本田 敏志

Context . Latitude distribution of stellar magnetic activity is not well constrained by observations, despite its importance for a better understanding of stellar dynamos and their effects on planetary environments. Aims . Our aim is to obtain an accurate reconstruction of the surface spot distribution on the young rapidly rotating K2 star PW Andromedae by combining spectroscopic and photometric diagnostics. In particular, we assess how the inclusion of continuous high-precision TESS photometry in parallel with high-resolution spectroscopy improves latitude recovery of starspots, especially at low latitudes and in the southern hemisphere, which are poorly constrained by Doppler imaging (DI) alone. We thereby explore the spatial origins of the observed white-light flares. Methods . We performed simultaneous Doppler imaging and light curve inversion (DI+LCI) using contemporaneous high-resolution GAOES-RV spectra from the 3.8 m Seimei telescope ( R ∼ 65 000) and high-precision TESS light curves. Surface reconstructions employed the SpotDIPy code to model both line profiles and continuum brightness variations. We compared DI+LCI maps with DI-only solutions, conducted artificial-spot simulations to evaluate the effects of latitude, phase coverage, and signal-to-noise ratio on reconstruction reliability. We also investigated the spatial correlation between the DI+LCI reconstructed map and flares detected in the TESS data. Results . The DI+LCI reconstruction reveals significant spot features at mid to low latitudes, equatorial regions, and even in the southern hemisphere. These are the regions where DI-only fails to provide reliable information. Meanwhile, the high-latitude spot features, which are already recovered by DI-only, remain present, though with a restructured distribution. The estimated spot coverage is approximately 9.9% of the area of the stellar surface visible to the observer. Simulations show that DI+LCI provides more accurate reconstructions than DI-only, especially under conditions of incomplete phase coverage and low signal-to-noise, by better recovering both spot latitudes and filling factors. A comparison between the DI+LCI map and the TESS flare timings also suggests a potential association between flare occurrence and reconstructed spot longitudes. Conclusions . Simultaneous DI and continuous photometry improves the inversion accuracy of starspot distributions, also improving flare localization.

Abstract Coronal mass ejections (CMEs) on the early Sun may have profoundly influenced the planetary atmospheres of early Solar System planets. Flaring young solar analogues serve as excellent proxies for probing the plasma environment of the young Sun, yet their CMEs remain poorly understood. Here we report the detection of multi-wavelength Doppler shifts of the far-ultraviolet and optical lines during a flare on the young solar analogue EK Draconis. During and before a Carrington-class (~10 ³²  erg) flare, warm far-ultraviolet lines (~10 ⁵  K) exhibited blueshifted emission at 300–550 km s ⁻¹ , indicative of a warm eruption. Then, 10 min later, the Hα line showed slow (70 km s ⁻¹ ), long-lasting (≳2 h) blueshifted absorptions, indicating a cool (~10 ⁴  K) filament eruption. This provides evidence of the multi-temperature and multi-component nature of a stellar CME. If Carrington-class flares or CMEs occurred frequently on the young Sun, they may have cumulatively impacted the early Earth’s magnetosphere and atmosphere.

White-light flares are explosive phenomena accompanied by brightening of continuum from near-ultraviolet to optical, which occur on the Sun and stars. In order to investigate the mechanism of white-light flares, we carried out simultaneous optical photometry (TESS : 6000-10000 Å) and spectroscopy (Seimei Telescope : 4100-8900 Å) of a M-dwarf EV Lac on 2019 September 14. We detected a flare with high-time-cadence ($\sim 50$ sec) spectroscopic observation. At the peak, the continuum of the flare component is well fitted by a blackbody spectrum with temperature of $T = 8122 \pm 273$ K, which is comparable with the results of previous studies that reported the spectral energy distribution of near-ultraviolet to optical during the flare could be approximated by single-temperature blackbody radiation at $T \sim 10^{4}$ K. We also estimated the time evolution of the flare temperature during the decay phase. The radiative energy of this flare within the optical range is $4.4 \times 10^{32}$ erg, taking into account the time-dependent variation in the decreasing flare temperature and expanding flare area. Furthermore, we detected a delayed increase in the flux of H$\alpha$ after the photometric flare peak, secondary increase, and gradual increase even after the white-light flare ended. Comparison of our results with light curves obtained by the Sun-as-a-star analysis of solar flares indicates that these signals may be due to postflare loops near the stellar limb. Our result about time evolution of white-light continuum will help to gain more insight into the mechanism of white-light flares both on the Sun and stars. Additionally, since extreme ultraviolet radiation from flare loops plays a key role in planetary atmospheric escape, the existence of postflare loops on stellar flares and its time evolution will help future studies about habitability of close-in planets.