A supernova is the brilliant end of a giant star. For a brief moment of cosmic time, a star makes one final effort to keep shining, only to fade and collapse on itself. The end result is either a neutron star or a black hole with stellar mass. We have generally assumed that all stars above about ten solar masses will end up as a supernova, but a new study suggests that this is not the case.
In contrast to the famous Type Ia supernovae, which can result from the merging or interaction of two stars, large stars go through a so-called core collapse supernova. Stars survive by balancing heat and pressure against gravity. As more elements merge, a large star must generate heat by merging heavier and heavier elements. This eventually forms a layer of regions where different elements are fused. But this chain can only be worn up to the iron. After that, merging heavier elements costs energy instead of releasing them. So the core collapses, creating a shock wave that tears the star apart.
The onion skin model of a dying star, not to scale. Photo credit: RJ Hall
In models of large dying stars, core collapse supernovae occur for stars over 9-10 solar masses up to about 40-50 solar masses. Above this mass, stars are so massive that they will likely collapse straight into a black hole without becoming a supernova. Extremely massive stars on the order of 150 solar masses or more could explode as a hypernova. These beasts do not explode because of a core collapse, but rather because of an effect known as pair instability, in which colliding photons generated in the core create pairs of electrons and positrons.
This new study suggests that the upper mass limit for core collapse supernovae could be much lower than we thought. The team studied the elemental abundances of a pair of colliding galaxies known as Arp 299. As the galaxies are colliding, the region is a breeding ground for supernovae. As a result, the elemental abundances of Arp 299 should largely depend on the elements dropped in supernova explosions. They measured the abundance ratio of iron to oxygen and the ratio of neon and magnesium to oxygen. They found that the Ne / O and Mg / O ratios were similar to the sun, while the Fe / O ratio was much lower than the sun’s level. Iron is most efficiently thrown into the universe through large supernovae.
A Hubble image of colliding galaxies known as Arp 299. Photo credit: NASA, ESA, Hubble Heritage Collaboration, and A. Evans
The ratios observed by the team did not agree with standard core collapse models, but they found that the data was in good agreement with supernova models when ruling out supernovae over about 23 to 27 solar masses. In other words, if stars collapse into black holes above about 27 solar masses, then models and observations will agree.
This work does not conclusively prove that the upper mass limit for supernovae is smaller than we thought. It is also possible that supernovae produce higher amounts of neon and magnesium than the models predict. Either way, it is clear that we still have a great deal to learn about the last dying breaths of great stars.
Reference: Mao, Junjie, et al. “Elemental abundance of the hot atmosphere of the glowing infrared galaxy Arp 299.” The Astrophysical Journal Letters 918.1 (2021): L17.