In a recent study, 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 the University of California, 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 astronomers 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 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 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 because the study's 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.”