644 research outputs found
Massive main-sequence stars evolving at the Eddington limit
Context. Massive stars play a vital role in the Universe, however, their evolution even on the main-sequence is not yet well understood.
Aims. Because of the steep mass-luminosity relation, massive main-sequence stars become extremely luminous. This brings their envelopes very close to the Eddington limit. We analyse stellar evolutionary models in which the Eddington limit is reached and exceeded, explore the rich diversity of physical phenomena that take place in their envelopes, and investigate their observational consequences.
Methods. We use published grids of detailed stellar models, computed with a state-of-the-art, one-dimensional hydrodynamic stellar evolution code using LMC composition, to investigate the envelope properties of core hydrogen burning massive stars.
Results. We find that the Eddington limit is almost never reached at the stellar surface, even for stars up to 500 M⊙. When we define an appropriate Eddington limit locally in the stellar envelope, we can show that most stars more massive than ~40 M⊙ actually exceed this limit, in particular, in the partial ionisation zones of iron, helium, or hydrogen. While most models adjust their structure such that the local Eddington limit is exceeded at most by a few per cent, our most extreme models do so by a factor of more than seven. We find that the local violation of the Eddington limit has severe consequences for the envelope structure, as it leads to envelope inflation, convection, density inversions, and, possibly to, pulsations. We find that all models with luminosities higher than 4 × 105L⊙, i.e. stars above ~40 M⊙ show inflation, with a radius increase of up to a factor of about 40. We find that the hot edge of the S Dor variability region coincides with a line beyond which our models are inflated by more than a factor of two, indicating a possible connection between S Dor variability and inflation. Furthermore, our coolest models show highly inflated envelopes with masses of up to several solar masses, and appear to be candidates for producing major luminous blue variable eruptions.
Conclusions. Our models show that the Eddington limit is expected to be reached in all stars above ~40 M⊙ in the LMC, even in lower mass stars in the Galaxy, or in close binaries or rapid rotators. While our results do not support the idea of a direct super-Eddington wind driven by continuum photons, the consequences of the Eddington limit in the form of inflation, pulsations and possibly eruptions may well give rise to a significant enhancement of the time averaged mass-loss rate
Discovery of a new Galactic bona fide luminous blue variable with Spitzer
We report the discovery of a circular mid-infrared shell around the emission-line star Wray 16-
137 using archival data of the Spitzer Space Telescope. Follow-up optical spectroscopy of
Wray 16-137 with the Southern African Large Telescope revealed a rich emission spectrum
typical of the classical luminous blue variables (LBVs) like P Cygni. Subsequent spectroscopic
and photometric observations showed drastic changes in the spectrum and brightness during
the last three years, meaning that Wray 16-137 currently undergoes an S Dor-like outburst.
Namely, we found that the star has brightened by ≈1 mag in the V and Ic bands, while its
spectrum became dominated by Fe II lines. Taken together, our observations unambiguously
show that Wray 16-137 is a new member of the family of Galactic bona fide LBVs
Determination of the parameters of semiconducting CdF2:In with Schottky barriers from radio-frequency measurements
Physical properties of semiconducting CdF_2 crystals doped with In are
determined from measurements of the radio-frequency response of a sample with
Schottky barriers at frequencies 10 - 10^6 Hz. The dc conductivity, the
activation energy of the amphoteric impurity, and the total concentration of
the active In ions in CdF_2 are found through an equivalent-circuit analysis of
the frequency dependencies of the sample complex impedance at temperatures from
20 K to 300 K. Kinetic coefficients determining the thermally induced
transitions between the deep and the shallow states of the In impurity and the
barrier height between these states are obtained from the time-dependent
radio-frequency response after illumination of the material. The results on the
low-frequency conductivity in CdF_2:In are compared with submillimeter (10^{11}
- 10^{12} Hz) measurements and with room-temperature infrared measurements of
undoped CdF_2. The low-frequency impedance measurements of semiconductor
samples with Schottky barriers are shown to be a good tool for investigation of
the physical properties of semiconductors.Comment: 9 pages, 7 figure
The self-consistent bounce: an improved nucleation rate
We generalize the standard computation of homogeneous nucleation theory at
zero temperature to a scenario in which the bubble shape is determined
self-consistently with its quantum fluctuations. Studying two scalar models in
1+1 dimensions, we find the self-consistent bounce by employing a two-particle
irreducible (2PI) effective action in imaginary time at the level of the
Hartree approximation. We thus obtain an effective single bounce action which
determines the rate exponent. We use collective coordinates to account for the
translational invariance and the growth instability of the bubble and finally
present a new nucleation rate prefactor. We compare the results with those
obtained using the standard 1-loop approximation and show that the
self-consistent rate can differ by several orders of magnitude.Comment: 28 pages, revtex, 7 eps figure
Bose-Einstein condensation and superfluidity of dilute Bose gas in a random potential
We develop the dilute Bose gas model with random potential in order to
understand the Bose system in random media such as 4He in porous glass. Using
the random potential taking account of the pore size dependence, we can compare
quantitatively the calculated specific heat with the experimental results,
without free parameters. The agreement is excellent at low temperatures, which
justifies our model. The relation between Bose condensation and superfluidity
is discussed. Our model can predict some unobserved phenomena in this system.Comment: 9 pages, 11 figures, accepted for publication in Phys. Rev.
