2016ApJ...817..181Y

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 R_g ∼ 4 kpc. Because this cluster has a typical cluster mass of ∼10³ M_☉ as opposed to very massive clusters, which have been observed in previous studies (>10⁴ M_☉), 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 United Kingdom Infrared Telescope Infrared Deep Sky Survey, we found eight HAeBe candidates in the cluster, and performed K-band medium-resolution (R ≡ Δλ/λ ∼ 800) spectroscopy for three of them with the Subaru 8.2 m telescope. These are found to have both Brγ 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 a conclusion should await future spectroscopic study of all candidates of cluster members.