Mass is a key parameter in understanding the evolution and eventual fate of hot, luminous stars. Mass loss through a wind driven by UV-scattering forces is already known to reduce the mass of such stars by 10−10 − 10−4 M⊙/yr over the course of their lifetimes. However, high-mass stars already drive such strong winds while they are still in their accretion epoch. Therefore, stellar UV-scattering forces will efficiently ablate material off the surface of their circumstellar disks, perhaps even shutting off the final accretion through the last several stellar radii and onto a massive protostar. By using a three-dimensional UV-scattering prescription, we here quantify the role of radiative ablation in controlling the disk’s accretion rate onto forming high-mass stars. Particular emphasis is given to the potential impact of this process on the stellar upper mass limit.

For 30 years, cyclical wind variability in OB stars has puzzled the astronomical community. Phenomenological models involving co-rotating bright spots provide a potential explanation for the observed variations, but the underlying physics remains unknown. We present recent results from hydrodynamical simulations constraining bright spot properties and compare them to what can be inferred from space-based photometry. We also explore the possibility that these spots are caused by magnetic fields and discuss the detectability of such fields.

Early-type stars are well-known to be sources of soft X-rays. However, this high-energy emission can be supplemented by bright and hard X-rays when magnetically confined winds are present. In an attempt to clarify the systematics of the observed X-ray properties of this phenomenon, a large series of Chandra and XMM observations was analyzed, over 100 exposures of 60% of the known magnetic massive stars listed recently by Petit et al. (2013). It is found that the X-ray luminosity is strongly correlated with mass-loss rate, in agreement with predictions of magnetically confined wind models, though the predictions of higher temperature are not always verified. We also investigated the behaviour of other X-ray properties (absorption, variability), yielding additional constraints on models. This work not only advances our knowledge of the X-ray emission of massive stars, but also suggests new observational and theoretical avenues to further explore magnetically confined winds.

Our Working Group (WG) studies massive, luminous stars, both individually and in resolved and unresolved populations, with historical focus on early-type (OB) stars, A-supergiants, and Wolf-Rayet stars. Our group also studies lower mass stars (e.g., central stars of planetary nebulae and their winds) which display features similar or related to those present in massive stars, and thus may improve our understanding of the physical processes occurring in massive stars. In recent years, massive red supergiants that evolve from hot stars have been included into our activities as well. We emphasize the role of massive stars in other branches of astrophysics, particularly regarding the First Stars, long duration Gamma-Ray bursts, formation of massive stars and their feedback on star formation in general, pulsations of massive stars, and starburst galaxies.

Perhpas the most extreme examples of “Active OB stars" are the subset of high-mass X-ray binaries – consisting of an OB star plus compact companion – that have recently been observed by Fermi and ground-based Cerenkov telescopes like HESS to be sources of very high energy (VHE; up to 30 TeV!) γ-rays. This paper focuses on the prominent γ-ray source, LS5039, which consists of a massive O6.5V star in a 3.9-day-period, mildly elliptical (e ≈ 0.24) orbit with its companion, assumed here to be a black-hole or unmagnetized neutron star. Using 3-D SPH simulations of the Bondi-Hoyle accretion of the O-star wind onto the companion, we find that the orbital phase variation of the accretion follows very closely the simple Bondi-Hoyle-Lyttleton (BHL) rate for the local radius and wind speed. Moreover, a simple model, wherein intrinsic emission of γ-rays is assumed to track this accretion rate, reproduces quite well Fermi observations of the phase variation of γ-rays in the energy range 0.1-10 GeV. However for the VHE (0.1-30 TeV) radiation observed by the HESS Cerenkov telescope, it is important to account also for photon-photon interactions between the γ-rays and the stellar optical/UV radiation, which effectively attenuates much of the strong emission near periastron. When this is included, we find that this simple BHL accretion model also quite thus making it a strong alternative to the pulsar-wind-shock models commonly invoked to explain such VHE γ-ray emission in massive-star binaries.

