The Initial Mass Function and Young Brown Dwarf Candidates in NGC 2264. IV. The Initial Mass Function and Star Formation History

We have studied the star formation history and the initial mass function (IMF) using the age and mass derived from spectral energy distribution (SED) fitting and from color-magnitude diagrams (CMDs).We also examined the physical and structural parameter

UBVI CCD Photometry of the Open Cluster NGC 4609 and Hogg 15

UBVI CCD photometry is obtained for the open clusters NGC 4609 and Hogg 15 in Crux. For NGC 4609, CCD data are presented for the first time. From new photometry we derive the reddening, distance modulus and age of each cluster - NGC 4609 : E(B-V) = 0.37 +/- 0.03, V_0 - M_V = 10.60 +/- 0.08, log tau = 7.7 +/- 0.1; Hogg 15 : E(B-V) = 1.13 +/- 0.11, V_0 - M_V = 12.50 +/- 0.15, log tau <= 6.6. The young age of Hogg 15 strongly implies that WR 47 is a member of the cluster. We have also determined the mass function of these clusters and have obtained a normal slope (Gamma = -1.2 +/- 0.3) for NGC 4609 and a somewhat shallow slope (Gamma = -0.95 +/- 0.5) for Hogg 15.Comment: 12 pages, 14 figures, JKAS, in pres

A Spitzer View of the Young Open Cluster NGC 2264

We have performed mid-IR photometry of the young open cluster NGC 2264 using the images obtained with the Spitzer Space Telescope IRAC and MIPS instruments and present a normalized classification scheme of young stellar objects in various color-color diagrams to make full use of the information from multicolor photometry. These results are compared with the classification scheme based on the slope of the spectral energy distribution (SED). From the spatial distributions of Class I and II stars, we have identified two subclusterings of Class I objects in the CONE region of Sung et al. The disked stars in the other star forming region S MON are mostly Class II objects. These three regions show a distinct difference in the fractional distribution of SED slopes as well as the mean value of SED slopes. The fraction of stars with primordial disks is nearly flat between log m = 0.2 -- -0.5, and that of transition disks is very high for solar mass stars. In addition, we have derived a somewhat higher value of the primordial disk fraction for NGC 2264 members located below the main pre-main sequence locus (so-called BMS stars). This result supports the idea that BMS stars are young stars with nearly edge-on disks. We have also found that the fraction of primordial disks is very low near the most massive star S Mon and increases with distance from S Mon.Comment: 38 pages, 21 figures, 5 tables (AJ accepted

Physical Parameters of the Old Open Cluster Trumpler 5

We present a study of the old open cluster Trumpler 5 (Tr 5), based on the CDS archival data. From the color-magnitude diagrams of Tr 5, we have found the positions of main-sequence turn-off (MSTO) and red giant clump (RGC) stars. Using the mean magnitude of the RGC stars, we have estimated the reddening toward Tr 5, E(B-V) = 0.60 +/- 0.10. Using the stars common in two data sets and the theoretical isochrones of Padova group, we have estimated the distance modulus V_0-M_V=12.64 +/- 0.20 (d = 3.4 +/- 0.3 kpc), the metallicity [Fe/H] = -0.30 +/- 0.10, and the age of 2.4 +/- 0.2 Gyr (log t=9.38). These metallicity and distance values are consistent with the relation between the metallicity and the Galactocentric distance of other old open clusters, for which we obtain the slope of Delta_[Fe/H]/R_gc= -0.064 +/- 0.010 dex/kpc.Comment: 9 pages(AASLaTeX), 8 Postscript figures, Accepted for publication in the Journal of the Korean Astronomical Society, 2003 March issu