Topic: Fundamental Parameters of Cool Stars and Brown Dwarfs
We investigate the variability properties of main sequence stars in Kepler data, using a new astrophysically robust correction for systematics (ARC). In McQuillan, Aigrain & Roberts (2012), we focussed on the first month of data and confirmed findings from previous studies, indicating that 60% of stars appear more active than the Sun. We defined low and high variability samples and compare the properties of the stars belonging to each sample, showing tentative evidence that more active stars have lower proper motions. We also investigated the frequency content of the variability, finding clear evidence for periodic or quasi-periodic behaviour in 16% of stars, and showing that there exist significant differences in the nature of variability between spectral types. Of the periodic objects, most A and F stars have short periods (<2 days) and highly sinusoidal variability, suggestive of pulsations, whilst G, K and M stars tend to have longer periods (>5 days, with a trend towards longer periods at later spectral types) and show a mixture of periodic and stochastic variability, indicative of activity. Finally, we used auto-regressive models to characterise the stochastic component of the variability, and show that its typical amplitude and time-scale both increase towards later spectral types, which we interpret as a corresponding increase in the characteristic size and life-time of active regions. In this talk I will present updated results based on the analysis of a more extended dataset, with a particular focus on the evolution of stellar activity signatures on longer timescales, and the determination of rotation periods for slowly rotating stars.