard-Smithsonian Center for Astrophysics; the National Optical Astronomy Observatory; The Space Telescope Science Instititute on the campus of The Johns Hopkins University and operated by the Association of Universities for Research in Astronomy for NASA; and Aarhus University in Denmark.


For more information:
Science contact: Ryan Foley
Assitant Professor, U. of I. Department of Astronomy
(217) 333-3090
rfoley@illinois.edu