Red giants are dying stars in the advanced stages of stellar evolution, which have depleted the hydrogen in their cores. In this work, published in Nature Communications, a team of astronomers, mainly from Instituto de Astrofísica e Ciências do Espaço (IA), 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:
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“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 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.”
Red giants are used in learning more about stellar chemical evolution
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 scientists must 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 below a certain stellar mass, measuring each star’s various properties and individual oscillation frequencies. They discovered that almost 7 percent of these stars exhibit structural discontinuities.
Two main theories 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, this study does not support the first scenario, 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 allows us to find a way to improve them.”
Provided by Institute of Astrophysics and Space Sciences [Note: Materials may be edited for content and length.]