Multiple components in the molecular outflow of the red supergiant NML
  Cyg
  Despite their large impact on stellar and galactic evolution, the properties
of outflows from red supergiants are not well characterised. We used the Onsala
20m telescope to perform a spectral survey at 3mm and 4mm (68 - 116 GHz) of the
red supergiant NML Cyg, alongside the yellow hypergiant IRC+10420. Our
observations of NML Cyg were combined with complementary archival data to
enable a search for signatures of morphological complexity in the circumstellar
environment, using emission lines from 15 molecular species. The recovered
parameters imply the presence of three distinct, coherent and persistent
components, comprised of blue-shifted and red-shifted components, in addition
to an underlying outflow centred at the stellar systemic velocity. Furthermore,
to reproduce CO emission with three-dimensional radiative transfer models
required a spherical outflow with three superposed conical outflows, one
towards and one away from the observer, and one in the plane of the sky. These
components are higher in density than the spherical outflow by up to an order
of magnitude. We hence propose that NML Cyg's circumstellar environment
consists of a small number of high-density large-scale coherent outflows
embedded in a spherical wind. This would make the mass-loss history similar to
that of VY CMa, and distinct from mu Cep, where the outflow contains many
randomly distributed smaller clumps. A possible correlation between stellar
properties, outflow structures and content is critical in understanding the
evolution of massive stars and their environmental impact.
