Spitzer Mid-infrared Study of Sh 2-208: Evolution of Protoplanetary
  Disks in Low-metallicity Environments
  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$10$\sigma$, 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.
