Rotation-Activity Relation of Ca II and Mg I Infrared Emission Lines of Young Stars

Abstract

To reveal detail of internal structure, relationship between chromospheric activity and Rossby number, N_R (= rotational period P / convective turnover time tau_c), has been extensively examined for main-sequence stars. The goal of our work is to apply same methods to pre-main-sequence (PMS) stars and identify appropriate model of tau_c for them. Yamashita et al. (2020) investigated relationship between N_R and strengths of Ca II infrared triplet (IRT; lambda 8498, 8542, 8662 A) emission lines of 60 PMS stars. Their equivalent widths are converted into emission line to stellar bolometric luminosity ratio (R'). The 54 PMS stars have N_R < 10^{-1.0} and show R' \sim 10^{-4.2} as large as maximum R' of zero-age main-sequence (ZAMS) stars. However, because all R' was saturated against N_R, it was not possible to estimate appropriate tau_c model for PMS stars. We noticed that Mg I emission lines at 8808 A is an optically thin chromospheric line, appropriate for determination of adequate tau_c for PMS stars. Using archive data of Anglo-Australian Telescope (AAT)/the University College London Echelle Spectrograph (UCLES), we investigated Mg I line of 52 ZAMS stars. After subtracting photospheric absorption component, Mg I line is detected as an emission line in 45 ZAMS stars, whose R' is between 10^{-5.9} and 10^{-4.1}. The Mg I line is not saturated yet in the saturated regime for Ca II emission lines, i.e. 10^{-1.6} < N_R < 10^{-0.8}. Therefore, adequate tau_c for PMS stars can be determined by measuring of their R' values.