5.4 Astrochemistry of evolved stars

Evolved stars undergo dramatic changes, expanding and cooling as they age. These changes drive complex chemical processes in their atmospheres, forming molecules and dust. This topic explores how stars' physical evolution impacts their chemical composition.

As stars shed mass through stellar winds, they enrich the surrounding space with newly formed elements and molecules. This material becomes part of the interstellar medium, providing building blocks for future stars and planets.

Chemical Processes in Evolved Stars

Physical Changes and Their Impact on Chemical Processes

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• As stars evolve off the main sequence, they undergo significant changes in their physical properties, such as increased luminosity, expanded radius, and cooler surface temperatures, which greatly impact the chemical processes occurring within them
• In the later stages of stellar evolution, particularly during the asymptotic giant branch (AGB) phase, stars experience mass loss through stellar winds, which expel material from their outer layers into the surrounding circumstellar envelope

Nuclear Fusion and Convection in Evolved Stars

• Nuclear fusion processes in the core of evolved stars, such as the triple-alpha process and the CNO cycle, produce heavier elements like carbon, nitrogen, and oxygen, which are transported to the outer layers through convection
• The triple-alpha process involves the fusion of three helium-4 nuclei to form carbon-12, while the CNO cycle is a catalytic cycle that converts hydrogen into helium using carbon, nitrogen, and oxygen as catalysts
• Convection in evolved stars efficiently mixes the newly synthesized elements from the core to the outer layers, altering the chemical composition of the stellar atmosphere

Pulsations, Shocks, and Molecule Formation

• Evolved stars exhibit pulsations and shock waves in their atmospheres, which can trigger chemical reactions and facilitate the formation of molecules and dust grains
• Pulsations cause the stellar atmosphere to expand and contract periodically, leading to changes in temperature and density that can drive chemical processes
• Shock waves, generated by the pulsations or other instabilities, can compress and heat the gas, providing the energy needed to overcome reaction barriers and form new molecules
• The cool, extended atmospheres of evolved stars provide favorable conditions for the formation of a wide variety of molecules, including simple diatomic species like CO, CN, and TiO, as well as more complex polyatomic molecules like H2O, SiO, and HCN

Molecule and Dust Formation in Evolved Stars

Chemical Equilibrium and Non-Equilibrium Processes

• The formation of molecules in the atmospheres of evolved stars is governed by a combination of chemical equilibrium and non-equilibrium processes, which depend on factors such as temperature, density, and the availability of atomic and molecular species
• In the inner regions of the stellar atmosphere, where temperatures are higher, chemical equilibrium dominates, and the composition is determined by the minimization of the Gibbs free energy
• As the gas expands and cools in the outer layers of the atmosphere, non-equilibrium processes become more important, and the formation of molecules is driven by kinetic reactions, often involving neutral-neutral or ion-molecule interactions

Dust Formation and Growth

• Dust formation occurs in the cooler outer regions of the stellar atmosphere, where the temperature drops below the condensation temperature of various materials, such as silicates, carbon, and metal oxides (iron, magnesium)
• The formation of dust grains typically involves a two-step process: nucleation, where small seed particles form from the condensation of gas-phase species, followed by grain growth, where additional material condenses onto the surface of the seed particles
• Nucleation can occur homogeneously, where the seed particles form directly from the gas phase, or heterogeneously, where the seed particles form on the surface of pre-existing grains or molecules
• Grain growth proceeds through various processes, such as physical adsorption, chemical reactions on the grain surface, and coagulation of smaller grains into larger ones
• The presence of shocks and pulsations in the atmospheres of evolved stars can enhance the formation of dust grains by providing additional compression and cooling of the gas, as well as by facilitating the mixing of material from different regions of the atmosphere

Role of Shocks and Pulsations

• Shocks and pulsations in the atmospheres of evolved stars play a crucial role in the formation and processing of molecules and dust grains
• Shocks can compress and heat the gas, triggering chemical reactions that would not occur under equilibrium conditions, such as the formation of complex organic molecules (methanol, formaldehyde)
• Pulsations can lead to periodic changes in the temperature and density of the atmosphere, which can drive the formation and destruction of molecules and dust grains
• The mixing of material from different regions of the atmosphere, facilitated by shocks and pulsations, can bring together species that would otherwise be spatially separated, enabling new chemical pathways and enhancing the overall molecular and dust formation rates

