Protoplanetary Disk Evolution around the Triggered Star Forming Region
  Cepheus B
  The Cepheus B (CepB) molecular cloud and a portion of the nearby CepOB3b OB
association, one of the most active regions of star formation within 1 kpc,
have been observed with the IRAC detector on board the Spitzer Space Telescope.
The goals are to study protoplanetary disk evolution and processes of
sequential triggered star formation in the region. Out of ~400 pre-main
sequence (PMS) stars selected with an earlier Chandra X-ray Observatory
observation, 95% are identified with mid-infrared sources and most of these are
classified as diskless or disk-bearing stars. The discovery of the additional
>200 IR-excess low-mass members gives a combined Chandra+Spitzer PMS sample
complete down to 0.5 Mo outside of the cloud, and somewhat above 1 Mo in the
cloud. Analyses of the nearly disk-unbiased combined Chandra+Spitzer selected
stellar sample give several results. Our major finding is a spatio-temporal
gradient of young stars from the hot molecular core towards the primary
ionizing O star HD 217086. This strongly supports the radiation driven
implosion (RDI) model of triggered star formation in the region. The empirical
estimate for the shock velocity of 1 km/s is very similar to theoretical models
of RDI in shocked molecular clouds...ABRIDGED... Other results include: 1.
agreement of the disk fractions, their mass dependency, and fractions of
transition disks with other clusters; 2. confirmation of the youthfulness of
the embedded CepB cluster; 3. confirmation of the effect of suppression of
time-integrated X-ray emission in disk-bearing versus diskless systems.
