Letters of Intent received in 2018

LoI 2020-2019
The Origin of Outflows in Evolved Stars

Date: 7 September 2020 to 11 September 2020
Category: Non-GA Symposium
Location: Leuven, Belgium
Contact: Leen Decin (leen.decin@kuleuven.be)
Coordinating division: Division G Stars and Stellar Physics
Other divisions: Division B Facilities, Technologies and Data Science
Division C Education, Outreach and Heritage
Division H Interstellar Matter and Local Universe
Chair of SOC: Leen Decin (University of Leuven)
Chair of LOC: Clio Gielen (University of Leuven)



1) Origin of stellar outflows: Theory
a. Low-mass stars (1/2 day)
b. Massive stars (1/2 day)
2) Origin of stellar outflows: Observational diagnostics
a. Low-mass stars (1/2 day)
b. Massive stars (1/2 day)
3) Interstellar enrichment by stellar winds (1/2 day)
a. Heavy-element enrichment
b. Chemical lifecycle of galaxies
4) Binarity (1/2 day)
a. Role in total loss of mass – including gravitational-wave events
b. Role in shaping the outflow
5) Astrochemistry (1/2 day)
a. As diagnostic tool – gas and dust
b. Laboratory and quantum-chemical Astrophysics
6) Observing facilities (1/2 day)
a. Current and Future
b. Through wavelength range (from optical to millimeter)
7) Hot topics (1/2 day)
8) Education (1/2 day)
a. Focus on PhD students and post-docs
i. Theoretical Astrophysics
ii. Instrumentation and proposal writing
iii. Computational astrophysics (state-of-the art numerical codes exploiting HPC facilities)
b. Outreach
i. Children and Teenagers
ii. General Public



The prime parameter determining the evolution of a star is its mass. Any modification to the stellar mass over time has large repercussions on its evolutionary path. Both low-mass and massive stars are known to power strong stellar winds at the end of their life. Such winds carry away both mass and momentum from the star’s surface, with rates that vary as a function of stellar luminosity, evolutionary phase, and chemical composition (i.e. metallicity). The mass-loss rate determines the type of the stellar end product and the amount by which these stars contribute to the chemical enrichment of the interstellar medium, hence providing the building blocks of planets and life. A proper understanding of stellar evolution and of the chemical make-up of the pristine building blocks in the Universe near and far can thus not be achieved without a detailed understanding of the wind physics during the late stages of stellar evolution as a function of the cosmic environment.

Observations and theory provide strong evidence for the fact that the outflows of these evolved stars are the dominant sources for the chemical enrichment of the interstellar medium (ISM). Key species such as carbon, nitrogen, and oxygen are synthesized in the core of low-mass stars with the s-process being responsible for the origin of approximately half the atomic nuclei heavier than iron. In massive stars, the temperature and pressure in the core can reach high enough values for fusion of carbon, oxygen, and then even heavier elements like neon, magnesium, and silicon. Low-mass stars will slowly eject their atmosphere via a stellar wind, transiting through the post-AGB phase into a planetary nebula. Massive stars eject mass via a fast stellar outflow and finally via a supernova explosion, the latter providing one of the sources for r-process elements. While elements are further processed into a large variety of molecules and dust species in the stellar outflows of low-mass stars, an analogous chemistry does not occur in the winds of massive stars. The wind driving mechanism between low-mass and massive stars is also highly different with atomic line driving being the cause for the winds of massive stars, dust continuum driving for low-mass stars, and potentially molecular line driving for red supergiant winds. And while the dynamical chemistry seems highly different in low-mass and massive evolved stars, their outflows share various common characteristics including non-local thermodynamic equilibrium conditions, density inhomogeneities, radiative forces and hence acceleration, creation of bow shocks at the interface with the ISM, etc.

In addition, recent observations provide support for 50-90% of all stars being part of a binary (or multiple) system. Binary interaction impacts the evolution of stars and hence can play a decisive role for the stellar end product that is produced. The ground-braking detection of the first gravitational-wave event in September 2015, which was caused by the merging of a binary black hole system with surprisingly high inferred masses, is just one of numerous examples indicating our limited understanding of winds of massive stars. Recent high spatial resolution observations also have set aside the long-standing idea that the winds of low-mass stars are spherically symmetric with clear evidence of stellar and planetary companions shaping the wind’s morphology.

