The surprising chemistry at the core of a white dwarf

NASA’s Kepler space observatory, which has circled the Sun for nearly nine years in an Earth-trailing orbit, is best known as a planet hunter. By continuously observing variations in the brightness of 150 000 stars in its fixed field of view, it can detect the telltale periodic dimming caused by transiting exoplanets. But Kepler‘s observations are also a treasure trove for asteroseismologists, who analyze stars’ intrinsic pulsations to tease out the physical properties of stellar interiors. Such studies have yielded precise estimates of stellar masses, temperatures, and internal rotation speeds. (See the article by Conny Aerts, Physics Today, May 2015, page 36 .)

Now Noemi Giammichele and her coworkers at the University of Toulouse in France and the University of Montreal have used asteroseismology to determine the chemical makeup of the core of a white dwarf—a star in the slow cooling phase that marks the end of the life cycle of all but the most massive stars. Giammichele and her colleagues analyzed nearly two years of Kepler data on the white dwarf KIC 08626021 to find out how carbon and oxygen, biproducts of the star’s helium-burning phase, mixed and settled in the core. Using sophisticated mathematical techniques, the team deduced that the ratio of oxygen to carbon at the star’s core is roughly six to one, far higher than the two-to-one ratio predicted by conventional stellar evolution theory. The result suggests that the theory underestimates the star’s internal mixing during the helium-burning phase, underestimates certain nuclear reaction rates, or both. The high oxygen–carbon ratio could have implications for how astronomers interpret the spectra of type 1a supernovae, which occur whenever a matter-accreting white dwarf grows beyond a critical mass. (N. Giammichele et al., Nature 554, 73, 2018 .)