Low-metallicity Young Clusters in the Outer Galaxy I. Sh 2-207

TL;DR

This study investigates a young star cluster in the low-metallicity environment of the outer Galaxy, analyzing its properties and comparing its initial mass function and disk fraction to those in the solar neighborhood.

Contribution

First detailed analysis of a young cluster in a low-metallicity outer Galaxy environment, including age, distance, and initial mass function estimation.

Findings

Cluster age estimated at 2-3 Myr

Initial mass function similar to solar metallicity clusters

Low disk fraction (<10%) in the young cluster

Abstract

To study star formation in low metallicity environments ([M/H] ~ -1 ,dex), we obtained deep near-infrared (NIR) images of Sh 2-207 (S207), which is an HII region in the outer Galaxy with spectroscopically determined metallicity of [O/H] ~= -0.8 dex. We identified a young cluster in the western region of S207 with a limiting magnitude of Ks =19.0 mag (10 sigma) that corresponds to a mass detection limit of <~0.1 M_sun and enables the comparison of star-forming properties under low metallicity with those of the solar neighborhood. From the fitting of the K-band luminosity function (KLF), the age and distance of S207 cluster are estimated at 2-3Myr and ~4 kpc, respectively. The estimated age is consistent with the suggestion of small extinctions of stars in the cluster (Av ~ 3 mag) and the non-detection of molecular clouds. The reasonably good fit between observed KLF and model KLF suggests that the underlying initial mass function (IMF) of the cluster down to the detection limit is not significantly different from the typical IMFs in the solar metallicity. From the fraction of stars with NIR excesses, a low disk fraction (<10 %) in the cluster with relatively young age is suggested, as we had previously proposed.