Chemical Composition of Circumstellar Envelopes

Oxygen-Rich and Carbon-Rich Envelopes

• Circumstellar envelopes, which form around evolved stars due to mass loss, exhibit a wide range of chemical compositions depending on the initial mass and evolutionary stage of the central star
• In oxygen-rich circumstellar envelopes, which arise from stars with initial masses less than about 8 solar masses, the dominant molecules include H2O, CO, SiO, TiO, and OH, while the dust is primarily composed of silicates and metal oxides
• Carbon-rich circumstellar envelopes, which form around stars with initial masses between about 2 and 4 solar masses that have experienced third dredge-up events, are characterized by an abundance of carbon-bearing molecules like CO, HCN, C2H2, and C2H4, and dust grains composed of amorphous carbon and silicon carbide

Chemical Composition of Planetary Nebulae

• Planetary nebulae, which represent the final stage of evolution for low- and intermediate-mass stars, exhibit a complex interplay between the hot, ionized gas and the cooler, neutral and molecular components
• The ionized regions of planetary nebulae are characterized by emission lines from various atomic species, such as hydrogen (Hα, Hβ), helium (He I, He II), oxygen (O III), nitrogen (N II), and sulfur (S II, S III)
• The neutral and molecular components of planetary nebulae contain a wide range of molecules, including CO, HCN, HNC, CN, and HCO+, which can be observed through their rotational and vibrational transitions in the radio and infrared wavelengths
• The chemical composition of planetary nebulae is influenced by the previous evolutionary history of the central star, as well as by the interaction between the expanding nebula and the surrounding interstellar medium
• The central stars of planetary nebulae, which are hot white dwarfs, emit intense UV radiation that can drive photochemical reactions and ionize the surrounding gas, leading to the formation of photodissociation regions (PDRs) and ionization fronts

Evolved Stars and Interstellar Enrichment

Contributions to the Interstellar Medium

• Evolved stars, particularly AGB stars and planetary nebulae, play a crucial role in the chemical evolution of galaxies by injecting newly synthesized elements and complex molecules into the interstellar medium
• Through their intense mass loss, evolved stars contribute a significant fraction of the total gas and dust in galaxies, providing the raw materials for future generations of star and planet formation
• AGB stars are estimated to contribute about 50-80% of the total dust in the interstellar medium, with the remainder coming from supernovae and other sources
• The outflows from evolved stars contain a wide variety of molecules, ranging from simple species like CO and SiO to more complex organic molecules like acetylene (C2H2), methanol (CH3OH), and polyaromatic hydrocarbons (PAHs)

Processing in the Interstellar Medium

• The dust grains formed in the atmospheres of evolved stars can act as catalysts for chemical reactions in the interstellar medium, providing surfaces upon which atoms and molecules can adsorb and interact
• The surfaces of dust grains can facilitate the formation of molecules through various processes, such as atom addition reactions (H + CO -> HCO), radical-radical reactions (OH + CO -> HOCO), and UV photolysis (H2O + hν -> OH + H)
• The material expelled by evolved stars undergoes further processing in the interstellar medium, where it is subjected to various physical and chemical processes, such as shocks, UV radiation, and cosmic rays, which can lead to the formation of even more complex molecules
• Shocks in the interstellar medium can compress and heat the gas, leading to the formation of molecules like water (H2O), ammonia (NH3), and methanol (CH3OH) through gas-phase and grain-surface reactions
• UV radiation from nearby stars can photodissociate molecules and ionize atoms, driving a complex network of photochemical reactions that can produce a wide range of species, including ions, radicals, and complex organic molecules

Implications for Star and Planet Formation

• The complex molecules and dust particles produced by evolved stars are incorporated into new generations of stars and planets, contributing to the chemical diversity observed in these systems and potentially playing a role in the emergence of life
• The dust grains expelled by evolved stars can serve as the building blocks for planetesimals and planets, providing the solid material needed for their formation and growth
• The organic molecules and ices produced by evolved stars and processed in the interstellar medium can be delivered to planetary surfaces through cometary impacts and meteoritic infall, potentially providing the precursors for prebiotic chemistry and the origin of life
• The chemical composition of protoplanetary disks and exoplanetary atmospheres is influenced by the material inherited from the interstellar medium, which has been enriched by the products of evolved stars over billions of years of galactic evolution