513 research outputs found
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
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