This paper presents results obtained from Stokes I and V spectra of the B2Vp star sigma Ori E, observed by both the Narval and ESPaDOnS spectropolarimeters. Using Least-Squares Deconvolution, we investigate the longitudinal magnetic field at the current epoch, including period analysis exploiting current and historical data. σ Ori E is the prototypical helium-strong star that has been shown to harbor a strong magnetic field, as well as a magnetosphere, consisting of two clouds of plasma forced by magnetic and centrifugal forces to co-rotate with the star on its 1.19 day period. The Rigidly Rotating Magnetosphere (RRM) model of Townsend & Owocki (2005) approximately reproduces the observed variations in longitudinal field strength, photometric brightness, Hα emission, and various other observables. There are, however, small discrepancies between the observations and model in the photometric light curve, which we propose arise from inhomogeneous chemical abundances on the star's surface. Using Magnetic Doppler Imaging (MDI), future work will attempt to identify the contributions to the photometric variation due to abundance spots and due to circumstellar material.

The Magnetism in Massive Stars (MiMeS) Project is a consensus collaboration among many of the foremost international researchers of the physics of hot, massive stars, with the basic aim of understanding the origin, evolution and impact of magnetic fields in these objects. At the time of writing, MiMeS Large Programs have acquired over 950 high-resolution polarised spectra of about 150 individual stars with spectral types from B5-O4, discovering new magnetic fields in a dozen hot, massive stars. The quality of this spectral and magnetic matériel is very high, and the Collaboration is keen to connect with colleagues capable of exploiting the data in new or unforeseen ways. In this paper we review the structure of the MiMeS observing programs and report the status of observations, data modeling and development of related theory.

There are currently two optical interferometry recombiners that can provide spectral resolutions better than 10000, AMBER/VLTI operating in the H-K bands, and VEGA/CHARA, recently commissioned, operating in the visible. These instruments are well suited to study the wind activity of the brightest AB supergiants in our vicinity, in lines such as Hα or Brγ. We present here the first observations of this kind, performed on Rigel (B8Ia) and Deneb (A2Ia). Rigel was monitored by AMBER in two campaigns, in 2006-2007 and 2009-2010, and observed in 2009 by VEGA; whereas Deneb was monitored in 2008-2009 by VEGA. The extension of the Hα and Brγ line forming regions were accurately measured and compared with CMFGEN models of both stars. Moreover, clear signs of activity were observed in the differential visibility and phases. These pioneer observations are still limited, but show the path for a better understanding of the spatial structure and temporal evolution of localized ejections using optical interferometry.

The spinup of massive stars induced by evolution of the stellar interior can bring the star to near-critical rotation. In critically rotating stars the decrease of the stellar moment of inertia must be balanced by a net loss of angular momentum through an equatorial decretion disk. We examine the nature and role of mass loss via such disks. In contrast to the usual stellar wind mass loss set by exterior driving from the stellar luminosity, such decretion-disk mass loss stems from the angular momentum loss needed to keep the star near and below critical rotation, given the interior evolution and decline in the star's moment of inertia. Because the specific angular momentum in a Keplerian disk increases with the square root of the radius, the decretion mass loss associated with a required level of angular momentum loss critically depends on the outer radius for viscous coupling of the disk, and can be significantly less than the spherical, wind-like mass loss commonly assumed in evolutionary calculations.

During the course of their evolution, massive stars lose a substantial fraction of their initial mass, both through steady winds and through relatively brief eruptions during their Luminous Blue Variable (LBV) phase. This talk reviews the dynamical driving of this mass loss, contrasting the line-driving of steady winds to the potential role of continuum driving for eruptions during LBV episodes when the star exceeds the Eddington limit. A key theme is to emphasize the inherent limits that self-shadowing places on line-driven mass loss rates, whereas continuum driving can in principle drive mass up to the “photon-tiring” limit, for which the energy to lift the wind becomes equal to the stellar luminosity. We review how the “porosity” of a highly clumped atmosphere can regulate continuum-driven mass loss, but also discuss recent time-dependent simulations of how base mass flux that exceeds the tiring limit can lead to flow stagnation and a complex, time-dependent combination of inflow and outflow regions. A general result is thus that porosity-mediated continuum driving in super-Eddington phases can explain the large, near tiring-limit mass loss inferred for LBV giant eruptions.

This report covers the activity period 2002–2005 of the IAU Working Group on Massive Stars

Our group studies active early-type (OB) stars, with historical focus on classical Be stars, but extending in recent years to include Slowly Pulsating B-stars (SPB), Beta-Cephei stars, the strongly magnetic Bp stars, Luminous Blue Vairiable (LBV) stars, and B[e] stars. An overall goal is to understand the nature, origin, and consequences of this activity, in terms of both the stellar structure and evolution, as well as the distribution and dynamics of circumstellar material and mass outflows.