Stars in the final stages of stellar development are known as red giants. They burn the hydrogen in the surrounding shell because the hydrogen in their core is depleted. The star enters the clump phase as soon as the helium in the core fuses. Clumps of stars are often used in astrophysical studies as indicators of distance, galaxy density, galaxy extinction, and chemical evolution of stars, as they have similar observable characteristics.

In a new study, a team of astronomers, mainly from Instituto de Astrofísica e Ciências do Espaço (IA), performed the detailed observational characterization of the deepest layers of clump stars. Their study provides new evidence that red giant stars experience “glitches” — sharp structural variations — in their inner cores. This work provides the first characterization of structural discontinuities in the cores of red giant stars, making it possible for the first time to clearly understand the physical processes taking place in this region.

evolution from a main sequence star to a red giant
Scheme of the evolution from a main-sequence star to a red giant. The stars in this study are at the end of the evolutionary track shown and are experiencing helium nuclear fusion. The different stages of evolution are not to scale. (Credit: Thomas Kallinger, U. of British Columbia and U. of Vienna)

The team used data from the Kepler Space Telescope (NASA) to detect and study waves propagating to the deepest layers of evolved stars.

Lead author Mathieu Vrard, currently a postdoctoral research associate in astronomy at Ohio State University, said: “By analyzing these variations, we can not only obtain the global parameters of the star, but also information about the precise structure of these objects.”

The team analyzed a sample of 359 red giants below a given stellar mass. They measured different properties and individual oscillation frequencies of each star. They found that nearly 7% of these stars show structural discontinuities.

The two basic theories explaining how these perturbations could work are as follows. According to the former, there are “glitches” during the evolution of the star, but they are usually very small and fall short of what astronomers consider to be a true discontinuity.

The second suggests that these irregularities are “smoothed out” by an unknown physical process that later leads to changes in the structure of the star’s core.

Diego Bossini (IA) explains: “It turns out that the first scenario is not supported by this study, but more precise data is needed before scientists can confidently subscribe to the second. This study shows the limits of our models and allows us to develop a way to improve them.”

Magazine reference:

  1. Mathieu Vrard et al., Evidence of structural discontinuities in the inner core of red giant stars, Nature Communications (2022). DOI: 10.1038/s41467-022-34986-z