Analysis of the acoustic cut-off frequency and high-frequency peaks in six Kepler stars with stochastically excited pulsations^⋆

¹ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife Spain
e-mail: ajm@iac.es
² Universidad de La Laguna, Dpto. de Astrofísica, 38206 Tenerife, Spain
³ Laboratoire AIM, CEA/DSM – CNRS – Univ. Paris Diderot – IRFU/SAp, Centre de Saclay, 91191 Gif-sur-Yvette Cedex, France
⁴ Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO 80301, USA

Received: 5 May 2015
Accepted: 12 August 2015

Abstract

Gravito-acoustic modes in the Sun and other stars propagate in resonant cavities with a frequency below a given limit known as the cut-off frequency. At higher frequencies, waves are no longer trapped in the stellar interior and become traveller waves. In this article, we study six pulsating solar-like stars at different evolutionary stages observed by the NASA Kepler mission. These high signal-to-noise targets show a peak structure that extends at very high frequencies and are good candidates for studying the transition region between the modes and interference peaks or pseudo-modes. Following the same methodology successfully applied on Sun-as-a-star measurements, we uncover the existence of pseudo-modes in these stars with one or two dominant interference patterns depending on the evolutionary stage of the star. We also infer their cut-off frequency as the midpoint between the last eigenmode and the first peak of the interference patterns. Using ray theory we show that, while the period of one of the interference patterns is very close to half the large separation, the period of the other interference pattern depends on the time phase of mixed waves, thus carrying additional information on the stellar structure and evolution.