Volume 556, August 2013
|Number of page(s)||10|
|Section||Galactic structure, stellar clusters and populations|
|Published online||05 August 2013|
Spectroscopic observations of blue stars with infrared excesses in NGC 6611
Dipartimento di Fisica e ChimicaUniversità di Palermo,
P.zza del Parlamento 1,
2 INAF – Osservatorio Astronomico di Palermo, P.zza del Parlamento 1, 90134 Palermo, Italy
3 Smithsonian Astrophysical Observatory − SAO, 60 Garden St, Cambridge, MA02138, USA
Received: 4 March 2013
Accepted: 5 June 2013
Context. The young open cluster NGC 6611 includes a group of peculiar objects with interesting properties among its candidate members: blue stars with infrared (IR) excesses. These stars show excesses in IR bands, a signature of the presence of a circumstellar disk, but optical colors typical of older field stars. To confirm their membership in the cluster, it is therefore important to use new spectroscopic observations, together with previous photometric data.
Aims. We aim to confirm the membership of these objects and investigate their physical properties to verify whether the observed colors are intrinsic or altered by the disk or by the accretion processes.
Methods. We analyzed the intermediate-resolution spectroscopic data obtained for a subsample of blue stars in NGC 6611 with FLAMES. In particular, we focused on the study of 1) the profile of the Hα emission line, to select stars with accretion and outflow activity; 2) the Li absorption line, used as a youth indicator; 3) the radial velocity.
Results. Using the spectroscopic analysis, it has been possible to investigate the Li absorption line, as well as to distinguish between stars with inert or active disks. In particular, from the analysis of the Hα 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.
Conclusions. From our spectroscopic analysis, we conclude that half of the sample of blue stars (10/20) are confirmed members of NGC 6611 (with 6 more stars that could also be possible members). In conclusion, our results indicate that members of young clusters can also be found in an anomalous region of the color−magnitude diagram, i.e., outside of the pre-main sequence locus where most of the cluster members lie.
Key words: stars: formation / stars: pre-main sequence / accretion, accretion disks
© ESO, 2013
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