Multiple components in the molecular outflow of the red supergiant NML Cyg

Abstract

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 3 mm and 4 mm (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 12 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 𝜇 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.