Scientific Rationale for the IAU Symposium
Astrophysical Dynamics: From Stars to Galaxies

We live in a dynamic Universe. Wherever we point our ground-based or space-born telescopes, we see clear evidence of intricate, multi-scale, time-dependent phenomena. The Sun exhibits intense convective and magnetic activity that impacts the entire heliosphere, including our own planet Earth. Other stars, whether they are being formed, live on the main sequence or are on the verge of dying, likewise show intense activity, and the most massive ones die in tremendous energetic events. Their host galaxies are equally active and dynamic. Throughout their formation and subsequent evolution they may undergo dramatic mergers, create and destroy bars, accrete from surrounding cosmological filaments or emit powerful jets detectable as radio lobes. These phenomena and many others occurring in the cosmos undeniably demonstrate that in order to understand our Universe, astronomers in the 21st century must face the complexity of nonlinear dynamical systems head-on with multi-wavelength, multi-scale observations, sophisticated theoretical models, and high-resolution numerical simulations on modern high-performance computing platforms.
This is a very challenging task, but thanks to the universality of the laws of physics and to common fundamental physical processes at work in most astronomical objects, such as gravity, turbulence, mixing, magnetism, dynamo action and dynamical instabilities, progress can be made by appealing to a comprehensive theoretical framework.
Astrophysical dynamics, which encompasses astrophysical fluid dynamics, clearly constitutes such a framework and has already led to substantial progress in modern astronomy. To list just a few notable achievements, we are now able to model the convective layers of stars and their large-scale differential rotation, to describe aspects of the formation of stars and the dynamics of accretion disks and jets, to study the formation of bars and density waves in galaxies, and to simulate galactic mergers. Still, many fundamental challenges remain, such as elucidating the ultimate origins of the 22-year solar activity cycle, the stellar initial mass function, and the bi-modal galaxy color distribution. How do subtle nonlinear interactions among gravitation, magnetism, and inertia give rise to such a diversity of observed phenomena?

The purpose of IAU Symposium 271 on Astrophysical Dynamics: From Stars to Galaxies supported by IAU Division IV and Commissions 10,27,28,35 is to bridge the gap between stars and galaxies, emphasizing commonalities in physical processes, observational techniques and modelling strategies.By bringing together leading experts in solar and stellar physics, galaxy structure and evolution, astrophysical fluid dynamics, and dynamical systems theory, we expect to promote a deeper understanding of complex astrophysical systems and in particular, to educate and inspire young scientists, preparing them for the challenges that lie ahead. The technical issues we will address are often neglected in topical meetings because it is felt that they are too specialized. In this symposium researchers will interact directly and compare their methods and approaches in many cases they are solving the same equations! The lecturers will be encouraged to identify points of contact between apparently disparate systems and to explore promising observing and modelling strategies that may be generalized to broader applications. Ample time will be allocated to such interdisciplinary exchanges in order to promote cross-fertilization.

This conference will also be the occasion to celebrate Professor Juri Toomre's 70th birthday and great achievements in Astrophysical Dynamics.

More specifically we intend to address the following subjects :

The Sun and stars:

The Sun is an enigmatic star, and its proximity allows us to obtain accurate high-resolution observations of its multi-scale dynamics. It is an incredible challenge to explain the latest findings of satellites such as SoHO, Hinode and Stereo, such as torsional oscillations and fluctuating meridional flows, flux emergence at all scales from bright points up to active regions, seething horizontal fields in the solar photosphere, fast reconnection and the dynamic corona.Other low mass stars also demonstrate intense activity as revealed by observational proxies such as soft X-ray emission. The general understanding of such activity is linked to their convective envelopes and coronae, although a detailed description is still forthcoming. Further, all stars rotate, and differential rotation and meridional circulation play a key role in their overall dynamics.

