Scientific Rationale, Topics and Sessions
A) ASYMMETRY IN PLANETARY NEBULAE AND BACK TO THE AGB: THE KEY OBSERVATIONAL FACTS
We will first deal with the observational facts. Crucial observations should try to answer the following key questions:
- When, where and how does the asymmetry start?
- What is the observational evidence for proposed mechanisms for the origin of asymmetry (e.g., disks, magnetic fields, fast stellar rotation, binarity) from the AGB to the PN phase?
- Are there critical correlations between asphericity and the main stellar evolution parameters (mass and chemical composition of progenitors, their distribution and population size in different kind of galaxies)?
- What can we learn from similar nebulae in other classes of objects, and which of them are linked to PNe via evolution?
SESSION 1) ASYMMETRY IN THE PN PHASE ...
PNe have amazing shapes, a large fraction of which show more or less marked deviations from the spherical symmetry, up to the most extreme cases showing highly collimated geometries, point-symmetrical structures, small-scale features, etc. In this session, we will review the observational picture, present the new observations, and discuss key objects. Asymmetry is certainly the result of complex shaping mechanisms, but clear and conclusive observations supporting one or the other models proposed so far are missing and needed. Also, clear links to the stellar evolution parameters of their progenitor stars (core mass, metallicity, binarity, ...) are needed, as well as a better understanding of how morphology evolves with the age of the nebulae.
The session includes: multiwavelength observations of the different symmetries, collimated outflows and jets, microstructures, spherical shells around aspherical PNe, correlations with central stars/progenitors parameters in the Galaxy and the MCs.
SESSION 2) ... IN THE PRE-PN PHASE ...
Pre-Planetary Nebulae (PPNe) have traditionally been understood as rare objects representing a transitory phase in the evolution of AGB stars to PNe, and for a long time, only a few objects, like the Egg Nebula, were well-studied. However, in recent years, mainly due to high-resolution imaging surveys with HST, it has become possible to start studying the detailed physical properties for a sufficient number of these objects. PPNe hold the key to understanding the transition from spherical AGB stars to aspherical PNe. In this session we will provide a detailed overview of what we know about the physical structure of PPNe from observations, including key discoveries of PPNe with quadrupolar/multipolar morphologies, and/or very optically-thick but geometrically-thin waists, large expansion ages, low luminosities, and Halpha lines with very broad wings.
Key issues to be addressed will include identifying the physical mechanisms which can produce elongated lobes, collimated outflows and jets, and dusty waists in PPNe, and how and why these get triggered, apparently just as the central primary star starts its post-AGB journey.
SESSION 3) ... AND BACK TO THE AGB
The main question to be addressed in this session is: "Does asymmetry already start on the AGB? If so, to what extent and in which form?". Past observations have often provided contradictory answers, although radio, IR and optical observations over the past decade have shown that, at least in certain cases, the mode of mass loss can indeed change abruptly during the AGB or at its end, often from spherical to more complex shapes, higher flow momenta, and sometimes multiple symmetry axes. In this session we aim to review and update the observational picture. An important goal is to identify trends in morphologies in order to constrain classes of models of stellar mass outflows.
Includes: observations of AGB envelopes and their geometry at different wavelengths and spatial scales, measurements of magnetic fields and rotational velocities in AGB stars, haloes in PNe.
SESSION 4) LESSONS FROM RELATED OBJECTS
Several classes of objects show nebulae with morphological and dynamical resemblance to aspherical PNe. The goal of this session is to understand what we can learn from them about the physical processes shaping PNe, discuss which of these classes of objects are evolutionary linked to PNe, which are not, and the amount of misclassification presently existing among the different groups.
Includes: symbiotic stars, supersoft X-ray systems, YSO with slow jets, Eta Carinae and aspherical nebulae around massive stars, the Red Rectangle and post-AGB binary stars.
B) UNDERSTANDING THE ORIGIN OF ASYMMETRY: CONFRONTING THE HYPOTHESES
The second part of the conference deals with the intepretation of the observations. Key questions are:
- How do different theories fit observations? In other words, which - among the various physical processes proposed - play the main role in shaping PNe?
- How can theory guide future observations by proposing critical observational tests?
SESSION 5) PROCESSES AT WORK DURING THE AGB: STARS INSIDE AND OUT
The ultimate astronomical value of studying the asymmetric morphologies of PNe is simple: how to probe or constrain models of the structure and rapid evolution of AGB stars, the precursors of PNe and the sources of their mass. In this session we hear from experts on the inner workings of AGB stars and the factors that are likely to influence their evolution, such as their initial abundances, viscosity, rotations, thermal pulses, pulsations and other types of instabilities, external torques (from a nearby companion), and the pressure and dynamical effects of their internal magnetic fields.
A critical secondary goal is to determine how observers of the outflows from these stars can best focus their tools to make critical tests of these models, perhaps by observing the surface dynamics and/or fields, or the chemical or isotopic abundances of the resulting nebula.
SESSION 6) BINARY (VS. SINGLE STARS) EVOLUTION
One of the most fundamental questions in the field is what role binary interactions have had in the formation and shaping of PNe. Did most, or even all asymmetric PNe suffer the action of a companion, or are companions only responsible for a subset of the asymmetric PNe? The most direct way to address this issue is to determine the binary fraction and period distribution of central stars of PNe. This is, however, proving difficult observational task and the answer might not be forthcoming.
There are, however, other methods to understand the impact of binarity in shaping PNe. Among these more indirect methods, two stand out. First, we can determine the numbers and parameters of related systems (those that are the immediate progenitors or progeny to the binary central stars). Such systems are white dwarfs in close binaries, subdwarf O and B binaries, AGB and post-AGB binaries, symbiotic stars and all systems that suffered a common envelope phase. Second, we can carry out hydrodynamic simulations aimed at determining the dynamics of envelope gas under the presence of an interacting companion. The pursuit of all three research venues (the central star binary fraction, the characterization of related classes and hydrodynamic modelling), will eventually lead us to an answer to this problem.
Includes: theory for common-envelope systems; theory for detached interacting binaries; population synthesis models for binary and single stars; progenitors and progeny of PNe with central binary stars
SESSION 7) SHAPING MODELS, AND THE ROLE OF MAGNETIC FIELDS AND DISKS
Models describing the shaping of planetary nebulae have become quite sophisticated, as computational power increases and computational techniques improve. It is important to assess the successes, failures, potential, and drawbacks of the various PN shaping models that have been put forward thus far. All such models involve magnetic fields, stellar rotation, binary companions, or circumstellar disks. Some models invoke one of these elements independent of the others, whereas some rely on more than one element to describe a given system.
Which of these elements are truly necessary, and for which systems? What are the key observations that can be used to constrain PN shaping models? Can PN shaping theory be informed by recent progress in formulating and testing models of other disk/jet/outflow systems, ranging from T Tauri stars to active galactic nuclei?
Includes: HD models; theory and MHD modeling; disks and jets formation theories.