Magnetohydrodynamics of Mira's cometary tail

Aims. The asymptotic giant-branch, long-period variable star Mira exhibits a 4 parsec long cometary tail in the far-ultraviolet. We address the issue of the origin of this structure and its emission process by simulating the transition of this star from the interstellar medium to the Local Bubble, which is a tenuous, high-pressure medium. Methods. We use the hydrodynamic and the magnetohydrodynamic modules of the PLUTO astrophysical code to carry out our simulations. We study the system without a cooling function, with a simplified exponential cooling function, and with a simplified nonequilibrium cooling function. Results. We find evidence that magnetohydrodynamics constrain the shape of the cometary tail and explain features of its far-ultraviolet emission. We suggest an emission process that involves C-0 excitation through inelastic electron collisions and a two-photon continuum to explain the luminosity of Mira's tail.