Near-infrared spectroscopy of candidate red supergiant stars in clusters ^{⋆,⋆⋆,⋆⋆⋆}

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: messineo@mpifr-bonn.mpg.de
² European Space Agency (ESA), The Astrophysics and Fundamental Physics Missions Division, Research and Scientific Support Department, Directorate of Science and Robotic Exploration, ESTEC, Postbus 299, 2200 AG Noordwijk, The Netherlands
³ Chester F. Carlson Center for Imaging Science, Rochester Institute of Technology, 54 Lomb Memorial Drive, Rochester, NY 14623-5604, USA
⁴ Key Laboratory for Researches in Galaxies and Cosmology, University of Science and Technology of China, Chinese Academy of Sciences, Hefei, 230026 Anhui, PR China
⁵ European Southern Observatory, Alonso de Cordoba 3107, Santiago, Chile
⁶ Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead, Wirral, CH41 1LD, UK
⁷ Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA

Received: 13 March 2014
Accepted: 25 August 2014

Abstract

Context. Clear identifications of Galactic young stellar clusters farther than a few kpc from the Sun are rare, despite the large number of candidate clusters.

Aims. We aim to improve the selection of candidate clusters rich in massive stars with a multiwavelength analysis of photometric Galactic data that range from optical to mid-infrared wavelengths.

Methods. We present a photometric and spectroscopic analysis of five candidate stellar clusters, which were selected as overdensities with bright stars (K_s< 7 mag) in GLIMPSE and 2MASS images.

Results. A total of 48 infrared spectra were obtained. The combination of photometry and spectroscopy yielded six new red supergiant stars with masses from 10 M_⊙ to 15 M_⊙. Two red supergiants are located at Galactic coordinates (l,b) = (16.°7, −0.°63) and at a distance of about ~3.9 kpc; four other red supergiants are members of a cluster at Galactic coordinates (l,b) = (49.°3, + 0.°72) and at a distance of ~7.0 kpc.

Conclusions. Spectroscopic analysis of the brightest stars of detected overdensities and studies of interstellar extinction along their line of sights are fundamental to distinguish regions of low extinction from actual stellar clusters. The census of young star clusters containing red supergiants is incomplete; in the existing all-sky near-infrared surveys, they can be identified as overdensities of bright stars with infrared color–magnitude diagrams characterized by gaps.