UBV RI and Hα photometry has been performed for the open cluster NGC 6531. A total of 56 bright main sequence (MS) members were selected from their positions in photometric diagrams. We also classified 7 pre-main sequence (PMS) stars and 6 PMS candidates with Hα emission from Hα photometry. We determined a reddening of < E(B - V) >= 0.29 ± 0.03 and a distance modulus of Vo - Mv = 10.5 for the cluster. From the comparison of our photometric results to theoretical evolution models, we derived a MS turnoff age of 7.5 Myr and a PMS age spread of ~4 Myr. The IMF slope Γ, calculated in the mass range of 0.45 ≤ log m ≤ 1.35 is a steep value of Γ = -1.8 ±0.6.

• BYEONG-GON PARK(Korea Astronomy Observatory)
• HWANKYUNG SUNG(SEES, Seoul National University)
• YONG HEE KANG(Department of Earth Science Education, Kyungpook National University)