The accretion of migrating giant planets

Most studies concerning the growth and evolution of massive planets focus either on their accretion or their migration only. In this work we study both processes concurrently to investigate how they might mutually affect each other. We modeled a 2-dimensional disk with a steady accretion flow onto the central star and embed a Jupiter mass planet at 5.2 au. The disk is locally isothermal and viscosity is modeled using a constant $\alpha$. The planet is held on a fixed orbit for a few hundred orbits to allow the disk to adapt and carve a gap. After this period, the planet is released and free to move according to the gravitational interaction with the gas disk. The mass accretion onto the planet is modeled by removing a fraction of gas from the inner Hill sphere, and the removed mass and momentum can be added to the planet. Our results show that a fast migrating planet is able to accrete more gas than a slower migrating planet. Utilizing a tracer fluid we analyzed the origin of the accreted gas which comes predominantly originating from the inner disk for a fast migrating planet. In case of slower migration the fraction of gas from the outer disk increases. We also found that even for very high accretion rates in some cases gas crosses the planetary gap from the inner to the outer disk. Our simulations show that the crossing of gas changes during the migration process as the migration rate slows down. Therefore classical type II migration where the planet migrates with the viscous drift rate and no gas crosses the gap is no general process but may only occur for special parameters and at a certain time during the orbital evolution of the planet.Comment: 9 pages, 14 figures, accepted for publication in A&

Migration of massive planets in accreting disks

Massive planets that open a gap in the accretion disk are believed to migrate with exactly the viscous speed of the disk, a regime termed type II migration. Population synthesis models indicate that standard type II migration is too rapid to be in agreement with the observations. We study the migration of massive planets between $2\times10^{-4}$ and $2\times10^{-3} M_\odot$ corresponding to 0.2 to 2 Jupiter masses $M_J$. in order to estimate the migration rate in comparison to type II migration. We follow the evolution of planets embedded in two-dimensional, locally isothermal disks with non-zero mass accretion which is explicitly modelled using suitable in- and outflow boundary conditions to ensure a specific accretion rate. After a certain relaxation time we release the planet and measure its migration through the disk and the dependence on parameters such as viscosity, accretion rate and planet mass. We study accreting and non-accretion planets. The inferred migration rate of the planet is determined entirely by the disk torques acting on it and is completely independent of the viscous inflow velocity, so there is no classical type II migration regime. Depending on the local disk mass the migration rate can be faster or slower than type II migration. From the torques and the accretion rate profile in the disk we see that the gap formed by the planet does not separate the inner from the outer disk as necessary for type II migration, rather gas crosses the gap or is accreted onto the planet.Comment: 10 pages, 16 figures, accepted for publication in A&

