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Giant planet formation around HR8799

Wallsby, Dan LU (2017) In Lund Observatory Examensarbeten ASTM31 20171
Lund Observatory
Department of Astronomy and Theoretical Physics
Abstract
In the thriving field of exoplanet research new discoveries are made all the time, and while most of the observed systems can be explained with classical planet formation models - some are much harder to explain. When stars form they are often surrounded by the remaining material of the nebulae they formed from. Some of this remaining material forms into a protoplanetary disk around the protostars due to the conservation of angular momentum and the intrinsic movement of the gas, in this protoplanetary disk planets can form. Planet formation depend strongly on the environment in which they form, so the properties and evolution of protoplanetary disks is an important part of understanding planet formation. In the case of stars that are more... (More)
In the thriving field of exoplanet research new discoveries are made all the time, and while most of the observed systems can be explained with classical planet formation models - some are much harder to explain. When stars form they are often surrounded by the remaining material of the nebulae they formed from. Some of this remaining material forms into a protoplanetary disk around the protostars due to the conservation of angular momentum and the intrinsic movement of the gas, in this protoplanetary disk planets can form. Planet formation depend strongly on the environment in which they form, so the properties and evolution of protoplanetary disks is an important part of understanding planet formation. In the case of stars that are more massive then the Sun this field is largely unexplored.

In this project we look closer to one of these systems, HR8799. That, as will be shown, is not obvious how it formed and evolved. We explore the protoplanetary disk and its evolution around a young 1.47 solar mass star using a temperature structure as a function of radius and hydrodynamic simulations. This disk structure will serve as an environment for planet formation to recreate HR8799, which is an observed exoplanet system with four super jovian planets (5+ Jupiter mass) on wide orbits(14.5, 24, 38 and 68AU). To grow and evolve the modeled planets we use a N-body code in a parameterized space that represents the protoplanetary disk, in which we place planet seeds and let them grow by pebble and gas accretion while including migration, interactions with the protoplanetary disk and dynamics between the planets.

Starting the simulation with four planets, we define a stable system as one where no collisions or ejections of planets occur. We find that the survival rate of systems is below 10% and there are multiple parameters that influence the survival rate. Our search focused on the viscosity parameter alpha as it greatly affects the disk structure, that in turn affects the planets growth and migration. We also varied the disk age and the pebble metallicity. We do not find an exact match for HR8799 in our survivors, but we do find stable systems with four giant planets, over three Jupiter masses on wide orbits, where the outer planet is on >50 AU orbit. In this thesis I will show that it is possible to grow and evolve stable systems with giant planets on wide orbits via pebble accretion as main core growth mechanism. (Less)
Popular Abstract (Swedish)
I det blomstrande forskningsfältet om planeter och exoplaneter, det vill säga planeter utanför vårat eget Solsystem hittar vi konstant nya planeter och system som alla ger oss pusselbitar till att förstå hur planeter formas och utvecklas. Så för att förstå de observationer vi gör och bedöma hur sannolikt det är att det kan finnas liv på en planet, måste vi förstå alla olika typer av system och hur de formas.

I det är projektet är målet att återskapa ett observerat exoplanetsystem, HR8799. Det är ett ovanligt system med fyra väldigt massiva planeter på vida omloppsbanor runt sin stjärna. Kombinationen av dessa är ovanligt då massiva planeter både har en tendens att göra systemet instabilt, som leder till att planeter kolliderar eller... (More)
I det blomstrande forskningsfältet om planeter och exoplaneter, det vill säga planeter utanför vårat eget Solsystem hittar vi konstant nya planeter och system som alla ger oss pusselbitar till att förstå hur planeter formas och utvecklas. Så för att förstå de observationer vi gör och bedöma hur sannolikt det är att det kan finnas liv på en planet, måste vi förstå alla olika typer av system och hur de formas.

I det är projektet är målet att återskapa ett observerat exoplanetsystem, HR8799. Det är ett ovanligt system med fyra väldigt massiva planeter på vida omloppsbanor runt sin stjärna. Kombinationen av dessa är ovanligt då massiva planeter både har en tendens att göra systemet instabilt, som leder till att planeter kolliderar eller blir utkastade. Eller att de massiva planeterna migrerar väldigt nära sin stjärna och blir en så kallad "het Jupiter", en klass av planeter som vi inte har i Solsystemet med är vanlig kring andra stjärnor.

