Starspots, chromospheric emission lines, and flares of zero-age main-sequence stars

Abstract

Abstract Zero-age main-sequence (ZAMS) stars are considered to have enormous starspots and show strong chromospheric emission lines because of their strong surface magnetic field. We discuss the dynamo activities of ZAMS stars with respect to their periodic light variation caused by a starspot and with respect to the strength of the chromospheric emission lines. The light curves of 33 ZAMS stars in IC 2391 and IC 2602 were obtained from TESS (Transiting Exoplanet Survey Satellite) photometric data. The light curves can be grouped into the following four categories: single frequency, possible shape changer, beater, and complex variability. The amplitudes of the light curves are 0.001–0.145 mag, similar to those of ZAMS stars in Pleiades. The starspot coverages are $0.1\%$–$21\%$. We found that the light variations and Ca ii emission line strength of ZAMS stars in IC 2391, IC 2602, and the Pleiades cluster are as large as those of the most active superflare stars and two orders larger than those of the Sun, and are located on the extensions of the superflare stars. These results suggest that superflare stars link the properties of the Sun to those of the ZAMS stars of ages between 30 and 120 Myr. ZAMS stars with a single frequency or possible shape change in the light curve tend to have both large light variation, indicating large spot coverage, and saturated Ca ii emission line strength. ZAMS stars with beat or complex variability have small spot coverage and a faint Ca ii emission line. We also detected 21 flares in the TESS light curves of 12 ZAMS stars in IC 2391 and IC 2602, where most of these stars have saturated chromospheric Ca ii emission lines. The energies of the flares are estimated to be ∼1033–1035 erg, which is comparable with the energy of a superflare.