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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 3 - new-ppd-environments (First-principles global MHD disc simulations: Defining planet-forming environments in early solar systems)

Teaser

Summary

Planets have fascinated professional astronomers as well as casual stargazers for centuries. Since ancient times, we have literally only known a handful of planets, all orbiting our own star, the Sun. However, during the last two decades, and with the help of small ground-based telescopes and enormous space missions like Kepler and CoRoT alike, astronomers have discovered thousands of so-called exo-planets orbiting distant stars in our neighborhood of the Milky Way.

In their role as planet nurseries, protoplanetary discs are of key interest to planet formation theory. Their dynamical, radiative and thermodynamic properties critically define the environment for embedded solids: dust grains, pebbles and planetesimals. In short, the building blocks of planet formation. The discs\' dynamics and structure in turn depend critically on the influence of magnetic fields that couple to tenuously ionised and low-density regions. Being comparatively cold and dense, the ionisation state of the disc plasma is dominated by external far-UV, X-Ray, and cosmic-ray radiation, leading to a layered vertical structure - with turbulent, magnetised surface layers and a magnetically-decoupled midplane. This classic dead-zone picture is now turned upside-down by previously ignored micro-physical effects.

The aim of my research project is to create the most realistic computer simulations of protoplanetary discs of gas and dust, thus defining the environment that shapes the early development of planetary systems.

Work performed

I have established the research team, and beyond networking, supervision of team members, administrative tasks, and involvement in the organisation of various scientific meetings and a proceedings book, I have focussed on providing the technical foundation of the computational work. I have moreover successfully applied for compute time via PRACE Tier-0, which beyond our own local resources provides us with 9M CPUh to be used during a one-year period that started in September 2016. The development of the numerical infrastructure generally is on track: I have implemented and tested simplified radiation physics (including an efficient flux-limited diffusion solver and stellar irradiation) into the NIRVANA code during the first reporting period (Gressel 2016). This effort will much increase the realism of the magneto-thermal wind launching simulations, and will help to understand its effect on disk evolution in a more quantitative manner. I have furthermore implemented drag-force-coupled dust fluids, that will be used to study the evolution of small dust up to mm-sized grains, whose mutual distribution with the gas is crucial in interpreting (sub-)mm dust-continuum emission and establish an understanding of the most important physical effects at play in shaping planet-forming regions of circumstellar disks. In a complementary effort, I have implemented finite-Stokes-number coupled tracer particles that can be used to illustrate trajectories of entrained particles, giving clues about the evolution of, for instance, chondrules. To establish the foundation for one of the key aims of the ERC project, the study of full non-ideal MHD (comprised by Ohmic resistivity, ambipolar diffusion and the Hall effect in the one-fluid paradigm), I have implemented the missing third non-ideal effect, namely the Hall effect, in the NIRVANA MHD code (the former two effects were already present in the code at the time of submitting the ERC application). Recently, I have modified the NIRVANA code to support a complex thermodynamic equation of state, including the dissociation and ionisation of Hydrogen-Helium mixtures, important calorimetric effects that will determine the temperature structure of strongly heated gas (for instance, gas falling onto an embedded proto-planet). To put this ground-laying work in context, all of these implementations by themselves could be considered the focus of a post-doc term. In their entirety, the set of implemented features together will allow us to compete at international level in the forthcoming years.

Final results

Protoplanetary disks accrete onto their central T Tauri star via magnetic stresses. When the effect of ambipolar diffusion is included, and in the presence of a vertical magnetic field, the disk remains laminar in its planet-forming inner region, and a magnetocentrifugal disk wind forms that provides an important mechanism for removing angular momentum. We have performed global MHD simulations where the time-dependent gas-phase electron and ion fractions are computed under FUV and X-ray ionization with a simplified recombination chemistry. To investigate whether the mass loading of the wind is potentially affected by the limited vertical extent of our existing simulations, we attempt to develop a model of a realistic disk atmosphere. To this end, by accounting for stellar irradiation and diffuse reprocessing of radiation, we aim at improving our models towards more realistic thermodynamic properties. Determining the accretion rates of T Tauri systems from first principles is a long-standing problem in astrophysics. Providing robust estimates of typical accretion rates based on ab-initio calculations will have a major impact on related research fields, and will provide useful constraints for timescales available for planet formation.