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 WR star, it must shed its H envelope, but new estimates of the effects of clumping in winds indicate that line driving is vastly insufficient. We discuss massive stars above roughly 40-50 Msun, 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 eta Car, when the star shed 12-20 Msun or more in less than a decade. Other examples are circumstellar nebulae of LBVs, extragalactic eta Car analogs (``supernova impostors''), and massive shells around SNe and GRBs. We do not yet fully understand what triggers LBV outbursts, but they occur nonetheless, and present a fundamental mystery in stellar astrophysics. Since line opacity from metals becomes too saturated, the extreme mass loss probably arises from a continuum-driven 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 fate of massive metal-poor stars in the early universe. If they occur in these Population III stars, such eruptions would profoundly affect the chemical yield and types of remnants from early SNe and hypernovae.Comment: 4 pages, 1 figure, accepted by ApJ Letter

We present numerical magnetohydrodynamic (MHD) simulations of the effect of stellar dipole magnetic fields on line-driven wind outflows from hot, luminous stars. Unlike previous fixed-field analyses, the simulations here take full account of the dynamical competition between field and flow, and thus apply to a full range of magnetic field strength, and within both closed and open magnetic topologies. A key result is that the overall degree to which the wind is influenced by the field depends largely on a single, dimensionless, `wind magnetic confinement parameter', $\eta_{\ast}$ ($ = B_{eq}^2 R_\ast^2/{\dot M} v_\infty$), which characterizes the ratio between magnetic field energy density and kinetic energy density of the wind. For weak confinement $\eta_{\ast} \le 1$, the field is fully opened by the wind outflow, but nonetheless for confinements as small as $\eta_{\ast}=1/10$ can have a significant back-influence in enhancing the density and reducing the flow speed near the magnetic equator. For stronger confinement $\eta_{\ast} > 1$, the magnetic field remains closed over a limited range of latitude and height about the equatorial surface, but eventually is opened into a nearly radial configuration at large radii.Comment: 39 pages, 9 figures, for better quality figures visit: http://www.bartol.udel.edu/~owock

We argue that, in general, observational studies of Be‐star rotation have paid insufficient attention to the effects of equatorial gravity darkening.

We present two-dimensional hydrodynamical simulations of corotating stream structure in the wind from a rotating O star, together with resulting synthetic line profiles showing discrete absorption components (DACs). An azimuthal variation is induced by a local increase or decrease in the radiative driving force, as would arise from a bright or dark ``star spot'' in the equatorial plane. Since much of the emergent wind structure seems independent of the exact method of perturbation, we expect similar morphology in winds perturbed by localized magnetic fields or nonradial pulsations, as well as by either rotationally-modulated structure or transient mass ejections. We find that bright spots with enhanced driving generate high-density, low-speed streams, while dark spots generate low-density, high-speed streams. Corotating interaction regions (CIRs) form where fast material collides with slow material -- e.g. at the leading (trailing) edge of a stream from a dark (bright) spot, often steepening into shocks. The unperturbed supersonic wind obliquely impacts the high-density CIR and sends back a nonlinear signal which takes the form of a sharp propagating discontinuity (``kink'' or ``plateau'') in the radial velocity gradient. These features travel inward in the co-moving frame at the radiative-acoustic characteristic speed, and thus slowly outward in the star's frame. We find that these slow kinks, rather than the CIRs themselves, are more likely to result in high-opacity DACs in the absorption troughs of unsaturated P Cygni line profiles.

We examine radiatively driven mass loss from stars near and above the Eddington limit (Ledd). We begin by reviewing the instabilities that are expected to form extensive structure near Ledd. We investigate how this "porosity" can reduce the effective coupling between the matter and radiation. Introducing a new "porosity-length'' formalism, we derive a simple scaling for the reduced effective opacity, and use this to derive an associated scaling for the porosity-moderated, continuum-driven mass loss rate from stars that formally exceed Ledd. For a simple super-Eddington model with a single porosity length that is assumed to be on the order of the gravitational scale height, the overall mass loss is similar to that derived in previous porosity work. This is much higher than is typical of line-driven winds, but is still only a few percent of the photon tiring limit--for which the luminosity becomes insufficient to carry the flow out of the gravitational potential. To obtain still stronger mass loss that approaches observationally inferred values near this limit, we introduce a power-law-porosity model in which the associated structure has a broad range of scales. We show that the mass loss rate can be enhanced by a factor that increases with the Eddington parameter Gamma, such that for moderately large Gamma (> 3-4), mass loss rates could approach the photon tiring limit. Together with the ability to drive quite fast outflow speeds, the derived mass loss could explain the large inferred mass loss and flow speeds of giant outbursts in eta Carinae and other LBV stars.Comment: 17 pages, 6 figures, to appear in Ap