Archival photographs from NASA’s Hubble Space Telescope have been used to uncover the progenitor to the supernova SN 2012Z – the binary star system containing a helium star transferring material to a white dwarf.
This image shows the spiral galaxy NGC 1309; the X-shaped feature at the top of the image of the galaxy marks the location of the supernova SN 2012Z; the inset panel from 2013 shows the supernova; archival Hubble data from 2005 and 2006 show the progenitor system for the supernova. Image credit: NASA / ESA / C. McCully and S. Jha, Rutgers University / R. Foley, University of Illinois / Z. Levay, STScI / Hubble Heritage Team / A. Riess, JHU / STScI.
SN 2012Z was seen in January 2012 in the spiral galaxy NGC 1309, located about 110 million light-years from Earth. It is a member of a rare class of supernovae called Type Iax.
Type Iax supernovae are less energetic and fainter than their well-known cousins, Type Ia supernovae, which also originate from exploding white dwarfs in binary systems.
Scientists originally thought these weaker stellar blasts were unique Type Ia supernovae. So far, they have identified more than 30 of these explosions, which occur at one-fifth the rate of Type Ia supernovae.
“Astronomers have been searching for decades for Type Ia’s progenitors. 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,” said Dr Saurabh Jha of Rutgers University in Piscataway, New Jersey, who is a co-author of the paper published in the journal Nature.
“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.”
NASA’s Hubble Space Telescope observed NGC 1309 – the SN 2012Z’s host galaxy – in 2005, 2006, and 2010, before the supernova outburst.
Dr Saurabh Jha and his colleagues from Denmark and the United States 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,” said lead author Curtis McCully of Rutgers University.
“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.”
After studying the object’s colors and computer simulations showing possible Type Iax progenitor systems, the astronomers 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.
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.
The astronomers said 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, they plan to use Hubble again in 2015 to observe the area when the supernova’s light has dimmed enough to show any possible white dwarf and helium companion.
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Curtis McCully et al. A luminous, blue progenitor system for the type Iax supernova 2012Z. Nature 512, 54–56; doi: 10.1038/nature13615
