Advancing cool-dwarf asteroseismology with Extreme Precision Radial Velocities

Figure 1: Stellar radius–effective temperature diagram highlighting seismic detections from Kepler and TESS photometry, and radial-velocity campaigns.

Thanks to space-based photometry, asteroseismology is vastly benefiting the study of solar-type and red-giant stars, which exhibit convection-driven, solar-like oscillations. Despite that, space-based asteroseismology still faces a challenge regarding K dwarfs; whose oscillation amplitudes are extremely small (below a few ppm or 10 cm/s) and thus hard to detect, even with multi-year Kepler or multi-sector TESS photometry. As a result, only a few dwarfs cooler than the Sun have detected oscillations in photometry, and none cooler than about 5000 K (see Fig. 1).

In this regard, one should also not forget the pioneering role of pre-Kepler radial-velocity (RV) campaigns with then state-of-the-art spectrographs such as HARPS and UVES. They helped us set a lower effective temperature bound on cool-dwarf asteroseismology. However, insufficient RV precision and/or relatively small apertures meant that these early campaigns would remain limited to the very brightest dwarfs.

Figure 2: RV (top) and TESS (middle and bottom) power spectra for the K0 dwarf σ Draconis. From Hon et al. (2024).

Stellar “noise” associated with granulation is nevertheless substantially lower in Doppler than it is in photometry, which explains why RV observations have a higher signal-to-noise ratio (SNR) over the typical oscillation timescales. This is brilliantly illustrated in Fig. 2 by means of the RV and TESS power spectra for the K0 dwarf σ Draconis, despite the RV time series being almost 2000 times shorter than TESS observations.

Fast forward to the present day, advances in Doppler precision, driven by the search for Earth-like planets, are now providing a renewed opportunity for detecting low-amplitude oscillations, with Extreme Precision Radial Velocity (EPRV) techniques routinely reaching the 20 cm/s precision level in a single exposure.

The short oscillation periods in K dwarfs (of a few minutes) necessarily impose a fast observational cadence. And so, this calls for the optimal combination of a large collecting area and instrumental stability in order to reach the desired RV precision per data point on exposures that can be as short as 30 s. Therefore, this leaves ESPRESSO at the VLT as well as the Keck Planet Finder at the Keck Observatory among the very few instrumental setups capable of achieving such performance.

Figure 3: Power spectra of all 5 K dwarfs for which oscillations have thus far been detected with ESPRESSO and KPF, ordered from top to bottom by increasing effective temperature.

Thanks to these two instruments, the last couple of years have witnessed a resurgence of observing campaigns on K dwarfs. Figure 3 displays the power spectra of all 5 K dwarfs for which oscillations have thus far been detected with ESPRESSO and KPF, ordered from top to bottom by increasing effective temperature. In particular, the 6-night pilot campaign on the K5 dwarf ε Indi demonstrated the potential of EPRVs for cool-dwarf asteroseismology, enabling detection of solar-like oscillations with a peak amplitude of 2.6 cm/s. ε Indi is still, as of today, the coolest seismic dwarf observed.

The results presented here demonstrate that precise asteroseismology is now possible down to at least the mid-K regime, effectively opening up a new domain in observational asteroseismology.