Sejong Open Cluster Survey (SOS) - IV. The Young Open Clusters NGC 1624 and NGC 1931

TL;DR

This study investigates the properties, distances, ages, and initial mass functions of young open clusters NGC 1624 and NGC 1931 in the outer Galaxy, revealing their star formation characteristics and environmental differences from the solar neighborhood.

Contribution

It provides detailed photometric analysis and IMF determination for NGC 1624 and NGC 1931, highlighting their unique reddening laws and confirming their young ages and distances.

Findings

NGC 1624 is approximately 6 kpc away from the Sun.

NGC 1931 is approximately 2.3 kpc away from the Sun.

Both clusters have steep IMFs with slopes around -2.0.

Abstract

Young open clusters located in the outer Galaxy provide us with an opportunity to study star formation activity in a different environment from the solar neighborhood. We present a UBVI and H alpha photometric study of the young open clusters NGC 1624 and NGC 1931 that are situated toward the Galactic anticenter. Various photometric diagrams are used to select the members of the clusters and to determine the fundamental parameters. NGC 1624 and NGC 1931 are, on average, reddened by <E(B-V)> = 0.92 +/- 0.05 and 0.74 +/- 0.17 mag, respectively. The properties of the reddening toward NGC 1931 indicate an abnormal reddening law (Rv,cl = 5.2 +/- 0.3). Using the zero-age main sequence fitting method we confirm that NGC 1624 is 6.0 +/- 0.6 kpc away from the Sun, whereas NGC 1931 is at a distance of 2.3 +/- 0.2 kpc. The results from isochrone fitting in the Hertzsprung-Russell diagram indicate the ages of NGC 1624 and NGC 1931 to be less than 4 Myr and 1.5 - 2.0 Myr, respectively. We derived the initial mass function (IMF) of the clusters. The slope of the IMF (Gamma_NGC 1624 = -2.0 +/- 0.2 and Gamma_NGC 1931 = -2.0 +/- 0.1) appears to be steeper than that of the Salpeter/Kroupa IMF. We discuss the implication of the derived IMF based on simple Monte-Carlo simulations and conclude that the property of star formation in the clusters seems not to be far different from that in the solar neighborhood.