TY - BOOK

T1 - Planet Formation

T2 - The Roles of Pebble Accretion, Radiative and Convective Energy Transport

AU - Popovas, Andrius

PY - 2018

Y1 - 2018

N2 - This thesis deals with the early stages of rocky planet formation, when nascentplanets are still embedded in a protoplanetary disk, which consists mostlyof hydrogen, helium gas and dust grains. Hydrostatic equilibrium betweenthe gravitating planetary embryo and the surrounding gas forms an envelope.This envelope acts as a buffer between the embryo and the disk. Using ournewly developed DISPATCH framework, we construct high resolution nestedgridhydrodynamic simulations to investigate near-planet gas dynamics andhow it affects the accretion of pebbles, which are the main building blocksof planets. Only a small fraction of all the pebbles that cross into the planet’sregion of gravitational influence – the Hill sphere – are accreted. The pebbleaccretion rates scale linearly with the size of the pebbles and are, due to cancellationeffects, nearly independent of disk surface density, if the dust-to-gassurface density ratio is constant. With the measured accretion rates, we estimateaccurate 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 yearsfor an Earth mass planet at 1 AU and 1 million years for a Mars mass planetat 1.5 AU. For larger size particles or enhanced ratios of dust-to-gas surfacedensity, 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 convectivemotions, which significantly alter the gas dynamics inside 40 radiiof an Earth size embryo. Convective motions do not, however, result in a netchange in the transport of mass and the systematic inward drift of alreadygravitationally-bound pebbles continues as in the non-convective case. Toinvestigate what effect the radiative energy transport has on the local environmentof a forming planet, I have implemented a hybrid-characteristics radiativetransfer module in the DISPATCH framework. We find that althoughthe envelopes are generally opaque, they are locally optically thin and thusradiative heat transport has significant effects on the near-planet gas thermodynamics.The intensity of the convective motions are increased by radiativecooling of the atmosphere, which tends to increase the super-adiabatic temperaturegradient that drives convection. This does not, however, affect the netpebble accretion rates, but smaller pebbles do spend more time in the innerlayers of the atmosphere. Here, secondary effects, such as pebble destructionvia ablation, which would result in gas enrichment with heavy elements andenvelope replenishment with the disk gas rates, may be important. Consideringthe ongoing efforts to understand planet formation, and the importanceof realistically treating all of the relevant physical mechanisms, this thesisprovides a good start and a significant stepping stone to build future researchupon.

AB - This thesis deals with the early stages of rocky planet formation, when nascentplanets are still embedded in a protoplanetary disk, which consists mostlyof hydrogen, helium gas and dust grains. Hydrostatic equilibrium betweenthe gravitating planetary embryo and the surrounding gas forms an envelope.This envelope acts as a buffer between the embryo and the disk. Using ournewly developed DISPATCH framework, we construct high resolution nestedgridhydrodynamic simulations to investigate near-planet gas dynamics andhow it affects the accretion of pebbles, which are the main building blocksof planets. Only a small fraction of all the pebbles that cross into the planet’sregion of gravitational influence – the Hill sphere – are accreted. The pebbleaccretion rates scale linearly with the size of the pebbles and are, due to cancellationeffects, nearly independent of disk surface density, if the dust-to-gassurface density ratio is constant. With the measured accretion rates, we estimateaccurate 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 yearsfor an Earth mass planet at 1 AU and 1 million years for a Mars mass planetat 1.5 AU. For larger size particles or enhanced ratios of dust-to-gas surfacedensity, 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 convectivemotions, which significantly alter the gas dynamics inside 40 radiiof an Earth size embryo. Convective motions do not, however, result in a netchange in the transport of mass and the systematic inward drift of alreadygravitationally-bound pebbles continues as in the non-convective case. Toinvestigate what effect the radiative energy transport has on the local environmentof a forming planet, I have implemented a hybrid-characteristics radiativetransfer module in the DISPATCH framework. We find that althoughthe envelopes are generally opaque, they are locally optically thin and thusradiative heat transport has significant effects on the near-planet gas thermodynamics.The intensity of the convective motions are increased by radiativecooling of the atmosphere, which tends to increase the super-adiabatic temperaturegradient that drives convection. This does not, however, affect the netpebble accretion rates, but smaller pebbles do spend more time in the innerlayers of the atmosphere. Here, secondary effects, such as pebble destructionvia ablation, which would result in gas enrichment with heavy elements andenvelope replenishment with the disk gas rates, may be important. Consideringthe ongoing efforts to understand planet formation, and the importanceof realistically treating all of the relevant physical mechanisms, this thesisprovides a good start and a significant stepping stone to build future researchupon.

UR - https://soeg.kb.dk/permalink/45KBDK_KGL/fbp0ps/alma99122355317705763

M3 - Ph.D. thesis

BT - Planet Formation

PB - The Niels Bohr Institute, Faculty of Science, University of Copenhagen

ER -