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 due to the loss of hydrogen in their core. Astronomers from the Institute de Astrofsica e Cincias do Espao have found new evidence that red giant stars experience "glitches" in their inner core.

It is not possible to see inside a star. An indirect glimpse of stellar interiors can be provided by a technique called asteroseismology. The frequencies and paths of gravity and sound waves can be affected by theglitches.

It is possible to detect minute stellar brightness variations with highly precise space-based instruments. The physical properties of the stellar interiors are revealed by the waves that travel through them.

Animation 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

Waves propagating to the deepest layers of evolved stars were detected and studied by the team.

The first characterization of structural discontinuities in the core of red-giant stars is presented in this work, which allows for the first time to precisely sound the physical processes.

By analyzing these variations, we can get information on the precise structure of these objects, as well as the global parameters of the star.

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

To learn more about the physical processes behind stellar chemical evolution, low-mass red giants are often used in astrophysical studies as probes of distance. Scientists need to model them correctly and understand why discontinuities occur.

The team analyzed a sample of 358 red giants that were below a certain stellar mass, measuring various properties and individual frequencies. Some stars have structural discontinuities.

Astronomers discover irregularities in the core of red giants
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, University of British Columbia and University of Vienna

Two main theories explain how these disruptions might work. glitches are present throughout the star's evolution, but are weak and below the threshold for what an astronomer would consider a true discontinuity

The second suggests that the star's core may change due to a physical process that smoothes outIrregularities.

The first scenario is not supported by the study but more precise data is needed before scientists can subscribe to the second. The study shows the limits of our models and gives us an opportunity to improve them.

There is evidence of structural discontinuities in the inner core of red-giant stars.

Journal information: Nature Communications

Centro Ciência VivawavesInstituto de AstrofísicaAstrofísica e Ciências
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