  We investigated the chromospheric activity of 60 pre-main-sequence (PMS)
stars in four molecular clouds and five moving groups. It is considered that
strong chromospheric activity is driven by the dynamo processes generated by
the stellar rotation. In contrast, several researchers have pointed out that
the chromospheres of PMS stars are activated by mass accretion from their
protoplanetary disks. In this study, the Ca II infrared triplet (IRT) emission
lines were investigated utilizing medium- and high-resolution spectroscopy. The
observations were conducted with Nayuta/MALLS and Subaru/HDS. Additionally,
archive data obtained by Keck/HIRES, VLT/UVES, and VLT/X-Shooter was used. The
small ratios of the equivalent widths indicate that Ca II IRT emission lines
arise primarily in dense chromospheric regions. Seven PMS stars show broad
emission lines. Among them, four PMS stars have more than one order of
magnitude brighter emission line fluxes compared to the low-mass stars in young
open clusters. The four PMS stars have a high mass accretion rate, which
indicates that the broad and strong emission results from a large mass
accretion. However, most PMS stars exhibit narrow emission lines. No
significant correlation was found between the accretion rate and flux of the
emission line. The ratios of the surface flux of the Ca II IRT lines to the
stellar bolometric luminosity, R'_IRT, of the PMS stars with narrow emission
lines are as large as the largest R'_IRT of the low-mass stars in the young
open clusters. This result indicates that most PMS stars, even in the classical
T Tauri star stage, have chromospheric activity similar to zero-age
main-sequence stars.
