Texas Tech University

Newly Discovered Merging Binary System Shines Light on Interstellar Explosions

Glenys Young

July 12, 2021

Texas Tech astronomer Thomas Kupfer is part of an international collaboration studying the universe’s expansion.

Astronomers have made the rare sighting of two stars spiraling to their doom by spotting the tell-tale signs of a teardrop-shaped star.

The tragic shape is caused by a massive nearby white dwarf distorting the star with its intense gravity, which also will be the catalyst for an eventual supernova that will consume both. Found by an international team of astronomers and astrophysicists led by the University of Warwick, it is one of only a very small number of star systems discovered that will one day see a white dwarf star reignite its core. Texas Tech University's Thomas Kupfer, an assistant professor in the Department of Physics & Astronomy, is part of the collaboration.

New research published by the team today (July 12) in the journal Nature Astronomy confirms that the two stars are in the early stages of a spiral that will likely end in a Type Ia supernova, a type that helps astronomers determine how fast the universe is expanding.

Artist's impression of the HD265435 system at around 30 million years from now, with the smaller white dwarf distorting the hot subdwarf into a distinct teardrop shape. Credit: University of Warwick/Mark Garlick

“When it was discovered about two decades ago, the accelerated expansion of the universe was one of the most striking and unexpected findings,” Kupfer said. “The discovery was made possible by using one of the most luminous explosions in the universe: Type Ia supernovae. The only thing we know about Type Ia supernovae is that they originate in a thermonuclear explosion of a white dwarf. When a white dwarf reaches a critical mass limit, called the Chandrasekhar limit, this triggers the collapse and explosion. However, we still don't know for sure, how does the white dwarf reaches the critical mass, and is the critical mass unique among white dwarfs?

“This study discovered a system that will reach the Chandrasekhar limit when the objects merge together; therefore, this is a great example of a progenitor system for a Type Ia supernova. It brings us one step closer to understanding how white dwarfs explode to produce Type Ia supernovae and closer to an understanding of the accelerated expansion of the universe.”

The binary system, called HD265435, is located roughly 1,500 light-years away and comprises a hot subdwarf star and a white dwarf star orbiting each other closely at a rate of around 100 minutes per revolution. White dwarfs are essentially “dead” stars that have burnt out all their fuel and collapsed in on themselves, making them small but extremely dense.

A Type Ia supernova is generally thought to occur when a white dwarf star's core reignites, leading to a thermonuclear explosion. There are two scenarios where this can happen. In the first, the white dwarf gains enough mass to reach the Chandrasekhar limit, 1.4 times the mass of our Sun. HD265435 fits in the second scenario, in which the total mass of a close stellar system of multiple stars is near or above this limit. Only a handful of other star systems have been discovered that will reach this threshold and result in a Type Ia supernova.

“We don't know exactly how these supernovae explode, but we know it has to happen because we see it happening elsewhere in the universe,” said lead author Ingrid Pelisoli from the University of Warwick.

“One way is if the white dwarf accretes enough mass from the hot subdwarf, so as the two of them are orbiting each other and getting closer, the matter will start to escape the hot subdwarf and fall onto the white dwarf. Another way is that, because they are losing energy to gravitational wave emissions, they will get closer until they merge. Once the white dwarf gains enough mass from either method, it will go supernova.”

Using data from NASA's Transiting Exoplanet Survey Satellite (TESS), the team was able to observe the hot subdwarf, but not the white dwarf as the hot subdwarf is much brighter. However, that brightness varies over time which suggested the star was being distorted into a teardrop shape by a nearby massive object. Using radial velocity measurements from the Palomar Observatory and the W.M. Keck Observatory, and by modeling its effect on the hot subdwarf, Kupfer and his fellow astronomers could confirm that the hidden white dwarf is as heavy as our Sun but just slightly smaller than the Earth's radius.

Combined with the mass of the hot subdwarf, which is a little more than 60% of the mass of our Sun, both stars have the mass needed to cause a Type Ia supernova. As the two stars are already close enough to begin spiraling closer together, the white dwarf will inevitably go supernova in around 70 million years. Theoretical models produced specifically for this study predict that the hot subdwarf will contract to become a white dwarf star as well before merging with its companion.

Type Ia supernovae are important for cosmology because of their use as “standard candles.” Their brightness is constant and of a specific type of light, which means astronomers can compare what luminosity they should be with what we observe on Earth, and from that, work out how distant they are with a good degree of accuracy. By observing supernovae in distant galaxies, astronomers combine what they know of how fast this galaxy is moving with our distance from the supernova and calculate the expansion of the universe.

“The more we understand how supernovae work, the better we can calibrate our standard candles,” Pelisoli said. “This is very important at the moment because there's a discrepancy between what we get from this kind of standard candle and what we get through other methods.

“The more we understand about how supernovae form, the better we can understand whether this discrepancy we are seeing is because of new physics that we're unaware of and not considering, or simply because we're underestimating the uncertainties in those distances.

“There is another discrepancy between the estimated and observed galactic supernovae rate and the number of progenitors we see. We can estimate how many supernovae are going to be in our galaxy through observing many galaxies, or through what we know from stellar evolution, and this number is consistent. But if we look for objects that can become supernovae, we don't have enough. This discovery was very useful to put an estimate of what hot subdwarf and white dwarf binaries can contribute. It still doesn't seem to be a lot, none of the channels we observed seem to be enough.”

This research received funding from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) and the Science and Technology Facilities Council, part of U.K. Research and Innovation.

Kupfer recently was awarded a separate $264,318 grant from NASA to facilitate the search for, and detailed study of, similar systems. The grant will partly support the work of a graduate student and postdoctoral researcher so scientists may gain a better understanding of these systems as a whole.

“This great scientific discovery not only makes an important scientific contribution to astrophysics by understanding more about how supernovae form and work but also by publishing the study in Nature Astronomy, a top-level journal with a high impact factor,” said Sung-Won Lee, chair of the Department of Physics & Astronomy. “It is also meaningful for our department to be recognized for its international status. Dr. Kupfer is a promising astrophysicist and I have no doubt that he will continue his scientific quest to understand many scientific phenomena that have yet to be solved based on this discovery. I sincerely congratulate him on behalf of the Department of Physics & Astronomy.”

About The Science and Technology Facilities Council

The Science and Technology Facilities Council (STFC) is part of U.K. Research and Innovation – the U.K. body which works in partnership with universities, research organizations, businesses, charities and government to create the best possible environment for research and innovation to flourish. For more information, visit the U.K. Research and Innovation website.

STFC funds and supports research in particle and nuclear physics, astronomy, gravitational research and astrophysics, and space science. It also operates a network of five national laboratories, including the Rutherford Appleton Laboratory and the Daresbury Laboratory, as well as supporting U.K. research at a number of international research facilities including CERN, FERMILAB, the ESO telescopes in Chile and many more. Visit the STFC's website for more information.