289 research outputs found
Interior Models of Saturn: Including the Uncertainties in Shape and Rotation
The accurate determination of Saturn's gravitational coefficients by Cassini
could provide tighter constrains on Saturn's internal structure. Also,
occultation measurements provide important information on the planetary shape
which is often not considered in structure models. In this paper we explore how
wind velocities and internal rotation affect the planetary shape and the
constraints on Saturn's interior. We show that within the geodetic approach
(Lindal et al., 1985, ApJ, 90, 1136) the derived physical shape is insensitive
to the assumed deep rotation. Saturn's re-derived equatorial and polar radii at
100 mbar are found to be 54,445 10 km and 60,36510 km, respectively.
To determine Saturn's interior we use {\it 1 D} three-layer hydrostatic
structure models, and present two approaches to include the constraints on the
shape. These approaches, however, result in only small differences in Saturn's
derived composition. The uncertainty in Saturn's rotation period is more
significant: with Voyager's 10h39mns period, the derived mass of heavy elements
in the envelope is 0-7 M. With a rotation period of 10h32mns, this
value becomes , below the minimum mass inferred from
spectroscopic measurements. Saturn's core mass is found to depend strongly on
the pressure at which helium phase separation occurs, and is estimated to be
5-20 M. Lower core masses are possible if the separation occurs
deeper than 4 Mbars. We suggest that the analysis of Cassini's radio
occultation measurements is crucial to test shape models and could lead to
constraints on Saturn's rotation profile and departures from hydrostatic
equilibrium.Comment: Accepted for publication in Ap
Giant Planets
We review the interior structure and evolution of Jupiter, Saturn, Uranus and
Neptune, and giant exoplanets with particular emphasis on constraining their
global composition. Compared to the first edition of this review, we provide a
new discussion of the atmospheric compositions of the solar system giant
planets, we discuss the discovery of oscillations of Jupiter and Saturn, the
significant improvements in our understanding of the behavior of material at
high pressures and the consequences for interior and evolution models. We place
the giant planets in our Solar System in context with the trends seen for
exoplanets.Comment: This chapter is to be published in the second edition of the Treatise
on Geophysics (Eds. T. Spohn, G. Schubert). 42 pages, 16 figures. Accepted 25
February 201
A non-grey analytical model for irradiated atmospheres. I: Derivation
Context. Semi-grey atmospheric models (with one opacity for the visible and
one opacity for the infrared) are useful to understand the global structure of
irradiated atmospheres, their dynamics and the interior structure and evolution
of planets, brown dwarfs and stars. But when compared to direct numerical
radiative transfer calculations for irradiated exoplanets, these models
systematically overestimate the temperatures at low optical depth,
independently of the opacity parameters. We wish to understand why semi-grey
models fail at low optical depths, and provide a more accurate approximation to
the atmospheric structure by accounting for the variable opacity in the
infrared. Our analytical irradiated non-grey model is found to provide a range
of temperatures that is consistent with that obtained by numerical
calculations. We find that even for slightly non-grey thermal opacities the
temperature structure differs significantly from previous semi-grey models. For
small values of beta (expected when lines are dominant), we find that the
non-grey effects are confined to low-optical depths. However, for beta larger
than 0.5 (appropriate in the presence of bands with a wavelength-dependence
smaller or comparable with the width of the Planck function), we find that the
temperature structure is affected even down to infrared optical depths unity
and deeper as a result of the so-called blanketing effect. The expressions that
we derive may be used to provide a proper functional form for algorithms that
invert the atmospheric properties from spectral information. Because a full
atmospheric structure can be calculated directly, these expressions should be
useful for simulations of the dynamics of these atmospheres and of the thermal
evolution of the planets. Finally, they should be used to test full radiative
transfer models and improve their convergence.Comment: Accepted by A&A, model available at
http://www.oca.eu/parmentier/nongre
Formation of dust-rich planetesimals from sublimated pebbles inside of the snow line
Content: For up to a few millions of years, pebbles must provide a
quasi-steady inflow of solids from the outer parts of protoplanetary disks to
their inner regions. Aims: We wish to understand how a significant fraction of
the pebbles grows into planetesimals instead of being lost to the host star.
