4,660 research outputs found
Stellar mass-loss near the Eddington limit. Tracing the sub-photospheric layers of classical Wolf-Rayet stars
Towards the end of their evolution hot massive stars develop strong stellar
winds and appear as emission line stars, such as WR stars or LBVs. The
quantitative description of the mass loss in these important pre-SN phases is
hampered by unknowns such as clumping and porosity due to an in-homogeneous
wind structure, and by an incomplete theoretical understanding of optically
thick stellar winds. In this work we investigate the conditions in deep
atmospheric layers of WR stars to find out whether these comply with the theory
of optically thick winds, and whether we find indications of clumping in these
layers. We use a new semi-empirical method to determine sonic-point optical
depths, densities, and temperatures for a large sample of WR stars of the
carbon (WC) and oxygen (WO) sequence. Based on an artificial model sequence we
investigate the reliability of our method and its sensitivity to uncertainties
in stellar parameters. We find that the WR stars in our sample obey an
approximate relation with P_rad/P_gas~80 at the sonic point. This 'wind
condition' is ubiquitous for radiatively driven, optically thick winds, and
sets constraints on possible wind/envelope solutions affecting radii, mass-loss
rates, and clumping properties. Our results suggest that the presence of an
optically thick wind may force many stars near the Eddington limit to develop
clumped, radially extended sub-surface zones. The clumping in these zones is
most likely sustained by the non-linear strange-mode instability, and may be
the origin of the observed wind clumping. The properties of typical late-type
WC stars comply with this model. Solutions without sub-surface clumping and
inflation are also possible but demand for compact stars with comparatively low
mass-loss rates. These objects may resemble the small group of WO stars with
their exceptionally hot stellar temperatures and highly ionized winds.Comment: accepted by A&
Narrow He II emission in star-forming galaxies at low metallicity. Stellar wind emission from a population of Very Massive Stars
In a recent study star-forming galaxies with HeII emission between redshifts
2 and 4.6 have been found to occur in two modes, distinguished by the width of
their HeII emission lines. Broad HeII emission has been attributed to stellar
emission from a population of evolved Wolf-Rayet (WR) stars while narrow HeII
emission has been attributed to nebular emission excited by a population of
very hot PopIII stars formed in pockets of pristine gas at moderate redshifts.
In this work we propose an alternative scenario for the origin of the narrow
HeII emission, namely very massive stars (VMS) at low metallicity (Z) which
form strong but slow WR-type stellar winds due to their proximity to the
Eddington limit. We estimate the expected HeII line fluxes and equivalent
widths based on wind models for VMS and population synthesis models, and
compare the results with recent observations of star-forming galaxies at
moderate redshifts. The observed HeII line strengths and equivalent widths are
in line with what is expected for a population of VMS in one or more young
super-clusters located within these galaxies. In our scenario the two observed
modes of HeII emission originate from massive stellar populations in distinct
evolutionary stages at low Z. If this interpretation is correct there is no
need to postulate the existence of PopIII stars at moderate redshifts to
explain the observed narrow HeII emission. An interesting possibility is the
existence of self-enriched VMS with similar WR-type spectra at extremely low Z.
Stellar HeII emission from such very early generations of VMS may be detectable
in future studies of star-forming galaxies at high redshifts with the James
Webb Space Telescope. The fact that the HeII emission of VMS is largely
neglected in current population synthesis models will generally affect the
interpretation of the integrated spectra of young stellar populations.Comment: 4 pages, 1 figure, A&A letters (accepted
An explanation for the curious mass loss history of massive stars: from OB stars, through Luminous Blue Variables to Wolf-Rayet stars
The stellar winds of massive stars show large changes in mass-loss rates and
terminal velocities during their evolution from O-star through the Luminous
Blue Variable phase to the Wolf-Rayet phase. The luminosity remains
approximately unchanged during these phases. These large changes in wind
properties are explained in the context of the radiation driven wind theory, of
which we consider four different models. They are due to the evolutionary
changes in radius, gravity and surface composition and to the change from
optically thin (in continuum) line driven winds to optically thick radiation
driven winds.Comment: Accepted for publication in Astronomy and Astrophysics (Letter to the
Editor
Fluids with quenched disorder: Scaling of the free energy barrier near critical points
In the context of Monte Carlo simulations, the analysis of the probability
distribution of the order parameter , as obtained in simulation
boxes of finite linear extension , allows for an easy estimation of the
location of the critical point and the critical exponents. For Ising-like
systems without quenched disorder, becomes scale invariant at the
critical point, where it assumes a characteristic bimodal shape featuring two
overlapping peaks. In particular, the ratio between the value of at
the peaks () and the value at the minimum in-between ()
becomes -independent at criticality. However, for Ising-like systems with
quenched random fields, we argue that instead should be observed, where is the
"violation of hyperscaling" exponent. Since is substantially non-zero,
the scaling of with system size should be easily detectable in
simulations. For two fluid models with quenched disorder, versus
was measured, and the expected scaling was confirmed. This provides further
evidence that fluids with quenched disorder belong to the universality class of
the random-field Ising model.Comment: sent to J. Phys. Cond. Mat
Jets as diagnostics of the circumstellar medium and the explosion energetics of supernovae: the case of Cas A
We present hydrodynamical models for the Cassiopeia A (Cas A) supernova
remnant and its observed jet / counter-jet system. We include the evolution of
the progenitor's circumstellar medium, which is shaped by a slow red supergiant
wind that is followed by a fast Wolf-Rayet (WR) wind.
