Volume 547, November 2012
|Number of page(s)||19|
|Section||Galactic structure, stellar clusters and populations|
|Published online||19 October 2012|
Red supergiants around the obscured open cluster Stephenson 2⋆,⋆⋆
Departamento de Física, Ingeniería de Sistemas y Teoría de la
SeñalUniversidad de Alicante,
2 Centro de Astrobiología (INTA-CSIC), Departamento de Astrofísica, PO Box 78, 28691, Villanueva de la Cañada, Madrid, Spain
3 Spanish Virtual Observatory, Spain
4 Saint Louis University, Madrid Campus, Division of Science and Engineering, Avda. del Valle 34, 28003 Madrid, Spain
5 Department of Physics and Astronomy, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
6 Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
7 Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna Tenerife, Spain
Received: 4 May 2012
Accepted: 3 August 2012
Context. Several clusters of red supergiants have been discovered in a small region of the Milky Way close to the base of the Scutum-Crux Arm and the tip of the Long Bar. Population synthesis models indicate that they must be very massive to harbour so many supergiants. Amongst these clusters, Stephenson 2, with a core grouping of 26 red supergiants, is a strong candidate to be the most massive young cluster in the Galaxy.
Aims. Stephenson 2 is located close to a region where a strong over-density of red supergiants had been found. We explore the actual cluster size and its possible connection to this over-density.
Methods. Taking advantage of Virtual Observatory tools, we have performed a cross-match between the DENIS, USNO-B1 and 2MASS catalogues to identify candidate obscured luminous red stars around Stephenson 2, and in a control nearby region. More than 600 infrared bright stars fulfill our colour criteria, with the vast majority having a counterpart in the I band and >400 being sufficiently bright in I to allow observation with a 4-m class telescope. We observed a subsample of ~250 stars, using the multi-object, wide-field, fibre spectrograph AF2 on the WHT telescope in La Palma, obtaining intermediate-resolution spectroscopy in the 7500–9000 Å range. We derived spectral types and luminosity classes for all these objects and measured their radial velocities.
Results. Our targets turned out to be G and K supergiants, late (≥ M4) M giants, and M-type bright giants (luminosity class II) and supergiants. We found ~35 red supergiants with radial velocities similar to Stephenson 2 members, spread over the two areas surveyed. In addition, we found ~40 red supergiants with radial velocities incompatible in principle with a physical association.
Conclusions. Our results show that Stephenson 2 is not an isolated cluster, but part of a huge structure likely containing hundreds of red supergiants, with radial velocities compatible with the terminal velocity at this Galactic longitude (and a distance ~6 kpc). In addition, we found evidence of several populations of massive stars at different distances along this line of sight.
Key words: stars: evolution / supergiants / open clusters and associations: individual: Stephenson 2 / Galaxy: structure / virtual observatory tools
Tables 2 and 3 are available in electronic form at http://www.aanda.org
© ESO, 2012
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