Study Sheds Light on the Explosion of White Dwarfs

Astrophysicists have detected a striking flash of ultraviolet (UV) light that accompanies the explosion of a white dwarf. This is the second study of its kind to be performed, to date.

The event, which is a very rare form of supernova, is set to provide a better understanding of several mysteries that have persisted for a long time, for example, how does dark energy speed up the cosmos? What causes the explosion of white dwarfs? And how does the universe produce heavy metals, like iron?

The UV flash is telling us something very specific about how this white dwarf exploded. As time passes, the exploded material moves farther away from the source. As that material thins, we can see deeper and deeper. After a year, the material will be so thin that we will see all the way into the center of the explosion.

Adam Miller, Study Lead and Astrophysicist, Northwestern University

At that juncture, the research group will learn more about the explosion of this white dwarf and all other white dwarfs, which are thick fragments of dead stars, Miller added.

The study was recently published in The Astrophysical Journal on July 23^rd, 2020.

Miller is also a fellow in Northwestern’s Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and program director of the Legacy Survey of Space and Time Corporation Data Science Fellowship Program.

A Common Event with a Rare Twist

The scientists used the Zwicky Transient Facility in California and initially detected the unusual supernova back in December 2019—just a day following its explosion. Called SN2019yvq, the event took place in a comparatively adjacent galaxy situated 140 million light-years from the planet Earth, extremely close to the tail of the Draco constellation that is shaped like a dragon.

Within a matter of hours, astrophysicists used the Neil Gehrels Swift Observatory of NASA to investigate this phenomenon in X-ray and UV wavelengths. They instantly categorized the SN2019yvq event as a type Ia (pronounced “one-A”) supernova. This event occurs quite frequently during the explosion of a white dwarf.

These are some of the most common explosions in the universe. But what’s special is this UV flash. Astronomers have searched for this for years and never found it. To our knowledge, this is actually only the second time a UV flash has been seen with a type Ia supernova.

Adam Miller, Study Lead and Astrophysicist, Northwestern University

Heated Mystery

The occasional flash, which persisted for two days, denotes that something nearby or within the white dwarf was extremely hot. Since white dwarfs become increasingly cooler as they age, astronomers were puzzled over the influx of heat.

“The simplest way to create UV light is to have something that’s very, very hot,” added Miller. “We need something that is much hotter than our sun—a factor of three or four times hotter. Most supernovae are not that hot, so you don’t get the very intense UV radiation. Something unusual happened with this supernova to create a very hot phenomenon.”

According to Miller and his research group, this is a curial clue to understand the reason behind the explosion of white dwarfs. Such explosions have remained a mystery for a long time in this research area.

At present, there are numerous competing theories but Miller is specifically interested in analyzing four different hypotheses, which corresponds with his group’s data analysis from the SN2019yvq event.

The following are possible situations that could cause the explosion of a white dwarf with a UV flash:

1. After consuming material from its companion star, a white dwarf becomes very huge and unstable and ultimately explodes. The materials of the white dwarf and the companion star crash together, resulting in a flash of UV emission.
2. Carbon inside the white dwarf is ignited by an outer helium layer, resulting in a massive hot double explosion accompanied by a UV flash.
3. Very hot radioactive material in the core of the white dwarf combines with its external layers, causing the exterior shell to reach temperatures that are higher than normal.
4. A pair of white dwarfs combine together, causing an explosion with colliding ejecta that produce UV rays.

“Within a year,” added Miller, “we’ll be able to figure out which one of these four is the most likely explanation.”

Earth-Shattering Insights

Once the scientists find out the reason behind the explosion, they will use those findings to know more about dark energy and the formation of planets.

Since a majority of the iron present in the universe is produced by type Ia supernovae, a deeper insight into this occurrence could shed more light about Earth. For instance, Iron resulting from the exploded stars formed the center of all rocky planets, such as Earth.

If you want to understand how the Earth formed, you need to understand where iron came from and how much iron was needed. Understanding the ways in which a white dwarf explodes gives us a more precise understanding of how iron is created and distributed throughout the universe.

Adam Miller, Study Lead and Astrophysicist, Northwestern University

Illuminating Dark Energy

White dwarfs have already contributed significantly to physicists’ present interpretation of dark energy. Physicists believe that all white dwarfs have the same level of brightness upon explosion. Hence, type Ia supernovae are believed to be “standard candles,” enabling astronomers to accurately estimate the distance of the explosions from Earth.

The use of supernovae to quantify the distances resulted in the detection of dark energy, a discovery that earned the Nobel Prize in Physics in 2011.

“We don’t have a direct way to measure the distance to other galaxies,” explained Miller. “Most galaxies are actually moving away from us. If there is a type Ia supernova in a distant galaxy, we can use it to measure a combination of distance and velocity that allows us to determine the acceleration of the universe.”

He continued, “Dark energy remains a mystery. But these supernovae are the best way to probe dark energy and understand what it is.”

Miller believes that a better understanding of white dwarfs may help gain a deeper insight into dark energy and how rapidly it causes the acceleration of the universe.

“At the moment, when measuring distances, we treat all of these explosions as the same, yet we have good reason to believe that there are multiple explosion mechanisms. If we can determine the exact explosion mechanism, we think we can better separate the supernovae and make more precise distance measurements,” concluded Miller.

The study titled, “The spectacular ultraviolet flash from the peculiar type Ia supernova 2019yvq” was partly funded by the Large Synoptic Survey Telescope Corporation, the Moore Foundation, and the Brinson Foundation.

Journal Reference:

Miller, A. A., et al. (2020) The Spectacular Ultraviolet Flash from the Peculiar Type Ia Supernova 2019yvq. Astrophysical Journal. doi.org/10.3847/1538-4357/ab9e05.