Analysis of far-infrared fine-structure emissions from the red supergiant µ Cephei

Context . The complex structure and dynamics of red supergiant stars (RSGs) and their variability hinder the spectral analysis of the conditions above their photosphere. RSGs share these features with their lower-mass equivalents on the asymptotic giant branch, but they are less well understood. Since RSGs are the direct progenitors of core collapse (Type II) supernovae, a better characterization is desirable. Aims . This work aims to create a concordant model unifying stellar parameters, elemental abundances, and chromospheric properties of µ Cephei, which is among the most luminous RSGs of the Galaxy. Its distance is weakly constrained (reported values range between 390 and 870 pc), and the immediate vicinity of the photosphere is still largely uncharted territory. Methods . For this purpose, we conducted high-resolution spectroscopy of the transparent far-infrared fine-structure line emissions from the electronic ground states of O I , C II , S I , and Fe II , with the EXES and GREAT instruments flown aboard the Stratospheric Observatory for Infrared Astronomy (SOFIA). Line-area ratios were compared with predictions from a grid of model chromospheres around an early-M-type RSG. We used a distance of 421 −42 +57 pc, obtained by linking spectroscopic estimators for µ Cephei to its nearinfrared interferometry, which are both available in the literature. The uptake of the observed species by the thermal-equilibrium chemistry in the inner circumstellar envelope was modeled including 523 gaseous and 190 solid species. We applied a dedicated code for the mixed radiative transfer of spectral-line and continuum emissions. The underlying opacities were obtained from statistical-equilibrium calculations and a two-component dust model comprised of silicate and alumina. The velocity structure required to match the observed line profiles accounts for bulk motions and turbulence in the stellar wind, and for the onset of a radiation-pressure-driven wind at the dust condensation zone. Results . We identify two solutions differing in chromospheric temperature and density. The resulting thermal pressures are within a factor of four. For a clumpy medium, these models reproduce the measured line fluxes, but they require an additional source for the C II abundance, which is most likely the photo-chemistry of carbon. It is driven by far-UV emissions from shocks forming in response to, for example, encounters of outflowing and inflowing gas. A self-consistent model calls for more accurate elemental abundances at the stellar surface. For the other species, the corrections are smaller. Our models also reproduce the H I λ 21 cm flux from the recent literature.