Video: Historic Observation Of Rare Type 1a Supernova, A One In A Thousand Discovery

In the August 24 issue of Science, the multi-institutional Palomar Transient Factory (PTF) team presents the first-ever direct observations of a Type 1a supernova progenitor system. Astronomers have collected evidence indicating that the progenitor system of a Type 1a supernova, called PTF 11kx, contains a red giant star. They also show that the system previously underwent at least one much smaller nova eruption before it ended its life in a destructive supernova. The system is located 600 million light years away in the constellation Lynx.

By comparison, indirect observations of another Type 1a supernova progenitor system (called SN 2011fe, conducted by the PTF team last year) showed no evidence of a red giant star. Taken together, these observations unequivocally show that just because Type 1a supernovae look the same, that doesn’t mean they are all born the same way.

“We know that Type 1a supernovae vary slightly from galaxy to galaxy, and we’ve been calibrating for that, but this PTF 11kx observation is providing the first explanation of why this happens,” says Peter Nugent, a senior scientist at the Lawrence Berkeley National Laboratory (Berkeley Lab) and a co-author on the paper. “This discovery gives us an opportunity to refine and improve the accuracy of our cosmic measurements.”

“It’s a total surprise to find that thermonuclear supernovae, which all seem so similar, come from different kinds of stars,” says Andy Howell, a staff scientist at the Las Cumbres Observatory Global Telescope Network (LCOGT) and a co-author on the paper. “How could these events look so similar, if they had different origins?”

A One in a Thousand Discovery, Powered by Supercomputers

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 more rare. “You maybe find one of these systems in a sample of 1,000 Type 1a supernovae,” he says. “The Palomar Transient Factory Real-Time Detection Pipeline was crucial to finding PTF 11kx.”

The PTF survey uses a robotic telescope mounted on the 48-inch Samuel Oschin Telescope at Palomar Observatory in southern California to scan the sky nightly. 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 supercomputers, a high-speed parallel filesystem 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 January 16, 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. These observations revealed incredibly strong calcium signals in the gas and dust surrounding the supernova, which is extremely unusual.

The signals were so peculiar that Nugent and his UC Berkeley colleagues, Alex Filippenko and Joshua Bloom, triggered a Target of Opportunity (ToO) observation using the Keck Telescope in Hawaii. “We basically called up a fellow UC observer and interrupted their observations in order to get time critical spectra,” Nugent explains.

The supernova PTF 11kx can be seen as the blue dot on the galaxy. The image was taken when the supernova was near maximum brightness by the Faulkes Telescope North. The system is located approximately 600 million light years away in the constellation Lynx.

Credit: BJ Fulton (Las Cumbres Observatory Global Telescope Network)

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 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.

In the months following the supernova, the PTF team watched the calcium signal drop and eventually vanish. Then, 58 days after the supernova went off, Berkeley Lab Scientist Nao Suzuki who was observing the system with the Lick telescope noticed a sudden, strong burst in calcium coming from the system, indicating that the new supernova material had finally collided with the old material.

“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 researchers and lead author of the study.

A New Kind of Type 1a Supernova

According to Dilday, it is not unusual for a star to undergo nova eruptions more than once. 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 is close enough that astronomers can tell that it consists of a compact white dwarf star (the corpse of a sun-like 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, or novas, in this case, about every 20 years.

Astronomers predict that in recurring novas, 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.

“Because we’ve looked at thousands of systems and PTF 11kx is the only one that we’ve found that looks exactly like this, 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,” says Silverman.

The Palomar Transient Factory’s Real-Time detection pipeline is made possible with support from the DOE Office of Science, NASA, and the National Science Foundation.

 In the August 24 issue of the journal Science, astronomers show for the first time that at least some thermonuclear (Type Ia) supernovae come from a recurrent nova. The results of the study, led by Ben Dilday, a postdoctoral researcher in physics at UC Santa Barbara and at Las Cumbres Observatory Global Telescope Network (LCOGT), are surprising because previous indirect –– but strong –– evidence had pointed to the merger of two white dwarf stars as the originators of other Type Ia supernovae.

The authors conclude that there are multiple ways to make a Type Ia supernova –– a finding that could have implications for understanding the differences seen in these “standard candles,” that were used to reveal the presence of dark energy.

Supernova PTF 11kx

Supernova PTF 11kx was discovered by the Palomar Transient Factory (PTF) in a galaxy 600 million light years away –– relatively nearby in astronomical terms, but like all Type Ia supernovae, much too far away to make out the details of the stars before they exploded. However, the astronomers could discern that the supernova was surrounded by shells of gas, some of it containing hydrogen that had likely been cast off in previous nova eruptions, decades before the supernova occurred. These are much more frequent weak explosions that do not destroy the star. While similar shells of material had been seen before in a handful of Type Ia supernovae, their origin was debated, and they had never before been firmly linked to novae. Some doubted that the material was even near to the supernova at all.

