When small dense stars, known as white dwarfs explode, they create type Ia supernovae which can outshine a galaxy. With a new robotic observing system, known as the intermediate Palomar Transient Factory (iPTF), a team of astronomers discovered a Type Ia supernova, designated iPTF14atg, in nearby galaxy IC 831, located 300 million light-years away. The information that was immediately collected by the iPTF team lends support to one of two competing theories about the origin of white dwarf supernovae and also suggests the possibility that there are actually two distinct populations of this type of supernova.
Both theories start with the same general scenario: the white dwarf that eventually explodes is one of a pair of stars orbiting around a common center of mass. The interaction between these two stars, the theories say, is responsible for triggering supernova development. What is the nature of that interaction? At this point, the theories diverge.
According to one theory, the so-called double-degenerate model, the companion to the exploding white dwarf is also a white dwarf, and the supernova explosion initiates when the two similar objects merge.
However, in the second theory, called the single-degenerate model, the second star is instead a sun-like star - or even a red giant, a much larger type of star. In this model, the white dwarf's powerful gravity pulls material from the second star. This process increases the temperature and pressure in the center of the white dwarf until a runaway nuclear reaction begins, ending in a dramatic explosion.
The difficulty in determining which model is correct stems from the facts that supernova events are very rare - occurring about once every few centuries in our galaxy - and that the stars involved are very dim before the explosions.
That is where the iPTF comes in. From Palomar Mountain in southern California, where it is mounted on the 48-inch Samuel Oschin Telescope, the project's fully automated camera optically surveys roughly 1,000 square degrees of sky per night looking for transients, including Type Ia supernovae, whose brightness changes over timescales that range from hours to days.
"My colleagues and I spent many sleepless nights on designing our system to search for luminous ultraviolet emission from baby Type Ia supernovae," Caltech graduate student and iPTF team member Yi Cao said, according to the press release. "As you can imagine, I was fired up when I first saw a bright spot at the location of this supernova in the ultraviolet image. I knew this was likely what we had been hoping for."
UV radiation has higher energy than visible light, so it is particularly suited to observing very hot objects like supernovae (although such observations are possible only from space, because Earth's atmosphere and ozone later absorbs almost all of this incoming UV). Swift measured a pulse of UV radiation that declined initially but then rose as the supernova brightened. Because such a pulse is short-lived, it can be missed by surveys that scan the sky less frequently than the iPTF.
This observed ultraviolet pulse is consistent with a formation scenario in which the material ejected from a supernova explosion slams into a companion star, generating a shock wave that ignites the surrounding material. In other words, the data are in agreement with the single-degenerate model.
Although the data from supernova iPTF14atg 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 SN2011fe, another Type Ia supernova discovered in the nearby galaxy Messier 101 by PTF (the precursor to the iPTF), appeared to rule out the single-degenerate model for that particular supernova. And that means that both theories actually may be valid, explained Caltech professor of theoretical astrophysics Sterl Phinney, who was not involved in the research. "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."
"Both rapid discovery of supernovae in their infancy by iPTF, and rapid follow-up by the Swift satellite, were essential to unveil the companion to this exploding white dwarf. Now we have to do this again and again to determine the fractions of Type Ia supernovae akin to different origin theories," said iPTF team member Mansi Kasliwal, according to the press release.
The details are outlined in a paper appearing in the May 21 issue of the journal Nature.
The iPTF project is a scientific collaboration between Caltech; Los Alamos National Laboratory; the University of Wisconsin-Milwaukee; the Oskar Klein Center in Sweden; the Weizmann Institute of Science in Israel; the TANGO Program of the University System of Taiwan; and the Kavli Institute for the Physics and Mathematics of the Universe in Japan. The Caltech team is funded in part by the National Science Foundation.
This work was supported by the U. S. government, the National Science Foundation; the EU/FP7 via an ERC grant; the "Quantum Universe" I-Core program, ISF, a Minerva grant, a Weizmann-UK grant, the Kimmel Award; the Carnegie-Princeton fellowship.
