1,362 research outputs found
Thermodynamics of Giant Planet Formation: Shocking Hot Surfaces on Circumplanetary Disks
The luminosity of young giant planets can inform about their formation and
accretion history. The directly imaged planets detected so far are consistent
with the "hot-start" scenario of high entropy and luminosity. If nebular gas
passes through a shock front before being accreted into a protoplanet, the
entropy can be substantially altered. To investigate this, we present high
resolution, 3D radiative hydrodynamic simulations of accreting giant planets.
The accreted gas is found to fall with supersonic speed in the gap from the
circumstellar disk's upper layers onto the surface of the circumplanetary disk
and polar region of the protoplanet. There it shocks, creating an extended hot
supercritical shock surface. This shock front is optically thick, therefore, it
can conceal the planet's intrinsic luminosity beneath. The gas in the vertical
influx has high entropy which when passing through the shock front decreases
significantly while the gas becomes part of the disk and protoplanet. This
shows that circumplanetary disks play a key role in regulating a planet's
thermodynamic state. Our simulations furthermore indicate that around the shock
surface extended regions of atomic - sometimes ionized - hydrogen develop.
Therefore circumplanetary disk shock surfaces could influence significantly the
observational appearance of forming gas-giants.Comment: 5 pages, 3 figures, 1 table, accepted for publication at MNRAS
Letter
Migration and giant planet formation
We extend the core-accretion model of giant gaseous planets by Pollack et al.
(\cite{P96}) to include migration, disc evolution and gap formation. Starting
with a core of a fraction of an Earth's mass located at 8 AU, we end our
simulation with the onset of runaway gas accretion when the planet is at 5.5 AU
1 Myr later. This timescale is about a factor ten shorter than the one found by
Pollack et al. (\cite{P96}) even though the disc was less massive initially and
viscously evolving. Other initial conditions can lead to even shorter
timescales. The reason for this speed-up is found to result from the fact that
a moving planet does not deplete its feeding zone to the extend of a static
planet. Thus, the uncomfortably long formation timescale associated with the
core-accretion scenario can be considerably reduced and brought in much better
agreement with the typical disc lifetimes inferred from observations of young
circumstellar discs.Comment: 9 pages, 2 figures, published in A&A Letter
Characterization of exoplanets from their formation III: The statistics of planetary luminosities
This paper continues a series in which we predict the main observable
characteristics of exoplanets based on their formation. In Paper I we described
our global planet formation and evolution model. In Paper II we studied the
planetary mass-radius relationship. Here we present an extensive study of the
statistics of planetary luminosities during both formation and evolution. Our
results can be compared with individual directly imaged (proto)planets as well
as statistical results from surveys. We calculated three synthetic planet
populations assuming different efficiencies of the accretional heating by gas
and planetesimals. We describe the temporal evolution of the planetary
mass-luminosity relation. We study the shock and internal luminosity during
formation. We predict a statistical version of the post-formation mass versus
entropy "tuning fork" diagram. We find high nominal post-formation luminosities
for hot and cold gas accretion. Individual formation histories can still lead
to a factor of a few spread in the post-formation luminosity at a given mass.
However, if the gas and planetesimal accretional heating is unknown, the
post-formation luminosity may exhibit a spread of as much as 2-3 orders of
magnitude at a fixed mass covering cold, warm, and hot states. As a key result
we predict a flat log-luminosity distribution for giant planets, and a steep
increase towards lower luminosities due to the higher occurrence rate of
low-mass planets. Future surveys may detect this upturn. During formation an
estimate of the planet mass may be possible for cold gas accretion if the gas
accretion rate can be estimated. Due to the "core-mass effect" planets that
underwent cold gas accretion can still have high post-formation entropies. Once
the number of directly imaged exoplanets with known ages and luminosities
increases, the observed distributions may be compared with our predictions.Comment: 44 pages, 26 figures (journal format). A&A in print. Language
correction only relative to V
Grain opacity and the bulk composition of extrasolar planets. II. An analytical model for the grain opacity in protoplanetary atmospheres
Context. We investigate the grain opacity k_gr in the atmosphere of
protoplanets. This is important for the planetary mass-radius relation since
k_gr affects the H/He envelope mass of low-mass planets and the critical core
mass of giant planets. Aims. The goal of this study is to derive an analytical
model for k_gr. Methods. Our model is based on the comparison of the timescales
of microphysical processes like grain settling in the Stokes and Epstein
regime, growth by Brownian motion coagulation and differential settling, grain
evaporation, and grain advection due to envelope contraction. With these
timescales we derive the grain size, abundance, and opacity. Results. We find
that the main growth process is differential settling. In this regime, k_gr has
a simple functional form and is given as 27 Q/8 H rho in the Epstein regime and
as 2 Q/H rho for Stokes drag. Grain dynamics lead to a typical radial structure
of k_gr with high ISM-like values in the top layers but a strong decrease in
the deeper parts where the grain-free molecular opacities take over.
