A dynamical magnetosphere model for periodic Halpha emission from the slowly rotating magnetic O star HD191612

TL;DR

This paper models the cyclic Halpha emission of the magnetic O-star HD191612 using radiation magneto-hydrodynamical simulations, revealing how dynamical magnetospheres cause observed spectral variability.

Contribution

It introduces a dynamical magnetosphere model to explain periodic emission in slow-rotating magnetic massive stars, supported by detailed simulations.

Findings

Synthetic spectra match observed Halpha variability.

Dynamical magnetospheres cause overdense regions leading to emission.

Contrast with centrifugal magnetospheres in rapid rotators.

Abstract

The magnetic O-star HD191612 exhibits strongly variable, cyclic Balmer line emission on a 538-day period. We show here that its variable Halpha emission can be well reproduced by the rotational phase variation of synthetic spectra computed directly from full radiation magneto-hydrodynamical simulations of a magnetically confined wind. In slow rotators such as HD191612, 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.