Building on results from the Magnetism in Massive Stars ( MiMeS ) project , this paper shows how a two-parameter classification of massive-star magnetospheres in terms of the magnetic wind confinement ( which sets the Alfvén radius R _ { \mathrm { A } } ) and stellar rotation ( which sets the Kepler co-rotation radius R _ { \mathrm { K } } ) provides a useful organisation of both observational signatures and theoretical predictions .
We compile the first comprehensive study of inferred and observed values for relevant stellar and magnetic parameters of 64 confirmed magnetic OB stars with T _ { \mathrm { eff } } \ga 16 kK .
Using these parameters , we locate the stars in the magnetic confinement-rotation diagram , a log-log plot of R _ { \mathrm { K } } vs . R _ { \mathrm { A } } .
This diagram can be subdivided into regimes of centrifugal magnetospheres ( CM ) , with R _ { \mathrm { A } } > R _ { \mathrm { K } } , vs . dynamical magnetospheres ( DM ) , with R _ { \mathrm { K } } > R _ { \mathrm { A } } .
We show how key observational diagnostics , like the presence and characteristics of H \alpha emission , depend on a star ’ s position within the diagram , as well as other parameters , especially the expected wind mass-loss rates .
In particular , we identify two distinct populations of magnetic stars with H \alpha emission : namely , slowly rotating O-type stars with narrow emission consistent with a DM , and more rapidly rotating B-type stars with broader emission associated with a CM .
For O-type stars , the high mass-loss rates are sufficient to accumulate enough material for line emission even within the relatively short free-fall timescale associated with a DM : this high mass-loss rate also leads to a rapid magnetic spindown of the stellar rotation .
For the B-type stars , the longer confinement of a CM is required to accumulate sufficient emitting material from their relatively weak winds , which also lead to much longer spindown timescales .
Finally , we discuss how other observational diagnostics , e.g .
variability of UV wind lines or X-ray emission , relate to the inferred magnetic properties of these stars , and summarise prospects for future developments in our understanding of massive-star magnetospheres .