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The fate of pebbles and planetesimals entering protoplanetary envelopes

Zadera, Emil LU (2019) In Lund Observatory Examensarbeten ASTM31 20191
Lund Observatory - Undergoing reorganization
Abstract
Planetary embryos grow by the accretion of solid dust-material, ranging from cm- to m-sized pebbles up to km-sized planetesimals. However, the underlying size-distribution of the accreted material is poorly understood. When the pebbles and planetesimals encounter a protoplanet, they are subjected to the gaseous environment of the protoplanetary envelope. Because of the drag-force from the gas, the pebble- and the planetesimal-trajectories change significantly from their initial Keplerian orbits, and so does the evolution of their surface temperatures and the ablation rates. It is consequently of interest to track the evolution for a large range of particle sizes that encounter protoplanets, from small pebbles to large planetesimals.... (More)
Planetary embryos grow by the accretion of solid dust-material, ranging from cm- to m-sized pebbles up to km-sized planetesimals. However, the underlying size-distribution of the accreted material is poorly understood. When the pebbles and planetesimals encounter a protoplanet, they are subjected to the gaseous environment of the protoplanetary envelope. Because of the drag-force from the gas, the pebble- and the planetesimal-trajectories change significantly from their initial Keplerian orbits, and so does the evolution of their surface temperatures and the ablation rates. It is consequently of interest to track the evolution for a large range of particle sizes that encounter protoplanets, from small pebbles to large planetesimals. Depending on which particle size is responsible for the growth of protoplanets, and the corresponding thermal evolution and ablation, the protoplanetary core and its envelope will evolve differently. If all the accreted particles are ablated, we expect the envelopes of protoplanets to be polluted and with less massive cores. On the other hand, if the ablation is inefficient, protoplanetary cores are expected to grow efficiently. Effectively, the form in which material is accreted sets constraints on protoplanetary interior and atmospheric evolution models.

In this project, I study the evolution of both pebbles and planetesimals that encounter a protoplanet. This is done by simulating the trajectories of the in-falling solids in the protoplanetary envelope while tracing their thermal evolution, dynamical pressure, and ablation. The mass loss of the particles is further related to the accretion rates onto protoplanets of different mass, where both the solid and the ablated mass are accounted for.

From the results, I can conclude that pebbles are efficiently ablated above protoplanetary cores with masses of 0.5 M_\oplus. Small planetesimals, between 10^3-10^4 cm in size, are fully ablated for core masses about 1-5 M_\oplus. For sizes of 10^5-10^6 cm, the core masses have to reach between 5-10 M_\oplus. Finally, for planetesimals on the order of 10^7-10^8 cm, several tenths of Earth-masses are required to fully ablate the impactors. This means that if protoplanets grow predominantly by pebble accretion, they grow into, so called, vapour-blobs, already at 0.5 M_\oplus. The evolution of the protoplanetary interior structure and the pollution of the envelope and its chemical composition is consequently determined by the internal gas-flows, dust settling, and the interchange of material between the envelope and the protoplanetary disc.

I also find that the latent heat is cooling the surface of the impactors efficiently, limiting the ablation rates. Planetesimals, that have a large reservoir of volatiles, can remain cold as they pass through the envelope by only ablating a small fraction of their total mass. I further find that the sum of the accreted solid mass and the ablated material follows the classical core accretion model, where no ablation of the particles is included. Thus, the results obtained in classical core accretion simulations are in the larger picture unaffected by ablation. (Less)
Popular Abstract (Swedish)
De mest framgångsrika planetbildningsmodellerna som forskare jobbar med idag härstammar från en idé som kom fram för nästan 300 års sedan, under 1700-talet. Det var den svenske forskaren Emanuel Swedenborg och den tyske filosofen Immanuel Kant som kom fram med en hypotes som vi idag kallar Solnebulosan. Idén bygger på att solen bildades från ett gigantiskt stoftmoln som kollapsade, på grund av gravitation, till stjärnor som omringas av roterande diskformade strukturer av gas och stoft där planeter formas. Därefter har de disklika strukturerna som vi idag observerar kring unga stjärnor blivit nämnda protoplanetära skivor.

Dock var det inte förrän år 1969 som själva modellen angående planetbildningen, som är grund för dagens forskning,... (More)
De mest framgångsrika planetbildningsmodellerna som forskare jobbar med idag härstammar från en idé som kom fram för nästan 300 års sedan, under 1700-talet. Det var den svenske forskaren Emanuel Swedenborg och den tyske filosofen Immanuel Kant som kom fram med en hypotes som vi idag kallar Solnebulosan. Idén bygger på att solen bildades från ett gigantiskt stoftmoln som kollapsade, på grund av gravitation, till stjärnor som omringas av roterande diskformade strukturer av gas och stoft där planeter formas. Därefter har de disklika strukturerna som vi idag observerar kring unga stjärnor blivit nämnda protoplanetära skivor.

