Chromospheric Mg I Emission Lines of Pre-Main-Sequence Stars

To reveal the detail of the internal structure, the relationship between chromospheric activity and the Rossby number, $N_{\rm R}$(= rotational period $P$ / convective turnover time $\tau_{\rm c}$), has been extensively examined for main-sequence stars. For active premain sequence (PMS) stars, the activity will be determined by using optically thin emission lines such as Mg I. To detect the Mg I chromospheric emission lines ($\lambda 8807\ \mathrm{\AA}$) from PMS stars, we analyzed high-resolution optical spectra of the $64\ $PMS stars obtained with VLT/UVES and X-Shooter. We distinguish whether the Mg I emission lines were activated by dynamo activity or mass accretion from protoplanetary disks. The strengths of the Mg I emission lines of the PMS stars with no veiling are comparable to those of zero-age main-sequence (ZAMS) stars if both kinds of stars have similar Rossby numbers. The Mg I emission lines are considered to be formed by the dynamo activity similar to the ZAMS stars. The strengths of Mg I emission lines of the PMS stars with veiling are larger than those of the ZAMS stars. It suggests that the mass accretion from the protoplanetary disk makes a shock near the photosphere, and heats the chromosphere. The chromosphere of the PMS stars are activated not only by the dynamo process but also by mass accretion. 

theme: New Insights into Star Formation and Evolution, Cool Stars as Stellar Systems