Abstract:
The first CCD photometric UBVRI observations of the open clusters NGC 1857 (Czernik 20, OCl 427) and Czernik 25 (OCl 493) are presented. The observations were obtained on 28 November 2003 with the 2m Himalayan Chandra Telescope at Hanle using a LN2 cooled 2k ×2k CCD. The colour excess E(B-V) is found to be non-uniform in the line of sight of NGC 1857 ranging between 0.38 and 0.60 mag, while it is 0.69 mag for Cz 25. Further, the clusters NGC 1857 and Cz 25 are estimated to be at a distance of 5.75±0.8 kpc and 4.79±0.5 kpc respectively. As the log (age) of NGC 1857 is estimated to be in the range of 8.00 to 8.25 and that of Cz 25 is 8.45, they may be considered to be of intermediate age. The luminosity function (LF) and the mass function (MF) for both the clusters have been derived. The MF slopes for the clusters NGC 1857 and Cz 25 are -2.39±0.26 and -2.70±0.25 respectively. These values are in fair agreement with that given by Salpeter. Mass segregation is observed in both the clusters and it could be due to the dynamical evolution of the clusters or the imprint of star formation process or both.
