Herbig Ae/Be candidate stars in the innermost Galactic disk: Quartet cluster

TL;DR

This study investigates Herbig Ae/Be stars in the Quartet cluster within the Galactic disk, revealing a protoplanetary disk fraction of about 25%, which is slightly higher than similar clusters nearby, providing insights into environmental effects on disk evolution.

Contribution

First spectroscopic confirmation of Herbig Ae/Be candidates in the Quartet cluster, and estimation of its intermediate-mass disk fraction in the inner Galaxy.

Findings

Identified eight Herbig Ae/Be candidates in the cluster.

Spectroscopy confirms protoplanetary disks around three candidates.

Estimated the cluster's disk fraction to be approximately 25%.

Abstract

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.