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We suggest that the mass lost during the evolution of very massive stars may be dominated by optically thick, continuum-driven outbursts or explosions, instead of by steady line-driven winds. In order for a massive star to become a Wolf-Rayet star, it must shed its hydrogen envelope, but new estimates of the effects of clumping in winds from O-type stars indicate that line driving is vastly insufficient. We discuss massive stars above roughly 40-50 M , , which do not become red supergiants and for which the best alternative is mass loss during brief eruptions of luminous blue variables (LBVs). Our clearest example of this phenomenon is the 19th century outburst of h Carinae, when the star shed 12-20 M , or more in less than a decade. Other examples are circumstellar nebulae of LBVs and LBV candidates, extragalactic h Car analogs (the so-called supernova impostors), and massive shells around supernovae and gamma-ray bursters. We do not yet fully understand what triggers LBV outbursts or what supplies their energy, but they occur nonetheless, and they present a fundamental mystery in stellar astrophysics. Since line opacity from metals becomes too saturated, the extreme mass loss probably arises from a continuumdriven wind or a hydrodynamic explosion, both of which are insensitive to metallicity. As such, eruptive mass loss could have played a pivotal role in the evolution and ultimate fate of massive metal-poor stars in the early universe. If they occur in these Population III stars, such eruptions would also profoundly affect the chemical yield and types of remnants from early supernovae and hypernovae thought to be the origin of long gamma-ray bursts.

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 A ) and stellar rotation (which sets the Kepler co-rotation radius R 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 eff 16 kK. Using these parameters, we locate the stars in the magnetic confinement-rotation diagram, a log-log plot of R K vs. R A . This diagram can be subdivided into regimes of centrifugal magnetospheres (CM), with R A > R K , vs. dynamical magnetospheres (DM), with R K > R A . We show how key observational diagnostics, like the presence and characteristics of Hα 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 Hα 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.

We argue that, in general, observational studies of Be-star rotation have paid insufficient attention to the effects of equatorial gravity darkening. We present new line-profile calculations that emphasize the insensitivity of line width to rotation for fast rotators. Coupled with a critical review of observational procedures, these calculations suggest that the observational parameter v sin(i) may systematically underestimate the true projected equatorial rotation velocity, ve sin(i), by some tens of per cent for rapid rotators. The crucial implication of this work is that Be stars may be rotating much closer to their critical velocities than is generally supposed, bringing a range of new processes into contention for the elusive physical mechanism responsible for the circumstellar disk thought to be central to the Be phenomenon.Comment: 8 pages, 3 figures, shortened Sec. 6.3, new Sec. 6.4. Accepted by MNRA