Rapid evolution of the innermost dust disc of protoplanetary discs surrounding intermediate-mass stars

Abstract

We derived the intermediate-mass (~=1.5--7 M_sun) disk fraction (IMDF) in the near-infrared JHK photometric bands as well as in the mid-infrared (MIR) bands for young clusters in the age range of 0 to ~10 Myr. From the JHK IMDF, the lifetime of the innermost dust disk (~0.3 AU; hereafter the K disk) is estimated to be ~3 Myr, suggesting a stellar mass (M*) dependence of K-disk lifetime proportional to M*^-0.7. However, from the MIR IMDF, the lifetime of the inner disk (~5 AU; hereafter the MIR disk) is estimated to be ~6.5 Myr, suggesting a very weak stellar mass dependence (proportional to M*^-0.2). The much shorter K-disk lifetime compared to the MIR-disk lifetime for intermediate-mass (IM) stars suggests that IM stars with transition disks, which have only MIR excess emission but no K-band excess emission, are more common than classical Herbig Ae/Be stars, which exhibit both. We suggest that this prominent early disappearance of the K disk for IM stars is due to dust settling/growth in the protoplanetary disk, and it could be one of the major reasons for the paucity of close-in planets around IM stars.