Low-metallicity Young Clusters in the Outer Galaxy. II. Sh 2-208

TL;DR

This study investigates a young, low-metallicity star cluster in Sh 2-208, revealing its age, mass distribution, and disk fraction, and compares its star-forming properties to those in the solar neighborhood, suggesting similar star formation processes.

Contribution

First detailed analysis of a low-metallicity young cluster in the outer Galaxy, showing comparable initial mass function but faster disk dispersal compared to solar metallicity regions.

Findings

Cluster age ~0.5 Myr and distance ~4 kpc.

IMF similar to solar neighborhood in shape.

Disk fraction significantly lower (~27%) than in solar metallicity regions.

Abstract

We obtained deep near-infrared images of Sh 2-208, one of the lowest-metallicity HII regions in the Galaxy, [O/H] = -0.8 dex. We detected a young cluster in the center of the HII region with a limiting magnitude of K = 18.0 mag (10sigma), which corresponds to a mass detection limit of ~0.2 M_sun. This enables the comparison of star-forming properties under low metallicity with those of the solar neighborhood. We identified 89 cluster members. From the fitting of the K-band luminosity function (KLF), the age and distance of the cluster are estimated to be ~0.5 Myr and ~4 kpc, respectively. The estimated young age is consistent with the detection of strong CO emission in the cluster region and the estimated large extinction of cluster members (Av ~ 4--25 mag). The observed KLF suggests that the underlying initial mass function (IMF) of the low-metallicity cluster is not significantly different from canonical IMFs in the solar neighborhood in terms of both high-mass slope and IMF peak (characteristic mass). Despite the very young age, the disk fraction of the cluster is estimated at only 27pm6 %, which is significantly lower than those in the solar metallicity. Those results are similar to Sh 2-207, which is another low-metallicity star-forming region close to Sh 2-208 with a separation of 12 pc,suggesting that their star-forming activities in low-metallicity environments are essentially identical to those in the solar neighborhood, except for the disk dispersal timescale. From large-scale mid-infrared images, we suggest that sequential star formation is taking place in Sh 2-207, Sh 2-208 and the surrounding region, triggered by an expanding bubble with a ~30 pc radius.