Doppler imaging combined with high-cadence photometry

I. Revisiting the surface of a pre-main-sequence flare star PW Andromedae

¹ Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan
² Department of Astronomy and Space Sciences, Faculty of Science, Ankara University, Beşevler 06100, Ankara, Türkiye
³ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
⁴ Department of Social Data Science, Hitotsubashi University, 2-1 Naka, Kunitachi, Tokyo 186-8601, Japan
⁵ The Hakubi Center for Advanced Research, Kyoto University, Yoshida-Honmachi, Sakyo, Kyoto 606-8501, Japan
⁶ Department of Physics, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo, Kyoto 606-8502, Japan
⁷ NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA
⁸ Department of Physics, The Catholic University of America, 620 Michigan Ave NE, Washington, DC 20064, USA
⁹ National Astronomical Observatory of Japan, NINS, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹⁰ Nishi-Harima Astronomical Observatory, University of Hyogo, 407-2 Nishigaichi, Sayo-cho, Sayo, Hyogo 679-5313, Japan
¹¹ Department of Physical Sciences, Ritsumeikan University, 1-1-1 Noji-higashi, Kusatsu, Shiga 525-8577, Japan
¹² Astrobiology Center, NINS, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹³ Okayama Branch Office, Subaru Telescope, National Astronomical Observatory of Japan, NINS, 3037-5 Honjo, Kamogata, Asakuchi, Okayama 719-0232, Japan
¹⁴ Department of Earth and Planetary Sciences, Institute of Science Tokyo, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan

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Received: 27 July 2025
Accepted: 26 December 2025

Abstract

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.