KIC5950759: a high-amplitude $\delta$ Sct star with amplitude and frequency modulation near the terminal age main sequence

TL;DR

This study investigates KIC5950759, a rare high-amplitude delta Scuti star near the end of its main sequence phase, revealing unprecedented period changes and suggesting it is a metal-poor star possibly related to SX Phe variables.

Contribution

It provides detailed observational and asteroseismic analysis of KIC5950759, highlighting its rapid period change and evolutionary status, which challenges existing stellar evolution models.

Findings

Largest measured period change in a delta Scuti star to date

KIC5950759 is a metal-poor star near the terminal-age main sequence

Period change rate exceeds predictions by two orders of magnitude

Abstract

Amongst the intermediate mass pulsating stars known as $\delta$ Sct stars is a subset of high-amplitude and predominantly radial-mode pulsators known as high-amplitude $\delta$ Sct (HADS) stars. From more than 2000 $\delta$ 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 $\delta$ 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 $\delta$ Sct stars and warrants further study to ascertain why its pulsation modes are evolving remarkably faster than predicted by stellar evolution.