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

TL;DR

This study used deep near-infrared imaging to analyze two young, low-metallicity clusters in the outer Galaxy, revealing their large sizes, very young ages, typical IMFs, and rapid disk dispersal, which informs star formation in such environments.

Contribution

It provides the first detailed analysis of large, very young low-metallicity clusters, showing they have IMFs similar to solar neighborhood clusters but with faster disk loss.

Findings

Clusters are larger than previously studied low-metallicity clusters.

IMFs are consistent with those of solar neighborhood clusters.

Disk fractions are lower and suggest rapid disk dispersal in low-metallicity environments.

Abstract

In deep near-infrared imaging of the low-metallicity (${\rm [O/H]}=-0.7$ 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$\sigma$), corresponding to a mass detection limit of $\sim$0.2 $M_\odot$. 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 $A_V\simeq20$ 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 $28\% \pm 3\%$ for S127A and $40\% \pm 4\%$ for S127B are significantly lower than those of similarly aged solar neighborhood clusters ($\sim$50$\%$-60$\%$). The disk fraction for S127B is higher than those of previously studied low-metallicity clusters ($<$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.