5.4 Astrochemistry of evolved stars
7 min read•august 14, 2024
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 , 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 , , and
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 (, )
- 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, , and , 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), (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
Key Terms to Review (24)
Ammonia: Ammonia (NH₃) is a simple nitrogen-containing molecule crucial in astrochemistry, serving as a fundamental building block for more complex organic compounds. Its presence in various astronomical environments, such as interstellar clouds and planetary atmospheres, provides key insights into chemical processes that shape celestial bodies and the evolution of the universe.
Asymptotic Giant Branch Star: An asymptotic giant branch (AGB) star is a late-stage stellar evolutionary phase characterized by significant expansions and increases in luminosity as a star exhausts its nuclear fuel. During this phase, these stars experience thermal pulses, leading to the creation of heavy elements through nucleosynthesis, and play a critical role in enriching the interstellar medium with these newly formed materials.
C2H2: C2H2, also known as acetylene, is a colorless gas that is the simplest alkyne, consisting of two carbon atoms and two hydrogen atoms connected by a triple bond. In the context of evolved stars, acetylene plays a significant role in the chemical processes that occur during stellar evolution and is an important molecule in the study of interstellar chemistry and star formation.
C2H4: C2H4, also known as ethylene, is a simple hydrocarbon that plays a crucial role in astrochemistry, particularly in the chemistry of evolved stars. It is a colorless gas at room temperature and is known for its role as a plant hormone and in various industrial processes. In the context of evolved stars, C2H4 is significant due to its formation in the complex chemical reactions that occur during the later stages of stellar evolution, particularly in carbon-rich environments.
Carbon burning: Carbon burning refers to the process occurring in the cores of massive stars where temperatures exceed approximately 600 million Kelvin, enabling carbon nuclei to undergo fusion into heavier elements such as neon, magnesium, and sodium. This process is critical in the lifecycle of evolved stars, marking the transition from helium burning to the formation of even heavier elements, contributing to the complex chemical enrichment of the universe.
Carbon monoxide: Carbon monoxide (CO) is a colorless, odorless gas that plays a crucial role in astrochemistry as a key molecular species in the interstellar medium and various astrophysical environments. It is significant for understanding chemical processes and interactions among molecules, particularly in regions where star formation occurs and around evolved stars.
Carl Sagan: Carl Sagan was an influential American astronomer, cosmologist, author, and science communicator who played a key role in popularizing science. He is best known for his work on the search for extraterrestrial intelligence and the study of planetary atmospheres, as well as for his iconic television series 'Cosmos.' Sagan's passion for exploration of the universe and the quest for understanding life beyond Earth connects deeply with the exploration of biosignatures and the potential existence of extraterrestrial life.
Formaldehyde: Formaldehyde is a simple organic compound with the chemical formula CH₂O, consisting of a carbonyl group bonded to two hydrogen atoms. This compound is significant in astrochemistry as it is one of the simplest aldehydes and plays a crucial role in the formation of complex organic molecules in space, influencing various processes including those related to the historical context of astrochemical discoveries and the study of interstellar molecules.
H2O: H2O, or water, is a simple molecule composed of two hydrogen atoms covalently bonded to one oxygen atom. It is essential for life and plays a significant role in various chemical processes in astrophysical environments, influencing the formation and evolution of celestial bodies and the chemistry of evolved stars.
Harlow Shapley: Harlow Shapley was an American astronomer known for his work in the early 20th century, particularly in mapping the Milky Way and determining the location of the Sun within it. His contributions to the understanding of stellar populations and the structure of our galaxy laid the groundwork for modern astrophysics, especially in the study of evolved stars and their chemical compositions.
HCN: HCN, or hydrogen cyanide, is a colorless, highly toxic gas that can exist in various chemical environments, including space. In astrochemistry, it is particularly important as it serves as a building block for more complex organic molecules and is found in the atmospheres of evolved stars and interstellar clouds. Its detection through spectroscopic techniques reveals insights into the chemical processes occurring in space and the potential for organic chemistry in the universe.
