Abstract
A&A, 683, L16 (2024) Fuelled by space photometry, asteroseismology is vastly benefitting the study
of cool main-sequence stars, which exhibit convection-driven solar-like
oscillations. Even so, the tiny oscillation amplitudes in K dwarfs continue to
pose a challenge to space-based asteroseismology. A viable alternative is
offered by the lower stellar noise over the oscillation timescales in Doppler
observations. In this letter we present the definite detection of solar-like
oscillations in the bright K5 dwarf $\epsilon$ Indi based on time-intensive
observations collected with the ESPRESSO spectrograph at the VLT, thus making
it the coolest seismic dwarf ever observed. We measured the frequencies of a
total of 19 modes of degree $\ell=0$--2 along with $\nu_{\rm
max}=5305\pm176\:{\rm \mu Hz}$ and $\Delta\nu=201.25\pm0.16\:{\rm \mu Hz}$. The
peak amplitude of radial modes is $2.6\pm0.5\:{\rm cm\,s^{-1}}$, or a mere
${\sim} 14\%$ of the solar value. Measured mode amplitudes are ${\sim} 2$ times
lower than predicted from a nominal $L/M$ scaling relation and favour a scaling
closer to $(L/M)^{1.5}$ below ${\sim} 5500\:{\rm K}$, carrying important
implications for our understanding of the coupling efficiency between
pulsations and near-surface convection in K dwarfs. This detection conclusively
shows that precise asteroseismology of cool dwarfs is possible down to at least
the mid-K regime using next-generation spectrographs on large-aperture
telescopes, effectively opening up a new domain in observational
asteroseismology.