Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - AEROSOL (Astrochemistry of old stars:direct probing of unique chemical laboratories)

Teaser

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...

Summary

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. Thanks to dredge-up events and convective patterns, these new elements are brought to the stellar surface. At the end of a star’s life, a strong stellar wind or a supernova explosion inject these newly manufactured elements into the interstellar medium, out of which new stars and planets are born. As time evolves, the metal content of the universe increases and elements such as carbon reach large enough abundances for life to eventually develop on newborn planets.

The dominant chemical factory in the universe are `old’ stars. More than about 90% of all stars with initial mass above 0.8 solar masses, 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. Besides carbon, AGB stars are considered to be the major producers in the universe of nitrogen and of various elements heavier than iron (such as barium and lead). They also make important contributions to the production of fluor and magnesium. In the cool extended stellar atmospheres, these atoms combine into molecules and large gas-phase clusters. If densities are high enough and temperatures low enough, the molecules condense and form 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 is to unravel the chemical and dynamical evolution of all evolved stars with initial mass between 0.8 to ~20 solar masses. We therefore need 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 stars which are already in the AGB phase. This is the only way to enable scientists to resolve, for example, a long-standing puzzle on the future evolution of our own Sun. The Sun is now halfway its stellar evolution and will become an AGB star in ~4.5 billion years. However, we still cannot predict the kind and amount of molecular and dust species that our Sun will manufacture in its stellar wind, we do not know yet how strong the mass-loss rate will be or the speed of the wind which will inject the material into the solar neighborhood. Knowing how the Sun will evolve will tell us about the potential future of our own Earth. As of yet, it is not clear whether (or how long) the Earth will survive this dramatic change of the Sun into a gigantic giant star. The outcome of the Earth’s future is, amongst many aspects, dependent on the stellar wind the Sun will develop. This is only one example out of numerous that we wish to tackle with this ERC-CoG grant. Other examples include the puzzle of the origin of non-homogeneous mass loss in evolved stars, the quest for the first dust seeds that will form (both composition and size), the chemical pathways followed for the next steps in the dust formation and growth, the physical cause for the transformation of a roughly spherically symmetric wind during the AGB phase to the more complex (often bipolar or multipolar) morphology seen for their descendants, etc.

Answering these multitude of questions asks 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 reac

Work performed

The various disciplinary work packages (four in total, linked to the four specific objectives formulated above) have each obtained already several exciting scientific results. At this stage, we are midway of the project and the intersection between the work packages (WPs) starts to flourish. As usual, each WP first needed to establish firm grounds before the cross-breeding to other WPs could start.

With the specific aim of understanding the chemistry of dust precursors and dust formation in the winds of AGB stars, as well as a more general aim of identifying chemical processes applicable to other astrophysical environments (including novae, supernovae, protoplanetary nebulae, interstellar shocks), the main choice of targets for this ERC-CoG grant are oxygen-rich stellar winds (having a carbon-to-oxygen ratio smaller than 1). Aiming to quantify the total enrichment of the interstellar medium, a fraction of carbon-rich stars is added to the sample. An ensemble of observing techniques is used to unravel the stellar winds of the selected targets, including ALMA, Herschel, SPHERE, MIDI, IRAM, PdBI, GALEX etc. so to have both low and high-spectral resolution data, photometric data, line profiles, images, and interferometric data. The main focus is on the infrared-millimeter wavelength domain due to the strong gas and dust diagnostics in this region. The ERC-CoG team has been very successful in writing peer-reviewed observing proposals, the majority of which has been accepted. To top it all, an ALMA Large Programme (113 observing hours, PI. L. Decin, acronym ATOMIUM) centering on the theme of this ERC-CoG was accepted in July 2018! For the first time, an ALMA Large Programme in the field of stellar evolution has been accepted, and the amount of allocated time is unparalleled. Almost a Petabyte of ALMA data will be received, reduced and analyzed. This is a unique event stressing the importance of evolved stars within the broad scope of astrophysics. Of equal importance is the fact that it builds directly on the interdisciplinary approach of this ERC-CoG where the collaboration between astronomers, physicists, chemists, and mathematicians is key to achieve scientific results significantly surpassing the state of the art. While an ERC grant is granted to a single PI, it is upon him/her to raise a strong and diverse team which is successful in this type of high risk/high gain (observing) proposals.