Migration of Massive Planets

This thesis studies the migration of massive planets with mass bigger than half the mass of Jupiter. Planetary migration is a process that changes the semi-major axis of the planetary orbit. The gravitational forces of disturbances in the disk density created by the planet result in a torque changing the angular momentum of the planet and thus reducing or increasing its distance to the star. Planets this massive also open an annular gap in the disk. Because of the gap the torques close to the planet are reduced resulting in a slowdown of the migration. At the same time the reduced density due to the gap will have effects on the growth of the planet. For this work and the embedded publications numerical simulations were conducted with different parameters as disk density, planet mass, strength of viscosity and accretion rate in order to study the migrational behavior in detail. The results show that type II migration is not working as the simple models used today suggest. The migration rate depends on the exact properties of the disk and is not just the viscous radial speed of the gas which can be slower or faster than the migration in this models. The reason is that the gap is not able to separate the inner from the outer disk because gas can cross the gap in both directions. Even though, accretion onto the planet will not prohibit gas crossing the planet's orbit. Furthermore, the simulations show the migration and accretion rate depend mutually on each other. Planets migrating faster can accrete more gas because more gas must cross the gap. At the same time accretion leads to a deeper gap which then reduces the torques from the region close to the planet and this way leads to slower migration. Hence, to study type II migration in numerical models both migration and accretion should be considered at the same time. Under certain circumstances this might slow down migration and thus solve current problems of giant gas planet formation.Diese Arbeit beschäftigt sich mit der Migration von massereichen Planeten mit einer Planetenmasse größer als eine halbe Jupitermasse. Planetenmigration bezeichnet die Veränderung der großen Halbachse des Orbits eines Planeten. Die Gravitationskräfte von durch den Planeten verursachten Störungen in der Scheibe bewirken ein Drehmoment, welches den Drehimpuls des Planeten verändert, wodurch sich dessen Orbit verkleinert oder vergrößert. Diese massereichen Planeten öffnen dabei eine ringförmige Lücke in der Gasscheibe. Diese bewirkt, dass die Drehmomente abnehmen und der Planet langsamer migriert. Gleichzeitig hat die verringerte Dichte in der Nähe des Planeten Auswirkungen auf sein Wachstum. Im Rahmen dieser Arbeit und der eingebetteten Publikationen wurden numerische Simulationen durchgeführt, um für verschiedene Parameter wie Dichte der Scheibe, Masse des Planeten, Stärke der Viskosität und der Akkretionsrate das genaue Verhalten zu untersuchen. Die Ergebnisse zeigen, dass sich Typ II Migration nicht so verhält wie bisher verwendete einfache Modelle nahelegen. Die Migrationsrate hängt von den genauen Eigenschaften der Scheibe ab und entspricht nicht einfach der viskosen radialen Strömungsgeschwindigkeit, die in diesen Modellen sowohl unter- als auch überschritten werden kann. Grund dafür ist, dass die Lücke in der Scheibe nicht geeignet ist die innere von der äußeren Scheibe zu trennen, da Gas die Lücke in beide Richtungen passieren kann. Dies ist auch für akkretierende Planeten unter bestimmten Umständen noch der Fall. Die Simulationen zeigen auch, dass die Migrationsrate und die Akkretionsrate des Planeten miteinander wechselwirken. Schneller migrierende Planeten können, da mehr Gas die Lücke passieren muss, schneller wachsen. Gleichzeitig sorgt Akkretion für eine tiefere Lücke, wodurch die Drehmomente in der direkten Umgebung des Planeten reduziert werden und die Migrationsrate sinkt. Für Modelle, die Typ II Migration untersuchen sollen, ist also eine gleichzeitige Berücksichtigung beider Effekte notwendig. Unter geeigneten Bedingungen könnte so die Migration verlangsamt werden, um bestehende Probleme bei der Entstehung von Gasriesenplaneten zu lösen

Ice Particles Trapped by Temperature Gradients at mbar Pressure

In laboratory experiments we observe that ice particles (\leq100 \mu m) entrained in a low pressure atmosphere (~1 mbar) get trapped by temperature gradients between three reservoirs at different tempertature. Confining elements are a peltier element at 250 K (bottom), a liquid nitrogen reservoir at 77 K (top) and the surrounding vacuum chamber at 293 K. Particle levitation and trapping is modeled by an interplay of thermophoresis, photophoresis and gravity. A number of ice particles are trapped simultaneously in close spatial distance to each other at least up to minutes and are accessible for further experiments.Comment: Published in Review of Scientific Instruments 82, 2011, 4 pages, 6 figure

Biological Experiments in BIOLAB Facility on board Columbus

BIOLAB is a multi-user payload facility for biological experiments, that was launched on board Space Shuttle Atlantis with the Columbus laboratory on 7 February 2008. Since then the facility is accessible to the scientific community, with a number of experiments by different project teams have been performed and research is still ongoing. BIOLAB is operated under ESA contract by the Microgravity User Support Center (MUSC) at DLR in Cologne, Germany. BIOLAB offers two centrifuges inside an incubator (18°C up to 40°C) to allow experiments to be undertaken in micro gravity but also under simulated gravity at 1g, to compare results. Most scientists also perform a ground reference experiment at their laboratories. For the experiments two types of experiment containers are available a standard experiment container or an advanced experiment container. On-orbit, experiment containers are inserted into Biolab for processing. A typical experiment can run from one day to three months. The Facility Responsible Centre for Biolab, has the overall responsibility to operate it according to the needs of the scientists. The individual experiment container providers can monitor the processing of experiments from own User Home Bases. After an experiment run two temperature-controlled units are available for post cold stowage from -20°C to +10°C until there is a download possibility. Several experiments were executed in BIOLAB, e.g.: • CYTOSKELETON: Measurements of morphology, cytoskeleton, gene expression and signaling of mammalian fibroblast and osteoblasts. • Arthrospira-B: Measurements of growth, oxygen production and photosynthesis of the cyanobacteria Arthrospira. • TripleLux A&B: Bioluminescence measurement of reactive oxygen species production in mammalian and invertebrate immune cells during immune response. • Cold stowage of the experiments Extremophiles, CrISStal and Seedling Growth-3. Future experiments such as Suture in Space, Lux in Space, WAPS or Arthrospira-C are still planned in the near term future. The paper will give a survey on BIOLAB operations over the last decade and provide an outlook for future BIOLAB planning