Amplitude modulation in delta Scuti stars: statistics from an ensemble of Kepler targets
Dominic Bowman (University of Central Lancashire), Donald W. Kurtz (Jeremiah Horrocks Institute, University of Central Lancashire, Preston PR1 2HE, UK), Michel Breger (Department of Astronomy, University of Texas, Austin, TX 78712, USA), Simon J. Murphy (Sydney Institute for Astronomy (SIfA), School of Physics, The University of Sydney, NSW 2006, Australia), Daniel L. Holdsworth (Jeremiah Horrocks Institute, University of Central Lancashire, Preston PR1 2HE, UK)

Kepler data provide unprecedented views of light variability in pulsating stars. Intermediate-mass pulsators, such as delta Scuti stars, lie in a transition region between radiative cores and thick convective envelopes in low-mass stars, and large convective cores and thin convective envelopes in high-mass stars. Mode coupling and nonlinearity are predicted in delta Scuti stars, which give rise to variable pulsation amplitudes. The delta Scuti star KIC 7106205 was shown to have a single amplitude-modulated pulsation mode that decreased in amplitude by more than a factor of 20 over 6 yr, whilst all other pulsation mode frequencies remained constant in amplitude and phase (Bowman et al. 2015). A thorough and statistical search for amplitude modulation in approximately 1000 delta Scuti stars that were continuously observed by Kepler for 4 yr has been carried out. We present these results and demonstrate that diverse pulsational behaviour, in particular amplitude modulation, is common among delta Scuti stars with the majority of these stars exhibiting at least one pulsation mode that varies significantly in amplitude over 4 yr (Bowman et al. 2016). We construct models and use case studies to distinguish among different scenarios: beating of close-frequency pulsation modes; pure amplitude modulation; mode coupling and nonlinearity. Our study shows that time spans of years and decades are important for delta Scuti stars and that amplitude modulation is not restricted to a small region of the HR diagram. The study of approximately 1000 delta Scuti stars using the Kepler data set by Bowman et al. (2016) will be useful for studying similar stars with K2 and TESS, as the 4-yr time span will not be surpassed for some time.