Herbig Ae/Be candidate stars in the innermost Galactic disk: Quartet
  cluster
  In order to investigate the Galactic-scale environmental effects on the
evolution of protoplanetary disks, we explored the near-infrared (NIR) disk
fraction of the Quartet cluster, which is a young cluster in the innermost
Galactic disk at the Galactocentric radius Rg ~ 4 kpc. Because this cluster has
a typical cluster mass of ~10^3 M_sun as opposed to very massive clusters,
which have been observed in previous studies (>10^4 M_sun), we can avoid
intra-cluster effects such as strong UV field from OB stars. Although the age
of the Quartet is previously estimated to be 3-8 Myr old, we find that it is
most likely ~3-4.5 Myr old. In moderately deep JHK images from the UKIDSS
survey, we found eight HAeBe candidates in the cluster, and performed K-band
medium-resolution ($R \equiv \Delta \lambda / \lambda ~ 800$) spectroscopy for
three of them with the Subaru 8.2 m telescope. These are found to have both
Br\gamma absorption lines as well as CO bandhead emission, suggesting that they
are HAeBe stars with protoplanetary disks. We estimated the intermediate-mass
disk fraction (IMDF) to be ~25 % for the cluster, suggesting slightly higher
IMDF compared to those for young clusters in the solar neighborhood with
similar cluster age, although such conclusion should await future spectroscopic
study of all candidates of cluster members.
