Investigation of Star Formation toward the Sharpless 155 H II region
  We present a comprehensive study of star formation toward the H II region
S155. Star-formation activities therein were investigated based on
multi-wavelength data from optical to the far-infrared. The surface density
distribution of selected 2MASS sources toward S155 indicates the existence of a
compact cluster, which is spatially consistent with the position of the
exciting source of the H II region, HD 217086. A sample of more than 200
excessive emission sources in the infrared were selected based on their 2MASS
color indices. The spatial distribution of the sample sources reveals the
existence of three young sub-clusters in this region, among which, sub-cluster
A is spatially coincident with the bright rim of the H II region. In addition,
photometric data from the WISE survey were used to identify and classify young
stellar objects (YSOs). To further explore the evolutionary stages of the
candidate YSOs, we fit the spectral energy distribution (SEDs) of 44 sources,
which led to the identification of 14 Class I, 27 Class II, and 3 Class III
YSOs. The spatial distribution of the classified YSOs at different evolutionary
stages presents a spatio-temporal gradient, which is consistent with a scenario
of sequential star formation. On the other hand, Herschel PACS observations
toward the interface between S155 and the ambient molecular cloud disclose an
arc-shaped dust layer, the origin of which could be attributed to the UV
dissipation from the early type stars e.g. HD 217061 in S155. Four dusty cores
were revealed by the Herschel data, which hints for new generations of star
formation.
