Evolution of Emission-Line Activity in Intermediate-Mass Young Stars

We present optical spectra of 45 intermediate-mass Herbig Ae/Be stars. Together with the multiepoch spectroscopic and photometric data compiled for a large sample of these stars and ages estimated for individual stars by using pre-main-sequence evolutionary tracks, we have studied the evolution of emission-line activity in them. We find that, on average, the Hα emission line strength decreases with increasing stellar age in Herbig Ae/Be stars, indicating that the accretion activity gradually declines during the pre-main-sequence phase. This would hint at a relatively long-lived (a few Myr) process being responsible for the cessation of accretion in Herbig Ae/Be stars. We also find that the accretion activity in these stars drops substantially by ~3 Myr. This is comparable to the timescale in which most intermediate-mass stars are thought to lose their inner disks, suggesting that inner disks in intermediate-mass stars are dissipated rapidly after the accretion activity has fallen below a certain level. We further find a relatively tight correlation between strength of the emission line and near-infrared excess due to inner disks in Herbig Ae/Be stars, indicating that the disks around Herbig Ae/Be stars cannot be entirely passive. We suggest that this correlation can be understood within the framework of the puffed-up inner rim disk models if the radiation from the accretion shock is also responsible for the disk heating.