Spectroscopic observations of blue stars with infrared excesses in NGC 6611

TL;DR

This study uses spectroscopic data to confirm the membership and analyze the physical properties of blue stars with infrared excesses in NGC 6611, focusing on accretion, youth indicators, and binarity.

Contribution

It provides new spectroscopic evidence to confirm cluster membership and investigates the accretion activity and binarity of these peculiar blue stars.

Findings

Confirmed membership of some blue stars via spectroscopic analysis.

Identified accretion and outflow activity through Ha emission line profiles.

Detected Li absorption as an indicator of stellar youth.

Abstract

The young open cluster NGC 6611 includes among its candidate members a class of peculiar objects with interesting properties: blue stars with infrared IR excesses. These stars show excesses in IR bands, signature of the presence of a circumstellar disk, but optical colors typical of older field stars. In order to confirm their membership to the cluster, it is therefore important to use new spectroscopic observations, together with previous photometric data. We aim at confirming the membership of these objects and at investigating their physical properties to verify whether the observed colors are intrinsic or altered by the presence of the disk or by the accretion processes. We analyze the intermediate resolution spectroscopic data obtained for a subsample of blue stars in NGC 6611 with FLAMES. In particular, we focus on the study of: 1) the profile of the Ha emission line, to select stars with accretion and outflow activity; 2) the Li absorption line, used as a youth indicator; 3) the radial velocity. Using the spectroscopic analysis, it has been possible to investigate the presence of the Li absorption line as well as to discriminate between stars with inert or active disk. In particular, from the analysis of the Ha emission line we were able to infer the activity due to the accretion and outflow processes and the variability of the emission. We also investigated the binarity of the blue stars and their membership to NGC 6611.