Devendra Ojha1, Motohide Tamura2 and SIRIUS Team 1 Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai 400 005, India 2 National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan
Abstract. We have conducted deep near-infrared surveys of the Sh-2 255, W3 Main and NGC 7538 massive star forming regions using simultaneous observations of the JHKs -band with the near-infrared camera SIRIUS on the UH 88-inch telescope. The near-infrared surveys cover a total area of ~72 square arcmin of three regions with 10-sigma limiting magnitudes of ~19.5, 18.4 and 17.3 in J, H and Ks-band, respectively. Based on the colour-colour and colour- magnitude diagrams and their clustering properties, the candidate young stellar objects are identified and their luminosity functions are constructed in Sh-2 255, W3 Main and NGC 7538. A large number of previously unreported red sources (H - K > 2) have also been detected around these regions. We argue that these red stars are most probably pre-main sequence stars with intrinsic colour excesses. The detected young stellar objects show a clear clustering pattern in each region: the Class I-like sources are mostly clustered in molecular cloud region, while the Class II-like sources in or around more evolved optical H II regions. We find that the slopes of the Ks -band luminosity functions of Sh-2 255, W3 Main and NGC 7538 are lower than the typical values reported for the young embedded clusters and their stellar populations are primarily composed of low mass pre-main sequence stars. From the slopes of the Ks -band luminosity functions, we infer that Sh-2 255, W3 Main and NGC 7538 star forming regions are rather young (age 1 Myr).
Keywords: ISM: clouds -- stars: formation -- stars: pre-main sequence -- open clusters and associations: general -- infrared: stars