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.
This video shows an 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 though 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.
(Photo Credit: Romano Corradi and the Instituto de Astrofísica de Canarias)
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.
(Photo Credit: BJ Fulton (Las Cumbres Observatory Global Telescope Network))