112 research outputs found
The Free-Free Opacity in Warm, Dense, and Weakly Ionized Helium
We investigate the ionization and the opacity of warm, dense helium under
conditions found in the atmospheres of cool white dwarf stars. Our particular
interest is in densities up to and temperatures from 1000K to
10000K. For these physical conditions various approaches for modeling the
ionization equilibrium predict ionization fractions that differ by orders of
magnitudes. Furthermore, estimates of the density at which helium
pressure-ionizes vary from to . In this context, the
value of the electron-atom inverse bremsstrahlung absorption is highly
uncertain. We present new results obtained from a non-ideal chemical model for
the ionization equilibrium, from Quantum Molecular Dynamics (QMD) simulations,
and from the analysis of experimental data to better understand the ionization
fraction in fluid helium in the weak ionization limit.Comment: 4 pages, 3 figures, 1 table. Accepted for publication in the
Proceedings of the 14th APS Topical Conference on Shock Compression of
Condensed Matter, Baltimore, M
Melting and metallization of silica in the cores of gas giants, ice giants and super Earths
The physical state and properties of silicates at conditions encountered in
the cores of gas giants, ice giants and of Earth like exoplanets now discovered
with masses up to several times the mass of the Earth remains mostly unknown.
Here, we report on theoretical predictions of the properties of silica,
SiO, up to 4 TPa and about 20,000K using first principle molecular dynamics
simulations based on density functional theory. For conditions found in the
Super-Earths and in ice giants, we show that silica remains a poor electrical
conductor up to 10 Mbar due to an increase in the Si-O coordination with
pressure. For Jupiter and Saturn cores, we find that MgSiO silicate has not
only dissociated into MgO and SiO, as shown in previous studies, but that
these two phases have likely differentiated to lead to a core made of liquid
SiO and solid (Mg,Fe)O.Comment: 5 pages, 4 figure
A new equation of state for dense hydrogen-helium mixtures
This is the final version. Available from American Astronomical Society via the DOI in this recordWe present a new equation of state (EOS) for dense hydrogen/helium mixtures that covers a range of densities
from 10−8 to 106 g cm-3, pressures from 10−9 to 1013 GPa, and temperatures from 102 to 108 K. The calculations
combine the EOS of Saumon, Chabrier & van Horn in the low-density, low-temperature molecular/atomic domain,
the EOS of Chabrier & Potekhin in the high-density, high-temperature fully ionized domain, the limits of which
differ for H and He, and ab initio quantum molecular dynamics calculations in the regime of intermediate density
and temperature, characteristic of pressure dissociation and ionization. The EOS for the H/He mixture is based on
the so-called additive volume law and thus does not take into account the interactions between the two species. A
major improvement of the present calculations over existing ones is that we calculate the entropy over the entire
density–temperature domain, a necessary quantity for calculations of stellar or planetary evolution. The EOS
results are compared with existing experimental data, namely Hugoniot shock experiments for pure H and He, and
with first-principles numerical simulations for both the single elements and the mixture. This new EOS covers a
wide range of physical and astrophysical conditions, from Jovian planets to solar-type stars, and recovers the
existing relativistic EOS at very high densities, in the domains of white dwarfs and neutron stars. All the tables are
made publicly available.Programme National de Planétologie (PNP
Ab initio based equation of state of dense water for planetary and exoplanetary modeling
This is the author accepted manuscript. The final version is available from EDP Sciences via the DOI in this record.As a first step toward a multi-phase equation of state for dense water, we develop a temperature-dependent equation of state for dense water covering the liquid and plasma regimes and extending to the super-ionic and gas regimes. This equation of state covers the complete range of conditions encountered in planetary modeling. We use first principles quantum molecular dynamics simulations and its Thomas-Fermi extension to reach the highest pressures encountered in giant planets several times the size of Jupiter. Using these results, as well as the data available at lower pressures, we obtain a parametrization of the Helmholtz free energy adjusted over this extended temperature and pressure domain. The parametrization ignores the entropy and density jumps at phase boundaries but we show that it is sufficiently accurate to model interior properties of most planets and