830 research outputs found
Logarithmically modified scaling of temperature structure functions in thermal convection
Using experimental data on thermal convection, obtained at a Rayleigh number
of 1.5 , it is shown that the temperature structure functions
, where is the absolute value of the temperature
increment over a distance , can be well represented in an intermediate range
of scales by , where the are the scaling
exponents appropriate to the passive scalar problem in hydrodynamic turbulence
and the function . Measurements are made in the
midplane of the apparatus near the sidewall, but outside the boundary layer
Multiscale SOC in turbulent convection
Using data obtained in a laboratory thermal convection experiment at high
Rayleigh numbers, it is shown that the multiscaling properties of the observed
mean wind reversals are quantitatively consistent with analogous multiscaling
properties of the Bak-Tang-Wiesenfeld prototype model of self-organized
criticality in two dimensions
Fluctuations of temperature gradients in turbulent thermal convection
Broad theoretical arguments are proposed to show, formally, that the
magnitude G of the temperature gradients in turbulent thermal convection at
high Rayleigh numbers obeys the same advection-diffusion equation that governs
the temperature fluctuation T, except that the velocity field in the new
equation is substantially smoothed. This smoothed field leads to a -1 scaling
of the spectrum of G in the same range of scales for which the spectral
exponent of T lies between -7/5 and -5/3. This result is confirmed by
measurements in a confined container with cryogenic helium gas as the working
fluid for Rayleigh number Ra=1.5x10^{11}. Also confirmed is the logarithmic
form of the autocorrelation function of G. The anomalous scaling of
dissipation-like quantities of T and G are identical in the inertial range,
showing that the analogy between the two fields is quite deep
Can a charged ring levitate a neutral, polarizable object? Can Earnshaw's Theorem be extended to such objects?
Stable electrostatic levitation and trapping of a neutral, polarizable object
by a charged ring is shown to be theoretically impossible. Earnshaw's Theorem
precludes the existence of such a stable, neutral particle trap.Comment: 11 pages, 1 figur
The Galactic WN stars: Spectral analyses with line-blanketed model atmospheres versus stellar evolution models with and without rotation
CONTEXT: Very massive stars pass through the Wolf-Rayet (WR) stage before
they finally explode. Details of their evolution have not yet been safely
established, and their physics are not well understood. Their spectral analysis
requires adequate model atmospheres, which have been developed step by step
during the past decades and account in their recent version for line blanketing
by the millions of lines from iron and iron-group elements. However, only very
few WN stars have been re-analyzed by means of line-blanketed models yet.
AIMS: The quantitative spectral analysis of a large sample of Galactic WN
stars with the most advanced generation of model atmospheres should provide an
empirical basis for various studies about the origin, evolution, and physics of
the Wolf-Rayet stars and their powerful winds.
METHODS: We analyze a large sample of Galactic WN stars by means of the
Potsdam Wolf-Rayet (PoWR) model atmospheres, which account for iron line
blanketing and clumping. The results are compared with a synthetic population,
generated from the Geneva tracks for massive star evolution. RESULTS: We obtain
a homogeneous set of stellar and atmospheric parameters for the Galactic WN
stars, partly revising earlier results.
CONCLUSIONS: Comparing the results of our spectral analyses of the Galactic
WN stars with the predictions of the Geneva evolutionary calculations, we
conclude that there is rough qualitative agreement. However, the quantitative
discrepancies are still severe, and there is no preference for the tracks that
account for the effects of rotation. It seems that the evolution of massive
stars is still not satisfactorily understood.Comment: 19 pages, 11 figures, A&A, in press, additional Online-material on
http://www.astro.physik.uni-potsdam.de/abstracts/galwn.htm
Parameter constraints for high-energy models of colliding winds of massive stars: the case WR 147
We explore the ability of high energy observations to constrain orbital
parameters of long period massive binary systems by means of an inverse Compton
model acting in colliding wind environments. This is particular relevant for
(very) long period binaries where orbital parameters are often poorly known
from conventional methods, as is the case e.g. for the Wolf-Rayet (WR) star
binary system WR 147 where INTEGRAL and MAGIC upper limits on the high-energy
emission have recently been presented. We conduct a parameter study of the set
of free quantities describing the yet vaguely constrained geometry and
respective effects on the non-thermal high-energy radiation from WR 147. The
results are confronted with the recently obtained high-energy observations and
with sensitivities of contemporaneous high-energy instruments like Fermi-LAT.
For binaries with sufficient long periods, like WR 147, gamma-ray attenuation
is unlikely to cause any distinctive features in the high-energy spectrum. This
leaves the anisotropic inverse Compton scattering as the only process that
reacts sensitively on the line-of-sight angle with respect to the orbital
plane, and therefore allows the deduction of system parameters even from
observations not covering a substantial part of the orbit.
Provided that particle acceleration acts sufficiently effectively to allow
the production of GeV photons through inverse Compton scattering, our analysis
indicates a preference for WR 147 to possess a large inclination angle.
Otherwise, for low inclination angles, electron acceleration is constrained to
be less efficient as anticipated here.Comment: 33 pages, 9 figures; accepted by Ap
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