Organisers

• Jean-Piere Bertoglio (Ecole Centrale de Lyon, France)
• Rolf Walder (Ecole Normale Superieure de Lyon, France)
• Baraffe Isabelle (Ecole Normale Superieure de Lyon, France)
• Chabrier Gilles (Ecole Normale Superieure de Lyon, France)
• Alexei M. Khokhlov (University of Chicago, USA)
• Ewald Mueller (Max Plank Institute for Astrophysics, Garching, Germany, Germany)
• Richard Klein (University of California, Berkeley, USA)

Supports

   CECAM

Description

Computational astrophysics has rapidly developed in the recent years, enabling, since only recently, multi-scale, multi-physics simulations of entire stars, planets, galaxies, and cosmic explosions (see e.g. Mueller 2004). The main goal of the proposed workshop is to address several physical processes and computational methods which are common to the study of very different astrophysical problems. The main concerned fields are (i) multi-dimensional stellar structure and evolution, (ii) planetary atmospheric circulation, and (iii) explosive fate of stars. The common ground of these ields are multi-scale, magneto-hydrodynamical flows and turbulence in different facets. And all the topics are interdisciplinary with strong connections between different research areas. The research on all these fields is currently very active and fashionable, characterized by important investment from the national and international communities devoted to both observational (ground- and space- based instruments) and computational developments (developments of large computer facilities, such as IDRIS on the French national level, or PRACE on an European scale).

Within the past decade, computer facilities and development of computational methods have literally exploded, allowing huge progress in our understanding of star and planet formation, a complex field which combines radiative- and magneto-hydrodynamics, the physics of shocks, supersonic turbulence and elaborated computational methods such as adaptive mesh refinement techniques (see e.g Chabrier et al. 2007).  Also, recent numerical development devoted to the study of multi-dimensional hydrodynamical processes, such as turbulent convection, rotation, magnetic field and dynamo generation, mixing processes, now allows a better understanding of stellar structures through the development of new generations of multi-dimensional stellar models(Deaborn et al. 2006; Browning 2008; Mocak et al. 2009).

With the discovery of extrasolar planets, new laboratories for the physics of planet atmosphere has been found, opening new avenues for the understanding of atmospheric circulation and climatology, not only on Earth but also on these new worlds (Showman et al. 2008). This field is now extremely active and represents nowadays a perfect example of interdisciplinarity, requiring complementary expertises from fluid dynamics, radiative transfer, planetology, climatology and the development of sophisticated numerical simulations.

Cosmic explosion are driving the chemical evolution of the universe. There are two basic mechanism for this. The first one is based on nuclear detonation fronts in the degenerate matter of white dwarfs. The main computational problem here is to combine the flame scale with the global convection scale. To treat the flame propagation, different numerical techniques are applied, from direct numerical simulations (Zingale et al. 2005) to level set methods (Röpke, Woosley, & Hillebrandt 2007). The second mechanism is based on neutrino driven explosions of massive stars, requiring complex radiative transfer calculations on the basis of Boltzmann equations for photons (Janka et al. 2005) and a good treatment of the magnetic dynamo driven by the magnetic-rotational instability (Obergaulinger et al. 2006). A subclass of this process has a connection to Gamma-Ray-Bursts.

Turbulence is one of the physical processes present in all fields. Cosmic turbulence is is often highly compressible and even supersonic (e.g.  Kritsuket al. 2007). Turbulence models are starting to be developed for these environments (Schmidt et al 2006). While astrophysics can profit from the waste experience of engineering and fundamental physics, space provides an excellent laboratory for those regimes of turbulence not present on Earth.

The organizers have recognized expertises in different fields covered by the topics of the workshop, namely stellar physics and evolution (Chabrier & Baraffe 2000; Baraffe et al. 1998), star and planet formation (Chabrier et al. 2007), multi-dimensional simulations and computational astrophysics (Walder & Folini 2000; Walder et al. 2008) and code development (Walder & Folini 1999).

Scientific Objectives

In astrophysics, computational research is outstanding for several reasons. The latest generations of telescopes give an unprecedented amount of observational data. However, a proper interpretation of such data always depends on computational models which explore the origin and propagation of the observed photons. On the other hand, astrophysical objects are multi-scale, multi-physics objects. Flows, magnetic fields, and radiation are in perpetual dynamical interplay. Non-linear couplings among the different ingredients are crucial for the understanding of objects like stars, planets, galaxies, and cosmic explosions like supernovae.

