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 (10σ), corresponding to a mass detection limit of ∼0.2 M ⊙ . 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.

Spitzer Mid-infrared Study of Sh 2-208: Evolution of Protoplanetary Disks in Low-metallicity Environments

“…In a series of papers, we compiled a list of low-metallicity ([O/H] ≃ −1 dex) young clusters in the Galaxy and presented deep NIR imaging observations with a detection limit of ∼0.1 M ⊙ . The estimated disk fractions were relatively high for very young clusters, but they declined rapidly on the timescale of ∼1 Myr, suggesting that the lifetime of protoplanetary disks in such environments is shorter than that in clusters with solar metallicity (Yasui et al 2016a(Yasui et al , 2009(Yasui et al , 2010(Yasui et al , 2016b(Yasui et al , 2021. Although the results for the MIR disk fraction of the S208 cluster discussed herein appears to be contrary to our previous results, the difference can be attributed to the difference of the stellar mass range.…”

“…In addition to the MIR data, NIR JHK-band data are necessary in this study to estimate the extinction and mass for each source and to determine the SED slope for each source for categorization into evolutionary disk stages. The stellar density in S208 is relatively high (Yasui et al 2016a; see also Figure 12 of Yasui et al 2021); therefore, the archival data do not provide sufficiently accurate photometry (e.g., 2MASS) due to inadequate spatial resolution (FWHM of ∼2.5 arcsec). We used the results of NIR JHK-band photometry from images with spatial resolutions 1 arcsec in the literature (Yasui et al 2016a).…”

“…In a series of papers, we have compiled a list of low-metallicity ([O/H] ≃ −1 dex) young clusters in the Galaxy and have presented deep NIR imaging observations with a mass detection limit of ∼0.1 M ⊙ . The estimated fractions of stars with disks based on NIR continuum emission (NIR disks) in such clusters are lower than those in the solar neighborhood, suggesting that the lifetimes of protoplanetary disks in such environments are shorter than in clusters with solar metallicity (Yasui et al 2009(Yasui et al , 2010(Yasui et al , 2016b(Yasui et al ,a, 2021. Puga et al (2009), Cusano et al (2011), and Kalari & Vink (2015) studied another H II region in the Galaxy, Sh 2-284 (S284), which is located at a distance D ∼ 4 kpc, has a high-mass star-forming region, and is possibly in a low-metallicity environment.…”

“…Because the MIR disk traces a slightly larger radius in a protoplanetary disk (a stellocentric distance of approximately a few AU), compared to the NIR disk, which traces the inner disk edge (a stellocentric distance of ∼0.1 AU), it may be possible to investigate the properties of disks more precisely in lowmetallicity environments. From our compilation of star-forming clusters in low-metallicity environments, all of which have cluster radii of ∼1 pc (see Figure 12 in Yasui et al 2021), Sh 2-208 (S208) is found to have a relatively small distance (D ∼ 4 kpc). Among such regions, S208 turned out to be almost the only cluster with sensitive MIR imaging obtained with Spitzer/IRAC.…”

The statistical properties of protostellar discs and their dependence on metallicity

“…The disc fraction in young low metallicity star clusters is observed to be lower than in clusters of higher metallicity at a similar age (∼ 0.5 Myr), providing evidence that discs have a shorter lifetime at low metallicities (Yasui et al 2009(Yasui et al , 2016Yasui et al 2021). Ercolano & Clarke (2010) show that these observations may be due to the metallicity dependence of disc dispersal by photoevaporation.…”