3,150 research outputs found
The Interiors of Giant Planets: Models and Outstanding Questions
We know that giant planets played a crucial role in the making of our Solar
System. The discovery of giant planets orbiting other stars is a formidable
opportunity to learn more about these objects, what is their composition, how
various processes influence their structure and evolution, and most importantly
how they form. Jupiter, Saturn, Uranus and Neptune can be studied in detail,
mostly from close spacecraft flybys. We can infer that they are all enriched in
heavy elements compared to the Sun, with the relative global enrichments
increasing with distance to the Sun. We can also infer that they possess dense
cores of varied masses. The intercomparison of presently caracterised
extrasolar giant planets show that they are also mainly made of hydrogen and
helium, but that they either have significantly different amounts of heavy
elements, or have had different orbital evolutions, or both. Hence, many
questions remain and are to be answered for significant progresses on the
origins of planets.Comment: 43 pages, 11 figures, 3 tables. To appear in Annual Review of Earth
and Planetary Sciences, vol 33, (2005
Modeling Pressure-Ionization of Hydrogen in the Context of Astrophysics
The recent development of techniques for laser-driven shock compression of
hydrogen has opened the door to the experimental determination of its behavior
under conditions characteristic of stellar and planetary interiors. The new
data probe the equation of state (EOS) of dense hydrogen in the complex regime
of pressure ionization. The structure and evolution of dense astrophysical
bodies depend on whether the pressure ionization of hydrogen occurs
continuously or through a ``plasma phase transition'' (PPT) between a molecular
state and a plasma state. For the first time, the new experiments constrain
predictions for the PPT. We show here that the EOS model developed by Saumon
and Chabrier can successfully account for the data, and we propose an
experiment that should provide a definitive test of the predicted PPT of
hydrogen. The usefulness of the chemical picture for computing astrophysical
EOS and in modeling pressure ionization is discussed.Comment: 16 pages + 4 figures, to appear in High Pressure Researc
Polybenzoxazole-filled nitrile butadiene rubber compositions
An insulation composition that comprises at least one nitrile butadiene rubber (NBR) having an acrylonitrile content that ranges from approximately 26% by weight to approximately 35% by weight and polybenzoxazole (PBO) fibers. The NBR may be a copolymer of acrylonitrile and butadiene and may be present in the insulation composition in a range of from approximately 45% by weight to approximately 56% by weight of a total weight of the insulation composition. The PBO fibers may be present in a range of from approximately 3% by weight to approximately 10% by weight of a total weight of the insulation composition. A rocket motor including the insulation composition and a method of insulating a rocket motor are also disclosed
EPDM rocket motor insulation
A novel and improved EPDM formulation for a solid propellant rocket motor is described wherein hexadiene EPDM monomer components are replaced by alkylidene norbornene components and with appropriate adjustment of curing and other additives functionally-required rheological and physical characteristics are achieved with the desired compatibility with any one of a plurality of solid filler materials, e.g. powder silica, carbon fibers or aramid fibers, and with appropriate adhesion and extended storage or shelf life characteristics
Structure and evolution of the first CoRoT exoplanets: Probing the Brown Dwarf/Planet overlapping mass regime
We present detailed structure and evolution calculations for the first
transiting extrasolar planets discovered by the space-based CoRoT mission.
Comparisons between theoretical and observed radii provide information on the
internal composition of the CoRoT objects. We distinguish three different
categories of planets emerging from these discoveries and from previous
ground-based surveys: (i) planets explained by standard planetary models
including irradiation, (ii) abnormally bloated planets and (iii) massive
objects belonging to the overlapping mass regime between planets and brown
dwarfs. For the second category, we show that tidal heating can explain the
relevant CoRoT objects, providing non-zero eccentricities. We stress that the
usual assumption of a quick circularization of the orbit by tides, as usually
done in transit light curve analysis, is not justified a priori, as suggested
recently by Levrard et al. (2009), and that eccentricity analysis should be
carefully redone for some observations. Finally, special attention is devoted
to CoRoT-3b and to the identification of its very nature: giant planet or brown
dwarf ? The radius determination of this object confirms the theoretical
mass-radius predictions for gaseous bodies in the substellar regime but, given
the present observational uncertainties, does not allow an unambiguous
identification of its very nature. This opens the avenue, however, to an
observational identification of these two distinct astrophysical populations,
brown dwarfs and giant planets, in their overlapping mass range, as done for
the case of the 8 Jupiter-mass object Hat-P-2b. (abridged)Comment: 6 pages, 5 figures, accepted for publication in Astronomy and
Astrophysic
Distorted, non-spherical transiting planets: impact on the transit depth and on the radius determination
We quantify the systematic impact of the non-spherical shape of transiting
planets and brown dwarfs, due to tidal forces and rotation, on the observed
transit depth. Such a departure from sphericity leads to a bias in the
derivation of the transit radius from the light curve and affects the
comparison with planet structure and evolution models which assume spherical
symmetry. As the tidally deformed planet projects its smallest cross section
area during the transit, the measured effective radius is smaller than the one
of the unperturbed spherical planet. This effect can be corrected by
calculating the theoretical shape of the observed planet.
