Predictions of variable mass loss for Luminous Blue Variables

We present radiation-driven wind models for Luminous Blue Variables (LBVs) and predict their mass-loss rates. We study the effects of lower masses and modified abundances in comparison to the normal OB supergiants, and we find that the main difference in mass loss is due to the lower masses of LBVs. In addition, we find that the increase in helium abundance changes the mass-loss properties by small amounts (up to about 0.2 dex in log Mdot), while CNO processing is relatively unimportant for the mass-loss rate. A comparison between our mass loss predictions and the observations is performed for four relatively well-studied LBVs. The comparison shows that (i) the winds of LBVs are driven by radiation pressure on spectral lines, (ii) the variable mass loss behaviour of LBVs during their S Doradus-type variation cycles is explained by changes in the line driving efficiency, notably due to the recombination/ionisation of Fe IV/III and Fe III/II, and finally, (iii) the winds of LBVs can be used to derive their masses, as exemplified by the case of AG Car, for which we derive a present-day mass of 35 Msun.Comment: 12 pages; A&A accepte

On the nature of the bi-stability jump in the winds of early-type supergiants

We study the origin of the observed bi-stability jump in the terminal velocity of the winds of supergiants near spectral type B1. To this purpose, we have calculated a grid of wind models and mass-loss rates for these stars. The models show that the mass-loss rate 'jumps' by a factor of five around spectral type B1. Up to now, a theoretical explanation of the observed bi-stability jump was not yet provided by radiation driven wind theory. The models demonstrate that the subsonic part of the wind is dominated by the line acceleration due to Fe. The elements C, N and O are important line drivers in the supersonic part of the wind. We demonstrate that the mass-loss rate 'jumps' due to an increase in the line acceleration of Fe III below the sonic point. Finally, we discuss the possible role of the bi-stability jump on the mass loss during typical variations of Luminous Blue Variable stars.Comment: Accepted by A&A, 19 pages Latex, 10 figure

Bottling the champagne: dynamics and radiation trapping of wind-driven bubbles around massive stars

In this paper we make predictions for the behaviour of wind bubbles around young massive stars using analytic theory. We do this in order to determine why there is a discrepancy between theoretical models that predict that winds should play a secondary role to photoionisation in the dynamics of HII regions, and observations of young HII regions that seem to suggest a driving role for winds. In particular, regions such as M42 in Orion have neutral hydrogen shells, suggesting that the ionising radiation is trapped closer to the star. We first derive formulae for wind bubble evolution in non-uniform density fields, focusing on singular isothermal sphere density fields with a power law index of -2. We find that a classical "Weaver"-like expansion velocity becomes constant in such a density distribution. We then calculate the structure of the photoionised shell around such wind bubbles, and determine at what point the mass in the shell cannot absorb all of the ionising photons emitted by the star, causing an "overflow" of ionising radiation. We also estimate perturbations from cooling, gravity, magnetic fields and instabilities, all of which we argue are secondary effects for the conditions studied here. Our wind-driven model provides a consistent explanation for the behaviour of M42 to within the errors given by observational studies. We find that in relatively denser molecular cloud environments \around single young stellar sources, champagne flows are unlikely until the wind shell breaks up due to turbulence or clumping in the cloud.Comment: 17 pages, 10 figures, published in MNRA

Advances in mass-loss predictions

We present the results of Monte Carlo mass-loss predictions for massive stars covering a wide range of stellar parameters. We critically test our predictions against a range of observed mass-loss rates -- in light of the recent discussions on wind clumping. We also present a model to compute the clumping-induced polarimetric variability of hot stars and we compare this with observations of Luminous Blue Variables, for which polarimetric variability is larger than for O and Wolf-Rayet stars. Luminous Blue Variables comprise an ideal testbed for studies of wind clumping and wind geometry, as well as for wind strength calculations, and we propose they may be direct supernova progenitors.Comment: 3 pages, 3 figures, to appear in the proceedings of workshop 'Clumping in Hot Star Winds', eds. W.-R. Hamann, A. Feldmeier, & L. Oskinov

The rotation rates of massive stars: the role of binary interaction through tides, mass transfer and mergers

Rotation is thought to be a major factor in the evolution of massive stars, especially at low metallicity, with consequences for their chemical yields, ionizing flux and final fate. Determining the natal rotation-rate distribution of stars is of high priority given its importance as a constraint on theories of massive star formation and as input for models of stellar populations in the local Universe and at high redshift. Recently, it has become clear that the majority of massive stars interact with a binary companion before they die. We investigate how this affects the distribution of rotation rates. For this purpose, we simulate a massive binary-star population typical for our Galaxy assuming continuous star formation. We find that, because of binary interaction, 20^+5_-10% of all massive main-sequence stars have projected rotational velocities in excess of 200km/s. We evaluate the effect of uncertain input distributions and physical processes and conclude that the main uncertainties are the mass transfer efficiency and the possible effect of magnetic braking, especially if magnetic fields are generated or amplified during mass accretion and stellar mergers. The fraction of rapid rotators we derive is similar to that observed. If indeed mass transfer and mergers are the main cause for rapid rotation in massive stars, little room remains for rapidly rotating stars that are born single. This implies that spin down during star formation is even more efficient than previously thought. In addition, this raises questions about the interpretation of the surface abundances of rapidly rotating stars as evidence for rotational mixing. Furthermore, our results allow for the possibility that all early-type Be stars result from binary interactions and suggest that evidence for rotation in explosions, such as long gamma-ray bursts, points to a binary origin.Comment: 14 pages, 5 figures, accepted for publication in ApJ., no changes with v1 apart from fixed typos/ref

The nature of B supergiants: clues from a steep drop in rotation rates at 22000 K. The possibility of Bi-stability braking

The location of B supergiants in the Hertzsprung-Russell diagram (HRD) represents a long-standing problem in massive star evolution. Here we propose their nature may be revealed utilising their rotational properties, and we highlight a steep drop in massive star rotation rates at an effective temperature of 22000 K. We discuss two potential explanations for it. On the one hand, the feature might be due to the end of the main sequence, which could potentially constrain the core overshooting parameter. On the other hand, the feature might be the result of enhanced mass loss at the predicted location of the bi-stability jump. We term this effect "bi-stability breaking" and discuss its potential consequences for the evolution of massive stars.Comment: Accepted by A&A Letters (4 pages, 5 figures); typos correcte