SiO maser emission from red supergiants across the Galaxy: I. Targets in
  massive star clusters
  Aims. Red supergiants (RSGs) are among the most luminous of all stars, easily
detectable in external galaxies, and may ideally serve as kinematic tracers of
Galactic structure. Some RSGs are surrounded by circumstellar envelopes
detectable by their dust and molecular and, in particular, maser emission. This
study consists of a search for maser emission from silicon monoxide (SiO)
toward a significant number of RSGs that are members of massive stellar
clusters, many of which have only recently been discovered. Further, we aim to
relate the occurrence of maser action to properties of the host stars.
  Methods. Using the IRAM 30 meter telescope, we searched for maser emission in
the J = 2 - 1 rotational transition within the first vibrationally excited
state of SiO toward a sample of 88 RSGs.
  Results. With an average rms noise level of 0.25 Jy, we detected maser
emission in 15% of the sample, toward most of the sources for the first time in
this transition. The peak of the emission provides accurate radial velocities
for the RSGs. The dependence of the detection rate on infrared colors supports
a radiative pumping mechanism for the SiO masers.
