The enigmatic nature of the circumstellar envelope and bow shock surrounding Betelgeuse as revealed by Herschel

The interaction between stellar winds and the interstellar medium (ISM) can create complex bow shocks. We have studied the bow shock region around Betelgeuse using Herschel PACS images at 70, 100, and 160 μ m and SPIRE images at 250, 350, and 500 μ m. These data were complemented with ultraviolet GALEX data, near-infrared WISE data, and radio 21 cm GALFA-HI data. The infrared Herschel images of the environment around Betelgeuse are spectacular, showing the occurrence of multiple arcs at  ~6–7′ from the central target and the presence of a linear bar at  ~9′. Remarkably, no large-scale instabilities are seen in the outer arcs and linear bar. The dust temperature in the outer arcs varies between 40 and 140 K, with the linear bar having the same colour temperature as the arcs. The inner envelope shows clear evidence of a non-homogeneous clumpy structure (beyond 15′′). The non-homogeneous distribution of the material even persists until the collision with the ISM. A strong variation in brightness of the inner clumps at a radius of  ~2′ suggests a drastic change in mean gas and dust density  ~32 000 yr ago. Using hydrodynamical simulations (see van Marle & Decin, these proceedings), we try to explain the observed morphology of the bow shock around Betelgeuse. Different hypotheses, based on observational and theoretical constraints, are formulated to explain the origin of the multiple arcs and the linear bar and the fact that no large-scale instabilities are visible in the bow shock region. We infer that the two main ingredients for explaining these phenomena are a non-homogeneous mass-loss process and the influence of the Galactic magnetic field. The linear bar is probably an interstellar structure illuminated by Betelgeuse itself.