The ALMA ATOMIUM Large Programme is the most important observing proposal with data secured for this ERC-CoG. It is complemented by various (smaller/normal) observing projects with complementary instruments proposed by team members. The PI, PhD students and post-doctoral fellows focusing on the retrieval of the physical and chemical characteristics of the stellar winds have reached already some unprecedented results based on the data already obtained, including the detection of FeO, accurate determination of the sulfur budget, detection of AlO with direct implications for the presence of large gas-phase aluminium-oxide clusters (precursors to the first dust seeds), study of the cyanopolyynes and carbon chains in a carbon-rich wind, first detection with ALMA of a disk around an evolved star which possibly harbors an exoplanet, first detection of spirals around oxygen-rich stars pointing towards the action of a binary companion etc. These results are obtained from modeling the observations using a so-called retrieval approach. Centered around a radiative transfer module are various modules describing the physical or chemical setup in more (or less) detail. Sophisticated statistical tools are then used to scan the large parameter space and deduce the chemical and physical parameters of the stellar system.

Hand-in-hand goes the development of novel theoretical wind models. These models encompass much more physics and chemistry than the models used to retrieve the wind parameters. The basic idea is to develop an ab-initio theoretical model avoi

Final results

Several aspects of this project are well beyond the state of the art. This includes the set of high-quality observations for a large and well-chosen sample of targets, the use of adequate retrieval codes yielding unprecedented results on the chemical make-up of the stellar winds, the development of a novel theoretical wind model 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. More details are provided in the box above.

This ERC-CoG is now halfway and the four work packages have seen significantly progress during the first 2.5 years. The work package focusing on the chemical reaction rates and formation pathways encountered a slower start-up due to unforeseen circumstances, but it is now catching up with expectations in a very fast way.

Now that each work package is well established, they should and will culminate through the interdisciplinary approach envisioned for this ERC-CoG. While this interdisciplinary approach was identified as a high risk/high gain factor of the project, we are convinced that we will succeed. This statement is driven by the very successful ERC-AEROSOL meeting recently organized in Leuven (July 2018) where almost 20 scientists involved in or linked to the AEROSOL project have gathered. Time seemed too short to discuss all interesting ideas proposed by the astrophysicists and chemists around the table. Various interfaces with direct impact were identified and input was exchanged powering the cross-breeding between the work packages.

The most profound breakthrough expected for this project is the determination of the first dust condensate forming in the wind of (solar-type) evolved stars. At this moment, the theoretical wind models favor TiO2, while observations and considerations based on the analysis of meteorites seems to favor Al2O3, but neither of them is conclusive at this stage. To resolve this conundrum, (1) the ALMA ATOMIUM Large Programme (with an adequate set of complementary observations) and (2) the determination of new chemical pathways for the formation of aluminium-oxide and titanium-oxide species and nano-grains are two critical components. Both components feed directly the retrieval and forward wind models, but from a different end.

If we can determine the composition of the first dust condensate, we have the key in hand for a better estimation of (heterogeneous) dust growth in these environments, and the impact of it on the molecular content and the wind driving. This study will reveal with enormous detail how gas and dust interact with each other, how photons interact with the material and impact thermodynamical wind structure. The legacy value will be (1) the plethora of ground-breaking results in the field of evolved stars, astrochemistry, and the chemical life cycle of the interstellar medium; and (2) the newly acquired astrochemical knowledge that will strongly impact research on supernovae, protostars, interstellar clouds, active galactic nuclei, etc. — environments in which it is far more difficult to grasp the dust nucleation process. As stipulated above, these results will elucidate how the Sun will evolve once becoming an AGB star and how this evolution will impact (habitability) on the solar system planets.

To reach this overarching goal, several new results are expected from the different work packages including the acquirement of new observations linked to the ALMA ATOMIUM Large Programme, a better understanding of (i) the evolution of gas and dust content throughout the AGB evolution, (ii) the impact of density inhomogeneities (clumps, arcs, spirals, disks, …) on the chemical and dynamical wind structure, (iii) the impact of a binary companion on the wind structure, (iv) the formation pathways toward aluminium-oxide, titanium-oxide and silicate dust species, (v) temperature-dependent reaction rate constants for

Website & more info

More info: https://fys.kuleuven.be/ster/research-projects/aerosol.