We derive simple analytical expressions for the ellipsoidal shape of a fluid
object (star or planet) accounting for both tidal and rotational deformations
and calibratre it with fully numerical evolution models in the 0.3Mjup-75Mjup
mass range. Our calculations yield a 20% effect on the transit depth, i.e. a
10% decrease of the measured radius, for the extreme case of a 1Mjup planet
orbiting a Sun-like star at 0.01AU. For the closest planets detected so far (<
0.05 AU), the effect on the radius is of the order of 1 to 10%, by no means a
negligible effect, enhancing the puzzling problem of the anomalously large
bloated planets. These corrections must thus be taken into account for a
correct determination of the radius from the transit light curve.
Our analytical expressions can be easily used to calculate these corrections,
due to the non-spherical shape of the planet, on the observed transit depth and
thus to derive the planet's real equilibrium radius. They can also be used to
model ellipsoidal variations of the stellar flux now detected in the CoRoT and
Kepler light curves. We also derive directly usable analytical expressions for
the moment of inertia, oblateness and Love number (k_2) of a fluid planet as a
function of its mass.Comment: 19 pages, 6 figures, 5 tables. Published in A&A. Correction of minor
errors in Appendix B. An electronic version of the grids of planetary models
is available at
http://perso.ens-lyon.fr/jeremy.leconte/JLSite/JLsite/Exoplanets_Simulations.htm
Studying nonlinear effects on the early stage of phase ordering using a decomposition method
Nonlinear effects on the early stage of phase ordering are studied using
Adomian's decomposition method for the Ginzburg-Landau equation for a
nonconserved order parameter. While the long-time regime and the linear
behavior at short times of the theory are well understood, the onset of
nonlinearities at short times and the breaking of the linear theory at
different length scales are less understood. In the Adomian's decomposition
method, the solution is systematically calculated in the form of a polynomial
expansion for the order parameter, with a time dependence given as a series
expansion. The method is very accurate for short times, which allows to
incorporate the short-time dynamics of the nonlinear terms in a analytical and
controllable way.Comment: 11 pages, 1 figure, to appear in Phys Lett
A new model for mixing by double-diffusive convection (semi-convection): I. The conditions for layer formation
The process referred to as "semi-convection" in astrophysics and
"double-diffusive convection in the diffusive regime" in Earth and planetary
sciences, occurs in stellar and planetary interiors in regions which are stable
according to the Ledoux criterion but unstable according to the Schwarzschild
criterion. In this series of papers, we analyze the results of an extensive
suite of 3D numerical simulations of the process, and ultimately propose a new
1D prescription for heat and compositional transport in this regime which can
be used in stellar or planetary structure and evolution models.
In a preliminary study of the phenomenon, Rosenblum et al. (2011) showed
that, after saturation of the primary instability, a system can evolve in one
of two possible ways: the induced turbulence either remains homogeneous, with
very weak transport properties, or transitions into a thermo-compositional
staircase where the transport rate is much larger (albeit still smaller than in
standard convection).
In this paper, we show that this dichotomous behavior is a robust property of
semi-convection across a wide region of parameter space. We propose a simple
semi-analytical criterion to determine whether layer formation is expected or
not, and at what rate it proceeds, as a function of the background
stratification and of the diffusion parameters (viscosity, thermal diffusivity
and compositional diffusivity) only. The theoretical criterion matches the
outcome of our numerical simulations very adequately in the numerically
accessible "planetary" parameter regime, and can easily be extrapolated to the
stellar parameter regime.
Subsequent papers will address more specifically the question of quantifying
transport in the layered case and in the non-layered case.Comment: Submitted to Ap
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