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There is much observational evidence that active star formation is takingplace in the HII regions Sh 2-255 -- 257. We present a photometric study ofthis star forming region (SFR) using imaging data obtained in passbands fromthe optical to the mid-infrared in order to study the star formation process. Atotal of 218 members were identified using various selection criteria based ontheir observational properties. The SFR is reddened by at least E(B-V) = 0.8mag, and the reddening law toward the region is normal (R_V = 3.1). From thezero-age main sequence fitting method it is confirmed that the SFR is 2.1 +/-0.3 kpc from the Sun. The median age of the identified members is estimated tobe about 1.3 Myr from comparison of the Hertzsprung-Russell diagram (HRD) withstellar evolutionary models. The initial mass function (IMF) is derived fromthe HRD and the near-infrared (J, J-H) color-magnitude diagram. The slope ofthe IMF is about Gamma = -1.6 +/- 0.1, which is slightly steeper than that ofthe Salpeter/Kroupa IMF. It implies that low-mass star formation is dominant inthe SFR. The sum of the masses of all the identified members provides the lowerlimit of the cluster mass (169M_sun). We also analyzed the spectral energydistribution (SED) of pre-main sequence stars using the SED fitting tool ofRobitaille et al. and confirm that there is a significant discrepancy betweenstellar mass and age obtained from two different methods based on the SEDfitting tool and the HRD.

Sejong open cluster survey (SOS) - V. The active star forming region Sh 2-255 -- 257