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: 2 red-giant stars with solar-like oscillations, 21 main-sequence pulsators (16 δ Sct and 5 γ Dor stars) and 13 rotating variables. Three new γ Dor variables (one is a hot γ Dor star) are detected in the present work together with five new rotating variables. An in-depth frequency analysis of the δ Scuti and γ Doradus stars in the sample shows that the frequency spectra are very rich, in particular for the δ 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 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 δ Sct stars. One of the δ 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 which probably originate in the δ Sct component.

Stellar Rotation in the Gaia Era: Revised Open Clusters’ Sequences

“…Given their proximity, the Pleiades (distance of ≈ 136 pc; age of ∼ 125 Myr) and Praesepe (distance of ≈ 186 pc; age of ∼ 580 Myr) clusters have been used for rotation studies for several decades (e.g., Anderson et al 1966;Kraft 1967b;Dickens et al 1968;Stauffer et al 1984;Stauffer & Hartmann 1987;Queloz et al 1998;Terndrup et al 1999Terndrup et al , 2000Scholz & Eislöffel 2007;Delorme et al 2011;Agüeros et al 2011;Douglas et al 2014;Rebull et al 2016aRebull et al ,b, 2017Stauffer et al 2016), and the periods derived from the K2 observations comprise their state-of-the-art rotational samples. While the number of NGC 6811 stars with Kepler periods is lower compared to the aforementioned clusters, its old age (∼ 1000 Myr) makes it a remarkably interesting system for stellar rotation studies (e.g., Meibom et al 2011b;Curtis et al 2019a;Rodríguez et al 2020;Velloso et al 2020).…”