To reveal the detail of the internal structure, the relationship between chromospheric activity and the 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 the same methods to pre-main-sequence (PMS) stars and identify the appropriate model of tau_c for them. Yamashita et al. (2020) investigated the relationship between N_R and strengths of the Ca II infrared triplet (IRT; lambda 8498, 8542, 8662 A) emission lines of 60 PMS stars. Their equivalent widths are converted into the 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 the maximum R' of the zero-age main-sequence (ZAMS) stars. However, because all R' was saturated against N_R, it was not possible to estimate the appropriate tau_c model for the PMS stars. We noticed that Mg I emission lines at 8808 A is an optically thin chromospheric line, appropriate for determination of the adequate tau_c for PMS stars. Using the archive data of the Anglo-Australian Telescope (AAT)/the University College London Echelle Spectrograph (UCLES), we investigated the Mg I line of 52 ZAMS stars. After subtracting photospheric absorption component, the 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 the Ca II emission lines", i.e. 10^{-1.6} < N_R < 10^{-0.8}. Therefore, the adequate tau_c for PMS stars can be determined by measuring of their R' values.

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