Low-metallicity Young Clusters in the Outer Galaxy. II. Sh 2-208
  We obtained deep near-infrared images of Sh 2-208, one of the
lowest-metallicity HII regions in the Galaxy, [O/H] = -0.8 dex. We detected a
young cluster in the center of the HII region with a limiting magnitude of K =
18.0 mag (10sigma), which corresponds to a mass detection limit of ~0.2 M_sun.
This enables the comparison of star-forming properties under low metallicity
with those of the solar neighborhood. We identified 89 cluster members. From
the fitting of the K-band luminosity function (KLF), the age and distance of
the cluster are estimated to be ~0.5 Myr and ~4 kpc, respectively. The
estimated young age is consistent with the detection of strong CO emission in
the cluster region and the estimated large extinction of cluster members (Av ~
4--25 mag). The observed KLF suggests that the underlying initial mass function
(IMF) of the low-metallicity cluster is not significantly different from
canonical IMFs in the solar neighborhood in terms of both high-mass slope and
IMF peak (characteristic mass). Despite the very young age, the disk fraction
of the cluster is estimated at only 27pm6 %, which is significantly lower than
those in the solar metallicity. Those results are similar to Sh 2-207, which is
another low-metallicity star-forming region close to Sh 2-208 with a separation
of 12 pc,suggesting that their star-forming activities in low-metallicity
environments are essentially identical to those in the solar neighborhood,
except for the disk dispersal timescale. From large-scale mid-infrared images,
we suggest that sequential star formation is taking place in Sh 2-207, Sh 2-208
and the surrounding region, triggered by an expanding bubble with a ~30 pc
radius.
