Red supergiants around the obscured open cluster Stephenson 2
  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 [ABRIDGED].
