703 research outputs found
Elemental abundances and minimum mass of heavy elements in the envelope of HD 189733b
Oxygen (O) and carbon (C) have been inferred recently to be subsolar in
abundance from spectra of the atmosphere of the transiting hot Jupiter HD
189733b. Yet, the mass and radius of the planet coupled with structure models
indicate a strongly supersolar abundance of heavy elements in the interior of
this object. Here we explore the discrepancy between the large amount of heavy
elements suspected in the planet's interior and the paucity of volatiles
measured in its atmosphere. We describe the formation sequence of the icy
planetesimals formed beyond the snow line of the protoplanetary disk and
calculate the composition of ices ultimately accreted in the envelope of HD
189733b on its migration pathway. This allows us to reproduce the observed
volatile abundances by adjusting the mass of ices vaporized in the envelope.
The predicted elemental mixing ratios should be 0.15--0.3 times solar in the
envelope of HD 189733b if they are fitted to the recent O and C determinations.
However, our fit to the minimum mass of heavy elements predicted by internal
structure models gives elemental abundances that are 1.2--2.4 times oversolar
in the envelope of HD189733b. We propose that the most likely cause of this
discrepancy is irradiation from the central star leading to development of a
radiative zone in the planet's outer envelope which would induce gravitational
settling of elements. Hence, all strongly irradiated extrasolar planets should
present subsolar abundances of volatiles. We finally predict that the
abundances of nitrogen (N), sulfur (S) and phosphorus (P) are of , and relative to
H, respectively in the atmosphere of HD 189733b.Comment: Accepted for publication in Astronomy & Astrophysic
Gaseous Planets, Protostars And Young Brown Dwarfs : Birth And Fate
We review recent theoretical progress aimed at understanding the formation
and the early stages of evolution of giant planets, low-mass stars and brown
dwarfs. Calculations coupling giant planet formation, within a modern version
of the core accretion model, and subsequent evolution yield consistent
determinations of the planet structure and evolution. Because of the
uncertainties in the initial conditions, however, it is not possible to say
whether young planets are faint or bright compared with low-mass young brown
dwarfs. We review the effects of irradiation and evaporation on the evolution
of short period planets and argue that substantial mass loss may have occurred
for these objects. Concerning star formation, geometrical effects in protostar
core collapse are examined by comparing 1D and 3D calculations. Spherical
collapse is shown to overestimate the core inner density and temperature and
thus to yield incorrect initial conditions for PMS or young brown dwarf
evolution. Accretion is also shown to occur over a very limited fraction of the
protostar surface. Accretion affects the evolution of young brown dwarfs and
yields more compact structures for a given mass and age, thus fainter
luminosities. This can lead to severe misinterpretations of the mass and/or age
of young accreting objects from their location in the HR diagram. We argue that
newborn stars and brown dwarfs should appear rapidly over an extended area in
the HR diagram, depending on their accretion history, rather than on a well
defined birth line. Finally, we suggest that the distinction between planets
and brown dwarfs be based on an observational diagnostic, reflecting the
different formation mechanisms between these two distinct populations, rather
than on an arbitrary, confusing definition.Comment: Invited Review, Protostars and Planets V (Hawai, October 2005
Formation and structure of the three Neptune-mass planets system around HD69830
Since the discovery of the first giant planet outside the solar system in
1995 (Mayor & Queloz 1995), more than 180 extrasolar planets have been
discovered. With improving detection capabilities, a new class of planets with
masses 5-20 times larger than the Earth, at close distance from their parent
star is rapidly emerging. Recently, the first system of three Neptune-mass
planets has been discovered around the solar type star HD69830 (Lovis et al.
2006). Here, we present and discuss a possible formation scenario for this
planetary system based on a consistent coupling between the extended core
accretion model and evolutionary models (Alibert et al. 2005a, Baraffe et al.
2004,2006). We show that the innermost planet formed from an embryo having
started inside the iceline is composed essentially of a rocky core surrounded
by a tiny gaseous envelope. The two outermost planets started their formation
beyond the iceline and, as a consequence, accrete a substantial amount of water
ice during their formation. We calculate the present day thermodynamical
conditions inside these two latter planets and show that they are made of a
rocky core surrounded by a shell of fluid water and a gaseous envelope.Comment: Accepted in AA Letter
Chemical composition of Earth-like planets
Models of planet formation are mainly focused on the accretion and dynamical
processes of the planets, neglecting their chemical composition. In this work,
we calculate the condensation sequence of the different chemical elements for a
low-mass protoplanetary disk around a solar-type star. We incorporate this
sequence of chemical elements (refractory and volatile elements) in our
semi-analytical model of planet formation which calculates the formation of a
planetary system during its gaseous phase. The results of the semi-analytical
model (final distributions of embryos and planetesimals) are used as initial
conditions to develope N-body simulations that compute the post-oligarchic
formation of terrestrial-type planets. The results of our simulations show that
the chemical composition of the planets that remain in the habitable zone has
similar characteristics to the chemical composition of the Earth. However,
exist differences that can be associated to the dynamical environment in which
they were formed.Comment: 3 pages, 4 figures - Accepted for publication in the Bolet\'in de la
Asociaci\'on Argentina de Astronom\'ia, vol.5
Theoretical fits of the \delta Cephei light, radius and radial velocity curves
We present a theoretical investigation of the light, radius and radial
velocity variations of the prototype Cephei. We find that the best fit
model accounts for luminosity and velocity amplitudes with an accuracy better
than , and for the radius amplitude with an accuracy of .
The chemical composition of this model suggests a decrease in both helium (0.26
vs 0.28) and metal (0.01 vs 0.02) content in the solar neighborhood. Moreover,
distance determinations based on the fit of light curves agree at the
level with the trigonometric parallax measured by the Hubble Space
Telescope (HST). On the other hand, distance determinations based on angular
diameter variations, that are independent of interstellar extinction and of the
-factor value, indicate an increase of the order of 5% in the HST parallax.Comment: accepted for publication on ApJ Letter
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
Source blending effects on microlensing time-histograms and optical depth determination
Source blending in microlensing experiments is known to modify the Einstein
time of the observed events. In this paper, we have conducted Monte-Carlo
calculations, using the analytical relationships derived by Han (1999) to
quantify the effect of blending on the observed event time distribution and
optical depth. We show that short-time events are affected significantly by
source blending and that, for moderately blended sources, the optical depth
is globally overestimated, because of an underestimation of the
exposure. For high blending situations, on the opposite, blending leads to an
{\it under}estimation of the optical depth. Our results are in agreement with
the most recent optical depth determinations toward the Galactic Center of the
MACHO collaboration (Popowski et al. 2004) and the OGLE-II collaboration (Sumi
et al. 2005) that use clump giants (less affected by the blending effect) as
sources. The blending-corrected, lower optical depth toward the Galactic Bulge
is now in good agreement with the value inferred from galactic models,
reconciling theoretical and observational determinations.Comment: Accepted in Astronomy Astrophysics. Note that these calculations were
conducted in 2001, prior to the recent DIA analyses mentioned in the
references (see Alibert, Y. SF2A-conference, 2001
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