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Source: MIL-OSI Submissions
Source: University of Canterbury (UC)

University of Canterbury (UC) astronomers are part of an international team that has revealed how explosions on the surface of a white dwarf star can increase its brightness by thousands or millions of times making it look like a new star.

For many years astronomers have thought that nuclear fusion of material on the surface of a white dwarf directly powers all the light from a nova explosion, which happen about 10 times a year in our galaxy.

A nova, or stella nova – Latin for “new star” – is a sudden explosion on the surface of a white dwarf, which is the hot, burnt-out core of a star. It produces an incredible amount of energy and light, increasing the star’s brightness by thousands or even millions of times. If a nova occurs relatively close to earth it can appear as a new star to the naked eye.

In new research, a team of international astronomers has shown that “shock waves” from the nova explosion, rather than nuclear fusion, cause most of the brightness.

The team used NASA’s space-based telescopes and ground-based telescopes, including some at the UC Mt John Observatory in Tekapo, to observe a recent nearby nova in the constellation of Carina and proved that it is indeed shock waves that cause most of the nova’s brightness.

Their results are documented in a new paper called “Direct evidence for shock-powered optical emission in a nova” published this month in the international journal Nature Astronomy.

UC Associate Professor in Astronomy and Director of the University of Canterbury Mt John Observatory Karen Pollard, who co-authored the paper, was observing at UC’s Mt John Observatory using the McLellan telescope and HERCULES spectrograph a few days after the bright nova in Carina was reported.

“I was excited to observe it – a new bright novae in the galaxy is an important opportunity to make a detailed study of the nova’s properties and how these change with time. Using spectroscopy we were able to examine shock-produced emission and calculate how energetic the shock waves were and how fast the shocked material was moving,” she says.

Elias Aydi, a research associate in Michigan State University’s (MSU) Department of Physics and Astronomy and lead author of the paper, says the discovery leads to a new way of understanding the origin of the brightness of novae and other stellar explosions. “Our findings present the first direct observational evidence, from unprecedented space observations, that shocks play a major role in powering these events.”

When material blasts out from the white dwarf, he says it is ejected in multiple phases and at different speeds. These ejections collide with one another and create shocks, which heat the ejected material producing much of the light.

Another side effect of astronomical shocks are gamma-rays, the highest-energy kind of electromagnetic radiation. The astronomers detected bright gamma-rays from the star, known as nova V906 Carinae (ASASSN-18fv), whose explosion in the constellation Carina was first detected in March 2018.

An optical satellite happened to be looking at the part of the sky where the nova occurred. Comparing the gamma-ray and optical data, the astronomers noted that every time there was a fluctuation in gamma-rays, the light from the nova fluctuated as well.

The simultaneous fluctuations in both the visual and gamma-ray brightness confirmed that both were originating from shocks.

The research team estimates that V906 Car is about 13,000 light years from Earth. This means that when the nova was first detected in 2018, it had actually happened 13,000 years ago. The new information may also help explain how large amounts of light are generated in other stellar events, including supernovae and stellar mergers, when two stars collide with one another. Each nova explosion releases about 10,000 to 100,000 times the annual energy output of the Sun.

MIL OSI