A Way to Reopen the Window for Electroweak Baryogenesis
We reanalyse the sphaleron bound of electroweak baryogenesis when allowing
deviations to the Friedmann equation. These modifications are well motivated in
the context of brane cosmology where they appear without being in conflict with
major experimental constraints on four-dimensional gravity. While suppressed at
the time of nucleosynthesis, these corrections can dominate at the time of the
electroweak phase transition and in certain cases provide the amount of
expansion needed to freeze out the baryon asymmetry without requiring a
strongly first order phase transition. The sphaleron bound is substantially
weakened and can even disappear so that the constraints on the higgs and stop
masses do not apply anymore. Such modification of cosmology at early times
therefore reopens the parameter space allowing electroweak baryogenesis which
had been reduced substantially given the new bound on the higgs mass imposed by
LEP. In contrast with previous attempts to turn around the sphaleron bound
using alternative cosmologies, we are still considering that the electroweak
phase transition takes place in a radiation dominated universe. The universe is
expanding fast because of the modification of the Friedmann equation itself
without the need for a scalar field and therefore evading the problem of the
decay of this scalar field after the completion of the phase transition and the
risk that its release of entropy dilutes the baryon asymmetry produced at the
transition.Comment: 19 pages, 3 figures; v2: minor changes, remark added at end of
section 5 and in caption of figure 1; v3: references added, version to be
publishe
Hopping Conduction in Disordered Carbon Nanotubes
We report electrical transport measurements on individual disordered carbon
nanotubes, grown catalytically in a nanoporous anodic aluminum oxide template.
In both as-grown and annealed types of nanotubes, the low-field conductance
shows as exp[-(T_{0}/T)^{1/2}] dependence on temperature T, suggesting that
hopping conduction is the dominant transport mechanism, albeit with different
disorder-related coefficients T_{0}. The field dependence of low-temperature
conductance behaves an exp[-(xi_{0}/xi)^{1/2}] with high electric field xi at
sufficiently low T. Finally, both annealed and unannealed nanotubes exhibit
weak positive magnetoresistance at low T = 1.7 K. Comparison with theory
indicates that our data are best explained by Coulomb-gap variable range
hopping conduction and permits the extraction of disorder-dependent
localization length and dielectric constant.Comment: 10 pages, 5 figure
Theory for the excitation spectrum of High-T$_c superconductors : quasiparticle dispersion and shadows of the Fermi surface
Using a new method for the solution of the FLEX-equations, which allows the
determination of the self energy of the Hubbard
model on the real frequency axis, we calculate the doping dependence of the
quasi-particle excitations of High-T superconductors. We obtain new results
for the shadows of the Fermi surface, their dependence on the deformation of
the quasi particle dispersion, an anomalous -dependence of and a related violation of the Luttinger theorem.
This sheds new light on the influence of short range magnetic order on the low
energy excitations and its significance for photoemission experiments.Comment: 4 pages (REVTeX) with 3 figure
Glassy dynamics: effective temperatures and intermittencies from a two-state model
We show the existence of intermittent dynamics in one of the simplest model
of a glassy system: the two-state model, which has been used to explain the
origin of the violation of the fluctuation-dissipation theorem. The dynamics is
analyzed through a Langevin equation for the evolution of the state of the
system through its energy landscape. The results obtained concerning the
violation factor and the non-Gaussian nature of the fluctuations are in good
qualitative agreement with experiments measuring the effective temperature and
the voltage fluctuations in gels and in polymer glasses. The method proposed
can be useful to study the dynamics of other slow relaxation systems in which
non-Gaussian fluctuations have been observed.Comment: 4 pages, 4 figure
The R136 star cluster dissected with Hubble Space Telescope/STIS. II. Physical properties of the most massive stars in R136
We present an optical analysis of 55 members of R136, the central cluster in the Tarantula Nebula of the Large Magellanic Cloud. Our sample was observed with STIS aboard the Hubble Space Telescope, is complete down to about 40 M⊙, and includes 7 very massive stars with masses over 100 M⊙. We performed a spectroscopic analysis to derive their physical properties. Using evolutionary models we find that the initial mass function (IMF) of massive stars in R136 is suggestive of being top-heavy with a power-law exponent γ ≈ 2 ± 0.3, but steeper exponents cannot be excluded. The age of R136 lies between 1 and 2 Myr with a median age of around 1.6 Myr. Stars more luminous than log L/L⊙ = 6.3 are helium enriched and their evolution is dominated by mass loss, but rotational mixing or some other form of mixing could be still required to explain the helium composition at the surface. Stars more massive than 40 M⊙ have larger spectroscopic than evolutionary masses. The slope of the wind-luminosity relation assuming unclumped stellar winds is 2.41 ± 0.13 which is steeper than usually obtained (∼1.8). The ionising (log Q0 [ph/s] = 51.4) and mechanical (log LSW [erg/s] = 39.1) output of R136 is dominated by the most massive stars (>100 M⊙). R136 contributes around a quarter of the ionising flux and around a fifth of the mechanical feedback to the overall budget of the Tarantula Nebula. For a census of massive stars of the Tarantula Nebula region we combined our results with the VLT-FLAMES Tarantula Survey plus other spectroscopic studies. We observe a lack of evolved Wolf-Rayet stars and luminous blue and red supergiants
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