Acoustic oscillations and dynamo action in the G8 sub-giant EK Eri

Abstract

We present further evidence of the presence of acoustic oscillations on the slowly-rotating, over-active G8 sub-giant EK Eri. This star was observed with the 1-m Hertzsprung SONG telescope, at the Observatorio del Teide for two different runs of 8 and 13 nights, respectively, and separated by about a year. We determined a significant excess of power around $\hat{I}{½}_{\max }=253\pm 3\hat{I}¼$Hz in the first observing run and we were able to determine the large separation $\hat{I}”\hat{I}½=16.43\pm 0.22\hat{I}¼$Hz. No significant excess of power was instead detected in a subsequent SONG observing season, as also supported by our analysis of the simultaneous TESS photometric observations. We propose a new amplitude-luminosity relation in order to account for the missing power in the power spectrum. Based on the evolutionary stage of this object we argue that standard $\hat{I}{\pm }^2\hat{I}\text{c◯}$ dynamo cannot be excluded as a possible origin for the observed magnetic field.

8 pages, 8 figures, accepted for publication in A&A