14,567 research outputs found
Strange pulsation modes in luminous red giants
We show that the spectrum of radial pulsation modes in luminous red giants
consists of both normal modes and a second set of modes with periods similar to
those of the normal modes. These additional modes are the red giant analogues
of the strange modes found in classical Cepheids and RR Lyrae variables. Here,
we describe the behaviour of strange and normal modes in luminous red giants
and discuss the dependence of both the strange and normal modes on the outer
boundary conditions. The strange modes always appear to be damped, much more so
than the normal modes. They should never be observed as self-excited modes in
real red giants but they may be detected in the spectrum of solar-like
oscillations. A strange mode with a period close to that of a normal mode can
influence both the period and growth rate of the normal mode.Comment: 6 pages, 5 figures, accepted by MNRA
Growth rate of Rayleigh-Taylor turbulent mixing layers with the foliation approach
For years, astrophysicists, plasma fusion and fluid physicists have puzzled
over Rayleigh-Taylor turbulent mixing layers. In particular, strong
discrepancies in the growth rates have been observed between experiments and
numerical simulations. Although two phenomenological mechanisms (mode-coupling
and mode-competition) have brought some insight on these differences,
convincing theoretical arguments are missing to explain the observed values. In
this paper, we provide an analytical expression of the growth rate compatible
with both mechanisms and is valide for a self-similar, low Atwood
Rayleigh-Taylor turbulent mixing subjected to a constant or time-varying
acceleration. The key step in this work is the introduction of {\it foliated}
averages and {\it foliated} turbulent spectra highlighted in our three
dimensional numerical simulations. We show that the exact value of the
Rayleigh-Taylor growth rate not only depends upon the acceleration history but
is also bound to the power-law exponent of the {\it foliated} spectra at large
scales
Exoplanets imaging with a Phase-Induced Amplitude Apodization Coronagraph - I. Principle
Using 2 aspheric mirrors, it is possible to apodize a telescope beam without
losing light or angular resolution: the output beam is produced by
``remapping'' the entrance beam to produce the desired light intensity
distribution in a new pupil. We present the Phase-Induced Amplitude Apodization
Coronagraph (PIAAC) concept, which uses this technique, and we show that it
allows efficient direct imaging of extrasolar terrestrial planets with a
small-size telescope in space. The suitability of the PIAAC for exoplanet
imaging is due to a unique combination of achromaticity, small inner working
angle (about 1.5 ), high throughput, high angular resolution and
large field of view. 3D geometrical raytracing is used to investigate the
off-axis aberrations of PIAAC configurations, and show that a field of view of
more than 100 in radius is available thanks to the correcting
optics of the PIAAC. Angular diameter of the star and tip-tilt errors can be
compensated for by slightly increasing the size of the occulting mask in the
focal plane, with minimal impact on the system performance. Earth-size planets
at 10 pc can be detected in less than 30s with a 4m telescope. Wavefront
quality requirements are similar to classical techniques.Comment: 35 pages, 16 figures, Accepted for publication in Ap
Axion Cosmology Revisited
The misalignment mechanism for axion production depends on the
temperature-dependent axion mass. The latter has recently been determined
within the interacting instanton liquid model (IILM), and provides for the
first time a well-motivated axion mass for all temperatures. We reexamine the
constraints placed on the axion parameter space in the light of this new mass
function. We find an accurate and updated constraint f_a \le 2.8(\pm2)\times
10^{11}\units{GeV} or m_a \ge 21(\pm2) \units{\mu eV} from the misalignment
mechanism in the classic axion window (thermal scenario). However, this is
superseded by axion string radiation which leads to f_a \lesssim
3.2^{+4}_{-2} \times 10^{10} \units{GeV} or m_a \gtrsim 0.20 ^{+0.2}_{-0.1}
\units{meV}. In this analysis, we take care to precisely compute the effective
degrees of freedom and, to fill a gap in the literature, we present accurate
fitting formulas. We solve the evolution equations exactly, and find that
analytic results used to date generally underestimate the full numerical
solution by a factor 2-3. In the inflationary scenario, axions induce
isocurvature fluctuations and constrain the allowed inflationary scale .
Taking anharmonic effects into account, we show that these bounds are actually
weaker than previously computed. Considering the fine-tuning issue of the
misalignment angle in the whole of the anthropic window, we derive new bounds
which open up the inflationary window near . In particular,
we find that inflationary dark matter axions can have masses as high as
0.01--1\units{meV}, covering the whole thermal axion range, with values of
up to GeV. Quantum fluctuations during inflation exclude dominant
dark matter axions with masses above meV.Comment: 42 pages, 12 figures, version as accepted by Phys.Rev.
Thermoelectric properties of Co, Ir, and Os-Doped FeSi Alloys: Evidence for Strong Electron-Phonon Coupling
The effects of various transition metal dopants on the electrical and thermal
transport properties of Fe1-xMxSi alloys (M= Co, Ir, Os) are reported. The
maximum thermoelectric figure of merit ZTmax is improved from 0.007 at 60 K for
pure FeSi to ZT = 0.08 at 100 K for 4% Ir doping. A comparison of the thermal
conductivity data among Os, Ir and Co doped alloys indicates strong
electron-phonon coupling in this compound. Because of this interaction, the
common approximation of dividing the total thermal conductivity into
independent electronic and lattice components ({\kappa}Total =
{\kappa}electronic + {\kappa}lattice) fails for these alloys. The effects of
grain size on thermoelectric properties of Fe0.96Ir0.04Si alloys are also
reported. The thermal conductivity can be lowered by about 50% with little or
no effect on the electrical resistivity or Seebeck coefficient. This results in
ZTmax = 0.125 at 100 K, still about a factor of five too low for solid-state
refrigeration applications
- …