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

Abstract

This study presents sensitive MIR photometry obtained with the Spitzer/IRAC for a young cluster in Sh 2-208 (S208) located in one of the lowest-metallicity HII regions in the Galaxy, ${\rm [O/H]} = -0.8$ dex. Previous studies suggested that the cluster is $\sim$0.5-Myr old and has a distance of $D = 4$ kpc, which is consistent with the astrometric distance from Gaia EDR3. In $\sim$$3.5 \times 4$-arcmin field, 96 sources were detected in at least one MIR band at $\ge$10$σ$, covering intermediate-mass stars with $\sim$1.0-$M_\odot$ mass detection limit. Total 41 probable cluster members were identified based on the spatial distributions of spectral-energy-distribution slopes derived from the NIR $K_S$ and IRAC bands and extinctions of the sources. The cumulative distribution of the SED slopes for the S208 cluster was not significantly different from those of other clusters in solar-metallicity environments with approximately the same age for intermediate-mass stars, if one also considers non-detected MIR sources identified as S208 cluster members from NIR observations. This suggests that the degree of dust growth/settling does not significantly change with metallicities as different as $\sim$1 dex. The fraction of stars with MIR disk emissions for the cluster members with $\ge$1-$M_\odot$ mass was 64%-93%, which is comparable to the results in solar-metallicity environments. Although this may suggest that dominant disk-dispersal mechanisms for intermediate-mass stars have either no, or very weak, dependence on metallicity, it can be argued alternatively that this may suggest that the disk dispersal process does not work effectively at this young stage.