Abstract
In the second phase of the NASA Kepler mission (K2), the Kepler spacecraft observed fields along the ecliptic plane for about 80 days each to search for planetary transits and monitor stellar variability. We analyzed the light curves of thousands of main-sequence stars observed as part of the Kepler Guest Observer program. Here we summarize the statistics of discovery and properties of the pulsation amplitude spectra for 249 delta Scuti variable stars or candidates observed during K2 Campaigns 4 through 17. delta Scuti variables are core or shell-hydrogen burning stars about twice as massive as the Sun, pulsating in many simultaneous radial and non-radial modes, with periods of about two hours. The growing collection of long time-series high signal-to-noise photometric data from space missions such as Kepler, combined with constraints from ground and space-based data, will be useful to constrain the interior structure of these types of variables. We hope that this list of delta Sct candidates observed by Kepler may be useful as a starting point for identifying promising targets for asteroseismic investigations.
Highlights
The NASA Kepler mission was launched March 6, 2009, with a primary objective to use highprecision long time-series CCD photometry to search for exoplanets via planetary transits, and a secondary mission to study stellar variability and characterize exoplanet host stars (Borucki et al, 2010; Gilliland et al, 2010; Lundkvist et al, 2018)
We report on analyses of light curves from the K2 mission from Campaigns 4 through 17 obtained as part of the Kepler K2 Guest Observer program2
The algorithm results generally agreed with the by-eye determination of the amplitude and frequency of the maximum-amplitude δ Sct mode, it could quantify these values to higher accuracy than the by-eye estimate
Summary
The NASA Kepler mission was launched March 6, 2009, with a primary objective to use highprecision long time-series CCD photometry to search for exoplanets via planetary transits, and a secondary mission to study stellar variability and characterize exoplanet host stars (Borucki et al, 2010; Gilliland et al, 2010; Lundkvist et al, 2018). Stellar pulsations can be used to infer the interior structure of stars, and to determine their masses, radii, and ages, some of the goals of the field of asteroseismology (Aerts et al, 2010). K2 relied on solar radiation pressure to assist spacecraft orientation, and observed 19 new fields along the ecliptic plane for up to 80 days each. 1https://exoplanets.nasa.gov/news/1534/goodnight-kepler-final-commands-for-space-telescope 2https://keplerscience.arc.nasa.gov/k2-approved-programs.html δ Sct Candidates Observed by K2. Kepler photometry can detect brightness variations produced by radial and low-degree non-radial acoustic (p-mode) pulsations for which the variations do not average out over the unresolved disk. The δ Sct p-mode pulsations are driven by the κ effect (opacity valving) mechanism, whereas solar-like oscillations are excited stochastically (Aerts et al, 2010). Many interesting phenomena remain to be explained, for example, mode selection, pulsation amplitudes, frequency and mode amplitude variations, peculiar element abundances, and the prevalence of many hybrid stars showing both γ Dor gravitymode and δ Sct p-mode pulsations, unexpected according to preKepler theoretical predictions (see, e.g., Grigahcène et al, 2010; Uytterhoeven et al, 2011; Balona, 2018)
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