Amongst the intermediate mass pulsating stars known as $δ$ Sct stars is a subset of high-amplitude and predominantly radial-mode pulsators known as high-amplitude $δ$ Sct (HADS) stars. From more than 2000 $δ$ Sct stars observed by the Kepler space mission, only two HADS stars were detected. We investigate the more perplexing of these two HADS stars, KIC5950759. We study its variability using ground- and space-based photometry, determine its atmospheric parameters from spectroscopy and perform asteroseismic modelling to constrain its mass and evolutionary stage. From spectroscopy, we find that KIC5950759 is a metal-poor star, which is in agreement with the inferred metallicity needed to reproduce its pulsation mode frequencies from non-adiabatic pulsation models. Furthermore, we combine ground-based WASP and Kepler space photometry, and measure a linear change in period of order $\dot{P}/P \simeq 10^{-6}$ yr$^{-1}$ for both the fundamental and first overtone radial modes across a time base of several years, which is at least two orders of magnitude larger than predicted by evolution models, and is the largest measured period change in a $δ$ Sct star to date. Our analysis indicates that KIC5950759 is a metal-poor HADS star near the short-lived contraction phase and the terminal-age main sequence, with its sub-solar metallicity making it a candidate SX Phe star. KIC5950759 is a unique object amongst the thousands of known $δ$ Sct stars and warrants further study to ascertain why its pulsation modes are evolving remarkably faster than predicted by stellar evolution.
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Departamento de Física y Química. IES Levante（物理与化学系，莱万特中学）
Wildly Oscillating Stars -- Unexplained dense ridge-like frequency agglomerations in A and F type pulsators
DTU Space, Technical University of Denmark（丹麦技术大学空间中心）；Centre for Astrophysics, University of Southern Queensland（南昆士兰大学天体物理中心）；Centre for Space Research, North-West University（西北大学太空研究中心）；Jeremiah Horrocks Institute, University of Lancashire（兰开夏大学杰里米·霍罗克斯研究所）；Sydney Institute for Astronomy (SIfA), School of Physics, University of Sydney（悉尼大学物理学院悉尼天文研究所）；Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences（波兰科学院尼古拉·哥白尼天文学中心）；TAPIR, Mailcode 350-17, California Institute of Technology（加州理工学院TAPIR）；LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris（巴黎天文台LESIA实验室（隶属巴黎文理研究大学、法国国家科学研究中心、索邦大学、巴黎大学））；Instutute for Physics and Astronomy, Aarhus University（奥胡斯大学物理与天文学研究所）；STAR Institute, University of Liège（列日大学STAR研究所）
