Multiwavelength Campaign Observations of a Young Solar-type Star, EK Draconis. I. Discovery of Prominence Eruptions Associated with Superflares

Young solar-type stars frequently produce superflares, serving as a unique window into the young Sun-Earth environments. Large solar flares are closely linked to coronal mass ejections (CMEs) associated with filament/prominence eruptions, but observational evidence for stellar superflares remains scarce. Here, we present a 12-day, multiwavelength campaign observation of young solar-type star EK Draconis (G1.5V, 50-120 Myr age) utilizing the Transiting Exoplanet Survey Satellite, the Neutron star Interior Composition ExploreR, and the Seimei telescope. The star has previously exhibited blueshifted H alpha absorptions as evidence for a filament eruption associated with a superflare. Our simultaneous optical and X-ray observations identified three superflares of 1.5 x 1033-1.2 x 1034 erg. We report the first discovery of two prominence eruptions on a solar-type star, observed as blueshifted H alpha emissions at speeds of 690 and 430 km s-1 and masses of 1.1 x 1019 and 3.2 x 1017 g, respectively. The faster, massive event shows a candidate of post-flare X-ray dimming with the amplitude of up to similar to 10%. Several observational aspects consistently point to the occurrence of a fast CME associated with this event. The comparative analysis of the estimated length scales of flare loops, prominences, possible dimming region, and starspots provides the overall picture of the eruptive phenomena. Furthermore, the energy partition of the observed superflares in the optical and X-ray bands is consistent with flares from the Sun, M-dwarfs, and close binaries, yielding the unified empirical relations. These discoveries provide profound implications of the impact of these eruptive events on early Venus, Earth, and Mars and young exoplanets.