Multiwavelength Campaign Observations of a Young Solar-type Star, EK Draconis. III. Comparison between Starspot Mapping, Zeeman Doppler Imaging, and Multiwavelength Variability

Abstract

Recent simultaneous multiwavelength observations of a nearby young solar-type star EK Dra in the optical, H$α$ spectrum, and X-ray, have provided evidence for stellar prominence eruptions associated with superflares. The large prominence eruption is suggested to have been caused by a large mid-latitude spot on the polarity inversion lines near the stellar limb from the concurrent Zeeman Doppler Imaging (ZDI) and optical photometry by the TESS. In this study, we perform starspot mapping for the TESS data of EK Dra to investigate the relation of starspots and magnetic fields from the photometry and ZDI. We also explore the multiwavelength rotational variability ascribed to starspots and active regions for the TESS, B-band, H$α$, and X-ray light curves. As a result, we find that (i) spot locations deduced from the TESS light curve are mostly consistent with the intensity map from the ZDI except for a polar spot, and (ii) the H$α$ light curve exhibits clear periodicity with respect to the TESS light curve because the H$α$ line is radiated around spots in the chromosphere. The X-ray light curve does not show such association probably because of multiple spots on high activity level and extended spatial structure of coronal active regions. The results provide clues to explore their association with stellar flares at different heights of active regions in chromospheric and coronal lines. Our study also enables us to quantify the stellar XUV radiation from the magnetic fields of active stars toward understanding atmospheric evolution of exoplanets.

Figures (9)

• Figure 1: Multiwavelength light curves of the (a) TESS, (b) B-band, (c) H$\alpha$, and (d) X-ray, in the Sector 50 (red, blue, orange, and purple). We also exhibit the TESS and B-band light curves in the second half of the Sector 49 (gray). The vertical dashed lines in all the panels (gray) represent the time of the prominence eruptions E1, E2, and E3 [][]Namekata23. In the panel (a), the vertical lines (green) represent the time of obtaining the spectra with the TBL/Neo-NARVAL for the ZDI [Table 1 in][]Namekata24.
• Figure 2: The GLS periodogram for multiwavelength light curves of the (a) TESS, (b) B-band, (c) H$\alpha$, and (d) X-ray (red, blue, orange, and purple). The vertical lines show the periodicity of the TESS, B-band, H$\alpha$ light curves for 2.731, 2.654, and 2.913 days under the false alarm probability (FAP) smaller than 1%.
• Figure 3: The light curves of (a) B-band, (b) H$\alpha$, and (c) X-ray versus the TESS light curve (blue, orange, and purple). The black lines derived from the least squares are formulated by $1.639F_{\rm TESS}-0.637$, $3.279F_{\rm TESS}-0.927$, and $-30.92F_{\rm TESS}+37.36$, where $F_{\rm TESS}$ is the relative flux of the TESS, respectively. The Pearson correlation coefficients are calculated to be $r=0.788$, $0.593$, and $-0.278$, respectively. We note that the H$\alpha$ emission is defined to be negative, and the vertical axis is reversed to clarify the emission in the panel (b).
• Figure 4: (Top) Spot maps at the different rotational phases including the E1, E2, and E3, for the two-spot model. (Bottom) (a) the TESS light curve (black) and reproduced one (red). (b) the temporal variation of visible projected area of each spot (red and blue) and the total (black) relative to the stellar disk and solar hemisphere, (c) that of visible area relative to the stellar and solar photospheres, (d) the temporal variation of each spot radius (red and blue), and (e) the B-band light curve (black) and light curve reproduced with parameters derived from the two-spot model (blue).
• Figure 5: Same as Figure \ref{['fig:S2_spotmap']}, but for the three-spot model (red, blue, and green).