SCYON Abstract

Received on September 6 2012

Red supergiants around the obscured open cluster Stephenson 2

AuthorsI. Negueruela (1), A. Marco (1), C. Gonzélez-Fernández (1), F. Jiménez-Esteban (2), J.S. Clark (3), M. Garcia (4), and E. Solano (2)
Affiliation(1) Universidad de Alicante, Spain
(2) Spanish Virtual Observatory and CAB-CSIC
(3) Open University
(4) IAC
Accepted byAstronomy & Astrophysics
Contactignacio.negueruela@ua.es
URLhttp://arxiv.org/abs/1208.3282
Links Stephenson 2

Abstract

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. Among them, Stephenson 2, with a core grouping of 26 RSGs, is a strong candidate to be the most massive cluster in the Galaxy. It is located close to a region where a strong over-density of RSGs had been found. We explore the actual cluster size and its possible connection to this over-density. We have performed a cross-match between DENIS, USNO-B1 and 2MASS to identify candidate obscured luminous red stars around Ste 2, and in a control nearby region, finding >600 candidates. More than 400 are sufficiently bright in I to allow observation with a 4-m class telescope. We have observed a subsample of ~250 stars, using AF2 on the WHT telescope in La Palma, obtaining intermediate-resolution spectroscopy in the 7500--9000A range. We derive spectral types and luminosity classes for all these objects and measure their radial velocities. Our targets are G and K supergiants, late (≥M4) M giants, and M-type bright giants (luminosity class II) and supergiants. We find ~35 RSGs with radial velocities similar to Ste 2 members, spread over the two areas surveyed. In addition, we find ~40 RSGs with radial velocities incompatible in principle with a physical association. Our results show that Ste 2 is not an isolated cluster, but part of a huge structure likely containing hundreds of RSGs, with radial velocities compatible with the terminal velocity at this Galactic longitude (and a distance ~6kpc). In addition, we find evidence of several populations of massive stars at different distances along this line of sight.