Periodic stellar variability from almost a million NGTS light curves

TL;DR

This study analyzes nearly a million NGTS light curves using a novel autocorrelation method to extract stellar variability periods, revealing new insights into stellar rotation, pulsations, and binary systems across different spectral types.

Contribution

It introduces the G-ACF method for irregularly sampled data and provides a large-scale analysis of stellar variability periods across a broad spectral and period range.

Findings

Identified 16,880 stars with variability periods from 0.1 to 130 days.

Discovered a bi-modality in stellar rotation periods between 15 and 25 days.

Observed deviations in long-period M-dwarfs from existing rotational evolution models.

Abstract

We analyse 829,481 stars from the Next Generation Transit Survey (NGTS) to extract variability periods. We utilise a generalisation 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 days 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 days. 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.