The objectives of the proposed workshop are to evolve the state of the art of numerical simulations of multi-scale, multi-physics objects. The increasing power of modern computing facilities, the strong development of numerical algorithms, and the progress in data visualization and analysis tools now promises realistic simulations of such objects over the relevant time scales. Quantitative predictions for a direct comparison with observational data thus become possible. However, the new tools also pose new challenges. Codes for thousands of processors have to be developed. Strategies for the storage and the analysis of huge data are necessary. Much work is needed to bring together the advances on all these fronts, to construct the necessary computational codes and data analysis instruments, and to built up the experience of the scientists for a proper handling of such tools. The proposed workshop aims to support this process.

We want to highlight the raised points along three outstanding astrophysical questions: extrasolar planets and livable habitats, stellar structure and dynamics, and explosive events such as novae, supernovae (SNe), and gamma ray bursts (GRBs). The three fields are prominent in astrophysics and will allow to deepen our understanding on the origin and evolution of the chemical elements and of life in the universe. About half of the elements more massive then iron are built up in SNe and GRBs, in the so-called r-process. Moreover, such stellar explosions play also a key process in measuring the universe as they act as standard candles. And extrasolar planets are perhaps the most exciting field in today's science as they clearly will deepen to an unprecedented degree our understanding of live and will definitely redefine our view of human beings in the universe. 

The concentration on these three topics will help to make the discussion on the numerical issues concrete and focused on actual tasks. It will also help to assemble excellent scientists. As different as the three topics appear at first sight, they have in common many of the physical key ingredients: radiative and magnetic flows, questions of stability and turbulence, temporal and spatial scales which cover many orders of magnitudes. From the computational point of view all these fields are closely linked. They demand high performance, massively parallelized computational tools and complex data analysis. They are therefore ideal catalysts to discuss, develop, and test advanced computational techniques.

Another common aspect of the three topics is that they are all linked with other branches of science where scientists similarly attempt to make progress in the development of computations. Clearly, earth science, climatology and weather predictions are close to the exploration of extrasolar planets, where the astrophysicists are starting to develop dynamical models of planet atmospheres. From the point of view of flow theory, stellar structure and evolution poses similar problems as the investigation of atmospheres: flows of a highly stratified, rotating medium, driven by radiative energy input. Surprisingly, this is even true for explosive events like supernovae, where this basic flow type also plays a crucial role. There, in addition, high-energy particles are a key dynamical players. The proposed workshop will consider these similarities and invite outstanding scientists from the fields of climate and weather research, of earth dynamo, of particle simulation of accelerator devices, and of theoretical fluid dynamics. In addition, because all these scientific communities need more knowledge on numerical and informatic tools, we

also want to invite experts from these fields, in order to learn about new algorithms and techniques for visualization and data analysis. We are convinced that mutual benefit will result from a discussion with people confronted with similar problems.

In summary, the objectives of the proposed workshop are to discuss the demanding open questions of complex multi-scale, multi-physics simulations. We suggest to discuss this along outstanding research fields in astrophysics and by inviting scientists from related fields - to exchange ideas and developing solutions; for a mutual benefit. We are convinced that the proposed workshop will have a strong impact on the development of computational astrophysics and of the computational mastering of multi-scale, multi-physics in general.

References

Baraffe et al. 1998, A&A, 337, 403
Browning 2008, ApJ, 676, 1262
Chabrier & Baraffe 2000, ARA&A, 38, 337
Chabrier et al. 2007, Protostars and Planets V, p. 623
Janka, H.-T. et al. 2005, Nuclear Physics A, Vol. 758, p.19-26
Kritsuk, A. G.; Norman, M. L.; Padoan, P.; Wagner, R. 2007, ApJ 665, 416
Mocak et al. 2009, A&A, 501, 659
Müller, Ewald, 2004, Computer Physics Communications, Vol. 169
Obergaulinger, M.; Aloy, M. A.; Dimmelmeier, H.; Müller, E. 2006, A&A 457, 209
Röpke, F. K.; Woosley, S. E.; Hillebrandt, W. 2007, ApJ 660, 1344
Showman, A., Menou, K, Cho J. 2008, ASP Conference Series, Vol. 398, p. 419
Schmidt, W.; Niemeyer, J. C.; Hillebrandt, W.; Röpke, F. K. 2006 A&A 450, 283
Walder & Folini 2000, Ap&SS, 274, 343
Walder et al. 2008, A&A, 484, L9
Walder & Folini 1999, http://www.astro.phys.ethz.ch/staff/folini/private/research/a_maze/a_maze.html
Zingale, M.; Woosley, S. E.; Rendleman, C. A.; Day, M. S.; Bell, J. B. 2005, ApJ 632