Periodic stellar variability from almost a million NGTS light curves

Abstract

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

Magnetic Activity–Rotation–Age–Mass Relations in Late-pre-main-sequence Stars

“…Figure 3 shows that the ZTF periods are generally consistent with the periods (or half-periods) of Moraux et al (2013), with the exception of seven stars, for which our simulations prefer shorterperiod solutions. Similar discrepancies between stellar rotational periods derived from different data sets analyzed with different methods are shown in Briegal et al (2022).…”

Bridging the Gap—The Disappearance of the Intermediate Period Gap for Fully Convective Stars, Uncovered by New ZTF Rotation Periods

“…This means the gap cannot be formed at a single age, as was previously thought. 9 Furthermore, the persistence of the gap in all-sky surveys (e.g., Gordon et al 2021;Briegal et al 2022;this work) suggests this can only happen if the pause in star formation occurred on a large scale, throughout the local Galaxy, which has not been observed in other (nonrotation) age-dated samples. All recent studies rule strongly against this hypothesis.…”

“…After optimizing the pipeline and vetting criteria, we measured rotation periods from the ZTF archival light curves by first crossmatching the stars in ZTF with our Gaia target list (see Section 2.1), using an angular separation of 1 2. We found many potentially erroneous period measurements at P rot 1 day caused by the sampling alias (also seen in Briegal et al 2022). As a result, we only select stars with period measurements >1.1 days.…”

“…We take the period with the highest normalized LS peak power to be the measured period for each star. We want to warn the readers that ground-based systemics are more complicated due to the uneven sampling rate (see also Irwin et al 2011;Briegal et al 2022). We assumed periods that pass the vetting criteria and with agreeing measurements from different ZTF fields/CCDs are correct.…”

“…With more than three years of observation data from ZTF, we are now able to measure long rotation periods for faint, low-mass stars. Period catalogs using ground-based telescopes have already shown success in reproducing the Kepler period distribution and measuring rotation periods for fully convective stars (e.g., Irwin et al 2011;Newton et al 2016;Briegal et al 2022).…”