Tiniest ‘starquakes’ ever detected

We used one of the most advanced spectrographs in the world to detect solar-like oscillations in an orange dwarf star. The article “Expanding the frontiers of cool-dwarf asteroseismology with ESPRESSO. Detection of solar-like oscillations in the K5 dwarf ε Indi” was published today in the journal Astronomy & Astrophysics (DOI: 10.1051/0004-6361/202449197).

What's new? Why is this result important? And how it may change our view of the field?

The Sun and other Sun-like stars gently vibrate due to sound waves trapped in their interior. This sound is excited by convective motions near the stellar surface, as hot blobs of plasma rise and the cooling plasma sinks down (solar granules are a manifestation of these convective motions). The sound waves produced do not propagate through space, however they manifest themselves as oscillations (also known as solar-like oscillations) at the stellar surface with amplitudes of only a few tens of centimeters per second (slower than walking speed!). In particular, stars smaller and cooler than the Sun, characterized by proportionally smaller convective fluxes, find it increasingly more difficult to excite oscillations to the point that these can be detected by telescopes on Earth.

In this study, we present the detection of solar-like oscillations in the orange-hued dwarf star (what astronomers call a K dwarf) ε Indi based on time-intensive observations collected with the ESPRESSO spectrograph, mounted on the Very Large Telescope (VLT) at the European Southern Observatory (ESO), Paranal, Chile. ε Indi’s radius is 71% of the solar radius and its surface temperature is about 1000 degrees cooler than the Sun’s. This makes ε Indi at once the smallest and coolest dwarf star observed to date with confirmed solar-like oscillations. In other words, these are the tiniest ‘starquakes’ ever detected. To give a flavor of the extreme precision level of these observations, the peak amplitude of the detected oscillations is just 2.6 centimeters per second (or a mere 14% of the oscillation amplitudes measured for the Sun), an outstanding technological achievement.

Importantly, this detection conclusively shows that precise asteroseismology (i.e., the study of stellar oscillations) of cool dwarfs is possible down to at least the mid-K regime (corresponding to surface stellar temperatures of 4200 degrees Celsius) using state-of-the-art spectrographs on large-aperture telescopes, effectively opening up a new domain in observational astrophysics.

Stellar oscillations are unique probes of the deepest layers of stars (through sound, we can ‘see’ inside a star, as if submitting it to an echogram), making inferences on their internal structure and physical conditions possible, while leading to unrivaled precision in the estimation of their global properties (e.g., stellar mass and age). This study hence carries profound implications for stellar astrophysics in general, all the more so considering that K dwarfs are among the most common and long-lived stars in the Universe.

How did ESPRESSO uniquely enable us to achieve this result?

First, the tiny oscillation amplitudes in K dwarfs (below the 10 cm/s level or, equivalently, a few parts-per-million) are beyond the reach of current space-based photometric missions. Second, given the high observational cadence (faster than 60 s) needed to properly sample the short oscillation periods (3 minutes) in ε Indi, one required the right combination of a large collecting area, instrumental stability, and high spectral resolution in order to reach a radial-velocity precision per data point of 30 cm/s on exposures as short as 25 s (accounting for the readout time). Currently, ESPRESSO/VLT is the only instrumental setup (ESO or other) in the southern hemisphere capable of achieving such performance. We note that the KPF spectrograph at the Keck Observatory (Hawaii) is capable of achieving a similar performance in the northern hemisphere.

What led to this research? What were we hoping to discover?

Fueled by photometry provided by space missions like CoRoT, Kepler, and TESS, asteroseismology is vastly benefiting the study of cool main-sequence stars, which exhibit solar-like oscillations. Despite this success story, the tiny oscillation amplitudes in K dwarfs continue to pose a challenge to space-based asteroseismology, with only a handful of dwarfs cooler than the Sun having detected oscillations to date.

A viable alternative is offered by the lower granulation ‘noise’ in Doppler observations. With this study, we intended to push a decade-old frontier in the field of asteroseismology by observing a mid-K dwarf (ε Indi) for asteroseismology with ESPRESSO/VLT. Further to detecting the presence of solar-like oscillations in ε Indi, we hoped to gain insight into the typical oscillation amplitudes of K dwarfs. Oscillation amplitudes measured through Doppler observations can be converted to amplitudes in photometry. This information is key to accurately predict the seismic yield of ESA’s PLATO mission (with a planned 2026 launch) and can potentially influence the PLATO pipeline development strategy.

Moreover, we hoped to shed new light on the cool-dwarf mass-radius relation, regarding which there is a long-standing disagreement between theory and observations. In a nutshell, stellar evolution models are known to underestimate the radii of K dwarfs by 5-15% compared to radii obtained from empirical methods. K-dwarf asteroseismology should thus help us assess the deficiencies of current stellar models.

The ingredients that make this a truly pioneering endeavor are all in there, from the ingenious use of ESPRESSO’s unparalleled sensitivity to the risky (and daring) attempt at measuring cm/s-level quakes in a distant star. Having set a new frontier in cool-dwarf asteroseismology, now’s the time for the community to systematically start extending this type of study to other similar bright stars.

What aspect of this research could capture the public's imagination?

Owing to their stability and long lifespan, K dwarfs and their planetary systems have become a primary focus in searches for habitable worlds and extraterrestrial life. We have therefore demonstrated that the power of asteroseismology can now potentially be put to use in the detailed characterization of K exoplanet-host dwarfs and their habitable planets, with truly far-reaching implications. In particular, precise stellar ages from asteroseismology are a sought-after commodity, as the ages of nearby bright stars may be critical in interpreting biosignatures in directly imaged exoplanets.