🔭 Smallest “starquakes” ever detected

2024-04-03 04:00:11

An international team used one of the world’s most advanced spectrographs to detect the smallest “stellarquakes” ever recorded in an orange dwarf star, making it the smallest and coolest star observed to date with confirmed solar oscillations.

This study demonstrates that asteroseismology is a powerful technique for studying such stars, opening new perspectives in our understanding of stellar physics and, at the same time, exoplanets.

Artist’s impression of seismic waves propagating within the star at more or less deep layers depending on their frequencies. The study of these different modes of vibration at the surface of the star informs us about the structure and composition of the different stellar layers, just as an echogram allows us to see the interior of our body.
Credit: Tania Cunha (Planetário do Porto – Centro Ciência Viva)/Institute of Astrophysics and Space Sciences)

This study is the subject of a publication in the journal Astronomy & Astrophysics Letters: “Expanding the frontiers of cool-dwarf asteroseismology with ESPRESSO: Detection of solar-like oscillations in the K5 dwarf ε Indi”.

Incredibly precise measurements…

Located at a distance of 11.9 light years, Epsilon Indi (ε Indi) is an orange dwarf star (also known as a K dwarf) whose diameter is 71% that of the Sun. To observe this small star, the team of scientists used the powerful spectrograph ESPRESSOmounted on the Very Large Telescope (VLT) of the’European Southern Observatory (THAT).

The team then used a technique called asteroseismology to measure stellar tremors. These seismic oscillations, measurable only at the surface of the star by photometry or radial velocity, are rich in information because they propagate throughout all stars. They give a precise measurement of fundamental stellar parameters (mass, radius, age) as well as an indirect insight into the interior of the star, such as its structure and composition, like terrestrial earthquakes which tell us about the interior of the Earth.

In the case of ε Indi, the maximum amplitude of the detected oscillations is only 2.6 centimeters per second, or 14% of the amplitude of the solar oscillations, making it the smallest and coolest dwarf star observed to date with confirmed solar oscillations. These measurements are so precise that the speed detected is slower than the average speed of a sloth!

Infographic comparing the orange dwarf star? Indi in the Sun. The detection of such weak oscillations opens up numerous perspectives, both in stellar physics and in the study of exoplanets.
Credit: Paulo Pereira (Institute of Astrophysics and Space Sciences)

“The level of extreme precision of these observations is an exceptional technological achievement. Importantly, this detection conclusively shows that precise asteroseismology is possible down to cold dwarfs with surface temperatures as low as 4200 degrees Celsius, about 1000 degrees cooler than the surface of the Sun, thus opening indeed a new field in astrophysics observational“, comments Tiago Campante, searcher principal of the study and assistant professor in the Department of Physics andastronomy from the Faculty of SciencesUniversité of Porto (DFA-FCUP).

…which open up many new perspectives!

This level of precision could help scientists resolve a long-standing disagreement between theory and observations regarding the relationship between the mass and diameter of these cool dwarf stars. “Stellar evolution models tend to underestimate the diameter of K dwarfs by 5 to 15% compared to the diameter obtained by empirical methods. The study of oscillations in K dwarfs, via asteroseismology, will make it possible to identify the shortcomings of current stellar models and, thus, to improve them in order to eliminate this disparity“, explains researcher Margarida Cunha, from AI.

Despite initial skepticism about the possibility of detecting such oscillations beyond current instrumental capabilities, Mário João Monteiro (IA & DFA-FCUP) explains that: “In addition to detecting the presence of solar oscillations in ε Indi, we now hope to use the oscillations to study the complex physics of the surface layers of K dwarfs. These stars are cooler and more active than our Sun, making them laboratories important for probing key phenomena taking place in their surface layers that we have not yet studied in detail in other stars.“

Diagram of stellar radius as a function of temperature effective, highlighting seismic detections from the Kepler and TESS photometry campaigns (blue circles) and radial velocity (red diamonds). The dotted lines demarcate the spectral classes of the stars. ? Indi is the smallest and coldest star analyzed in asteroseismology.
Credit: Campante et al. 2024

Furthermore, because orange dwarf stars and their planetary systems have very long lifespans, they have recently become a main focal point in the search for habitable worlds and extraterrestrial life. This result demonstrates that asteroseismology can be brought to bear in the detailed characterization of such stars and their habitable planets, with truly far-reaching implications. Furthermore, the precise determination of the ages of nearby cool dwarfs made possible by asteroseismology can be crucial in the interpretation of biosignatures in directly imaged exoplanets.

Finally, these “star shakes” can also be used to help plan the future space telescope PLATO of the’European Space Agency (ESA), a mission in which the Department of Astrophysics is strongly involved. The amplitudes ofoscillation measured in this study are key information to help accurately predict the seismic performance of PLATO, scheduled to launch in 2026.

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