Rare nuclear transition provides evidence for supernova mechanism

The lives of the universe’s most massive stars end when they collapse under their own gravity to form a neutron star or a black hole and shed their outer layers in a supernova. (See, for example, the article by Hans Bethe, Physics Today, September 1990, page 24 .) However, for smaller stars in the range of 7–11 solar masses, gravitational collapse may not be the only possible route to supernova. If reactions in their neon- and oxygen-rich cores generate sufficient energy to counter gravitational collapse, those stars can expire in thermonuclear explosions, leaving behind a white dwarf remnant. Theory suggests that the core’s ²⁰Ne nuclei could provide that energy by capturing electrons in a reaction that ultimately releases gamma radiation and heats the core. But because that nuclear reaction occurs only in stellar conditions, its rate is difficult to determine. Now a team led by Oliver Kirsebom (now at Dalhousie University in Nova Scotia, Canada) and Gabriel Martínez-Pinedo (Technical University Darmstadt in Germany) has determined experimentally the rate at which ²⁰Ne captures electrons. The rate is higher than assumed by stellar evolution models, and it could imply that thermonuclear explosion is the common end for many of our galaxy’s stars.

At the temperatures and densities of the oxygen–neon stellar core, ground-state ²⁰Ne changes to ground-state fluorine-20 after capturing an electron from the degenerate electron gas. To learn more about that transition, Kirsebom and colleagues studied the reverse process that occurs on Earth, in which radioactive ²⁰F emits electrons and decays into ²⁰Ne. At the University of Jyväskylä accelerator laboratory in Finland, the researchers bombarded carbon foil with ²⁰F nuclei. They then measured the energy of each electron emitted by the nuclei that embedded in the foil. The researchers found that most decay events produced ²⁰Ne in an excited state. However, ²⁰Ne was produced in its ground state in about 1 in 250 000 events, a rate that should match that of the reverse process. Considering the extreme stellar plasma densities of about 10⁹ g/cm³, electron capture should occur frequently—specifically, eight orders of magnitude faster than assumed in previous calculations.

Numerical simulations of stellar evolution that take into account the newly measured neon–fluorine transition rate suggest that the cores begin to generate heat earlier than predicted by previous models. Once the core heats, oxygen can ignite; if ignition happens earlier, the star can die via thermonuclear explosion. A better understanding of how convection affects energy transport in stellar cores will provide more detailed insight into the stars’ life cycles and the chemical products they contribute to our and other galaxies. (O. S. Kirsebom et al., Phys. Rev. Lett. 123, 262701, 2019 ; O. S. Kirsebom et al., Phys. Rev. C 100, 065805, 2019 .)