Sejong Open Cluster Survey (SOS). III. The Young Open Cluster NGC 1893 in the H II Region

We present a UBV I and Hα photometric study of the young open cluster NGC 1893 in the H II region W8 (IC 410 or Sh 2-236). A total of 65 early-type members are selected from photometric diagrams. A mean reddening of the stars is 〈E(B − V) 〉 = 0.563 ± 0.083 mag. The published photometric data in the near- and mid-infrared passbands are used to test the reddening law toward the cluster, and we confirm that the reddening law is normal (RV = 3.1). Zero-age main sequence fitting gives a distance modulus of V0−MV = 12.7 ± 0.2 mag, equivalent to 3.5 ± 0.3 kpc. From Hα photometry 125 Hα emission stars and candidates are identified as pre-main sequence (PMS). The lists of young stellar objects and X-ray sources published by previous studies allow us to select a large number of PMS members down to 1M. Isochrone fitting in the Hertzsprung-Russell diagram gives a turn-off age of 1.5 Myr and the median age of 1.9 Myr from the PMS members with a spread of ∼ 5 Myr. We derive the initial mass function (IMF) for stars with mass larger than 1M. The slope of the IMF (Γ = −1.3 ± 0.1) is well consistent with the Salpeter/Kroupa IMF. A total mass of the cluster appears to be in excess of 1,300 M. Finally, we estimate the mass accretion rate of 82 PMS members in the mass range of 0.6M to 5M. Key words: accretion, accretion discs – circumstellar matter – stars: luminosity function, mass function – open clusters and associations: individual (NGC 1893)