A group of astronomers from the intermediate Palomar Transient Factory (iPTF) has witnessed a supernova smashing into a nearby white dwarf, shocking it, and creating an UV glow that reveals the size of the companion.
An image from a simulation in which a Type Ia supernova explodes, as shown in brown. The supernova material is ejected outward at a velocity of about 10,000 km per second and slams into its companion star, as shown in light blue. The collision produces an UV pulse, which is emitted from the conical hole carved out by the companion star. Image credit: Dan Kasen, University of California Berkeley.
Type Ia supernovae are commonly theorized to be the thermonuclear explosions of a white dwarf star that is part of a binary system – two stars that are physically close and orbit around a common center of mass.
But how this white dwarf goes from binary star system to Type Ia supernova is a matter of debate.
Now, the iPTF team finds strong evidence pointing toward one origin theory, called the single degenerate channel.
This theory hypothesizes that the white dwarf accretes matter from its companion star and the resulting increase in its central pressure and temperature reaches a tipping point and ignites a thermonuclear explosion.
In contrast, the double degenerate theory proposes that the orbit between two white dwarf stars shrinks until the lighter star’s path is disrupted and it moves close enough for some of its matter to be absorbed into the primary white dwarf and initiate an explosion.
In 2014, the astronomers discovered a Type Ia supernova, designated iPTF 14atg, in the nearby galaxy IC 831, located 300 million light-years away.
Looking at the event from NASA’s Swift Space Telescope observation records, they detected bright UV emission from iPTF 14atg.
Because UV radiation is higher energy than visible light, it is particularly suited to observing very hot objects like supernovae. Such an early UV pulse within days of a supernova’s explosion is unprecedented. This strong pulse of emission is consistent with theoretical expectations of collision between material being ejected from a supernova explosion and the companion star from which it has been accreting matter.
“This provides good evidence that at least some Type Ia supernovae arise from the single degenerate channel. Now we have to determine the fraction of Type Ia that are akin to iPTF 14atg,” said Dr Mansi Kasliwal from Observatories of the Carnegie Institution for Science in Pasadena, California, who is a co-author of a paper published in the journal Nature.
The discovery provides direct evidence for the existence of a companion star in a Type Ia supernova, and demonstrates that at least some Type Ia supernovae originate from the single-degenerate channel.
Although the data from supernova iPTF 14atg support it being made by a single-degenerate system, other Type Ia supernovae may result from double-degenerate systems.
“In fact, observations in 2011 of SN 2011fe, another Type Ia supernova discovered in the nearby galaxy Messier 101, appeared to rule out the single-degenerate model for that particular supernova. And that means that both theories actually may be valid,” said Prof Sterl Phinney, who was not involved in the study.
“The news is that it seems that both sets of theoretical models are right, and there are two very different kinds of Type Ia supernovae.”
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Yi Cao et al. 2015. A strong ultraviolet pulse from a newborn type Ia supernova. Nature 521, 328-331; doi: 10.1038/nature14440
