Letters of Intent received in 2020

LoI 2022-2129
Planetary Nebulae

Date: 20 June 2022 to 24 June 2022
Location: Krakow, Poland
Contact: Albert Zijlstra (albert.zijlstra@manchester.ac.uk)
Coordinating division: Division H Interstellar Matter and Local Universe
Other divisions: Division G Stars and Stellar Physics
Co-Chairs of SOC: Ryszard Szczerba (NCAC, Torun)
Orsola de Marco (Macquarie University)
Albert Zijlstra (University of Manchester)
Chair of LOC: Ryszard Szczerba (NCAC, Torun)



-Stellar evolution: the AGB-white dwarf connection; stellar winds; binary interaction; eruptive events

- Hydrodynamics: shaping of stellar winds, dusty winds, jet launching, jet-nebula interaction; the asterosphere; shaping by binary interactions

- Astrochemistry: molecular evolution; PAHs and fullerenes; dust formation and destruction; PDRs

- Abundances: atomic physics; photo-ionization and shocks; the forbidden-line vs recombination line abundance discrepancy; primary production and enrichment

- structure and evolution of galaxies: star formation histories, abundance gradients, structural components, galaxy dynamics, hierarchical mass assembly



Planetary nebulae 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 which reaches mass loss rates 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 planetary nebula.

Planetary nebulae are the brightest and most easily observed phase of the evolution of all but the highest mass stars. The mass ejection coincides with their most luminous phase, where the stellar luminosity is of order $10^4$ Lsun. Much of this luminosity comes out in bright emission lines. Planetary nebulae 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. Especially the [O~III] line can be detected to very large distances. Planetary nebulae are used to study galaxy dynamics, abundances and stellar populations in our Milky Way, in external galaxies out to 6 effective radii, and in nearby clusters out to 200 kpc from the cluster’s centers.

But they are more than population tracers. Planetary nebulae trace the most complex and least understood phase of evolution of stars in this mass range. It is the fastest phase of stellar evolution known apart from supernovae: almost all known planetary nebulae formed after the last glacial period, 10,000 years ago! The stellar heating rates have been measured directly from decade-long studies of the nebulae and the expansion of the nebulae has similarly been directly measured. They have opened up the time domain in our understanding of how stars evolve.

Planetary nebulae also trace special cases of evolution. Many central stars have been found to be post-common-envelope systems: they trace the immediate aftermath of this poorly understood phase, with meaningful connections to transients and gravitational wave sources. Thermal pulses (degenerate helium ignition) can occur close to the surface of the central star and lead to dramatic evolution and ejection of a H-poor nebula: several such cases are now known among planetary nebulae.

Abundance studies are based on both forbidden and recombination lines. They are used to measure accurate abundances of the light elements, but also higher-row elements including s-process products. Planetary nebulae have been used to study abundance gradients in galaxies, and the origin of galactic halos. The forbidden and recombination lines are now known to trace different parts of the nebulae, with different absolute abundances. The abundance discrepancy appears linked to binary interactions, although the mechanism is not understood.

Planetary nebulae 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 molecules that are observed 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. Planetary nebulae are the strongest PAH emitters known. Extraterrestrial fullerenes were first found in planetary nebulae, and the first graphene studies are now appearing. Planetary nebulae are active sites of both dust formation and dust destruction, and some presolar grains have a planetary nebula origin.

The shaping of planetary nebulae has become a focus of interest. The nebulae show a variety of morphologies from elliptical and round to bipolar and butterfly, and in addition contain many microstructures. Jets have been found to play a significant role. Very recently similar shapes have been found in the progenitor AGB stars where they appear to be related to binary interactions. Magnetic 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.

In galaxy dynamics, PNe are now used to trace the motions of stars in the outermost regions of galaxies where the dark matter dominates and allow to constrain the mass three dimensional distribution. Because they provide the kinematics out to such large radii in galaxies, it now becomes possible to compare the predictions from cosmological simulations with PNe based kinematics maps and angular momentum profiles. Last but not least, PNe are key tracers to establish the presence of intracluster light in the nearby clusters, whose large velocities indicate that there stars are orbiting in the gravitational potential of the clusters, rather than that from single galaxy halos.

Finally, planetary nebulae are important in astronomy outreach and development. The spectacular images attract widespread public interest. They provide excellent tools for STEM education. Because of the impact and brightness, planetary nebulae are popular as first light for new facilities. And the ease of observations means that planetary nebulae are accessible even to communities with limited observational capabilities.

Planetary nebulae thus exist at the interface of stellar and galactic evolution. The symposium aims to develop the connections between these different areas, and to put the research into planetary nebulae into the context of modern, integrated astrophysics.

The last conference covering the full research field of planetary nebulae was in 2016, in Beijing. In the mean time there have been two specialised meetings: a meeting on shaping of planetary nebulae in Hong Kong in 2017, and a meeting on observational programs held in Leiden in 2019.
Comprehensive meetings on planetary nebulae (IAU Symposia) have been held once every 5 years on average.

There have been major new developments since 2016. On the modeling front, the post-AGB tracks have been radically updated since 2016. The new tracks have three times faster heating rates. This has main consequences, from a new mass scale for the central stars which links to the white dwarf mass distribution, to star formation history, and to the extragalactic distance scale. The new models are now being incorporated into the modeling of the nebulae. The new integral field units have changed the basis of spectroscopic modeling. The new MUSE data especially is allowing 3-d photo-ionization models of the entire nebulae. ALMA has revolutionised our understanding of the earliest phases of planetary nebula formation, and these findings are beginning to be integrated into the evolutionary picture. In the near future, GAIA will provide distances to most of the galactic planetary nebulae, thus eliminating one of the large remaining obstacles to interpreting observations and the key to mining the vast amount of past data. TESS will study close binaries inside planetary nebulae. JWST will provide detailed dust chemistry. PNe as dynamical tracers are now used as benchmark to constraint cosmological simulations at the galaxy scale. The field is changing rapidly and this promises to be be an exciting meeting.