Methods:We examined analytically how the inward flow of pebbles is affected by
the snow line and under which conditions dust-rich (rocky) planetesimals form.
When calculating the inward drift of solids that is due to gas drag, we
included the back-reaction of the gas to the motion of the solids. Results: We
show that in low-viscosity protoplanetary disks (with a monotonous surface
density similar to that of the minimum-mass solar nebula), the flow of pebbles
does not usually reach the required surface density to form planetesimals by
streaming instability. We show, however, that if the pebble-to-gas-mass flux
exceeds a critical value, no steady solution can be found for the solid-to-gas
ratio. This is particularly important for low-viscosity disks (alpha < 10^(-3))
where we show that inside of the snow line, silicate-dust grains ejected from
sublimating pebbles can accumulate, eventually leading to the formation of
dust-rich planetesimals directly by gravitational instability. Conclusions:
This formation of dust-rich planetesimals may occur for extended periods of
time, while the snow line sweeps from several au to inside of 1 au. The
rock-to-ice ratio may thus be globally significantly higher in planetesimals
and planets than in the central star.Comment: 5 pages, 3 figures; accepted for publication in Astronomy and
Astrophysic
Suppression of type I migration by disk winds
Planets less massive than Saturn tend to rapidly migrate inward in
protoplanetary disks. This is the so-called type I migration. Simulations
attempting to reproduce the observed properties of exoplanets show that type I
migration needs to be significantly reduced over a wide region of the disk for
a long time. However, the mechanism capable of suppressing type I migration
over a wide region has remained elusive. The recently found turbulence-driven
disk winds offer new possibilities. We investigate the effects of disk winds on
the disk profile and type I migration for a range of parameters that describe
the strength of disk winds. We also examine the in situ formation of close-in
super-Earths in disks that evolve through disk winds. The disk profile, which
is regulated by viscous diffusion and disk winds, was derived by solving the
diffusion equation. We carried out a number of simulations and plot here
migration maps that indicate the type I migration rate. We also performed
N-body simulations of the formation of close-in super-Earths from a population
of planetesimals and planetary embryos. We define a key parameter, Kw, which
determines the ratio of strengths between the viscous diffusion and disk winds.
For a wide range of Kw, the type I migration rate is presented in migration
maps. These maps show that type I migration is suppressed over the whole
close-in region when the effects of disk winds are relatively strong (Kw <
100). From the results of N-body simulations, we see that type I migration is
significantly slowed down assuming Kw = 40. We also show that the results of
N-body simulations match statistical orbital distributions of close-in
super-Earths.Comment: 5 pages, 4 figures, accepted for publication in A&A Letter
The composition of transiting giant extrasolar planets
In principle, the combined measurements of the mass and radius a giant
exoplanet allow one to determine the relative fraction of hydrogen and helium
and of heavy elements in the planet. However, uncertainties on the underlying
physics imply that some known transiting planets appear anomalously large, and
this generally prevent any firm conclusion when a planet is considered on an
individual basis. On the basis of a sample of 9 transiting planets known at the
time, Guillot et al. A&A 453, L21 (1996), concluded that all planets could be
explained with the same set of hypotheses, either by large but plausible
modifications of the equations of state, opacities, or by the addition of an
energy source, probably related to the dissipation of kinetic energy by tides.
On this basis, they concluded that the amount of heavy elements in close-in
giant planets is correlated with the metallicity of the parent star.
