Dust particle growth in protoplanetary disks [Elektronische Ressource] / presented by Frithjof Brauer

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• Astronomie

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• Publié par: ruprecht-karls-universitat_heidelberg
• Publié le: 01 janvier 2009
• Nombre de lectures: 14
• Langue: English
• Poids de l'ouvrage: 3 Mo

Extrait

Dust particle growth
in protoplanetary disks
Frithjof Brauer2Dissertation
submitted to the
Combined Faculties for the Natural Sciences and for Mathematics
of the Ruperto-Carola University of Heidelberg (Germany)
for the degree of
Doctor of Natural Sciences
presented by
Diplom-Physicist
Frithjof Brauer
born in Eisenach (Germany)
Oral examination: January, 28th 20084Dust particle growth
in protoplanetary disks
Referees: Prof. Dr. Thomas Henning
Prof. Dr. Ralf S. Klessen6Abstract
This thesis deals with the initial stages of planet formation in protoplanetary disks. Par-
ticular interest lies on the growth process from sub-m-sized dust grains towards planetesi-
mals of km-size. To form such large objects in protoplanetary disks, the primary coagulation
mechanism has to circumvent at least two severe obstacles, namely the rapid loss of solid
material due to radial inward drift and particle fragmentation due to destructive collisions
with high velocities. These two hurdles together are called the ”Meter size barrier” for par-
ticle growth, whose investigation is the main subject of this thesis. We find that the initial
dust-to-gas ratio is essential for the particles to overcome the radial drift barrier, i.e. the first
part of the meter size barrier. If this ratio is increased by a factor of two compared with the
canonical value for the interstellar medium, planetesimals can form within short time scales
in the disk. Our simulation results also suggest that the fragmentation barrier, the second
part ofthe meter size barrier, is onlyovercome ifimplausible high critical threshold velocities
for particle fragmentation are assumed. For this reason, we investigate disk environments
which could favour planetesimal formation. We focus on non-turbulent regions, so-called
dead zones, around the disk midplane in the presence of the ice evaporation front. We find
that in this specific disk environment, particle fragmentation is no longer an obstacle and
boulders of km-size can form within only a few thousand years. One major conclusion of this
thesis is, therefore, that solid material can overcome the meter size barrier of particle growth
and that planetesimal formation due to hit-and-stick mechanisms is in fact possible.8
Zusammenfassung
Diese Doktorarbeit bescha¨ftigt sich mit den ersten Phasen der Planetenentstehung in proto-
planetaren Scheiben. Besonderes Interesse kommt dabei dem Wachstumsprozeß von sub-m
großen Staubteilchen hin zu Planetesimalen von Kilometer gro¨ße zu. Um solch große Ob-
jekte inprotoplanetaren Scheiben bildenzu k¨onnen, mußderanf¨angliche Koagulationsprozeß
zwei erhebliche Hu¨rden umgehen; zum einen den schnellen Massenverlust der Staubscheibe
auf Grund radialer Driftbewegungen von Staubteilchen zum Stern hin, zum anderen die
Zerst¨orung bereits gro¨ßerer Teilchen auf Grund von Kollisionen mit hoher Geschwindigkeit.
Diese beiden Hu¨rden zusammengenommen werden als die ”Meter-Barriere” des Wachstum-
sprozesses bezeichnet, dessen Untersuchung den Hauptbestandteil dieser Arbeit darstellt. In
derArbeitwirdgezeigt, daßdasanf¨anglicheStaub-Gas-Verha¨ltnisvonentscheidenderBedeu-
tung ist, um das Problem der radialen Teilchenbewegung, also den ersten Teil des Problems
der Meter-Barriere, zu umgehen. Wenn dieses Verha¨ltnis, verglichen mit jenem des inter-
stellaren Mediums, um einen Faktor zwei erho¨ht wird, so fu¨hrt dies in astronomisch gesehen
kurzer Zeit zur Bildung von Planetesimalen. Die Simulationsergebnisse weisen auch darauf
hin, daß die Fragmentations-Barriere, der zweite Teil des Problems der Meter-Barriere, nur
unter der Annahme unrealistisch hoher kritischer Fragmentationsgeschwindigkeiten umgan-
genwerdenkann. AusdiesemGrundewirdaußerdemuntersucht, obbestimmteUmgebungen
in der protoplanetaren Scheibe die Koagulation gu¨nstig beeinflussen k¨onnen. Wir unter-
suchen den Einfluß von Regionen mit sehr geringer Turbulenz, sogenannte ”Tote Zonen”, in
Gegenwart der Verdampfungsfront von Wassereis. Es zeigt sich, daß Teilchenfragmentation
in dieser spezifischen Umgebung nicht l¨anger ein Hindernis darstellt, da es in dieses Modell
in nur wenigen tausend Jahren zur Bildung von Planetesimalen kommt. Eine grundlegende
SchlußfolgerungdieserArbeitistsomit, daßesprotoplanetarem Staubmo¨glich ist, dieMeter-
Barriere zu durchbrechen, und daß die Bildung von Planetesimalen durch Stoßinduzierte
Wachstumsvorga¨nge tatsa¨chlich mo¨glich ist.9
Acknowledgements
This thesis would not have been possible without the help of various persons to whom I
owe thanks.
First of all, my direct supervisor, Kees Dullemond, who did not only guide me very thor-
oughly through these last years accompanied with dozens of handwritten lists of good ideas,
but also taught me how to stand on my own feet and how to do my own science. In the same
breath, I would like to thank my official supervisor, Thomas Henning, who’s door was always
open, and who helped me to recognize the important things in the big picture. Far beyond
science,Iowebothofthemagreatdealforsupportingmeespeciallyduringthelastsixmonth.
During these years, I learned a lot about turbulence from very fruitful discussions with
Anders Johansen, Natalia Dzyurkevich, Andrej Bicanski and Hubert Klahr. The proximity
of our two groups was very inspiring and led to great insight of each others topic. I also have
to thank Andras Zsom and Andrej Bicanski for proof-reading my script and, hence, improv-
ing it substantially. I also thank Andras for providing my the plot about stochastic velocity
fluctuationsin thisthesis. Thanksalso to Martin Hennemannwhohelpedmefindingmyway
through the coppice of the formal PhD requirements and defense dates. Though millions of
years away from my own PhD topic, I owe Martina Queck for making me reconsider many
aspects of my work which helped me to understand it in a better way. I would also like to
thank Juliet, Jelte and Holger for not talking about science with me.
I would not be here and writing my thesis without the comittment of Katrin Regenspurger
and Steffen Rosahl. I thank Katrin for standing up for me, for her open door at day and
night, and for her everlasting optimism which always brightens me up. Moreover, I have to
thank Steffen Rosahl for lots of hours of intense concentration, and for his dedication and
genuine honesty which I have learned to appreciate a lot.
Lastbutnotleast, Iwanttothankmyfamilyforsupportingmethroughtheseyears, forhelp-
ing me to move two times within Heidelberg, for waiting for me for hours in long corridors,
and for being there.10