Satoshi Honda

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 We report the discovery of two binary systems, each consisting of a slightly bloated G-type main-sequence star and an unseen companion, identified through photometric data from TESS and radial velocity variation from Gaia. High-resolution spectroscopy confirms orbital periods of 1.37 and $2.67$ d with circular orbits. The visible components have masses of ${\sim}0.9$–$1.0\, M_{\odot}$, while the minimum masses of the unseen companions are $1.078^{+0.058}_{-0.060}\, M_{\odot}$ and $0.943^{+0.048}_{-0.049}\, M_{\odot}$, respectively. Assuming tidal synchronization, we estimate the companion masses to be $1.13^{+0.11}_{-0.08}\, M_{\odot}$ and $1.05^{+0.15}_{-0.10}\, M_{\odot}$. The absence of detectable spectral features from the companions rules out main-sequence stars of these masses, suggesting that the unseen companions are likely O/Ne or C/O massive white dwarfs. The short orbital periods imply that these systems are post-common envelope binaries. Their subsequent evolution is uncertain, with possible outcomes including cataclysmic variables, Type Ia supernovae, or accretion-induced collapse, depending on the nature of future mass transfer.