Puerto de la Cruz (Tenerife, Spain) has hosted a Symposium of the International Astronomical Union on Planetary Nebulae (IAUS 283 Planetary Nebulae: an Eye to the Future; 25-29 July 2011).
The Planetary Nebula phase is a short stage in the late evolution of low- and intermediate-mass stars (roughly, 1 to 8 solar masses). When such stars run devoid of nuclear fuel in the core, they undergo a rapid evolution that culminates with the ejection of the outer envelopes of the star. As the envelope is dispersed into the space, the core of the star starts shrinking, getting hotter and hotter. The UV radiation emitted in this phase ionizes the expanding envelope, making it visible in emission lines. This basic pictures is complicated by a number of factors, such as binarity, magnetic fields, stellar rotation etc, all of which result in an incredible wealth of PN morphologies.
PNe provide key information for a number of different fields in astronomy. They can be used to constrain stellar evolution theories, shedding light on important but poorly known phenomena such as mass loss mechanisms, the efficiency of convection and chemical mixing in stellar interiors, and stellar nucleosynthesis. They are also laboratories in which hydrodynamic shaping mechanisms (such as binarity, magnetic fields and rotation) can be tested before being applied to more complex objects. PNe are key contributors to the chemical enrichment of galaxies, especially for what nitrogen and carbon are concerned, so they are a key ingredient in chemical evolution models. In addition, they map the current chemical composition in galaxies and can be used to determine abundance gradients. In outer galaxies, they can be used as standard candles.
The conference was structured around four main topics:
- New results from observations
- The stellar evolutionary connection
- The cosmic population of galactic and extragalactic PNe
- Future endeavours in the field
Each topic was introduced by a generous number of review talks which made it possible to rapidly plunge into each subject. A very short summary of the issues recurring across topics includes the following:
- As the data improve in quality and quantity, the definition of a what a PN is becoming more and more stringent. Current surveys dedicate a lot of effort to discriminate between true PNe and impostors.
- Morphology, a “classic” category in PNe studies, is still the focus of a large number of investigations, which aim to explain the diversity of shapes in terms of very different mechanisms, such as binarity, accretion disks, or projection effects.
- Technology is making it increasing possible to expand the sample of PNe known beyond our Galaxy and a number of talks at the Symposium have reported on large surveys in other galaxies.
- The accuracy of atomic data for low-weight ions is still a bleeding edge and a limiting factor in PNe studies. Particularly uncertain are low T dielectronic recombination and, to a lesser degree, charge transfer.
- Atomic data for heavy elements (in particular, s-process elements) are badly needed.
- While it is true that atomic data are needed to study PNe, PNe are natural laboratories to test atomic data, so it is should be of mutual interest of nebular astrophysicists and atomic physicists to understand which ions and which processes are crucial and prioritize the future computations accordingly.
Communicated by Valentina Luridiana and Jorge García Rojas from IAC, Tenerife, Spain.