On the relation between active-region lifetimes and the autocorrelation function of light curves

TL;DR

This study investigates how the autocorrelation function of stellar light curves relates to active-region lifetimes, finding that a linear decay model better captures the ACF behavior and can provide lower bounds on active-region durations.

Contribution

The paper introduces a linear decay model for the ACF, improving the understanding of how light curve autocorrelation relates to active-region lifetimes in stars.

Findings

Linear decay better models ACF than exponential decay

ACF-based lifetime estimates are limited by observation length

Differential rotation and spot evolution affect lifetime inference

Abstract

Rotational modulation of stellar light curves due to dark spots encloses information on spot properties and, thus, on magnetic activity. In particular, the decay of the autocorrelation function (ACF) of light curves is presumed to be linked to spot/active-region lifetimes, given that some coherence of the signal is expected throughout their lifetime. In the literature, an exponential decay has been adopted to describe the ACF. Here, we investigate the relation between the ACF and the active-region lifetimes. For this purpose, we produce artificial light curves of rotating spotted stars with different observation, stellar, and spot properties. We find that a linear decay and respective timescale better represent the ACF than the exponential decay. We therefore adopt a linear decay. The spot/active-region timescale inferred from the ACF is strongly restricted by the observation length of the light curves. For 1-year light curves our results are consistent with no correlation between the inferred and the input timescales. The ACF decay is also significantly affected by differential rotation and spot evolution: strong differential rotation and fast spot evolution contribute to a more severe underestimation of the active-region lifetimes. Nevertheless, in both circumstances the observed timescale is still correlated with the input lifetimes. Therefore, our analysis suggests that the ACF decay can be used to obtain a lower limit of the active-region lifetimes for relatively long-term observations. However, strategies to avoid or flag targets with fast active-region evolution or displaying stable beating patterns associated with differential rotation should be employed.