KIC 5950759: a high-amplitude δ Sct star with amplitude and frequency modulation near the terminal age main sequence

Abstract

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, KIC 5950759. 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 KIC 5950759 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 KIC 5950759 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. KIC 5950759 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.

Asteroseismology across the HR diagram

“…It is not surprising to find frequency variability in δ Sct stars, given their complex behavior with many modes, both radial and nonradial, simultaneously excited. This is illustrated here with two, of many, cases: that of 4 CVn for which Breger et al (2017) analyzed over 800 nights of ground-based photometry obtained over more than 40 yr; and that of KIC 5950759 (Bowman et al 2021) for which 4 yr of Kepler data were analyzed. Breger et al (2017) noted the complexity of the FT of 4 CVn and concentrated their attention on higher amplitude modes.…”

“…Lares-Martiz et al ( 2020) recognized this and developed a self-consistent method to extract combination frequencies. Bowman et al (2021) showed a plethora of combination frequencies and harmonics in the δ Sct star KIC 5950759, which Lares-Martiz et al ( 2020) used to illustrate their method.…”

“…Since the frequency separation of those modes is a measure of rotation in the pulsation cavity, their result suggests internal changes in the rotation of 4 CVn over the 40 yr of observations. Bowman et al (2021) combined photometry for KIC 5950759 from both ground-based WASP observations and a 4-yr Kepler data set, along with spectroscopy to determine fundamental parameters. KIC 5950759 is unusual: it is a high amplitude δ Sct (HADS) star (meaning ∆V > 0.3 mag peak-to-peak), and it has low metallicity, hence may also be an SX Phe star (Population II δ Sct star).…”

Detection of nonlinear resonances among gravity modes of slowly pulsating B stars: results from five iterative prewhitening strategies

“…It is also a well-established technique for unravelling nonlinearities in the light curves of oscillating white dwarf stars (e.g., Wu 2001;Zong et al 2016b), and hot B subdwarf stars (e.g., Zong et al 2016a). Yet, it has not received much attention for oscillating MS stars (Lee 2012;Bowman & Kurtz 2014;Kurtz et al 2015;Bowman et al 2016Bowman et al , 2021. In this work, we provide the identification of candidate direct resonances among the extracted oscillation frequencies in the light curves of 38 SPB stars, and test the impact of different iterative prewhitening strategies on the results.…”

Frequency Analysis of KIC 1573174: Shedding Light on the Nature of HADS Stars

“…The high-precision photometric data provided by Kepler (e.g., Borucki et al 2010;Koch et al 2010) provides an unprecedented opportunity to explore stellar interiors by using the natural oscillation mode of stars, thus greatly expanding the research field of asteroseismology (e.g., Aerts et al 2010;Chaplin et al 2010;Xue et al 2018;Bowman et al 2021). The ultra-high-precision photometric observations at the μ mag level have significantly advanced our understanding of several types of pulsating variable stars (e.g., Balona et al 2012;Silva Aguirre et al 2015;Stello et al 2022;Szewczuk et al 2022).…”