Type 1a Supernova May Be Linked To Nova Events
Type 1a supernova events are huge stellar explosions – ones with enough power to be seen across the Universe.Wow, that’s power!
Now, astronomers have – for the first time – found that a least some of these thermonuclear cataclysms may have originated from a from a recurrent nova. Many theories exist about how certain kinds of star systems could be involved, but no one has ever directly observed one… until now.
Published in the August 24 issue of “Science”, the multi-institutional Palomar Transient Factory (PTF) team has cataloged its first-ever direct observation of a Type 1a supernova progenitor system some 600 million light years away – one that also had at least one smaller nova event before the final destruction. Just prior to this observation last year, another indirect observation of a Type 1a supernova progenitor system by the PTF team revealed no evidence of a red giant star. Between these two observations, researchers are confident that Type 1a supernovae events may look the same, but weren’t created equally.
“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.”
Artist’s conception of a binary star system that produces recurrent novae, and ultimately, the supernova PTF 11kx. A red giant star (foreground) loses some of its outer layers through a stellar wind, and some of it forms a disk around a companion white dwarf star. This material falls onto the white dwarf, causing it to experience periodic nova eruptions every few decades. When the mass builds up to near the ultimate limit a white dwarf star can take, it explodes as a Type Ia supernova, destroying the white dwarf. (Animation credit: Romano Corradi and the Instituto de Astrofísica de Canarias)
“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?”
Just how unusual is it to spot a Type 1a progenitor system? Then think on this… A typical Type 1a supernova event only happens about once or twice a century in a typical galaxy. Finding a system like PTF 11kx even furthers the odds. Says Nugent: “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.”
Every night a robotic telescope mounted on the 48-inch Samuel Oschin Telescope at Palomar Observatory in southern California keeps watch over the skies. Its observations are cataloged and 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. From there, the Real-time Transient Detection Pipeline uses supercomputers, a high-speed parallel file system and sophisticated machine learning algorithms to sift the data and identify events for scientists to look more closely at.
On January 16, 2011 the pipeline picked up on the supernova and alerted Nugent. Along with UC Berkeley postdoctoral researcher Jeffrey Silverman, they immediately followed up on the event with spectroscopy observations from the Shane telescope at the University of California’s Lick Observatory. The spectroscopic observations revealed something unusual – elevated calcium signals in the gas and dust surrounding the supernova. These findings were so strange that the team expanded to UC Berkeley colleagues, Alex Filippenko and Joshua Bloom, which expanded into 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.
However, the Keck observations weren’t exactly “normal” either. The revealed clouds of gas and dust surrounding PTF 11kx were moving too slow to be part of the supernova, yet too fast to be stellar wind. The astronomers then surmised the star had possibly had an eruption, or gone nova expelling its shell outward. To further clarify, they also estimated the material as slowing down as it met with winds from a nearby red giant star.
However, for this theory to be complete, the material from the supernova event must eventually meet and collide with the material from the nova…. an observation backed up by the PTF team. They watched as the months went by and the calcium signal dropped into oblivion. Then, 58 days after the supernova began, 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.
Dilday isn’t done speculating, though. He’s confident that it isn’t unusual for star to have more than one nova eruptions and recurrent novae are an astronomical fact – ones such as RS Ophiuchi. This system is near enough for astronomers to positively identify a compact white dwarf star orbiting a red giant. We’ve been able to observe material being blown off the red giant star as stellar winds which impact the white dwarf. As this material accumulates, the white dwarf periodically explodes in a nova event… In this case, one that happens about every two decades.
From these types of observation, astronomers are able to predict that the white dwarf loses more mass during nova than it takes on from its red giant companion. According to the news release, a Type 1a supernovae occurs in systems where a white dwarf accretes mass from a nearby star until it can’t grow any further and explodes, and 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.
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.
Just how, indeed? Thanks to new studies we’re aware that Type 1a supernovae aren’t perfect standard candles. As a matter of fact, their brightness is highly dependent of what type of galaxy originated them. The reason behind this is still an enigma, but this recent discovery of different progenitors point to the fact that brightness may be a factor of whether or not the supernova originated from a nova or white dwarf merger.
Original Story Source: Berkeley Lab News Center.