Furthermore they showed that planets around metal-rich stars can possess large
amounts of heavy elements, up to 100 Earth masses. These results are confirmed
by studying the present sample of 18 transiting planets with masses between
that of Saturn and twice the mass of Jupiter.Comment: 13 pages, 6 figure
On the filtering and processing of dust by planetesimals 1. Derivation of collision probabilities for non-drifting planetesimals
Context. Circumstellar disks are known to contain a significant mass in dust
ranging from micron to centimeter size. Meteorites are evidence that individual
grains of those sizes were collected and assembled into planetesimals in the
young solar system. Aims. We assess the efficiency of dust collection of a
swarm of non-drifting planetesimals {\rev with radii ranging from 1 to
\,km and beyond. Methods. We calculate the collision probability of dust
drifting in the disk due to gas drag by planetesimal accounting for several
regimes depending on the size of the planetesimal, dust, and orbital distance:
the geometric, Safronov, settling, and three-body regimes. We also include a
hydrodynamical regime to account for the fact that small grains tend to be
carried by the gas flow around planetesimals. Results. We provide expressions
for the collision probability of dust by planetesimals and for the filtering
efficiency by a swarm of planetesimals. For standard turbulence conditions
(i.e., a turbulence parameter ), filtering is found to be
inefficient, meaning that when crossing a minimum-mass solar nebula (MMSN) belt
of planetesimals extending between 0.1 AU and 35 AU most dust particles are
eventually accreted by the central star rather than colliding with
planetesimals. However, if the disk is weakly turbulent ()
filtering becomes efficient in two regimes: (i) when planetesimals are all
smaller than about 10 km in size, in which case collisions mostly take place in
the geometric regime; and (ii) when planetary embryos larger than about 1000 km
in size dominate the distribution, have a scale height smaller than one tenth
of the gas scale height, and dust is of millimeter size or larger in which case
most collisions take place in the settling regime. These two regimes have very
different properties: we find that the local filtering efficiency
scales with (where is the orbital distance) in
the geometric regime, but with to in the settling regime.
This implies that the filtering of dust by small planetesimals should occur
close to the central star and with a short spread in orbital distances. On the
other hand, the filtering by embryos in the settling regime is expected to be
more gradual and determined by the extent of the disk of embryos. Dust
particles much smaller than millimeter size tend only to be captured by the
smallest planetesimals because they otherwise move on gas streamlines and their
collisions take place in the hydrodynamical regime. Conclusions. Our results
hint at an inside-out formation of planetesimals in the infant solar system
because small planetesimals in the geometrical limit can filter dust much more
efficiently close to the central star. However, even a fully-formed belt of
planetesimals such as the MMSN only marginally captures inward-drifting dust
and this seems to imply that dust in the protosolar disk has been filtered by
planetesimals even smaller than 1 km (not included in this study) or that it
has been assembled into planetesimals by other mechanisms (e.g., orderly
growth, capture into vortexes). Further refinement of our work concerns, among
other things: a quantitative description of the transition region between the
hydro and settling regimes; an assessment of the role of disk turbulence for
collisions, in particular in the hydro regime; and the coupling of our model to
a planetesimal formation model.Comment: Accepted for publication in A\&A. 31 pages, 29 figures. (Version
corrected by the A\&A Language Editor
Revisiting the pre-main-sequence evolution of stars I. Importance of accretion efficiency and deuterium abundance
Recent theoretical work has shown that the pre-main-sequence (PMS) evolution
of stars is much more complex than previously envisioned. Instead of the
traditional steady, one-dimensional solution, accretion may be episodic and not
necessarily symmetrical, thereby affecting the energy deposited inside the star
and its interior structure. Given this new framework, we want to understand
what controls the evolution of accreting stars. We use the MESA stellar
evolution code with various sets of conditions. In particular, we account for
the (unknown) efficiency of accretion in burying gravitational energy into the
protostar through a parameter, , and we vary the amount of deuterium
present. We confirm the findings of previous works that the evolution changes
significantly with the amount of energy that is lost during accretion. We find
that deuterium burning also regulates the PMS evolution. In the low-entropy
accretion scenario, the evolutionary tracks in the H-R diagram are
significantly different from the classical tracks and are sensitive to the
deuterium content. A comparison of theoretical evolutionary tracks and
observations allows us to exclude some cold accretion models () with
low deuterium abundances. We confirm that the luminosity spread seen in
clusters can be explained by models with a somewhat inefficient injection of
accretion heat. The resulting evolutionary tracks then become sensitive to the
accretion heat efficiency, initial core entropy, and deuterium content. In this
context, we predict that clusters with a higher D/H ratio should have less
scatter in luminosity than clusters with a smaller D/H. Future work on this
issue should include radiation-hydrodynamic simulations to determine the
efficiency of accretion heating and further observations to investigate the
deuterium content in star-forming regions. (abbrev.)Comment: Published in A&A. 16 pages, 14 figure
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