Det första steget i projektet är att först och bygga upp en modell av en protoplanetär disk runt stjärnan, som är resterna från nebulosan som stjärnan skapades ifrån. Genom bevaring av rörelsemängdsmoment och gasens rörelse plattas den ner till en roterande skiva runt stjärnan. I denna gasrika disk bildas planeter, asteroider och allt annat som kretsar kring en stjärna efter gasen har försvunnit.

Efter en diskmodell är uppbyggd kommer vi att plantera planetesimaler i storleksordning av några procent av en jordmassa och låta dem växa genom att krocka med och dra åt sig material från disken samtidigt som disken utvecklas och planeterna påverkar varandra via gravitation.

Vi finner att det är få (ca 8%) av systemen som överlever, men några gör det. Även om vi inte finner ett system exakt som HR8799 så hittar vi stabila system med fyra massiva planeter på vida omloppsbanor som är jämförbara med HR8799. Resultaten vi har berättar för oss att det är svårt, men möjligt att bygga multipla, massiva planeter på vida omloppsbanor med en modell där man först bygger upp en planetkärna och sen drar till sig en massiv gas atmosfär. (Less)
Please use this url to cite or link to this publication:
author
Wallsby, Dan LU
supervisor
organization
course
ASTM31 20171
year
type
H2 - Master's Degree (Two Years)
subject
keywords
HR8799, Planet formation, Protoplanetary disks, Giant planets, Astronomy, Migration, Dynamics
publication/series
Lund Observatory Examensarbeten
report number
2017-EXA111
language
English
id
8923405
date added to LUP
2017-08-23 16:10:17
date last changed
2017-08-23 16:10:17
@misc{8923405,
  abstract     = {In the thriving field of exoplanet research new discoveries are made all the time, and while most of the observed systems can be explained with classical planet formation models - some are much harder to explain. When stars form they are often surrounded by the remaining material of the nebulae they formed from. Some of this remaining material forms into a protoplanetary disk around the protostars due to the conservation of angular momentum and the intrinsic movement of the gas, in this protoplanetary disk planets can form. Planet formation depend strongly on the environment in which they form, so the properties and evolution of protoplanetary disks is an important part of understanding planet formation. In the case of stars that are more massive then the Sun this field is largely unexplored.

In this project we look closer to one of these systems, HR8799. That, as will be shown, is not obvious how it formed and evolved. We explore the protoplanetary disk and its evolution around a young 1.47 solar mass star using a temperature structure as a function of radius and hydrodynamic simulations. This disk structure will serve as an environment for planet formation to recreate HR8799, which is an observed exoplanet system with four super jovian planets (5+ Jupiter mass) on wide orbits(14.5, 24, 38 and 68AU). To grow and evolve the modeled planets we use a N-body code in a parameterized space that represents the protoplanetary disk, in which we place planet seeds and let them grow by pebble and gas accretion while including migration, interactions with the protoplanetary disk and dynamics between the planets.

Starting the simulation with four planets, we define a stable system as one where no collisions or ejections of planets occur. We find that the survival rate of systems is below 10% and there are multiple parameters that influence the survival rate. Our search focused on the viscosity parameter alpha as it greatly affects the disk structure, that in turn affects the planets growth and migration. We also varied the disk age and the pebble metallicity. We do not find an exact match for HR8799 in our survivors, but we do find stable systems with four giant planets, over three Jupiter masses on wide orbits, where the outer planet is on >50 AU orbit. In this thesis I will show that it is possible to grow and evolve stable systems with giant planets on wide orbits via pebble accretion as main core growth mechanism.},
  author       = {Wallsby, Dan},
  keyword      = {HR8799,Planet formation,Protoplanetary disks,Giant planets,Astronomy,Migration,Dynamics},
  language     = {eng},
  note         = {Student Paper},
  series       = {Lund Observatory Examensarbeten},
  title        = {Giant planet formation around HR8799},
  year         = {2017},
}