Expanding shells around young clusters -- S 171/Be 59
  Some HII regions that surround young stellar clusters are bordered by
molecular shells that appear to expand at a rate inconsistent with our current
model simulations. In this study we focus on the dynamics of Sharpless 171
(including NGC 7822), which surrounds the cluster Berkeley 59. We aim to
compare the velocity pattern over the molecular shell with the mean radial
velocity of the cluster for estimates of the expansion velocities of different
shell structures, and to match the observed properties with model simulations.
Optical spectra of 27 stars located in Berkeley 59 were collected at the Nordic
Optical Telescope, and a number of molecular structures scattered over the
entire region were mapped in $^{13}$CO(1-0) at Onsala Space Observatory. We
obtained radial velocities and MK classes for the cluster's stars. At least
four of the O stars are found to be spectroscopic binaries, in addition to one
triplet system. From these data we obtain the mean radial velocity of the
cluster. From the $^{13}$CO spectra we identify three shell structures,
expanding relative to the cluster at moderate velocity (4 km/s), high velocity
(12 km/s), and in between. The high-velocity cloudlets extend over a larger
radius and are less massive than the low-velocity cloudlets. We performed a
model simulation to understand the evolution of this complex. Our simulation of
the Sharpless 171 complex and Berkeley 59 cluster demonstrates that the
individual components can be explained as a shell driven by stellar winds from
the massive cluster members. However, our relatively simple model produces a
single component. Modelling of the propagation of shell fragments through a
uniform interstellar medium demonstrates that dense cloudlets detached from the
shell are decelerated less efficiently than the shell itself. They can reach
greater distances and retain higher velocities than the shell.
