NOEMA maps the CO J=2-1 environment of the red supergiant μ Cep

Red supergiant stars are surrounded by a gaseous and dusty circumstellar environment created by their mass loss, which spreads heavy elements into the interstellar medium. The structure and dynamics of this envelope are crucial to understand the processes driving the red supergiant mass loss and the shaping of the pre-supernova ejecta. We have observed the emission from the CO J = 2 - 1 line from the red supergiant star mu Cep with the NOEMA interferometer. In the line the synthesized beam was 0.92x0.72 arcsec (590x462 au at 641 pc). The continuum map shows only the unresolved contribution of the free-free emission of the star chromosphere. The continuum-subtracted channel maps reveal a very inhomogeneous and clumpy circumstellar environment. In particular, we detected a bright CO clump, as bright as the central source in the line, at 1.80 arcsec south-west from the star, in the blue channel maps. After a deprojection of the radial velocity assuming two different constant wind velocities, the observations were modelled using the 3D radiative transfer code LIME to derive the characteristics of the different structures. We determine that the gaseous clumps observed around mu Cep are responsible for a mass-loss rate of (4.9 +/- 1.0) x 10(-7) M-circle dot yr(-1), in addition to a spatially unresolved wind component with an estimated mass-loss rate of 2.0 x 10(-6) M-circle dot yr(-1). Therefore, the clumps have a significant role in mu Cep's mass loss (>= 25 per cent). We cannot exclude that the unresolved central outflow may be made of smaller unresolved clumps.