19 research outputs found
Planet Formation in Radiatively Inefficient Protoplanetary Discs.
PhDI examine the effects on planetary system formation of radiatively
inefficient disc models where positive corotation torques may counter
the rapid inward migration of low-mass planets driven by Lindblad
torques. I use N-body simulations coupled with algorithms to model
the evolution of the gas disc, type I migration, gap formation and type
II migration, planetary atmospheres that enhance the probability of
planetesimal accretion by protoplanets, gas accretion on to forming
planetary cores and gas disc dispersal.
The inclusion of entropy and vorticity related corotation torques can
lead to a net positive torque thus giving rise to outward migration of
planets. This can allow larger planets to survive for a longer period of
time, allowing some planets to accrete enough gas within the lifetime
of the disc to undergo runaway gas accretion thus forming gas giant
planets.
I review the current status of extrasolar planet observations and the
methods with which these observations are made, and provide a contextual
review of the theory of planet formation.
Using these models, I have successfully formed a number of gas giant
planets with semi-major axes ranging from 0.1 AU up to 75 AU and
masses from 100 Earth masses through to 700 Earth masses, as well
as a large number of terrestrial sized planets. In later simulations, a
large number of super-Earth, Neptune-mass and gas planets that are
too small to be considered giants were formed also
Orbital migration of interacting low-mass planets in evolutionary radiative turbulent models
The torques exerted by a locally isothermal disk on an embedded planet lead
to rapid inward migration. Recent work has shown that modeling the
thermodynamics without the assumption of local isothermality reveals regions
where the net torque on an embedded planet is positive, leading to outward
migration of the planet. When a region with negative torque lies directly
exterior to this, planets in the inner region migrate outwards and planets in
the outer region migrate inwards, converging where the torque is zero. We
incorporate the torques from an evolving non-isothermal disk into an N-body
simulation to examine the behavior of planets or planetary embryos interacting
in the convergence zone. We find that mutual interactions do not eject objects
from the convergence zone. Small numbers of objects in a laminar disk settle
into near resonant orbits that remain stable over the 10 Myr periods that we
examine. However, either or both increasing the number of planets or including
a correlated, stochastic force to represent turbulence drives orbit crossings
and mergers in the convergence zone. These processes can build gas giant cores
with masses of order ten Earth masses from sub-Earth mass embryos in 2-3 Myr.Comment: 15 pages, 11 figures. Accepted for publication in Ap
Global models of planetary system formation in radiatively-inefficient protoplanetary discs
(Abridged) We present the results of N-body simulations of planetary systems
formation in radiatively-inefficient disc models, where positive corotation
torques may counter the rapid inward migration of low mass planets driven by
Lindblad torques. The aim of this work is to examine the nature of planetary
systems that arise from oligarchic growth in such discs. We adapt the
commonly-used Mercury-6 symplectic integrator by including simple prescriptions
for planetary migration (types I and II), planetary atmospheres that enhance
the probability of planetesimal accretion by protoplanets, gas accretion onto
forming planetary cores, and gas disc dispersal. We perform a suite of
simulations for a variety of disc models with power-law surface density and
tempera- ture profiles, with a focus on models in which unsaturated corotation
torques can drive outward migration of protoplanets. In some models we account
for the quenching of corotation torques that arises when planetary orbits
become eccentric. Approximately half of our simulations lead to the successful
formation of gas giant planets with a broad range of masses and semimajor axes.
We conclude that convergent migration induced by corotation torques operating
during planet formation can enhance the growth rate of planetary cores, but
these often migrate into the central star because corotation torques saturate.
