Periodic stellar variability from almost a million NGTS light curves
Jackman, James A. G.; Wheatley, Peter J.; Bayliss, Daniel; Moyano, Maximiliano; Günther, Maximilian N.; Eigmüller, Philipp; Henderson, Beth A.; Casewell, Sarah L.; Gill, Samuel; Goad, Michael R.; Anderson, David R.; Armstrong, David J.; Burleigh, Matthew R.; Gillen, Edward; Jenkins, James S.; Lendl, Monika; West, Richard G.; Bryant, Edward M.; Queloz, Didier; Watson, Christopher A.; Smith, Gareth D.; Hodgkin, Simon; Briegal, Joshua T.; Acton, Jack S.; Battley, Matthew P.; Tilbrook, Rosanna H.; Costes, Jean C.; Kreutzer, Lars T.
United Kingdom, Germany, United States, Netherlands, Chile, Switzerland
Abstract
We analyse 829 481 stars from the Next Generation Transit Survey (NGTS) to extract variability periods. We utilize a generalization of the autocorrelation function (the G-ACF), which applies to irregularly sampled time series data. We extract variability periods for 16 880 stars from late-A through to mid-M spectral types and periods between ~0.1 and 130 d with no assumed variability model. We find variable signals associated with a number of astrophysical phenomena, including stellar rotation, pulsations, and multiple-star systems. The extracted variability periods are compared with stellar parameters taken from Gaia DR2, which allows us to identify distinct regions of variability in the Hertzsprung-Russell Diagram. We explore a sample of rotational main-sequence objects in period-colour space, in which we observe a dearth of rotation periods between 15 and 25 d. This 'bi-modality' was previously only seen in space-based data. We demonstrate that stars in sub-samples above and below the period gap appear to arise from a stellar population not significantly contaminated by excess multiple systems. We also observe a small population of long-period variable M-dwarfs, which highlight a departure from the predictions made by rotational evolution models fitted to solar-type main-sequence objects. The NGTS data spans a period and spectral type range that links previous rotation studies such as those using data from Kepler, K2, and MEarth.