Theory of Accretion Discs

Willy Kley †, Anna Penzlin, Thomas Rometsch, Lucas Jordan, Hugo Audiffren, Will Béthune, Christoph Schäfer

Hubble view of the Orion nebula showing five young
stars of which four are surrounded by protoplanetary discs (Courtesy: NASA)

Our research focusses on the physical structure and dynamics of accretion discs. Examples include circumstellar discs around T-Tauri stars, consisting mainly of gas and also a small amount of interstellar dust. In accretion discs the gas rotates with nearly Keplerian velocity around the host star, spirals slowly inwards and is accreted onto the central star. In our group we study various aspects of accretion disc physics.

• Turbulence
  It causes the gas to lose energy and angular momentum, and therefore drives the accretion process. The physical source of the turbulence is still unclear, but magnetorotational instability (MRI) is the most likely mechanism.
• Thermodynamics
  It governs the thermal structure of the disc. The temperature in the disc follows from an equilibrium between turbulent dissipation, pdV-work and radiative cooling. It determines the disc thickness in the vertical direction at a given location.
• Self-gravity
  It plays an important role in very massive discs. Depending on the efficiency of cooling, gravitational instabilities can occur and cause the disc to develop spiral arms or even to fragment.
• Companion stars
  A companion can influence the evolution of the disc by tidal forces and may cause the disc to become eccentric.

Planet Formation

Willy Kley †, Anna Penzlin, Thomas Rometsch, Lucas Jordan, Hugo Audiffren, Christoph Burger, Rafael Sfair, Christoph Schäfer

Simulated image of a self-gravitating disc undergoing fragmentation

Planet formation has been an area which has been studied in great detail with respect to the Solar System. The discovery of hundreds of extra-solar planets since the mid-1990s has put the field into a much larger context. A revision of the major aspects of classical planet formation theory is required. In our group we study various aspects of planet formation within the sequential accretion and gravitational instability scenario.

• Dust properties
  In the very beginning, planetesimal growth depends strongly on the material properties of the constituent dust particles. Using laboratory data for dusty material we perform ab initio simulations to calculate the continuum properties of dust agglomerates.
• Early dust growth
  The initial particle growth in the protoplanetary disc requires sticking upon individual collisions. Present models encounter great difficulties in growing particles beyond a decimeter due to destructive collisions. Using new data and methods we perform simulations to calculate the statistical outcome of collisions.
• Migration
  Planet-disc interactions cause a change in the orbital elements of embedded planets. For smaller planets of a few earth masses, the theoretically obtained migration rates are much too fast to match the observations. We perform detailed models of fully radiative discs with embedded planets to study this important phase.
• Gravitational Instability
  An alternative formation scenario for planet formation, at least a very large distances from the star, is provided through direct fragmentation of an initially unstable massive protoplanetary disc. We perform simulations to understand the efficiacy of such a process.
• Planets in binary stars
  Interestingly, a few dozen planets have been discovered in binary stars systems. As the dynamical action of a companion star perturbs the disc significantly, the planet formation process is more difficult. We follow the dynamics of such discs and study the impact on the formation process.