Sequential clustering of star formation in IC 1396
  We present in this paper a comprehensive study of the H II region IC 1396 and
its star formation activity, in which multi-wavelength data ranging from the
optical to the near- and far-infrared were employed. The surface density
distribution of all the 2MASS sources with certain detection toward IC 1396
indicates the existence of a compact cluster spatially consistent with the
position of the exciting source of the H II region, HD 206267. The spatial
distribution of the infrared excessive emission sources selected based on
archived 2MASS data reveals the existence of four sub-clusters in this region.
One is in association with the open cluster Trumpler 37. The other three are
found to be spatially coincident with the bright rims of the H II region. All
the excessive emission sources in the near infrared are cross-identified with
the AKARI IRC data, an analysis of the spectral energy distributions (SEDs) of
the resultant sample leads to the identification of 8 CLASS I, 15 CLASS II and
15 CLASS III sources in IC 1396. Optical identification of the sample sources
with R magnitudes brighter than 17 mag corroborates the results from the SED
analysis. Based on the spatial distribution of the infrared young stellar
objects at different evolutionary stages, the surrounding sub-clusters located
in the bright rims are believed to be younger than the central one. This is
consistent with a scenario of sequential star formation in this region. Imaging
data of a dark patch in IC 1396 by Herschel SPIRE, on the other hand, indicate
the presence of two far-infrared cores in LDN 1111, which are likely new
generation protostellar objects in formation. So we infer that the star
formation process in this H II region was not continuous but episodic.
