Thermonuclear power sets off a type of stellar explosion known as a classical nova. Now, new research explains the mechanisms that cause these blasts to light up.
Since antiquity, observers of the night sky have occasionally spotted a new star in the firmament, only to have it fade away over the course of weeks or months. Such an event is known as a nova (Latin for “new”).
When a white dwarf in a binary system draws too much hydrogen gas from its companion star, it ends up choking on its meal: The stolen-gas envelope becomes dense enough that hydrogen atoms begin to fuse, producing a runaway thermonuclear explosion. This detonation produces a blazing burst of light — a nova.
But some of these objects are so distant that nuclear power might not be enough to explain how bright they appear from Earth. Some other mechanism must be adding extra candles to the cake.
In some cases, these novae become brighter than what ought to be possible for a planet-sized fusion bomb. These puzzling events are called superluminous novae (not to be confused with supernovae). Astronomers haven’t been able to explain them, until now: A recent study published in the September Nature Astronomy argues that in one superluminous nova, the thermonuclear power source drives collisions between waves of escaping gas, forcing them to suddenly release their energy.
Kwan-Lok Li (Michigan State University) and colleagues used the Fermi Gamma-ray Space Telescope to observe powerful gamma-ray emissions from a nova called ASASSN-16ma as it simultaneously brightened in visible light. The only plausible explanation for the high-energy radiation is that shells of matter travelling at different speeds are bumping into one another. As they do so, they also emit visible light.
According to the researchers, as the nova goes off, the outer envelope of gas around the white dwarf puffs up, becoming an extended atmosphere. Then, as the white dwarf’s stellar wind drives a second wave of material off its surface, that layer smashes against the previously expelled gas. The collision shocks the material, forcing it to accelerate and heat up. This shock releases extra energy that astronomers see as gamma rays and additional visible light.
Researchers had used Fermi to detect gamma rays coming from novae several times since 2010, but those events weren’t bright enough for the astronomers to realize that the novae’s visual light was tracking the gamma-ray emission.
What makes this observation unique is that ASASSN-16ma shines brightly at both visible and gamma-ray wavelengths, so the activity could be better monitored.
Study coauthor Brian Metzger (Columbia University) had predicted that a nova’s gamma-ray activity might parallel what’s happening in visible light. He proposed in 2014 that most of a nova’s visible-light emission could originate in shocks rather than in nuclear burning.
The new results confirm this idea. “The fact that we see visual and gamma-ray emission which appear to track each other in time strongly suggests that both originate from the same source,” Metzger says. “Since we know the gamma-rays must come from the shockwaves produced when different shells collide, this suggests that the visual emission must as well.”
These findings not only explain the mystery of superluminous novae, but they also offer a testbed to understand other mighty astrophysical explosions powered by similar shockwaves, such as the superluminous supernovae that occur when massive or exotic stars reach the end of their lives. “These superluminous supernovae are very far away, and difficult to observe and understand in detail,” says coauthor Laura Chomiuk (Michigan State University). “On the other hand, novae explode 50 times a year in our galactic backyard.”
Amateur Astronomers Bring a Key Piece of the Puzzle
Li and colleagues relied on the American Association of Variable Star Observers (AAVSO) to make the visible-light observations used in this study.
This non-profit association helps amateur astronomers find and observe variable targets that professional astronomers don’t have the time or resources to monitor with the necessary frequency. AAVSO collects and maintains this data for research.
In the case of ASASSN-16ma, the object was first detected by the All Sky Automated Survey for SuperNovae (ASASSN), which prompted the AAVSO to send out an alert for further observations. AAVSO member Paul Luckas — who was credited as a collaborator in the new study — obtained spectral data that identified the object as a nova. This information enabled the authors to obtain observation time with Fermi.
Visible light curves were essential to the research itself, too, enabling astronomers to establish their alignment with gamma-ray emissions. “We wouldn't have been able to tell that without AAVSO observations,” says Chomiuk.
Kwan-Lok Li et al. “A Nova Outburst Powered by Shocks.” Nature Astronomy, September 2017.
Ackermann et al. “Fermi Establishes Classical Novae as a Distinct Class of Gamma-Ray Sources.” Science, August 2014.
Metzger et al. “Shocks in nova outflows. I. Thermal emission.” MNRAS, June 2014.