345 research outputs found
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
The Evolution of L and T Dwarfs in Color-Magnitude Diagrams
We present new evolution sequences for very low mass stars, brown dwarfs and
giant planets and use them to explore a variety of influences on the evolution
of these objects. We compare our results with previous work and discuss the
causes of the differences and argue for the importance of the surface boundary
condition provided by atmosphere models including clouds.
The L- to T-type ultracool dwarf transition can be accommodated within the
Ackerman & Marley (2001) cloud model by varying the cloud sedimentation
parameter. We develop a simple model for the evolution across the L/T
transition. By combining the evolution calculation and our atmosphere models,
we generate colors and magnitudes of synthetic populations of ultracool dwarfs
in the field and in galactic clusters. We focus on near infrared color-
magnitude diagrams (CMDs) and on the nature of the ``second parameter'' that is
responsible for the scatter of colors along the Teff sequence. Variations in
metallicity and cloud parameters, unresolved binaries and possibly a relatively
young population all play a role in defining the spread of brown dwarfs along
the cooling sequence. We find that the transition from cloudy L dwarfs to
cloudless T dwarfs slows down the evolution and causes a pile up of substellar
objects in the transition region, in contradiction with previous studies. We
apply the same model to the Pleiades brown dwarf sequence. Taken at face value,
the Pleiades data suggest that the L/T transition occurs at lower Teff for
lower gravity objects. The simulated populations of brown dwarfs also reveal
that the phase of deuterium burning produces a distinctive feature in CMDs that
should be detectable in ~50-100 Myr old clusters.Comment: Accepted for publication in the ApJ. 52 pages including 20 figure
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
Dense plasmas in astrophysics: from giant planets to neutron stars
We briefly examine the properties of dense plasmas characteristic of the
interior of giant planets and the atmospheres of neutron stars. Special
attention is devoted to the equation of state of hydrogen and helium at high
density and to the effect of magnetic fields on the properties of dense matter.Comment: Invited Review, Strongly Coupled Coulomb Systems, Moscow June 2005;
to appear in Journal of Physics
Near-Infrared Spectroscopy of the Y0 WISEP J173835.52+273258.9 and the Y1 WISE J035000.32-565830.2: the Importance of Non-Equilibrium Chemistry
We present new near-infrared spectra, obtained at Gemini Observatory, for two
Y dwarfs: WISE J035000.32-565830.2 (W0350) and WISEP J173835.52+273258.9
(W1738). A FLAMINGOS-2 R=540 spectrum was obtained for W0350, covering 1.0 <
lambda um < 1.7, and a cross-dispersed GNIRS R=2800 spectrum was obtained for
W1738, covering 0.993-1.087 um, 1.191-1.305 um, 1.589-1.631 um, and 1.985-2.175
um, in four orders. We also present revised YJH photometry for W1738, using new
NIRI Y and J imaging, and a re-analysis of the previously published NIRI H band
images. We compare these data, together with previously published data for
late-T and Y dwarfs, to cloud-free models of solar metallicity, calculated both
in chemical equilibrium and with disequilibrium driven by vertical transport.
We find that for the Y dwarfs the non-equilibrium models reproduce the
near-infrared data better than the equilibrium models. The remaining
discrepancies suggest that fine-tuning the CH_4/CO and NH_3/N_2 balance is
needed. Improved trigonometric parallaxes would improve the analysis. Despite
the uncertainties and discrepancies, the models reproduce the observed
near-infrared spectra well. We find that for the Y0, W1738, T_eff = 425 +/- 25
K and log g = 4.0 +/- 0.25, and for the Y1, W0350, T_eff = 350 +/- 25 K and log
g = 4.0 +/- 0.25. W1738 may be metal-rich. Based on evolutionary models, these
temperatures and gravities correspond to a mass range for both Y dwarfs of 3-9
Jupiter masses, with W0350 being a cooler, slightly older, version of W1738;
the age of W0350 is 0.3-3 Gyr, and the age of W1738 is 0.15-1 Gyr.Comment: Accepted on March 30 2016 for publication in Ap
Structure and evolution of super-Earth to super-Jupiter exoplanets: I. heavy element enrichment in the interior
We examine the uncertainties in current planetary models and we quantify
their impact on the planet cooling histories and mass-radius relationships.
These uncertainties include (i) the differences between the various equations
of state used to characterize the heavy material thermodynamical properties,
(ii) the distribution of heavy elements within planetary interiors, (iii) their
chemical composition and (iv) their thermal contribution to the planet
evolution. Our models, which include a gaseous H/He envelope, are compared with
models of solid, gasless Earth-like planets in order to examine the impact of a
gaseous envelope on the cooling and the resulting radius. We find that for a
fraction of heavy material larger than 20% of the planet mass, the distribution
of the heavy elements in the planet's interior affects substantially the
evolution and thus the radius at a given age. For planets with large core mass
fractions (\simgr 50%), such as the Neptune-mass transiting planet GJ436b,
the contribution of the gravitational and thermal energy from the core to the
planet cooling history is not negligible, yielding a 10% effect on the
radius after 1 Gyr. We show that the present mass and radius determinations of
the massive planet Hat-P-2b require at least 200 \mearth of heavy material in
the interior, at the edge of what is currently predicted by the core-accretion
model for planet formation. We show that if planets as massive as 25
\mjup can form, as predicted by improved core-accretion models, deuterium is
able to burn in the H/He layers above the core, even for core masses as large
as 100 \mearth. We provide extensive grids of planetary evolution
models from 10 \mearth to 10 M, with various fractions of heavy
elements.Comment: 20 pages, 12 figures. Accepted for publication in Astronomy and
Astrophysic
Oligarchic planetesimal accretion and giant planet formation
Aims. In the context of the core instability model, we present calculations
of in situ giant planet formation. The oligarchic growth regime of solid
protoplanets is the model adopted for the growth of the core. Methods. The full
differential equations of giant planet formation were numerically solved with
an adaptation of a Henyey-type code. The planetesimals accretion rate was
coupled in a self-consistent way to the envelope's evolution. Results. We
performed several simulations for the formation of a Jupiter-like object by
assuming various surface densities for the protoplanetary disc and two
different sizes for the accreted planetesimals. We find that the atmospheric
gas drag gives rise to a major enhancement on the effective capture radius of
the protoplanet, thus leading to an average timescale reduction of 30% -- 55%
and ultimately to an increase by a factor of 2 of the final mass of solids
accreted as compared to the situation in which drag effects are neglected. With
regard to the size of accreted planetesimals, we find that for a swarm of
planetesimals having a radius of 10 km, the formation time is a factor 2 to 3
shorter than that of planetesimals of 100 km, the factor depending on the
surface density of the nebula. Moreover, planetesimal size does not seem to
have a significant impact on the final mass of the core.Comment: 12 pages, 10 figures, accepted for publication in A&
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