There is much observational evidence that active star formation is taking place in the Hii regions Sh 2-255 – 257. We present a photometric study of this star forming region (SFR) using imaging data obtained in passbands from the optical to the mid-infrared, in order to study the star formation process. A total of 218 members were identified using various selection criteria based on their observational properties. The SFR is reddened by at least E(B −V ) = 0.8 mag, and the reddening law toward the region is normal (RV = 3.1). From the zero-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 to be about 1.3 Myr from a comparison of the Hertzsprung-Russell diagram (HRD) with stellar evolutionary models. The initial mass function (IMF) is derived from the HRD and the near-infrared (J, J −H) color-magnitude diagram. The slope of the IMF is about 􀀀 = −1.6 ± 0.1, which is slightly steeper than that of the Salpeter/Kroupa IMF. It implies that low-mass star formation is dominant in the SFR. The sum of the masses of all the identified members provides the lower limit of the cluster mass (169M⊙). We also analyzed the spectral energy distribution (SED) of pre-main sequence stars using the SED fitting tool of Robitaille et al., and confirm that there is a significant discrepancy between stellar mass and age obtained from two different methods based on the SED fitting tool and the HRD.

Abstract
1. INTRODUCTION
2. OBSERVATIONAL DATA
    2.1. Optical Imaging Data
    2.2. Optical Spectroscopic Data
    2.3. Archival Infrared Data
3. FUNDAMENTAL PARAMETERS
    3.1. Spectral Types of Two Ionizing Sources
    3.2. Membership Selection
    3.3. Interstellar Extinction and the Reddening Law
    3.4. Distance
    3.5. Age
4. TOTAL MASS AND THE INITIAL MASS FUNCTION
5. SPECTRAL ENERGY DISTRIBUTION ANALYSIS
6. SUMMARY
REFERENCES

• Beomdu Lim(Korea Astronomy and Space Science Institute) Corresponding author
• Hwankyung Sung(Department of Astronomy and Space Science, Sejong University)
• Hyeonoh Hur(Department of Astronomy and Space Science, Sejong University)
• Byeong-Cheol Lee(Korea Astronomy and Space Science Institute/Astronomy and Space Science Major, University of Science and Technology)
• Michael S. Bessell(Research School of Astronomy and Astrophysics, Australian National University)
• Jinyoung S. Kim(Steward Observatory, University of Arizona)
• Kang Hwan Lee(Gwacheon National Science Museum)
• Byeong-Gon Park(Korea Astronomy and Space Science Institute/Astronomy and Space Science Major, University of Science and Technology)
• Gwanghui Jeong(Korea Astronomy and Space Science Institute/Astronomy and Space Science Major, University of Science and Technology)