Observational constraints: helio and asteroseismology, magnetism and abundances

One way of assessing solar and stellar dynamics is through helio- and asteroseismology. These indirect techniques have allowed us to infer the sound speed, density and angular velocity profiles of the Sun. Progress is being made on the meridional circulation and for detecting gravity modes. For more distant stars, thanks to Corot and soon Kepler, the depth of the stars convection zone and radial profiles of key quantities (density, sound speed, preliminary rotation profile) are starting to be inferred. Mode identification is crucial in order to progress in probing the interior of stars since both fast rotation and strong magnetic fields can lead to mode mixing and overlapping. Observations of the magnetic activity and rotation profiles are also very important and instruments such as the spectropolarimeter Espadons are achieving important breakthrough. Abundance anomalies (such as the Lithium depletion in solar type stars or overabundance of rare earths in chemically peculiar stars) often indicate that a dynamical process is at work. Such observations serve to guide and constrain theoretical models.

Galaxies:

Galaxies are complex dynamical objects subject to both the gravitational forces of stars and dark matter and the influence of rotation. Their shape (elliptic or spiral) is the result of nonlinear evolution, mergers and interactions with the cold gas in the cosmic filaments in which they are embedded.Filament accretion in particular supplies fresh fuel that can trigger star bursts, promote the generation of bars, and feed active galactic nuclei (AGN). Galaxies possess both a disorganized magnetic field and a large-scale field of about a microgauss, and it is unclear how such a strong magnetic field is maintained. Understanding the internal nonlinear dynamics of galaxies, as well as their interactions, is crucial for a better comprehension of the large-scale evolution of the Universe.

Observational constraints: dynamics, magnetism and abundances 

In galaxies the chemical composition and the abundance of gas are as crucial to assessing their evolutionary stage as is their stellar population.The observation of large-scale magnetic fields through Faraday rotation for instance, places strong constraints on the galactic dynamo. Rotation curves are also important in order to assess the dynamical role of the galactic halo. Such observations serve to both guide and constrain theoretical models of internal galactic dynamics.

Nonlinear Astrophysics

The vast range of dynamical scales and nonlinear physical processes operating in both stars and galaxies poses a formidable challenge from a modelling perspective.High-resolution numerical simulations and dynamical systems theory have become essential tools to enable further progress, fuelled by continuing advances in high-performance computing technology and constrained by ongoing astronomical observations.

Astrophysical Turbulence, Convection, Rotation and Shear

Describing and understanding turbulence remains one of the most challenging problems in fluid dynamics and indeed, astrophysics. As a consequence of the extremely high Reynolds numbers that characterize them (due mostly to their large size), astrophysical objects are generally highly turbulent. Whether dealing with stars or galaxies, astronomers are continually confronted with the formidable task of characterizing isotropic or more generally, anisotropic turbulence, frequently driven or accompanied by thermal convection and rotational shear. International conferences provide a unique opportunity to promote a more comprehensive understanding of such a complex and ubiquitous nonlinear phenomenon.

Astrophysical Dynamos, MHD and instabilities

Magnetic fields are found in most celestial objects, since the most common state of matter in the Universe is ionised gas, i.e. plasmas. Like turbulence, magnetic fields play a key role in determining the evolution of many cosmic objects. Magnetism may trigger instabilities and mixing, may promote or inhibit the formation of complex structures, may power eruptive events such as coronal mass ejections and X-ray flares, and may apply large scale torques; in any case it is clear that understanding and describing magnetic fields, including their origin and topology, is of utmost importance for a complete vision of our dynamical Universe.
Directly connected to the existence of magnetic fields in the Universe is dynamo action. This fundamental process explains how magnetic fields can be maintained for long times against Ohmic dissipation. Understanding the subtle interplay between flows and magnetic fields leading to successful dynamo action is key to a better description of cosmic magnetism.

In the symposium Astrophysical Dynamics: From Stars to Galaxies, to be held in South-East France, in the city of Nice, between the 21st and the 25th of June of 2010, leading experts will review our current understanding of stars, galaxies and their dynamics, emphasizing the commonalities between the various fields and what projects or astrophysical problems could lead to cross-fertilization between the communities. Further previews of current challenges that may be amenable to observational and modelling techniques from other research fields will also be given.Astrophysical dynamics is the Astrophysics of the 21st century and 2010 undoubtedly is an opportune year to formulate a new prospective/perspective on such a vibrant and important research endeavour.