Letters of Intent received in 2021

LoI 2023-2139
Planetary Nebulae: a Universal Toolbox in the Era of Precision Astrophysics

Topics

-Planetary nebulae as tracers of stellar evolution: the giant branch-white dwarf connection; mass loss and stellar winds; binary interactions; eruptive events and transients
- Planetary nebulae as hydrodynamics events: shaping of stellar winds, dusty winds, jet launching, jet-nebula interaction; the asterosphere; outflow shaping by binary interactions
- Planetary nebulae as astrochemistry laboratories: molecular evolution; polycyclic aromatic hydrocarbons and fullerenes; dust formation and destruction; photo-dissociation regions
- Planetary nebulae as abundances decoders: atomic physics; photo-ionization and shocks; the forbidden-line vs recombination line abundance discrepancy; primary elemental production and ISM enrichment
- Planetary nebulae as unique tools to study the structure and evolution of galaxies: star formation histories, abundance gradients, structural components and dark matter, galaxy dynamics, hierarchical mass assembly

Rationale

Planetary nebulae (PN) trace 90% of all stellar death in the Universe. Low and intermediate-mass stars (1-8 Msun) expel their envelopes on the asymptotic giant branch. Between 50% and 80% of the stellar mass is ejected in a catastrophic wind that reaches mass loss rates of up to 10^-4 Msun per year. The remnant core of the star heats up and crosses the HR diagram to the blue, before nuclear burning ceases and the star enters the white dwarf cooling track. During this transition the ejected envelope is ionized, and becomes brightly visible as a PN.
PN are the brightest and most easily observed phase of the evolution of all but the highest mass stars. The PN mass ejection coincides with the brightest phase in the star's life (10^4 Lsun). Much of this luminosity is re-processed into bright nebular emission lines. PN are present in galaxies of all Hubble types. They are common in young and old stellar populations, and are seen even in old globular clusters. The prominent [OIII] line can be detected to very large distances, which makes PN useful tools to study galaxy dynamics, abundances and stellar populations not only in the Milky Way, but up to galaxies 100 Mpc away, where they are found out to 6 effective radii, and in nearby clusters out to 200 kpc from the cluster’s centres. The PN luminosity function provided a critical rung in the distance scale ladder, and is currently being adopted in a latest effort to resolve the Hubble constant discrepancy.
But PN they are much more than population tracers and cosmological tools. They also trace the most complex and least understood phase of the evolution of stars in this mass range. Because of their relatively high evolutionary speed (almost all known PN formed after the last glacial period, 10,000 years ago) stellar heating rates and nebular expansions have been measured directly from decade-long studies serving as stringent model constraints.
The binary origin of an increasing number of PN has been firmly established making us rethink the interpretation of many of the class characteristics in light of a possibly large fraction of binary interactions. This has also shown PN to probe the aftermath of binary interactions and connected them to common envelope transients and gravitational wave emitters. This turned PN, yet again, into a useful tools of binary interaction physics and outflow shaping.
Spectacular PN shapes, from elliptical and round to bipolar, most exhibiting distinct microstructures, have always been of interest. Recent improvements in imaging, interferometry and integral field spectroscopy have allowed to map the details of the shaping histories, the critical role of jets, and the impact of binary interactions. Similarities with other types of nebulae (from symbiotic nebulae to massive luminous blue variable nebulae) are enabling their use in a broad context of stellar outflows. The interplay between magnetic fields as shaping agents and binary interactions as the motor to sustain the fields are also proposed as movers and shapers of the stellar winds. The hydrodynamic models are directly applicable to the fast multiplying studies of mergers and interactions in the context of transients.
Forbidden and recombination emission lines have been used to measure accurate abundances of the elements, including higher-row elements such as s-process products. The forbidden and recombination lines are now known to trace different parts of the nebula, with different absolute abundance values. This abundance discrepancy is not understood, but it appears linked to binary interactions. Abundance measurements are used to understand aspects of stellar evolution and nucleosynthesis, but are also used to study abundance gradients in galaxies, and the origin of galactic halos. As such, a solution to the abundance discrepancy problems is urgently needed.
PN contain the primary products of nuclear burning, especially carbon, nitrogen and s-process elements. They are among the main drivers of galactic chemical evolution in these elements. The nebulae are rich in molecules and dust. All observed molecules have formed in the stellar ejecta themselves, and the nebulae are therefore direct laboratories for astrochemistry. The carbon-enrichment leads to a wealth of carbonaceous molecules. PN are the strongest PAH emitters known. Extraterrestrial fullerenes were first found in PN, and the first graphene studies are now appearing. PN are active sites of both dust formation and dust destruction, and some presolar grains have a PN origin.
Finally, PN are important in astronomy outreach and development. The spectacular images attract widespread public interest. They provide excellent tools for STEM education. Because of their visual impact and brightness, PN are popular as first light targets for new facilities, and their brightness means that they are accessible even to communities with limited observational capabilities.
PN thus exist at the interface of stellar and galactic evolution. The proposed symposium aims to develop the connections between these different areas, and to place PN research into the context of modern, integrated, precision astrophysics.
The last conference covering the full breath of the PN research field took place in 2016, in Beijing. In the meantime, there have been two specialised meetings: a meeting on shaping of PN in Hong Kong in 2017, and a Lorenz workshop aimed to build observational programs, held in Leiden in 2019. A meeting on shaping post-main sequence nebulae, including PN, is about to be held virtually (APN8e October 2021).

Comprehensive IAU symposia on PN have been held once every 5 years, on average.
Since the last PN IAU symposium in 2016 there have been major new developments. Of the 1243 papers with the word "planetary nebula" in the abstract published since, these are the highlights: the post-AGB stellar evolutionary tracks have been radically updated indicating heating rates that are three times faster. This has far reaching consequences, implying lower stellar masses overall. This links to our interpretation of the white dwarf mass distribution, to star formation history, and to the extragalactic distance scale. The new models are now being incorporated into the modelling of the nebulae. The new integral field units have drastically increased the ability of observations to constrain the abundance and chemistry modelling of the nebulae. For example, the new MUSE data is allowing 3-d photo-ionization models of the entire nebulae. ALMA has revolutionised our understanding of the earliest phases of PN formation, and these findings are beginning to be integrated into the evolutionary picture. GAIA is providing vastly improved distances to most galactic PN, thus eliminating one of the large remaining obstacles to interpreting observations and the key to mining the vast amounts of past data. Kepler and TESS data greatly increased the number of close binaries known inside PN further strengthening the connection between binary interaction, PN and transients. Collaborations have been established to exploit the power of the upcoming JWST data, that will soon provide unprecedented details of the molecular and dust chemistry. PNe as dynamical tracers are now used as benchmark to constrain cosmological simulations at the galaxy scale. The field is changing rapidly, with PN playing an increasing role in a range of astrophysical subdisciplines. The proposed symposium is therefore timely and it promises to enable the next generation of discoveries by showcasing recent results.