Helium burning: Helium burning is a nuclear fusion process that occurs in the cores of stars, where helium nuclei combine to form heavier elements like carbon and oxygen. This process typically happens after hydrogen in the core is exhausted and the star has evolved into a red giant or supergiant phase, marking a significant stage in stellar evolution and the chemical enrichment of the universe.
Infrared spectroscopy: Infrared spectroscopy is an analytical technique used to identify and study the molecular composition of substances by measuring their absorption of infrared light. This method is crucial for understanding molecular vibrations and can reveal information about functional groups in molecules, which connects it to various astronomical contexts, such as the detection of molecules in space and the study of celestial bodies.
Interstellar dust: Interstellar dust refers to tiny solid particles found in the space between stars, primarily composed of elements like carbon, silicon, and oxygen. These grains play a crucial role in various cosmic processes, such as star formation, chemical reactions, and the thermal balance of the interstellar medium.
Isotopic ratios: Isotopic ratios refer to the relative abundance of different isotopes of an element in a sample. These ratios are crucial for understanding various astrophysical processes, as they provide insights into the chemical composition and history of celestial objects, including their formation and evolution.
Metallicity: Metallicity refers to the abundance of elements heavier than hydrogen and helium in a celestial object, often expressed as a ratio of these heavier elements to hydrogen. This term is crucial for understanding the chemical evolution of galaxies, as higher metallicity usually indicates a more evolved and mature stellar population, influenced by processes such as supernovae and stellar nucleosynthesis. Metallicity also plays a significant role in the formation and composition of stars, particularly in high-redshift galaxies where conditions differ from those in the present universe.
Methanol: Methanol, also known as methyl alcohol, is a simple alcohol with the chemical formula CH₃OH. It plays a crucial role in astrochemistry, being one of the simplest organic molecules found in various astronomical environments, including interstellar space and comets, and is significant in understanding the chemical processes that occur during star formation and evolution.
Molecular Cloud Formation: Molecular cloud formation refers to the process by which dense regions in interstellar space accumulate gas and dust, leading to the creation of molecular clouds, which are essential for star and planet formation. These clouds are primarily composed of hydrogen molecules, along with other molecules and dust grains, playing a critical role in the evolution of the cosmos and the development of chemical complexity.
Nucleosynthesis: Nucleosynthesis is the process by which elements are formed through nuclear reactions, particularly in stars. This process is crucial for understanding the origin of elements in the universe, as it explains how light elements like hydrogen and helium were formed shortly after the Big Bang, while heavier elements are created within stars and during explosive events like supernovae.
Radio Astronomy: Radio astronomy is the branch of astronomy that studies celestial objects and phenomena through the detection of radio waves emitted by them. This technique allows scientists to observe and analyze various cosmic events, revealing information about the universe that is often invisible to optical telescopes. Radio astronomy plays a crucial role in understanding the components of the universe, including interstellar matter, star formation processes, and the chemical evolution of stars.
S-process nucleosynthesis: s-process nucleosynthesis refers to the process by which heavy elements are formed in stars through slow neutron capture, occurring primarily during the later stages of stellar evolution, particularly in asymptotic giant branch (AGB) stars. This process is responsible for creating about half of the heavy elements beyond iron in the periodic table, contributing significantly to the chemical enrichment of the universe as these stars evolve and expel their outer layers into space.
SiO: SiO, or silicon monoxide, is a simple molecule composed of one silicon atom and one oxygen atom. This compound plays a significant role in the chemistry of evolved stars, particularly during their late evolutionary stages, as it is often formed in the outflows of these stars. SiO can serve as a tracer for the physical conditions and processes occurring in stellar environments, helping astronomers understand the complex chemistry that occurs during stellar evolution.
Stellar winds: Stellar winds are streams of charged particles, primarily electrons and protons, that are ejected from the outer layers of stars into space. These winds play a crucial role in the evolution of stars and their surrounding environments, influencing star formation processes and contributing to the chemical enrichment of the interstellar medium.
Thermal pulsations: Thermal pulsations refer to the periodic expansion and contraction of a star's outer layers caused by changes in energy output, particularly in evolved stars during their later stages. These pulsations play a crucial role in the stellar evolution process, as they contribute to the mass loss of the star and influence the chemical composition of the surrounding environment through mechanisms like convection and nuclear reactions.