  Formation, evolution, and dispersal processes of protoplanetary disks are
investigated and the disk lifetime is estimated. Gravitational collapse of a
pre-stellar core forms both a central star and a protoplanetary disk. The
central star grows by accretion from the disk, and irradiation by the central
star heats up the disk and generates thermal wind, which results in the disk
dispersal. We calculate the evolution of protoplanetary disks from their parent
pre-stellar cores to dispersal of the disks. We find that the disk lifetimes of
typical pre-stellar cores are around 2--4 million years (Myr). A pre-stellar
core with high angular momentum forms a larger disk whose lifetime is long,
while a disk around a X-ray luminous star has a short lifetime. Integrating the
disk lifetimes under various mass and angular velocity of prestellar cores and
X-ray luminosities of young stellar objects, we obtain disk fraction at a given
stellar age and mean lifetime of the disks. Our model indicates that the mean
lifetime of protoplanetary disks is 3.7 Myr, which is consistent with the
observational estimate from young stellar clusters. We also find that the
dispersion of X-ray luminosity is needed to reproduce the observed disk
fraction.
