Low-metallicity Young Clusters in the Outer Galaxy. III. Sh 2-127

Abstract

In deep near-infrared imaging of the low-metallicity ([O/H] = −0.7 dex) H ii region Sh 2-127 (S127) with Subaru/MOIRCS, we detected two young clusters with 413 members (S127A) in a slightly extended H ii region and another with 338 members (S127B) in a compact H ii region. The limiting magnitude was K  = 21.3 mag (10 σ ), corresponding to a mass detection limit of ∼0.2 . These clusters are an order of magnitude larger than previously studied young low-metallicity clusters and larger than the majority of solar neighborhood young clusters. Fits to the K -band luminosity functions indicate very young cluster ages of 0.5 Myr for S127A and 0.1–0.5 Myr for S127B, consistent with the large extinction (up to A V  ≃ 20 mag) from thick molecular clouds and the presence of a compact H ii region and class I source candidates, and suggest that the initial mass function (IMF) of the low-metallicity clusters is indistinguishable from typical solar neighborhood IMFs. Disk fractions of 28% ± 3% for S127A and 40% ± 4% for S127B are significantly lower than those of similarly aged solar neighborhood clusters (∼50%–60%). The disk fraction for S127B is higher than those of previously studied low-metallicity clusters (<30%), probably due to S127B’s age. This suggests that a large fraction of very young stars in low-metallicity environments have disks, but the disks are lost on a very short timescale. These results are consistent with our previous studies of low-metallicity star-forming regions, suggesting that a solar neighborhood IMF and low disk fraction are typical characteristics for low-metallicity regions, regardless of cluster scales.