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

TL;DR

This study investigates magnetic activity on EK Draconis, a young solar analogue, by integrating century-scale photometry with 13 recent TESS sectors and 13 Doppler images derived from high-resolution spectroscopy. A persistent long-term activity cycle of $10.7$–$12.1$ years is identified, with an additional $7.3$–$8.2$ year signal in the latter half of the data, alongside a gradual brightness trend. Doppler-imaging reveals evolving mid-latitude spots and yields a solar-type surface differential rotation with $α_{\rm DR} = 0.030 \pm 0.008$, supported by photometric period changes. The flare energy distribution follows a broken power law with indices $1.466 \pm 0.007$ and $2.335 \pm 0.110$, and flare phases show no strong association with spot longitudes, highlighting a complex magnetic dynamo in a fast-rotating solar analogue with potential implications for early solar and planetary evolution.

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 $α_{DR} = 0.030 \pm 0.008$.

Figures (17)

• Figure 1: Short-term Fourier transform of the dataset used in Sect. \ref{['section:long-term-phot']}. A 10.7-12.1 years long cycle is seen through the whole plot, and in the more recent half of the light curve a 7.3-8.2 year-long period also appears. Features longer than 27 years are suppressed on the plot to 1/3 power. The peak in this range originates from the length of the available data and the gap between 1960-1970.
• Figure 2: TESS light curve of EK Dra from Sector 14 with the spot model. Top: The light curve from the full-frame images (gray) and the 3-spot-model fit (blue). Middle: The change in spot longitudes. The sizes of the dots are scaled by the spot radii. Bottom: The PDCSAP light curve along with the flares marked in red. Vertical lines indicate the times when the spots are in the line of sight.
• Figure 3: Rotational periods of EK Dra (blue dots) and their estimated error bars in consecutive TESS observation windows (adjacent sectors are considered continuous). Gray areas from left to right are sectors S14-16, S21-23, S41, S48-50, and S75-77. The horizontal dashed line marks the rotation period value adopted for Doppler imaging.
• Figure 4: Example of the polynomial fitting of the baseline of the flares. The data is marked gray, the black points were used for the fit, and red shows the fitted baseline. The flare in the figure is from Sector 48.
• Figure 5: The flare frequency distribution from TESS PDCSAP data. The black line denotes the fit with a power-law index of $1.466 \pm 0.007$ in the 10$^{33}$--10$^{34}$ erg energy range. The purple line is a fit above 10$^{34}$ erg, its power-law index is $2.335\pm 0.110$.