Abstract
This thesis deals with the early stages of rocky planet formation, when nascent
planets are still embedded in a protoplanetary disk, which consists mostly
of hydrogen, helium gas and dust grains. Hydrostatic equilibrium between
the gravitating planetary embryo and the surrounding gas forms an envelope.
This envelope acts as a buffer between the embryo and the disk. Using our
newly developed DISPATCH framework, we construct high resolution nestedgrid
hydrodynamic simulations to investigate near-planet gas dynamics and
how it affects the accretion of pebbles, which are the main building blocks
of planets. Only a small fraction of all the pebbles that cross into the planet’s
region of gravitational influence – the Hill sphere – are accreted. The pebble
accretion rates scale linearly with the size of the pebbles and are, due to cancellation
effects, nearly independent of disk surface density, if the dust-to-gas
surface density ratio is constant. With the measured accretion rates, we estimate
accurate growth times for specified particle sizes. For chondrule-size
(0.3–1 mm) particles, the growth time from a small seed is 1.5 million years
for an Earth mass planet at 1 AU and 1 million years for a Mars mass planet
at 1.5 AU. For larger size particles or enhanced ratios of dust-to-gas surface
density, the estimates are correspondingly shorter.
Accretion of solids onto the embryo releases a lot of potential energy,
which is converted to heat via the friction force. This extra heat drives convective
motions, which significantly alter the gas dynamics inside 40 radii
of an Earth size embryo. Convective motions do not, however, result in a net
change in the transport of mass and the systematic inward drift of already
gravitationally-bound pebbles continues as in the non-convective case. To
investigate what effect the radiative energy transport has on the local environment
of a forming planet, I have implemented a hybrid-characteristics radiative
transfer module in the DISPATCH framework. We find that although
the envelopes are generally opaque, they are locally optically thin and thus
radiative heat transport has significant effects on the near-planet gas thermodynamics.
The intensity of the convective motions are increased by radiative
cooling of the atmosphere, which tends to increase the super-adiabatic temperature
gradient that drives convection. This does not, however, affect the net
pebble accretion rates, but smaller pebbles do spend more time in the inner
layers of the atmosphere. Here, secondary effects, such as pebble destruction
via ablation, which would result in gas enrichment with heavy elements and
envelope replenishment with the disk gas rates, may be important. Considering
the ongoing efforts to understand planet formation, and the importance
of realistically treating all of the relevant physical mechanisms, this thesis
provides a good start and a significant stepping stone to build future research
upon.
planets are still embedded in a protoplanetary disk, which consists mostly
of hydrogen, helium gas and dust grains. Hydrostatic equilibrium between
the gravitating planetary embryo and the surrounding gas forms an envelope.
This envelope acts as a buffer between the embryo and the disk. Using our
newly developed DISPATCH framework, we construct high resolution nestedgrid
hydrodynamic simulations to investigate near-planet gas dynamics and
how it affects the accretion of pebbles, which are the main building blocks
of planets. Only a small fraction of all the pebbles that cross into the planet’s
region of gravitational influence – the Hill sphere – are accreted. The pebble
accretion rates scale linearly with the size of the pebbles and are, due to cancellation
effects, nearly independent of disk surface density, if the dust-to-gas
surface density ratio is constant. With the measured accretion rates, we estimate
accurate growth times for specified particle sizes. For chondrule-size
(0.3–1 mm) particles, the growth time from a small seed is 1.5 million years
for an Earth mass planet at 1 AU and 1 million years for a Mars mass planet
at 1.5 AU. For larger size particles or enhanced ratios of dust-to-gas surface
density, the estimates are correspondingly shorter.
Accretion of solids onto the embryo releases a lot of potential energy,
which is converted to heat via the friction force. This extra heat drives convective
motions, which significantly alter the gas dynamics inside 40 radii
of an Earth size embryo. Convective motions do not, however, result in a net
change in the transport of mass and the systematic inward drift of already
gravitationally-bound pebbles continues as in the non-convective case. To
investigate what effect the radiative energy transport has on the local environment
of a forming planet, I have implemented a hybrid-characteristics radiative
transfer module in the DISPATCH framework. We find that although
the envelopes are generally opaque, they are locally optically thin and thus
radiative heat transport has significant effects on the near-planet gas thermodynamics.
The intensity of the convective motions are increased by radiative
cooling of the atmosphere, which tends to increase the super-adiabatic temperature
gradient that drives convection. This does not, however, affect the net
pebble accretion rates, but smaller pebbles do spend more time in the inner
layers of the atmosphere. Here, secondary effects, such as pebble destruction
via ablation, which would result in gas enrichment with heavy elements and
envelope replenishment with the disk gas rates, may be important. Considering
the ongoing efforts to understand planet formation, and the importance
of realistically treating all of the relevant physical mechanisms, this thesis
provides a good start and a significant stepping stone to build future research
upon.
| Original language | English |
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| Publisher | The Niels Bohr Institute, Faculty of Science, University of Copenhagen |
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| Publication status | Published - 2018 |