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Astrochemistry of old stars: direct probing of unique chemical laboratories

Periodic Reporting for period 4 - AEROSOL (Astrochemistry of old stars:direct probing of unique chemical laboratories)

Reporting period: 2020-07-01 to 2021-12-31

Shortly after the Big Bang, the chemical make-up of the universe was dictated by hydrogen, helium, and a very small fraction of lithium. This chemical make-up was gradually enriched through time thanks to the birth and, very importantly, the death of stars. The hot and dense cores of stars serve as nuclear power plants in which elements such as carbon, oxygen, nitrogen, sulfur and phosphor are created. At the end of a star’s life, a strong stellar wind or a supernova explosion inject these new elements into the interstellar medium, out of which new stars and planets are born.

The dominant chemical factory in the universe are `old’ stars. More than about 90% of all stars, including our own Sun, will experience an Asymptotic Giant Branch (AGB) phase at the end of their lives, just before they run out of fuel for nuclear burning. AGB stars are considered to be the major producers in the universe including carbon, nitrogen, oxygen etc. In the cool extended stellar atmospheres, these atoms combine into molecules and dust grains. Through their stellar winds, these stars contribute ~85% of gas and ~35% of dust to the total enrichment of the interstellar medium. Understanding how much material is available and what type of material is formed, helps us to understand how the universe changes over time, as well as our place in it.

The central aim of this ERC grant was to unravel the chemical and dynamical evolution of evolved stars with initial mass between 0.8 to ~20 solar masses. We therefore needed to disentangle the intriguing coupling between the main micro-scale chemical and macro-scale dynamical processes throughout the complete wind region of a large population of AGB stars. This asked for a multi-disciplinary project involving (i) high-quality observations, (ii) novel theoretical models for AGB stellar winds, and (iii) targeted quantum chemical calculations and laboratory experiments to deduce the rate constants of key reactions occurring in the stellar winds. This intersection between astronomy, (astro)physics, mathematics and chemistry was crucial for this project to succeed.
This ERC project led to a myriad of new astronomical observations (WP1), maybe most importantly the high-impact ALMA Large Program ATOMIUM (PI Decin). This 3.4 million € project, with~45 international collaborators was the first ALMA Large Program accepted in the broad field of `Stellar evolution and the Sun' and lead to the discovery that stellar and planetary companions shape the winds of almost all evolved stars, and of which the first ground-breaking results were published in Science (Decin et al. 2020). A plethora of other papers based on observations of this ERC project has meanwhile been published or accepted for publication, including the high-impact Nature paper Montargès et al. (2021) on the Great Dimming of Betelgeuse, which reached a wide international audience.

These observations were the basis of a detailed analysis effort (WP2) to constrain the thermodynamic and chemical properties of these environments, allowing us to find new correlations and determine causal effects, thereby shedding new light on the prevailing mechanisms determining the late stages of stellar evolution of low and intermediate mass stars. These results were published in several papers including Danilovich et al. 2018, Van de Sande et al. 2018, Danilovich et al. 2020, Gottlieb et al. 2021.

During this ERC-CoG we developed a novel, worldwide unique, numerical model for simulating the winds of evolved stars (WP3), combining knowledge on (the temporal behavior of) the thermodynamic properties, chemical kinetics, and radiative transfer, and implementing novel ideas based on linear algebra, GPUs and machine learning.It is the combination of these sophisticated models with the rich observational data, which allowed for transformative research. Some relevant papers include Malfait et al. 2021, Maes et al. 2021, De Ceuster et al. 2020, El Mellah et al. 2020, De Ceuster et al. 2020, Van de Sande et al. 2019.

An important part of this project was the study of laboratory and theoretical (quantum)chemistry (WP4), and was mainly executed in collaboration with the colleagues from the University of Leeds. Here we tried to unravel the chemical pathways towards the formation of the first dust seeds in stellar winds, based on results from WP2 and WP3 to dictate the choice of relevant species to be studied. We performed complex quantum-chemical calculations to unravel the geometry and chemical activity of large gaseous clusters, being precursors of the dust grains. These theoretical calculations were complemented with laboratory experiments to determine the reaction rate coefficients at varying temperatures. Example publications include Mangan T. et al.; Gobrecht et al. 2021, Gobrecht et al. 2021, West N. et al. 2019.

This interdisciplinary approach, including collaboration with students and senior scientists from physics, chemistry, mathematics, computational sciences, and engineering sciences, proved invaluable, and made this ERC-CoG so successful, resulting in 70 peer-reviewed journal publications, including high-impact publications in flagship journals such as Science, Nature, Nature Astronomy, and Annual Review of Astronomy & Astrophysics.
Several aspects of this project are far beyond the state of the art. These include the set of high-quality observations for a large and well-chosen sample of targets, the in-house development of novel theoretical wind models unique in its kind and optimized for modern computer architectures, and new laboratory and theoretical results on the gas-phase reaction rate constants and formation pathways of cosmic nano-particles, of invaluable use for other fields in (astro)physics.

The most profound breakthrough of this CoG can be summarised as being `the completely new and unprecedented 3D view on the winds of evolved stars’, where we have shown that
- We need to understand the ‘3D life’ of molecules and solid-state aggregates to identify the first dust seeds formed in these environments
- (3D) clumps are omnipresent and pose a challenge for the modelling and for retrieving the hydrodynamical and chemical properties of the stellar winds
- Unrecognised binary interaction has a long-lasting impact on the (3D) wind morphology: current (1D) mass-loss rate formulae might be wrong by order(s) of magnitude, rendering contemporary stellar evolution predictions highly uncertain.

The importance of the novel results of this CoG were also recognised upon receiving an invitation to write a review paper for the premier international journal in our field, Annual Review of Astronomy & Astrophysics (Decin L., 2021, ARA&A 59, 337-389, + 15 pages of Supplementary material ). Moreover, these results formed one (of the two) pillars of an international IAUS Symposium (Chair: Leen Decin) - see iaus366.be that was held from 1-5 November 2021, with more than 350 international attendees, of which the outcome will be reflected in a proceedings book (to be published in 2022).
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