Protostars and Planets VI, Heidelberg, July 15-20, 2013

Poster 2S060

The Lifetime of Protoplanetary Disks Surrounding Intermediate-mass Stars

Yasui, Chikako (University of Tokyo)
Naoto, Kobayashi (University of Tokyo)
Tokunaga, Alan (University of Hawaii)
Saito, Masao (National Astronomical Observatory of Japan)

To quantitatively and comprehensively study the lifetime of protoplanetary disks surrounding intermediate-mass (IM) stars (~=1.5--7 M_sun), we derived the intermediate-mass disk fraction (IMDF) in the near-infrared JHK photometric bands as well as in the mid-infrared (MIR) bands for a large number (~20) of well-established nearby (D <~ 1.5 kpc) young clusters in the age range of 0 to ~10 Myr. The IM stars were selected from each cluster by making use of published spectral types in the literature, and the IMDF was derived assuming a single age per cluster. The derived JHK IMDF, which traces the innermost dust disk (hereafter the K disk), and MIR IMDF, which traces the inner disk at radii larger than the K disk (hereafter the MIR disk), appear to approximately follow an exponential decay with cluster age. The disk lifetime of the K disk for IM stars is estimated to be ~4 Myr, while that for low-mass (LM) stars is estimated to be ~10 Myr from the data in the literature. This suggests a stellar mass (M*) dependence of K-disk lifetime proportional to M*^{-0.5}. However, for the MIR disk, we found that the disk lifetimes for the IM stars (~7 Myr) and the LM stars (~9 Myr) are similar, so that the stellar mass dependence is very weak (proportional to M*^{-0.1}). The much shorter K-disk lifetime compared to the MIR-disk lifetime 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. If this is confirmed, our results set the time-scale of dust settling/growth in the K disk of the IM stars at ~3 Myr. We also discuss the possible implications of our results for the lack of close-in planets and the high frequency of Jupiter-mass planets around IM stars. Our results, the early dispersal of the K disk around IM stars, could be one of the major reasons for the paucity of close-in planets around IM stars.

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