Conclusions. In agreement with earlier results we find that k_gr is typically
much lower than in the ISM. The equations also show that a higher dust input in
the top layer does not strongly increase k_gr with two important implications.
First, for a formation of giant planet cores via pebbles, there could be the
issue that pebbles increase the grain input high in the atmosphere due to
ablation. This could potentially increase k_gr hindering giant planet
formation. Our study shows that this adverse effect should not occur. Second,
it means that a higher stellar [Fe/H] which presumably leads to a higher
surface density of planetesimals only favors giant planet formation without
being detrimental to it due to an increased k_gr. This corroborates the result
that core accretion explains the increase of the giant planet frequency with
[Fe/H].Comment: 33 pages, 19 figures. Accepted to A&A. Identical to V1 except for
updated reference
Theory of planet formation and comparison with observation: Formation of the planetary mass-radius relationship
The planetary mass-radius diagram is an observational result of central
importance to understand planet formation. We present an updated version of our
planet formation model based on the core accretion paradigm which allows to
calculate planetary radii and luminosities during the entire formation and
evolution of the planets. We first study with it the formation of Jupiter, and
compare with previous works. Then we conduct planetary population synthesis
calculations to obtain a synthetic mass-radius diagram which we compare with
the observed one. Except for bloated Hot Jupiters which can be explained only
with additional mechanisms related to their proximity to the star, we find a
good agreement of the general shape of the observed and the synthetic
mass-radius diagram. This shape can be understood with basic concepts of the
core accretion model.Comment: Proceedings Haute Provence Observatory Colloquium: Detection and
Dynamics of Transiting Exoplanets (23-27 August 2010). Edited by F. Bouchy,
R. F. Diaz & C. Moutou. Extended version: 17 pages, 8 figure
Grain opacity and the bulk composition of extrasolar planets. I. Results from scaling the ISM opacity
The opacity due to grains in the envelope of a protoplanet regulates the
accretion rate of gas during formation, thus the final bulk composition of
planets with primordial H/He is a function of it. Observationally, for
exoplanets with known mass and radius it is possible to estimate the bulk
composition via internal structure models. We first determine the reduction
factor of the ISM grain opacity f_opa that leads to gas accretion rates
consistent with grain evolution models. We then compare the bulk composition of
synthetic low-mass and giant planets at different f_opa with observations. For
f_opa=1 (full ISM opacity) the synthetic low-mass planets have too small radii,
i.e., too low envelope masses compared to observations. At f_opa=0.003, the
value calibrated with the grain evolution models, synthetic and actual planets
occupy similar mass-radius loci. The mean enrichment of giant planets relative
to the host star as a function of planet mass M can be approximated as
Z_p/Z_star = beta*(M/M_Jup)^alpha. We find alpha=-0.7 independent of f_opa in
synthetic populations in agreement with the observational result (-0.71+-0.10).
