Exploding stars called Type 1a supernovae are ideal for measuring cosmic distances because they are bright enough to spot across the Universe and have approximately the same luminosity everywhere. Although astronomers have many theories about the kinds of star systems involved in these explosions (or progenitor systems), no one has ever directly observed one — until now.
In the August 24 issue of Science, the multi-institutional Palomar Transient Factory (PTF) team presented the first-ever direct observations of a Type 1a supernova progenitor system. This discovery was made possible with a combination of super-fast research networks, sophisticated algorithms, and high-performance computers and will improve the accuracy of cosmic measurements today.
Astronomers have collected evidence indicating that the progenitor system of a Type 1a supernova called PTF 11kx contains a red giant star. The system is located 600 million light years away in the constellation Lynx.
“We know that Type 1a supernovae vary slightly from galaxy to galaxy, and we've been calibrating for that, but this PTF 11kx observation provides the first explanation of why this happens,” says Peter Nugent, who leads the Lawrence Berkeley National Laboratory's (Berkeley Lab's) Computational Cosmology Group and is a co-author on the paper.
A one in a thousand discovery
Although Type 1a supernovae are rare, occurring maybe once or twice a century in a typical galaxy, Nugent notes that finding a Type 1a progenitor system like PTF 11kx is even rarer. “You maybe find one of these systems in a sample of 1,000 Type 1a supernovae,” he says.
The PTF survey uses a robotic telescope mounted on the 48-inch Samuel Oschin Telescope at Palomar Observatory in southern California to scan the night sky. As the observations are taken, the data travels more than 400 miles via high-speed networks — including the National Science Foundation's High Performance Wireless Research and Education Network and the Department of Energy's Energy Sciences Network (ESnet) — to the National Energy Research Scientific Computing Center (NERSC), located at Berkeley Lab. There, the real-time transient detection pipeline uses high-performance computers, a high-speed parallel file system and sophisticated machine-learning algorithms to sift the data and identify events for scientists to follow up on.
According to Nugent, the pipeline detected the supernova on 16 January 2011. He and UC Berkeley postdoctoral researcher, Jeffrey Silverman, immediately followed up on the event with spectroscopy observations from the Shane telescope at the University of California's Lick Observatory.
From the Keck observations, astronomers noticed that the clouds of gas and dust surrounding PTF 11kx were moving too slowly to be coming from the recent supernova, but moving too quickly to be stellar wind. They suspected that maybe the star had erupted, or went nova, previously, propelling a shell of material outwards. The material, they surmised, must be slowing down as it collided with wind from a nearby red giant star. But for this theory to be true, the material from the recent supernova should eventually catch up and collide with gas and dust from the previous nova. That's exactly what the PTF team eventually observed.
“This was the most exciting supernova I've ever studied. For several months, almost every new observation showed something we'd never seen before,” says Ben Dilday, a UC Santa Barbra postdoctoral researcher and lead author of the study.
A new kind of supernova
According to Dilday, it is not unusual for a star to frequently undergo nova eruptions In fact, a 'recurrent nova' system called RS Ophiuchi exists within our own Milky Way Galaxy. Located about 5,000 light years away, the system consists of a compact white dwarf star orbiting a red giant. Material being blown off the red giant star in a stellar wind lands on the white dwarf. As the material builds up, the white dwarf periodically explodes about every 20 years.
Astronomers predict that in recurring novae, the white dwarf loses more mass in the nova eruption than it gains from the red giant. Because Type 1a supernovae occur in systems where a white dwarf accretes mass from a nearby star until it can't grow any further and explodes, many scientists concluded that recurrent nova systems could not produce Type 1a supernovae. They thought the white dwarf would lose too much mass to ever become a supernova. PTF 11kx is the first observational evidence that Type 1a supernovae can occur in these systems.
Silverman says, “Because we've looked at thousands of systems and PTF 11kx is the only one that we've found, we think it is probably a rare phenomenon. However, these systems could be somewhat more common, and nature is just hiding their signatures from us.”