A dynamical magnetosphere model for periodic Hα emission from the slowly rotating magnetic O star HD 191612

The magnetic O star HD 191612 exhibits strongly variable, cyclic Balmer line emission on a 538-d period. We show here that its variable Ha emission can be well reproduced by the rotational phase variation of synthetic spectra computed directly from full radiation magnetohydrodynamical simulations of a magnetically confined wind. In slow rotators such as HD 191612, wind material on closed magnetic field loops falls back to the star, but the transient suspension of material within the loops leads to a statistically overdense, low-velocity region around the magnetic equator, causing the spectral variations. We contrast such dynamical magnetospheres (DMs) with the more steady-state centrifugal magnetospheres of stars with rapid rotation, and discuss the prospects of using this DM paradigm to explain periodic line emission from also other non-rapidly rotating magnetic massive stars.