  Equivalent widths and line widths of Ca II infrared triplet emission lines
were measured in high-resolution optical spectra of 39 young stellar objects.We
found that the equivalent widths of the emission lines decrease with stellar
evolution. It has been often claimed that strong chromospheric activity is
generated by a dynamo process caused by fast rotation of the photosphere.
However, we found no clear correlation between the strength of the Ca II lines
and the stellar rotation velocity. Instead, we found that the objects with high
mass accretion rates had stronger Ca II emission lines. This correlation
supports the turbulent chromosphere model or the magnetic accretion theory for
classical T Tauri stars. We also noticed that the equivalent widths of Ca II
lines of transitional disk objects are one-tenth of those of classical T Tauri
stars, even if the masses of the circumstellar disks are comparable.