Astrochemistry provide us with unique tools to disentangle the phase-transition of atoms to small molecules and larger dust grains in environments unlike terrestrial laboratories. It provides us with crucial information to disentangle the prevailing chemical processes occurring in stellar atmospheres, stellar winds, and the surrounding medium. The study of molecular and dust absorption and emission is a key technique in modern astrophysics, particularly through the ability to probe physical environments otherwise hidden from view. In view of the exciting new and unexpected results from facilities such as ALMA, VLB(I), PdBI, SMA, VLT(I) which challenge our understanding of the dominant physical and chemical processes in evolved star’s outflows, it is timely to hold an IAU Symposium centering on this theme. Historically, IAU symposia have focused on one of the various classes of evolved stars exhibiting strong stellar winds and their contribution to the galactic evolution. In the past 30 years, there have been 18 Symposia devoted to Massive stars and supernovae, 7 Symposia on Planetary Nebulae, 3 Symposia on AGB stars, and none specifically oriented toward post-AGB or red supergiant stars. However, no IAU symposium has yet focused on evolved star’s outflows across stellar mass. As such, common ground on which significant progress can build has remained invisible, and hence not explored. Examples include new diagnostic and numerical methodologies of moving from a 1D to a 3D morphology, dealing with an ensemble of atomic/molecular lines, accurate assessment of radiative forces, solving coupled differential equations, exploiting current HPC facilities, diagnostic tools from detailed observations, statistical model fitting tools, retrieval and forward modeling, optimal use of current and future observing facilities etc. This IAU symposium aims to bridge the various communities dealing with the outflows of evolved stars with the prime goal being the creation of breeding grounds for new ideas to arise and new collaborations to grow across communities. As such, discussion sessions and education will be a crucial part of this symposium.

We have identified a number of themes that can be contained in a program held over 4.5 days, that is a morning or afternoon session on each, one of which will be on `hot topics’. Under each theme we shall review the field, presenting recent results, identify the challenges and seek to identify ways in which the field may advance including contributions that can be made by observational, theoretical and laboratory approaches. We will include speakers from the wider community to help integration and, in line with IAU guidance, will ensure a gender and geographic balance, as well as between junior and senior scientists, among our invited speakers.

In addition, at least ½ day will be devoted to education of PhDs and postdocs and to outreach. Training is a crucial aspect for young scientists. We therefore engage to offer various lectures on different themes (including theoretical astrophysics, instrumentation and proposal writing, binary evolution, astrochemistry, numerical coding etc.). Young scientists will get the possibility to follow at least 3 lectures (each being 1.5 hr). At the same time, scientists with proven capacities to teach young children and teenagers will give short (30 min.) lectures on stellar evolution. One evening (10/09/2020) will be devoted to a public lecture for the wide public aiming at reaching a few hundred interested citizens.

Nominations for candidate SOC members (with main expertise domain - location):
- Leen Decin (chair - AGB Stars – Leuven)
- Martha Boyer (AGB Stars – STScI Baltimore)
- Orsola de Marco, Albert Zijlstra (Planetary Nebulae - Sydney, Manchester)
- Hans Van Winckel (Post-AGB Stars - Leuven)
- Alex de Koter (Massive Stars - Amsterdam)
- Dinh-V-Trung (Hypergiants, Hanoi)
- Noam Soker (Binary Stars – Israel)
- Mike Barlow (Supernovae – London)
- Jeremy Yates (Computational Astrophysics - London)
- John Plane, Aki Takigawa (Laboratory Astrophysics – Leeds, Kyoto)
- Carl Gottlieb (Spectroscopy – Harvard)
- Tom Millar (Astrochemistry – Belfast)
- Shazrene Mohamed, Hyuosun Kim (Multi-D Modelling – Cape Town, Taipei)
- Margaret Meixner (Instrumentation – infrared – STScI Baltimore)
- Francisca Kemper (Instrumentation – submillimeter – Academia Sinica + ESO Garching)
- Gillian Wright (Instrumentation – future /JWST - Edinburgh)