In a tug of war between a binary system’s two massive stars, the heavier star can pull away the hydrogen-rich outer layer of the lighter star. Afterward, all that’s left is the compact helium-rich core. Stripped stars should be abundant in the universe: Theory predicts that about a third of all massive stars—those greater than 25 solar masses—should lose their hydrogen-rich outer layer. When that happens, stripped stars are capable of generating hydrogen-poor supernovae. But despite years of searching for midsized stripped helium stars, which range from 2 to 8 solar masses, scientists had yet to observe one. (Astronomers did find a 4-solar-mass candidate in 2008, but its mass estimate was recently revised to 2 solar masses.)
A new idea for how to look for them came in 2018. In the course of her PhD research, Ylva Götberg—now an assistant professor at the Institute of Science and Technology Austria—studied how some stripped binaries may emit and could be detected by UV light. Encouraged by that possibility, Götberg teamed up with the University of Toronto’s Maria Drout and some other colleagues to find the elusive stars. They have now found 25 of them, and the new analysis may help explain certain core-collapse supernovae.
Looking at the UV sky is challenging. Earth’s atmosphere absorbs a lot of UV light, which is good for people’s health but not for identifying hot stars. Another big problem is dust, which obscures UV emission from nearby massive stars in the Milky Way. So Drout, Götberg, and colleagues relied instead on NASA’s Neil Gehrels Swift Observatory, whose UV and optical telescope has collected thousands of images from two galaxies: the Large Magellanic Cloud and the Small Magellanic Cloud.
After using Götberg’s previous theoretical work to observe what appeared to be roughly 500 000 possible stars, the researchers selected a population of 25 for further analysis and gathered optical spectra from the Las Campanas Observatory in Chile. With those data, they conclusively identified all 25 as stripped binaries.
“Now with these observations, we can really begin to study and understand helium stars and improve our understanding of the whole universe,” says Jan Eldridge of the University of Auckland in New Zealand, who was not involved in the new work. Of particular interest to Drout and others are supernovae. The stripped binaries could, for example, experience core collapse, explode as hydrogen-poor supernovae, and produce neutron stars. Some could be sources of gravitational waves. Or if they merged, a kilonova event would occur and, through rapid neutron capture, fuse the heaviest elements. (M. R. Drout et al., Science 382, 1287, 2023.)