The main parameters of the simulations are the duration of the WR phase and
the jet energy. We find that the jet is destroyed if the WR phase is
sufficiently long and a massive circumstellar shell has formed. We therefore
conclude that the WR phase must have been short (a few thousand yr), if present
at all. Since the actual jet length of Cas A is not known we derive a lower
limit for the jet energy, which is ~10^{48} erg. We discuss the implications
for the progenitor of Cas A and the nature of its explosion.Comment: 9 pages, 5 figures, ApJ accepted. Version with high resolution
figures available at http://www.phys.uu.nl/~schure/CasA_jet.pd
Revealing the obscured supernova remnant Kes 32 with Chandra
I report here on the analysis and interpretation of a Chandra observation of
the supernova remnant Kes 32. Kes 32 is rather weak in X-rays due to a large
interstellar absorption, which is found to be ~4E22 cm^-2, larger than
previously reported. Spectral analysis indicates that the ionization age of
this object is very young, with n_e t ~ 4E9 cm^-3s, and a temperature of kT_e ~
1 keV. The X-ray emission peaks at a smaller radius than in the radio. The low
ionization age suggests that Kes 32 is a young remnant. However, a young age is
in contradiction with the relatively large apparent size, which indicates an
age of several thousand years, instead of a few hundred years. This problem is
discussed in connection with Kes 32's unknown distance and its possible
association with the Norma galactic arm.Comment: Accepted for publication in the Astrophysical Journal. 7 pages, 7
figure
Bladvlekken zomerbloemen : meer veroorzakers, dus extra alert reageren
Bladvlekkenziekten veroorzaken regelmatig problemen in de teelten van verschillende soorten zomerbloemen. Bladvlekken kunnen door verschillende ziekteverwekkende schimmels worden veroorzaakt
Bladvlekken in zomerbloemen 2 : vervolg op onderzoek 2007-2008
Bladvlekkenziekten veroorzaken regelmatig problemen in de teelten van verschillende soorten zomerbloemen. Bladvlekken kunnen door verschillende ziekteverwekkende schimmels worden veroorzaakt. Om een betrouwbaar en gericht advies te geven is het van belang om te weten welk organisme de bladvlekken veroorzaakt. In dit project zijn monsters met bladvlekken bij telers verzameld en is met behulp van de infectieproeven getracht de veroorzaker aan te wijzen
Accelerated Electrons in Cassiopeia A: An Explanation for the Hard X-ray Tail
We propose a model for the hard X-ray (> 10 keV) emission observed from the
supernova remnant Cas A. Lower hybrid waves are generated in strong (mG)
magnetic fields, generally believed to reside in this remnant, by shocks
reflected from density inhomogeneities. These then accelerate electrons to
energies of several tens of keV. Around 4% of the x-ray emitting plasma
electrons need to be in this accelerated distribution, which extends up to
electron velocities of order the electron Alfven speed, and is directled along
magnetic field lines. Bremsstrahlung from these electrons produces the observed
hard x-ray emission. Such waves and accelerated electrons have been observed in
situ at Comet Halley, and we discuss the viability of the extrapolation from
this case to the parameters relevant to Cas A.Comment: 20 pages, 3 figures, aasTeX502, accepted in Ap
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