Protoplanetary and Transitional Disks in the Open Stellar Cluster IC
  2395
  We present new deep UBVRI images and high-resolution multi-object optical
spectroscopy of the young (~ 6 - 10 Myr old), relatively nearby (800 pc) open
cluster IC 2395. We identify nearly 300 cluster members and use the photometry
to estimate their spectral types, which extend from early B to middle M. We
also present an infrared imaging survey of the central region using the IRAC
and MIPS instruments on board the Spitzer Space Telescope, covering the
wavelength range from 3.6 to 24 microns. Our infrared observations allow us to
detect dust in circumstellar disks originating over a typical range of radii ~
0.1 to ~ 10AU from the central star. We identify 18 Class II, 8 transitional
disk, and 23 debris disk candidates, respectively 6.5%, 2.9%, and 8.3% of the
cluster members with appropriate data. We apply the same criteria for
transitional disk identification to 19 other stellar clusters and associations
spanning ages from ~ 1 to ~ 18 Myr. We find that the number of disks in the
transitional phase as a fraction of the total with strong 24 micron excesses
([8] - [24] > 1.5) increases from 8.4 +\- 1.3% at ~ 3 Myr to 46 +\- 5% at ~ 10
Myr. Alternative definitions of transitional disks will yield different
percentages but should show the same trend.