PTF 11kx was different. The surrounding gas was moving too slowly to be from the supernova, but too fast to be a typical stellar wind. Lars Bildsten, director of UCSB’s Kavli Institute for Theoretical Physics, hypothesized that it was material shot out from a previous nova eruption, which had been slowed as it collided with the wind from the red giant star. Calculations from UCSB graduate student Kevin Moore showed this hypothesis to be plausible, and would lead to gas moving at speeds seen in the observations. Adding credence to the theory was the fact that the material moved at two different speeds –– faster-moving material interior to slower-moving material –– exactly as expected. The farthest-out material had been slowing for decades, while the interior material had less time to slow.

But if this was the case, the very fast-moving supernova ejecta should have eventually collided with the nova material. About two months after the explosion, this is exactly what happened. New observations showed that the supernova ejecta was smashing into the interior shell of material. It was impossible to doubt that this gas was near the supernova.

“This was the most exciting supernova I’ve ever studied,” said Dilday. “For several months, almost every new observation showed something we’d never seen before.”

The team was further convinced of the nova hypothesis based on its similarity to RS Ophiuchi, a recurrent nova in our own galaxy. Unlike distant supernovae, RS Oph is close enough that its properties are known: It is a compact white dwarf star (similar to what the core of the sun will become in 5 billion years) in orbit with a red giant star. Material that is being blown off the red giant in a stellar wind is landing on the white dwarf, and, as the material builds up, it periodically explodes as a nova eruption. RS Oph has these explosions every 20 years or so, with the last occurring in 2006. But theoretical studies indicated that white dwarfs lose more mass in a nova eruption than they gain from the red giant. Since a Type Ia supernova is thought to occur when the white dwarf grows in mass until it can’t grow any further, many astronomers concluded that novae could not produce Type Ia supernovae. The new study is the first observational evidence that they can.

Could novae explain all supernovae? The PTF team previously showed that SN 2011fe, the closest Type Ia supernova in 25 years, could not be a nova of the type suspected in PTF 11kx. And any red giant in the binary system is expected to survive the explosion of the white dwarf. A recent study found no such survivor in a nearby supernova remnant and concluded that it must have been caused by the merger of two white dwarf stars. Finally, other studies showed that some supernovae are so bright that they almost certainly result from the merger of two white dwarf stars.

Predicting the exact number of supernovae that may arise from novae is difficult, since the authors think they got lucky and saw the nova at just the right angle so that they could infer its presence. But novae look very different from different directions, so if they had seen the explosion from a different perspective, they might have missed it. Still, they estimate that novae give rise to more than one-tenth of a percent of all Type Ia supernovae, but less than 20 percent.

Andy Howell, second author on the study, said: “It is a total surprise to find that thermonuclear supernovae, which all seem so similar, come from different kinds of stars. It is like discovering that some humans evolved from ape-like ancestors, and others came from giraffes. How could they look so similar if they had such different origins?” Howell is the leader of the supernova group at LCOGT, and is an adjunct faculty member in physics at UCSB.

Recently, some studies have found that Type Ia supernovae are not perfect standard candles –– their brightness depends on the type of galaxy in which they were discovered. The reason is a mystery, but the finding that some Type Ia supernovae come from different progenitors would seem to suggest that the supernova’s ultimate brightness may be affected by whether or not it comes from a nova or a white dwarf merger.

“We don’t think this calls the presence of dark energy into question,” said Dilday. “But it does show that if we want to make progress understanding it, we need to understand supernovae better.”

DOE/Lawrence Berkeley National Laboratory
 
The Palomar Transient Factory is an international collaboration of scientists and engineers from Berkeley Lab, California Institute of Technology (Caltech) , NASA’s Infrared Processing and Analysis Center, UC Berkeley, Las Cumbres Observatory Global Telescope Network, the University of Oxford, Columbia University, the Weizmann Institute of Science in Israel, and Pennsylvania State University.http://www.astro.caltech.edu/ptf/.

About Berkeley Lab

Lawrence Berkeley National Laboratory addresses the world’s most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab’s scientific expertise has been recognized with 13 Nobel prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy’s Office of Science. For more, visit www.lbl.gov.

About NERSC

The National Energy Research Scientific Computing Center (NERSC) is the primary high-performance computing facility for scientific research sponsored by the U.S. Department of Energy’s Office of Science. Located at Lawrence Berkeley National Laboratory, the NERSC Center serves more than 4,000 scientists at national laboratories and universities conducting fundamental research in a wide range of disciplines.http://www.nersc.gov

About ESnet

The Energy Sciences Network (ESnet) provides the high-bandwidth, reliable connections that link scientists at national laboratories, universities and other research institutions, enabling them to collaborate on some of the world’s most important scientific challenges including energy, climate science, and the origins of the universe. Funded by the U.S. Department of Energy’s (DOE) Office of Science and located within the Scientific Networking Division at Lawrence Berkeley National Laboratory, ESnet provides scientists with access to unique DOE research facilities and computing resources. http://es.net