White Dwarf Measured Before it Exploded as a Supernova

Type Ia supernovae are an important tool for modern astronomy. They are thought to occur when a white dwarf star captures mass beyond the Chandrasekhar limit, triggering a cataclysmic explosion. Because that limit is the same for all white dwarfs, Type Ia supernovae all have about the same maximum brightness. Thus, they can be used as standard candles to determine galactic distances. Observations of Type Ia supernova led to the discovery of dark energy and that cosmic expansion is accelerating.

While these supernovae have revolutionized our understanding of the universe, they aren’t quite as standard as we first proposed. Some, such as SN 1991T are much brighter, while others, such as SN 1991bg are much dimmer. There is also a variation known as Type Iax, where the white dwarf isn’t completely destroyed by the explosion. We can generally take these variations into account when calculating stellar distances, but it would be good to have a better understanding of the mechanism behind their maximum brightness.

The progenitor of a Type Ia Supernova. Credit: NASA, ESA, and A. Field (STScI)

According to theoretical models, the maximum brightness of a Type Ia supernova depends upon the mass and central density of the white dwarf before it explodes. But how could these values be measured? After all, we typically only discover these stars after they explode. Fortunately, a new study in The Astrophysical Journal Letters shows how it can be done.

The study looked at a supernova remnant known as 3C 397. It’s about 33,000 light-years from Earth and probably exploded about 2,000 years ago. Because the supernova was relatively close and recent, astronomers can get a good view of the material cast out by the explosion. An earlier study of the remnant debris suggests that the original white dwarf star was very close to the Chandrasekhar limit when it exploded.

A comparison between the measured core density and theory. Credit: Ohshiro, et al

This study focused on the observations of particular isotopes within the debris, particularly those of titanium and chromium. It’s the first time titanium has been observed in a Type Ia remnant. When the team compared the amount of titanium and chromium to those of iron and nickel, they found an unexpectedly high ratio. This is important because the ratios of Ti/Ni and Cr/Ni are crucially dependent upon the core density of the progenitor star. Based on their observations, the team determined that the core of 3C 397 was 2-3 times higher than generally assumed for white dwarfs. Thus, the explosion was likely much brighter than a typical Type Ia supernovae.

While this is a single study of a single supernova, it shows how the ratio of elements can determine white dwarf core densities. This can be used to better calibrate the maximum brightness of Type Ia supernovae, better standardizing the candle for cosmologists.

Reference: Ohshiro, Yuken, et al. “Discovery of a Highly Neutronized Ejecta Clump in the Type Ia Supernova Remnant 3C 397.” The Astrophysical Journal Letters 913.2 (2021): L34.