Pulsation and rotation in NGC 6811: the Kepler short-cadence stars

Abstract

We have analysed a selected sample of 36 Kepler short-cadence stars in the field of NGC 6811. The results reveal that all the targets are variable: two red giant starswith solar-like oscillations, 21 main-sequence pulsators (16 delta Scuti and five. Doradus stars), and 13 rotating variables. Three new gamma Doradus (gamma Dor) variables (one is a hot gamma Dor star) are detected in this work together with five new rotating variables. An in-depth frequency analysis of the delta Scuti (delta Sct) and gamma Dor stars in the sample shows that the frequency spectra are very rich, in particular for the d Sct-type variables. They present very dense frequency distributions and wide diversity in frequency patterns, even for stars being members of the cluster and with very similar location in the Hertzsprung-Russell (H-R) diagram. Rotational modulation is found for a high percentage of these main-sequence pulsating stars, which is an indication of stellar activity being common on the surfaces of these hot stars. In some cases, activity dominates the luminosity variations. Significant amplitude variability is also detected in at least some of the pulsation modes of highest amplitude in almost all the delta Sct stars. One of the delta Sct pulsators is a member of a binary system with the pulsating component tidally distorted. This system also shows strong stellar activity, including several flares that probably originate in the delta Sct component.© 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society