  We present diffraction limited (0.6") 24.5micron Subaru/COMICS images of the
red supergiant mu Cep. We report the detection of a circumstellar nebula, that
was not detected at shorter wavelengths. It extends to a radius of at least 6"
in the thermal infrared. On these angular scales, the nebula is roughly
spherical, in contrast, it displays a pronounced asymmetric morphology closer
in. We simultaneously model the azimuthally averaged intensity profile of the
nebula and the observed spectral energy distribution using spherical dust
radiative transfer models. The models indicate a constant mass-loss process
over the past 1000 years, for mass-loss rates a few times 10^(-7) Msun/yr. This
work supports the idea that at least part of the asymmetries in shells of
evolved massive stars and supernovae may be due to the mass-loss process in the
red supergiant phase.
