2,438 research outputs found
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
Predictions for mass-loss rates and terminal wind velocities of massive O-type stars
Mass loss forms an important aspect of the evolution of massive stars, as
well as for the enrichment of the surrounding ISM. Our goal is to predict
accurate mass-loss rates and terminal wind velocities. These quantities can be
compared to empirical values, thereby testing radiation-driven wind models. One
specific issue is that of the "weak-wind problem", where empirically derived
mass-loss rates fall orders of magnitude short of predicted values. We employ
an established Monte Carlo model and a recently suggested new line acceleration
formalism to solve the wind dynamics consistently. We provide a new grid of
mass-loss rates and terminal wind velocities of O stars, and compare the values
to empirical results. Our models fail to provide mass-loss rates for
main-sequence stars below a luminosity of log(L/Lsun) = 5.2, where we run into
a fundamental limit. At luminosities below this critical value there is
insufficient momentum transferred in the region below the sonic point to
kick-start the acceleration. This problem occurs at the location of the onset
of the weak-wind problem. For O dwarfs, the boundary between being able to
start a wind, and failing to do so, is at spectral type O6/O6.5. The direct
cause of this failure is a combination of the lower luminosity and a lack of Fe
V lines at the wind base. This might indicate that another mechanism is
required to provide the necessary driving to initiate the wind. For stars more
luminous than log(L/Lsun) = 5.2, our new mass-loss rates are in excellent
agreement with the mass-loss prescription by Vink et al. 2000. This implies
that the main assumption entering the method of the Vink et al. prescriptions -
i.e. that the momentum equation is not explicitly solved for - does not
compromise the reliability of the Vink et al. results for this part of
parameter space (Abridged).Comment: 10 pages, 10 figures, Astronomy & Astrophysics (in press
Partner countries for Dutch bilateral development assistance 1962-2015
The Netherlands has been an active supporter of international development aid. Dutch development cooperation started in response to Truman's 'Four point programme' announced in 1949. It began as technical assistance, channelled through multilateral channels. Bilateral aid started in 1962 and was introduced by (then) Minister for Development Cooperation Berend-Jan Udink. Since then, the priorities, target countries and budget of Dutch development cooperation have continuously shifted. This thematic map illustrates how the partner countries for Dutch development cooperation have changed throughout the years
On the origin of microturbulence in hot stars
We present results from the first extensive study of convection zones in the
envelopes of hot massive stars, which are caused by opacity peaks associated
with iron and helium ionization. These convective regions can be located very
close to the stellar surface. Recent observations of microturbulence in massive
stars from the VLT-Flames survey are in good agreement with our predictions
concerning the occurrence and the strength of sub-surface convection in hot
stars. We argue further that convection close to the surface may trigger
clumping at the base of the stellar wind of massive stars.Comment: to appear in Comm. in Astroseismology - Proceedings of the 38th
LIAC/HELAS-ESTA/BAG, 200
Nitrogen chronology of massive main sequence stars
Rotational mixing in massive main sequence stars is predicted to
monotonically increase their surface nitrogen abundance with time. We use this
effect to design a method for constraining the age and the inclination angle of
massive main sequence stars, given their observed luminosity, effective
temperature, projected rotational velocity and surface nitrogen abundance. This
method relies on stellar evolution models for different metallicities, masses
and rotation rates. We use the population synthesis code STARMAKER to show the
range of applicability of our method. We apply this method to 79 early B-type
main sequence stars near the LMC clusters NGC 2004 and N 11 and the SMC
clusters NGC 330 and NGC 346. From all stars within the sample, 17 were found
to be suitable for an age analysis. For ten of them, which are rapidly rotating
stars without a strong nitrogen enhancement, it has been previously concluded
that they did not evolve as rotationally mixed single stars. This is confirmed
by our analysis, which flags the age of these objects as highly discrepant with
their isochrone ages. For the other seven stars, their nitrogen and isochrone
ages are found to agree within error bars, what validates our method.
Constraints on the inclination angle have been derived for the other 62
stars,with the implication that the nitrogen abundances of the SMC stars, for
which mostly only upper limits are known, fall on average significantly below
those limits. Nitrogen chronology is found to be a new useful tool for testing
stellar evolution and to constrain fundamental properties of massive main
sequence stars. A web version of this tool is provided.Comment: accepted by A&A, 15 pages, 16 figures, 6 table
Coordination in networked organizations : the Paradigm approach
Abstract An extension of the coordination specification language Paradigm is presented. In this set-up Paradigm models cater for multiple managers sharing the coordination of a set of common employees. A transition system semantics for the language is provided, that allows for reasoning about such constructions as delegation and self-management in matrix and general network organizations. An elaborated example illustrates the expressiveness of the proposed version of Paradigm. Key words: Paradigm, coordination, operational semantics, delegation, software architectur
Wind modelling of very massive stars up to 300 solar masses
Some studies have claimed a universal stellar upper-mass limit of 150 Msun. A
factor that is often overlooked is that there might be a difference between the
current and initial masses of the most massive stars, as a result of mass loss.
We present Monte Carlo mass-loss predictions for very massive stars in the
range 40-300 Msun, with large luminosities and Eddington factors Gamma. Using
our new dynamical approach, we find an upturn in the mass-loss vs. Gamma
dependence, at the point where the winds become optically thick. This coincides
with the location where wind efficiency numbers surpass the single-scattering
limit of Eta = 1, reaching values up to Eta = 2.5. Our modelling suggests a
transition from common O-type winds to Wolf-Rayet characteristics at the point
where the winds become optically thick. This transitional behaviour is also
revealed with respect to the wind acceleration parameter beta, which starts at
values below 1 for the optically thin O-stars, and naturally reaches values as
high as 1.5-2 for the optically thick Wolf-Rayet models. An additional finding
concerns the transition in spectral morphology of the Of and WN characteristic
He II line at 4686 Angstrom. When we express our mass-loss predictions as a
function of the electron scattering Gamma_e (=L/M) only, we obtain a mass-loss
Gamma dependence that is consistent with a previously reported power-law Mdot
propto Gamma^5 (Vink 2006) that was based on our semi-empirical modelling
approach. When we express Mdot in terms of both Gamma and stellar mass, we find
Mdot propto M^0.8 Gamma^4.8 for our high Gamma models. Finally, we confirm that
the Gamma-effect on the mass-loss predictions is much stronger than that of an
increased helium abundance, calling for a fundamental revision in the way mass
loss is incorporated in evolutionary models of the most massive stars.Comment: minor language changes (Astronomy & Astrophysics in press - 11 pages,
10 figures
The Eddington factor as the key to understand the winds of the most massive stars. Evidence for a Γ-dependence of Wolf-Rayet type mass loss
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