The absolute enrichment level decreases from beta=8.5 at f_opa=1 to 3.5 at
f_opa=0. At f_opa=0.003 one finds beta=7.2 which is similar to the
observational result (6.3+-1.0). We thus find observational hints that the
opacity in protoplanetary atmospheres is much smaller than in the ISM even if
the specific value of the grain opacity cannot be constrained here. The result
for the enrichment of giant planets helps to distinguish core accretion and
gravitational instability. In the simplest picture of core accretion where
first a critical core forms and afterwards only gas is added, alpha=-1. If a
core accretes all planetesimals inside the feeding zone, alpha=-2/3. The
observational result lies between these values, pointing to core accretion as
the formation mechanism.Comment: 21 pages, 15 figures. Accepted for A&
Planet Population Synthesis
With the increasing number of exoplanets discovered, statistical properties
of the population as a whole become unique constraints on planet formation
models provided a link between the description of the detailed processes
playing a role in this formation and the observed population can be
established. Planet population synthesis provides such a link. The approach
allows to study how different physical models of individual processes (e.g.,
proto-planetary disc structure and evolution, planetesimal formation, gas
accretion, migration, etc.) affect the overall properties of the population of
emerging planets. By necessity, planet population synthesis relies on
simplified descriptions of complex processes. These descriptions can be
obtained from more detailed specialised simulations of these processes. The
objective of this chapter is twofold: 1) provide an overview of the physics
entering in the two main approaches to planet population synthesis and 2)
present some of the results achieved as well as illustrate how it can be used
to extract constraints on the models and to help interpret observations.Comment: 23 pages, 8 figures, accepted for publication as a chapter in
Protostars and Planets VI, University of Arizona Press (2014), eds. H.
Beuther, R. Klessen, C. Dullemond, Th. Henning. Updated references relative
to v
Global Models of Planet Formation and Evolution
Despite the increase in observational data on exoplanets, the processes that
lead to the formation of planets are still not well understood. But thanks to
the high number of known exoplanets, it is now possible to look at them as a
population that puts statistical constraints on theoretical models. A method
that uses these constraints is planetary population synthesis. Its key element
is a global model of planet formation and evolution that directly predicts
observable planetary properties based on properties of the natal protoplanetary
disk. To do so, global models build on many specialized models that address one
specific physical process. We thoroughly review the physics of the sub-models
included in global formation models. The sub-models can be classified as models
describing the protoplanetary disk (gas and solids), the (proto)planet (solid
core, gaseous envelope, and atmosphere), and finally the interactions
(migration and N-body interaction). We compare the approaches in different
global models and identify physical processes that require improved
descriptions in future. We then address important results of population
synthesis like the planetary mass function or the mass-radius relation. In
these results, the global effects of physical mechanisms occurring during
planet formation and evolution become apparent, and specialized models
describing them can be put to the observational test. Due to their nature as
meta models, global models depend on the development of the field of planet
formation theory as a whole. Because there are important uncertainties in this
theory, it is likely that global models will in future undergo significant
modifications. Despite this, they can already now yield many testable
predictions. With future global models addressing the geophysical
characteristics, it should eventually become possible to make predictions about
the habitability of planets.Comment: 30 pages, 16 figures. Accepted for publication in the International
Journal of Astrobiology (Cambridge University Press
Impacts of planet migration models on planetary populations. Effects of saturation, cooling and stellar irradiation
Context: Several recent studies have found that planet migration in adiabatic
discs differs significantly from migration in isothermal discs. Depending on
the thermodynamic conditions, i.e., the effectiveness of radiative cooling, and
the radial surface density profile, planets migrate inward or outward. Clearly,
this will influence the semimajor axis - mass distribution of planets as
predicted by population synthesis simulations. Aims: Our goal is to study the
global effects of radiative cooling, viscous torque desaturation and gap
opening as well as stellar irradiation on the tidal migration of a synthetic
planet population. Methods: We combine results from several analytical studies
and 3D hydrodynamic simulations in a new semi-analytical migration model for
the application in our planet population synthesis calculations. Results: We
find a good agreement of our model with torques obtained in a 3D radiative
hydrodynamic simulations. We find three convergence zones in a typical disc,
towards which planets migrate from the in- and outside, affecting strongly the
migration behavior of low-mass planets. Interestingly, this leads to slow type
II like migration behavior for low-mass planets captured in those zones even
without an ad hoc migration rate reduction factor or a yet to be defined
halting mechanism. This means that the new prescription of migration including
non-isothermal effects makes the preciously widely used artificial migration
rate reduction factor obsolete. Conclusions: Outward migration in parts of a
disc makes some planets survive long enough to become massive. The convergence
zones lead to a potentially observable accumulations of low-mass planets at
certain semimajor axes. Our results indicate that further studies of the mass
where the corotation torque saturates will be needed since its value has a
major impact on the properties of planet populations.Comment: 18 pages, 15 figures. Accepted for A&
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