Dock var det inte förrän år 1969 som själva modellen angående planetbildningen, som är grund för dagens forskning, blev sammanfattad I ett verk som publicerades av den sovjetiske forskaren Victor Safronov. Han beskrev hur dammkorn med is- och sten lika egenskaper I den protoplanetära skivan växer via kollisioner mellan varandra, tills de bildat kilometerstora block som vi kallar planetesimaler. Vidare uppbyggnad av planetesimalerna sker sedan genom både kollisioner och anhopning av närliggande damm på grund av gravitation. Så småningom har protoplaneter bildats, vars massa ligger mellan en tiondels-, upp till tiotals gånger jordens massa. Protoplaneterna är då tillräckligt massiva så att de kan hålla ihop en atmosfär av gas som I vissa fall kan växa till sig så att de blir lika massiva som gasjättarna I vårt solsystem. Forskare vet idag att själva formationen av planetesimalerna är något mer komplex än bara via kollisioner. Men att protoplaneter växer till planeter genom attraktion av allt mellan dammkorn till planetesimaler är I stort sett en accepterad hypotes.

Aktiva forskningsområden idag angår distributionen av storlekarna på materialet som faller mot protoplaneterna och hur gasflödet runt en protoplanet ser ut. Men varför har detta betydelse? En anledning är att protoplaneternas tillväxttid inte är oberoende av storleken på byggnadsmaterialet. Vidare kommer protoplaneternas atmosfär medföra att det infallande materialet börjar förstöras innan det ens har nått dess yta, likt meteorer i jordens atmosfär. Slutligen avgör gasflödet, mellan den protoplanetära skivan och protoplanetens atmosfär, om materialet som förstördes bidrar till planetbildningen eller blir bortskickat.

I detta projekt har jag utvecklat en kod som simulerar evolutionen av enskilda partiklar som faller genom protoplanetära atmosfärer. Eftersom distributionen av storlekarna på det infallande materialet inte är väl känt, undersöker jag beteendet för allt mellan millimeter- till kilometer stora objekt. I simulationerna inkluderar jag allt från gasdynamik till meteorfysik för att förstå vad som händer med protoplaneterna och byggnadsmaterialet. (Less)
Please use this url to cite or link to this publication:
@misc{8989227,
  abstract     = {{Planetary embryos grow by the accretion of solid dust-material, ranging from cm- to m-sized pebbles up to km-sized planetesimals. However, the underlying size-distribution of the accreted material is poorly understood. When the pebbles and planetesimals encounter a protoplanet, they are subjected to the gaseous environment of the protoplanetary envelope. Because of the drag-force from the gas, the pebble- and the planetesimal-trajectories change significantly from their initial Keplerian orbits, and so does the evolution of their surface temperatures and the ablation rates. It is consequently of interest to track the evolution for a large range of particle sizes that encounter protoplanets, from small pebbles to large planetesimals. Depending on which particle size is responsible for the growth of protoplanets, and the corresponding thermal evolution and ablation, the protoplanetary core and its envelope will evolve differently. If all the accreted particles are ablated, we expect the envelopes of protoplanets to be polluted and with less massive cores. On the other hand, if the ablation is inefficient, protoplanetary cores are expected to grow efficiently. Effectively, the form in which material is accreted sets constraints on protoplanetary interior and atmospheric evolution models.

In this project, I study the evolution of both pebbles and planetesimals that encounter a protoplanet. This is done by simulating the trajectories of the in-falling solids in the protoplanetary envelope while tracing their thermal evolution, dynamical pressure, and ablation. The mass loss of the particles is further related to the accretion rates onto protoplanets of different mass, where both the solid and the ablated mass are accounted for.

From the results, I can conclude that pebbles are efficiently ablated above protoplanetary cores with masses of 0.5 M_\oplus. Small planetesimals, between 10^3-10^4 cm in size, are fully ablated for core masses about 1-5 M_\oplus. For sizes of 10^5-10^6 cm, the core masses have to reach between 5-10 M_\oplus. Finally, for planetesimals on the order of 10^7-10^8 cm, several tenths of Earth-masses are required to fully ablate the impactors. This means that if protoplanets grow predominantly by pebble accretion, they grow into, so called, vapour-blobs, already at 0.5 M_\oplus. The evolution of the protoplanetary interior structure and the pollution of the envelope and its chemical composition is consequently determined by the internal gas-flows, dust settling, and the interchange of material between the envelope and the protoplanetary disc.

I also find that the latent heat is cooling the surface of the impactors efficiently, limiting the ablation rates. Planetesimals, that have a large reservoir of volatiles, can remain cold as they pass through the envelope by only ablating a small fraction of their total mass. I further find that the sum of the accreted solid mass and the ablated material follows the classical core accretion model, where no ablation of the particles is included. Thus, the results obtained in classical core accretion simulations are in the larger picture unaffected by ablation.}},
  author       = {{Zadera, Emil}},
  language     = {{eng}},
  note         = {{Student Paper}},
  series       = {{Lund Observatory Examensarbeten}},
  title        = {{The fate of pebbles and planetesimals entering protoplanetary envelopes}},
  year         = {{2019}},
}