Expanding the frontiers of cool-dwarf asteroseismology with ESPRESSO

Abstract

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 $ along with $ max Hz $ and $ Hz $. The peak amplitude of radial modes is $2.6 $, or a mere $ sim 14<!PCT!>$ 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)^ $ below $ sim 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.