Red supergiants around the obscured open cluster Stephenson 2 , ⋆

Context. Several clusters of red supergiants have been discovered in a small region of the Milky Way close to the base of the ScutumCrux 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 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 the 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 have observed a subsample of ∼250 stars, using the multiobject, wide-field, fibre spectrograph AF2 on the WHT telescope in La Palma, obtaining intermediate-resolution spectroscopy in the 7500–9000Å range. We derive spectral types and luminosity classes for all these objects and measure their radial velocities. Results. Our targets turn out to be G and K supergiants, late (≥M4) M giants, and M-type bright giants (luminosity class II) and supergiants. We find ∼35 red supergiants with radial velocities similar to Stephenson 2 members, spread over the two areas surveyed. In addition, we find ∼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 find evidence of several populations of massive stars at different distances along this line of sight.