[P-EM11] Frontiers in solar physics

[PEM11-P12] Solar Flares on the Early Sun: Insights from TESS's Photometric Observations of a Young Sun-like Star, EK Dra.

*Daijiro Hitotsuyanagi¹, Daisuke Yamashiki¹, Hiroto Yamada¹, Kazuma Ishihara¹, Kosuke Namekata^1,2,3,4, Yosuke Yamashiki⁵ (1.National University Corporation Kyoto University, 2.National Astronomical Observatory of Japan, 3.National Goddard Space Flight Center, 4.Catholic University of America, 5.Graduate School of Advanced Integrated Studies in Human Survivability)

Keywords:Early Sun, Star Spot, Sun Spot, EK Dra, Flare

Stellar flares significantly impact a planet’s magnetic field, atmosphere, and potential habitability, making it essensial in astronomy to understand their underlying machanisms. Contemporary solar observations have shown that larger sunspot areas are associated with increased flare frequency and energy, and also many flares occur at mid-latitudes. However, it remains unclear whether a similar relationship existed in the young Sun, only a few hundred million years after its formation. In this study, we focus on the EK Dra—a young sun-like star about 100 million years old with mass and radius comparable to the Sun, making it excellent model for a young solar-type star.Our objective is to investigate the relationship between starspot area and flare frequency, as well as the latitudinal distribution of starspots on EK Dra. To this end, we analyzed photometric data from NASA’s TESS mission, which observed EK Dra in 12 sectors between 2019 and 2022, corresponding to roughly 12 months of coverage. For each sector, we calculated the flare frequency, starspot area, and its rotation period. While previous studies have compared the starspot area–flare frequency correlation across different stars, there is no precedent for a study of this correlation at different epochs for a single star, which is highly significant.We first examined the correlation between starspot area and its flare frequency. The starspot area was estimated from the amplitude of periodic brightness variations caused by the rotational modulation of starspots. while rapid luminosity changes were attributed to stellar flares. Futhermore, the frequency of stellar flares were estimated by examining the number of flares in areas of abrupt changes in luminosity, which we considered to be stellar flares. Our results reveal a positive correlation between starspot area and flare frequency, akin to that observed on the present-day Sun. This finding implies that large flares are driven by extensive starspot regions and suggests that insights from solar studies may be applicable to extreme events such as superflares.Next, to determine the latitudinal distribution of starspots on EK Dra, we analyzed the relationship between starspot area and the rotation period corresponding to the visibility of these starspots. Given that stars like the Sun exhibit differential rotation—with shorter rotation periods near the equator—the periodicity of brightness variations can be used to infer the latitudes at which starspots are located. We found that the starspot area increases when the rotation period is between 2.6 and 2.7 days. Considering that EK Dra’s average rotation period is 2.605 days, this indicates that starspot activity is most prominent at mid-latitudes, consistent with solar observations where sunspots and flares predominantly occur between 10° and 30°.In summary, our study shows that even in a young solar-type star like EK Dra, there is a positive correlation between starspot area and flare frequency, and both starspots and flares primarily occur at mid-latitudes. As the early Sun played a critical role in the development of planetary atmospheres and the emergence of life on Earth, understanding its activity is essential for insights into planet formation and the origins of life. The similarities between EK Dra and the present-day Sun suggest that our current understanding of solar activity can be applied to the early Sun, offering valuable clues to unravel the mysteries of our solar system’s past and the emergence of life.