Supernova Like No Other Shines A Light On The Explosive Demise Of White Dwarfs

A white dwarf stealing material from a red giant companion, a possible scenario for how Type Ia Supernovae form. Raymond Cassel/Shutterstock

Supernovae are the explosive ends for many stars, and they are divided into particular types depending on how their brightness changes over time. One particular kind, Type Ia (one-A), is of particular interest to astronomers because their peak luminosity is very consistent and can be seen from far away, making these supernovae ideal for measure distances.

There are still many questions about the origin of these events, but new research published in the Monthly Notices of the Royal Astronomical Society might help clear up some of them surrounding Type Ias.

The standard picture for this stellar explosion starts with a white dwarf, the naked and spent stellar core of a Sun-like star. White dwarfs do not have enough mass to explode so researchers believe that the extra weight comes from stellar material stolen by the white dwarf from a companion star. An alternative idea suggests two white dwarfs collide and then explode.

To find out more, researchers from the Carnegie Institution for Science carried out an ambitious observation campaign of Type Ias that have hydrogen signatures, looking for ones with high signatures. Despite being the most abundant element in the universe, hydrogen is mostly absent from Type Ia supernovas as it's thought the white dwarfs have pushed the hydrogen-rich layer that once enveloped them away. A detection of hydrogen could potentially indicate the presence of material from a companion star.

Most of the objects surveyed are in young galaxies where a lot of new stars are forming and are surrounded by clouds of hydrogen. But one event, known as ASASSN-18tb is from a galaxy made of old stars.

“It’s possible that the hydrogen we see when studying ASASSN-18tb is like these previous supernovae, but there are some striking differences that aren’t so easy to explain,” lead author Dr Juna Kollmeier said in a statement.

The major difference is the quantity of hydrogen. ASASSN-18tb is surrounded by hydrogen that makes up just 1/100th of the mass of the Sun. In the other objects in the survey, the quantity of hydrogen is tens of, and even up to 100, times more.

“One exciting possibility is that we are seeing material being stripped from the exploding white dwarf’s companion star as the supernova collides with it,” said Anthony Piro. “If this is the case, it would be the first-ever observation of such an occurrence.” 

While the researchers are certainly excited by the possibility, they admit that based on this object alone they can’t confirm the origin of the material without reasonable doubt. They argue that follow-up observations might be key to solving the mystery of where the hydrogen comes from.

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