Outward migration of planetary cores of modest mass can lead to the formation
of gas giant planets at large distances from the central star, similar to those
observed recently through direct imaging surveys. The excitation of planetary
eccentricities through planet-planet scat- tering during oligarchic growth may
quench the effects of corotation torques, however, such that inward migration
is driven by Lindblad torques.Comment: To be published in MNRA
On the corotation torque for low-mass eccentric planets
SMF acknowledges the support of an STFC PhD studentship. The simulations presented in this paper were performed on the QMUL HPC facility purchased under the SRIF initiatives
Recent developments in planet migration theory
Planetary migration is the process by which a forming planet undergoes a
drift of its semi-major axis caused by the tidal interaction with its parent
protoplanetary disc. One of the key quantities to assess the migration of
embedded planets is the tidal torque between the disc and planet, which has two
components: the Lindblad torque and the corotation torque. We review the latest
results on both torque components for planets on circular orbits, with a
special emphasis on the various processes that give rise to additional, large
components of the corotation torque, and those contributing to the saturation
of this torque. These additional components of the corotation torque could help
address the shortcomings that have recently been exposed by models of planet
population syntheses. We also review recent results concerning the migration of
giant planets that carve gaps in the disc (type II migration) and the migration
of sub-giant planets that open partial gaps in massive discs (type III
migration).Comment: 52 pages, 18 figures. Review article to be published in "Tidal
effects in Astronomy and Astrophysics", Lecture Notes in Physic
Planetary population synthesis
In stellar astrophysics, the technique of population synthesis has been
successfully used for several decades. For planets, it is in contrast still a
young method which only became important in recent years because of the rapid
increase of the number of known extrasolar planets, and the associated growth
of statistical observational constraints. With planetary population synthesis,
the theory of planet formation and evolution can be put to the test against
these constraints. In this review of planetary population synthesis, we first
briefly list key observational constraints. Then, the work flow in the method
and its two main components are presented, namely global end-to-end models that
predict planetary system properties directly from protoplanetary disk
properties and probability distributions for these initial conditions. An
overview of various population synthesis models in the literature is given. The
sub-models for the physical processes considered in global models are
described: the evolution of the protoplanetary disk, the planets' accretion of
solids and gas, orbital migration, and N-body interactions among concurrently
growing protoplanets. Next, typical population synthesis results are
illustrated in the form of new syntheses obtained with the latest generation of
the Bern model. Planetary formation tracks, the distribution of planets in the
mass-distance and radius-distance plane, the planetary mass function, and the
distributions of planetary radii, semimajor axes, and luminosities are shown,
linked to underlying physical processes, and compared with their observational
counterparts. We finish by highlighting the most important predictions made by
population synthesis models and discuss the lessons learned from these
predictions - both those later observationally confirmed and those rejected.Comment: 47 pages, 12 figures. Invited review accepted for publication in the
'Handbook of Exoplanets', planet formation section, section editor: Ralph
Pudritz, Springer reference works, Juan Antonio Belmonte and Hans Deeg, Ed
On the formation of planetary systems via oligarchic growth in thermally evolving viscous discs
GALC acknowledges the support of an STFC PhD studentship. The simulations presented in this paper were performed on the QMUL HPC facility purchased under the SRIF initiatives
The role of planetary formation and evolution in shaping the composition of exoplanetary atmospheres
Over the last twenty years, the search for extrasolar planets revealed us the
rich diversity of the outcomes of the formation and evolution of planetary
systems. In order to fully understand how these extrasolar planets came to be,
however, the orbital and physical data we possess are not enough, and they need
to be complemented with information on the composition of the exoplanets.
Ground-based and space-based observations provided the first data on the
atmospheric composition of a few extrasolar planets, but a larger and more
detailed sample is required before we can fully take advantage of it. The
primary goal of the Exoplanet Characterization Observatory (EChO) is to fill
this gap, expanding the limited data we possess by performing a systematic
survey of hundreds of extrasolar planets. The full exploitation of the data
that EChO and other space-based and ground-based facilities will provide in the
near future, however, requires the knowledge of what are the sources and sinks
of the chemical species and molecules that will be observed. Luckily, the study
of the past history of the Solar System provides several indications on the
effects of processes like migration, late accretion and secular impacts, and on
the time they occur in the life of planetary systems. In this work we will
review what is already known about the factors influencing the composition of
planetary atmospheres, focusing on the case of gaseous giant planets, and what
instead still need to be investigated.Comment: 26 pages, 9 figures, 1 table. Accepted for publication on
Experimental Astronomy, special issue on the M3 EChO mission candidat
Protoplanetary migration in non-isothermal discs with turbulence driven by stochastic forcing
International audienceLow-mass objects embedded in isothermal protoplanetary discs are known to suffer rapid inward Type I migration. In non-isothermal discs, recent work has shown that a decreasing radial profile of the disc entropy can lead to a strong positive corotation torque which can significantly slow down or reverse Type I migration in laminar viscous disc models. It is not clear however how this picture changes in turbulent disc models. The aim of this study is to examine the impact of turbulence on the torque experienced by a low-mass planet embedded in a non-isothermal protoplanetary disc. We particularly focus on the role of turbulence on the corotation torque whose amplitude depends on the efficiency of diffusion processes in the planet's horseshoe region. We performed 2D numerical simulations using a grid-based hydrodynamical code in which turbulence is modelled as stochastic forcing. In order to provide estimations for the viscous and thermal diffusion coefficients as a function of the amplitude of turbulence, we first set up non-isothermal disc models for different values of the amplitude of the turbulent forcing. We then include a low-mass planet and determine the evolution of its running time-averaged torque. We show that in non-isothermal discs, the entropy-related corotation torque can indeed remain unsaturated in the presence of turbulence. For turbulence amplitudes that do not strongly affect the disc temperature profile, we find that the running time-averaged torque experienced by an embedded protoplanet is in fairly good agreement with laminar disc models with appropriate values for the thermal and viscous diffusion coefficients. In discs with turbulence driven by stochastic forcing, the corotation torque therefore behaves similarly as in laminar viscous discs and can be responsible for significantly slowing down or reversing Type I migration