Red giants are dying stars, in advanced stages of stellar evolution, which have depleted the hydrogen in their cores. In this work, recently published in Nature Communications, a team of astronomers mainly from Instituto de Astrofísica e Ciências do Espaço (IA), at the University of Porto, have found new evidence that red giant stars experience “glitches” – sharp structural variations – in their inner core.

Unfortunately, it is impossible to look directly inside a star. However, a technique dubbed asteroseismology, which measures oscillations similar to “earthquakes” in stars, can provide indirect glimpses of stellar interiors. The “glitches” can affect these oscillations, or the frequencies and paths of gravity and sound waves traveling through the stellar interior.

IA researcher Margarida Cunha explains: “Waves propagating inside stars induce minute stellar brightness variations that can be detected with highly precise space-based instruments. These waves reveal the conditions of the medium where they propagate, which is to say, the physical properties of the stellar interiors.

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, who began this work at IA, but is currently a postdoctoral research associate in astronomy at the Ohio State University, adds: “By analyzing these variations, we can obtain not only the global parameters of the star, but also information on the precise structure of these objects”.

This work presents the first characterization of structural discontinuities present in the core of red-giant stars, therefore allowing, for the first time, to precisely sound the physical processes occurring in this region.
Mathieu Vrard

Low-mass red giants experiencing helium burning in their cores are often used in astrophysical studies as probes of distance, to measure aspects like galaxy density, and to learn more about the physical processes behind stellar chemical evolution. So it’s vital that scientists model them correctly which, in turn, requires that they understand why these discontinuities happen.

In this work the team analyzed a sample of 359 red giants that were below a certain stellar mass, measuring various properties and individual oscillation frequencies of each star. They discovered that almost 7% of these stars exhibit structural discontinuities.

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

There are two main theories which explain how these disturbances might work. The first states that “glitches” are present throughout the star’s evolution, but are generally very weak and below the threshold for what astronomers would categorize as a true discontinuity.

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

As it turns out, the first scenario is not supported by this study, but more precise data is needed before scientists can confidently subscribe to the second. Diego Bossini (IA) explains: “This study shows the limits of our models and it gives us an opportunity to find a way for improving them.