Mode identification in pulsating subdwarf B stars observed by Kepler and TESS

Abstract

The study of hot subdwarf B (sdB) stars gives valuable insights for understanding the internal physics of core He-burning (CHeB) stars. This master thesis first presents a review of the pulsating sdB stars observed by the Kepler space telescope to classify their pulsation types. One of these pulsators, KIC5807616 (Kepler-70 or KPD 1943+4058), is then selected as the focus of this work, with the primary goal of identifying its gravity (g-) mode pulsations. Mode identification consists in determining the geometrical properties of pulsation modes, a key asset for the future asteroseismic modeling of such stars. This thesis presents a methodology based on quantitative quality criteria, using period-spacing diagrams and rotational multiplets analysis, in order to reduce the part of arbitrary commonly faced in mode identification procedures. This methodology is preliminarily tested and validated on the sdB star TIC441725813 (TYC 4427-1021-1), observed by TESS, for which a recent and complete mode identification is available in the literature. Among the 18 sdB stars observed by Kepler, 14 are dominated by g-modes (7 pure and 7 hybrid pulsators), 2 are dominated by pressure (p-) modes (1 pure and 1 hybrid pulsators), and 2 could not be categorized. Concerning KIC5807616, the methodology developed in this thesis leads to the identification of 79 pulsation modes. Three possible identification configurations are discussed, including two configurations showing strong evidences of mode trapping. The rotation period of the envelope is also inferred from rotational splitting observed on p-modes: 40.0 ± 4.7 days.