Doppler imaging combined with high-cadence photometry. I. Revisiting the surface of a pre-main-sequence flare star

TL;DR

This paper demonstrates the first simultaneous Doppler imaging and light-curve inversion for PW Andromedae by combining high-resolution Seimei spectroscopy (DI) with continuous TESS photometry (LCI). The DI+LCI approach, implemented in SpotDIPy with a three-temperature surface model, yields a more complete latitude mapping than DI alone, revealing mid-to-high latitude spots, equatorial structures, and previously hidden southern-hemisphere features, and estimates a visible-surface spot coverage of about 9.9%. Simulations show DI+LCI outperforms DI-only under incomplete phase coverage and modest S/N, recovering latitudes and filling factors more reliably, while aiding interpretation of flare timing and distribution. The study finds that flares detected by TESS occur across a broad range of longitudes with no clear energy-spot correlation, highlighting the value of joint spectroscopic-photometric imaging for understanding stellar dynamos and flare production in young, active stars. The results support broader application of DI+LCI to young solar analogs and to multi-wavelength spectroscopic monitoring to link surface magnetic topology with chromospheric activity and superflares.

Abstract

Latitude distribution of stellar magnetic activity is not well constrained by observations, despite its importance for a better understanding of stellar dynamos. We aim to obtain an accurate reconstruction of the surface spot distribution on the young, rapidly rotating K2 star PW And by combining spectroscopic and photometric diagnostics. In particular, we seek to 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 explore the spatial origins of the observed white-light flares. 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~65000) and high-precision TESS light curves. Surface reconstructions employ the SpotDIPy code to model both line profiles and continuum brightness variations. We compare DI+LCI maps with DI-only solutions, conduct artificial-spot simulations to evaluate the effects of latitude, phase coverage, and S/N on reconstruction reliability. We also investigate the spatial correlation between the DI+LCI reconstructed map and flares detected in the TESS data. The DI+LCI reconstruction reveals significant spot features at mid-to-low latitudes, equatorial regions, and even in the southern hemisphere. Simulations show that DI+LCI provides more accurate reconstructions than DI-only, especially under conditions of incomplete phase coverage and low S/N, by better recovering both spot latitudes and filling factors. A comparison between the DI+LCI map and the TESS flare timings also suggests potential association between flare occurrence and reconstructed spot longitudes.

Figures (14)

• Figure 1: 2D grid search on the EW–$v\sin i$ plane. The colors show the loss function value.
• Figure 2: The epoch-ordered LSD profiles derived from the observed spectra, along with their associated error bars, are represented by black circles. The epochs, marked by green text, are computed as $(\mathrm{BJD}_{\mathrm{Mid}} - \mathrm{T}_0)$ / $P_{\rm rot}$. It should be noted that the error bars are too small to extend beyond the filled circles and are therefore hard to notice. The synthetic line profiles corresponding to the spotless case are shown as blue solid lines, while the best-fit models obtained through the reconstruction process are indicated by red solid lines.
• Figure 3: In the upper panel, the TESS light curve as a function of epoch is plotted with black filled circles representing the data points and their associated error bars, while the red solid line denotes the best-fit model obtained from the DI+LCI reconstruction process. The lower panel displays the residuals between the observations and the model fit.
• Figure 4: Surface brightness distribution maps in Mollweide projection obtained from the simultaneous reconstruction of the observed light curve and LSD profiles (top panel), from LSD profiles only (middle panel), and from the light curve only (bottom panel). Blue and green tick marks indicate the longitudinal positions covered by the spectral and photometric data, respectively, shown in terms of rotational phase ($\ell = 360 \times (1 - \phi)$). The gray-shaded region indicates the invisible part of the southern hemisphere owing to the axial inclination.
• Figure 5: The upper panel shows the normalized TESS SAP flux light curve of PW And from Sector 84 (October 1–26, 2024). Detrended light curve is shown as the blue line, where flare signals remain, but spot-induced modulation and instrumental effects have been filtered out. The red line denotes the model used for this detrending. Gray dashed vertical lines indicate GAOES-RV observations contemporaneous with the TESS monitoring. Flares, defined as events in which at least four consecutive data points exceed the 3$\sigma$ threshold, are marked by green points. Each detected flare is also indicated by a vertical green bar beneath the light curve. The lower panel shows an example of a detected flare, with the green dashed line representing the 3$\sigma$ threshold.