exoplanets. We produce an equation of state given in analytical form that is readily usable in planetary modeling codes and dynamical simulations {\bf (a fortran implementation can be found at http://www.ioffe.ru/astro/H2O/)}. The EOS produced is valid for the entire density range relevant to planetary modeling, {\bf for densities where quantum effects for the ions can be neglected, and for temperatures below 50,000K. We use this equation of state to calculate the mass-radius relationship of exoplanets up to 5,000M_Earth, explore temperature effects in ocean and wet Earth-like planets, and quantify the influence of the water EOS for the core on the gravitational moments of Jupiter.s. Part of this work was supported by the SNR grant PLANETLAB
12-BS04-0015 and the Programme National de Planetologie (PNP) of
CNRS-INSU co-funded by CNES. Funding and support from Paris Sciences et
Lettres (PSL) university through the project origins and conditions for the emergence
of life is also acknowledged. This work was performed using HPC resources
from GENCI- TGCC (Grant 2017- A0030406113
The orientational dichroism in (e, 2e) collisions: interplay between geometrical and dynamical effects
Abstract. The fully differential cross sections for the electron-impact ionization of atomic targets, prepared in a given circular state using laser-pumping, reveal a dependence on the inversion of the helicity of the exciting photon. This 'dichroism' effect was shown to be strongly dependent on the geometrical arrangements of the experiment both theoretically and experimentally. In addition, as shown theoretically in this paper, the dichroism may also vanish at certain 'non-geometrical' points that can be deduced analytically within the first Born approximation. More elaborate calculations using the distorted-wave Born approximation confirm this analysis. On the basis of this study, we further suggest a possible explanation for a structure observed recently in the state-resolved fully differential cross sections for a sodium target. A typical (e, 2e) experiment measures the cross section for the single ionization of a target in its ground state following the impact of an electron beam of well defined momentum p 0 . The experimental set-up simultaneously resolves the vector momenta p a and p b of the two receding electrons, the scattered and the electron ejected from the target, i.e. the energies and emission angles of these two electrons are determined in coincidence. Depending upon the kinematical conditions of the experiment, various types of information can be extracted from the (e, 2e) signal. For example, under favourable situations, the (e, 2e) technique has been exploited to study the target's electronic structure (McCarthy and Weigold 1976), final-state interactions One relatively recent facet of the (e, 2e) reaction is the dependence of the fully differential cross section on the initial orientation of the atomic target. This dependence, termed orientational dichroism, implies that the initial orientation of the bound atomic electron is transferred dynamically to the two outgoing continuum electrons. Experimentally, the initialstate orientation is achieved by optical pumping of the atomic targets with a laser of a given circular polarization. The information that such an experiment yields has been expressed in terms of an irreducible set of tensorial parameters whose number is determined by the symmetry of the initially prepared target state (1) 0 is given by (the L · S interaction is neglected) 0953-4075/99/153965+08$30.0
Plasma formation from ultracold Rydberg gases
Recent experiments have demonstrated the spontaneous evolution of a gas of
ultracold Rydberg atoms into an expanding ultracold plasma, as well as the
reverse process of plasma recombination into highly excited atomic states.
Treating the evolution of the plasma on the basis of kinetic equations, while
ionization/excitation and recombination are incorporated using rate equations,
we have investigated theoretically the Rydberg-to-plasma transition. Including
the influence of spatial correlations on the plasma dynamics in an approximate
way we find that ionic correlations change the results only quantitatively but
not qualitatively
Neutral Plasma Oscillations at Zero Temperature
We use cold plasma theory to calculate the response of an ultracold neutral
plasma to an applied rf field. The free oscillation of the system has a
continuous spectrum and an associated damped quasimode. We show that this
quasimode dominates the driven response. We use this model to simulate plasma
oscillations in an expanding ultracold neutral plasma, providing insights into
the assumptions used to interpret experimental data [Phys. Rev. Lett. 85, 318
(2000)].Comment: 4.3 pages, including 3 figure
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