A RED SUPERGIANT NEBULA AT 25 μm: ARCSECOND-SCALE MASS-LOSS ASYMMETRIES OF μ CEPHEI

We present diffraction-limited (0.6 '') 24.5 mu m Subaru COMICS images of the red supergiant mu Cep. We report the detection of a circumstellar nebula, which 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 ( SED) using spherical dust radiative transfer models. The models indicate a constant mass-loss process over the past 1000 years, for mass-loss rates of a few x 10(-7) M(circle dot) yr(-1). 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.