Astronomers discover irregularities in the core of red giants

Artist's impression of waves, with different frequencies, travelling in the inner layers of a star. P-modes or pure sound (pressure) waves, in white, spend most of their time in the outer envelope and are directly visible at the stars' surface. G-modes or pure gravity waves, in black, propagate in the radiative interior and hold information on the stellar core. In red giants, the two are coupled together and the resulting waves sense all layers, thus revealing information of the stars' deepest interior. (Credit: Tania Cunha (Planetário do Porto - Centro Ciência Viva)/Instituto de Astrofísica e Ciências do Espaço)

Red giants are dying stars, in advanced stages of stellar evolution, which have depleted the hydrogen in their cores. In this work1, published today in Nature Communications, a team2 of astronomers mainly from Instituto de Astrofísica e Ciências do Espaço (IA3), 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 asteroseismology4, 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”.

“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

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”.

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.

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)

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.

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”.

Notes

  1. The article “Evidence of structural discontinuities in the inner core of red-giant stars”, was published today in Nature Communications (DOI: 10.1038/s41467-022-34986-z).
  2. The team is: Mathieu Vrard, Margarida S. Cunha, Diego Bossini, Pedro P. Avelino, Enrico Corsaro & Benoît Mosser.
  3. The Instituto de Astrofísica e Ciências do Espaço (Institute of Astrophysics and Space Sciences – IA) is the reference Portuguese research unit in this field, integrating researchers from the University of Lisbon, the University of Coimbra and the University of Porto, and encompasses most of the field’s national scientific output. It was evaluated as “Excellent” in the last evaluation of research and development units undertaken by Fundação para a Ciência e Tecnologia (FCT). IA’s activity is funded by national and international funds, including FCT/MCES (UIDB/04434/2020 e UIDP/04434/2020).
  4. Asteroseismology is the study of stellar interiors, by measuring seismic oscillations at the star’s surface. In seismology, the different vibration modes from an earthquake can be used to study the Earth’s interior, in order to get data from the composition and depth of its different layers. In a similar fashion, oscillations at the star’s surface can be used to infer its internal structure and composition.

Contacts
Margarida Cunha, Mathieu Vrard

Science Communication Group
Ricardo Cardoso Reis; Sérgio Pereira; Filipe Pires (coordenação, Porto); João Retrê (coordenação, Lisboa)