Magnetic activity on the young Sun: a case study of EK Draconis

TL;DR

This study investigates the magnetic activity and surface features of the young Sun analog EK Draconis over a century, revealing a long-term activity cycle, short-term spot evolution, and solar-like differential rotation.

Contribution

The paper provides a comprehensive analysis of EK Dra's magnetic activity, combining century-long photometry, TESS data, and Doppler imaging to uncover its activity cycles and surface dynamics.

Findings

A 10.7-12.1 year activity cycle detected over 120 years.

Surface spots have a lifetime of 10-15 days.

EK Dra exhibits solar-like differential rotation with shear parameter 0.030 ± 0.008.

Abstract

Context. Young, solar analogue stars provide key insights into the early stages of stellar evolution, particularly in terms of magnetic activity and rotation. Their rapid rotation, high flaring rate, and enhanced surface activity make them ideal laboratories for testing stellar models or even the solar dynamo. Aims. Using long-term photometric data, we investigated the cyclic behaviour of EK Dra over the last century. We analyze its short-term activity based on 13 Transiting Exoplanet Survey Satellite (TESS) sectors. Applying Doppler imaging on high-resolution spectral data we investigate short and long-term spot evolution and surface differential rotation. Methods. We use Short-term Fourier-transform on a 120 years long archival photometric data in order to search for activity cycles. The short-term space photometry data is fitted with an analytic three-spot model, and we hand-select flares from it to analyze their phase and frequency distribution. Spectral synthesis is used to determine the astrophysical parameters of EK Dra. Using the iMap multi-line Doppler imaging code, we reconstruct 13 Doppler images. Differential rotation is derived by cross-correlating consecutive Doppler maps. Results. Long-term photometric data reveal a 10.7-12.1 year cycle that was persistently present for 120 years. In the more recent half of the light curve a 7.3-8.2 years-long signal is also visible. The distribution of the 142 flares in the TESS data shows no correlation with the rotational phase or with the spotted longitudes. The reconstructed Doppler images show a surface that varies from rotation to rotation, putting the lower limit of the spot lifetime between 10-15 days. Based on the cross-correlation of the Doppler maps, EK Dra has a solar-type differential rotation with a surface shear parameter of $\alpha_{DR} = 0.030 \pm 0.008$.