🌠Astrochemistry Unit 4 – Star Formation and Astrochemistry

Key Concepts and Definitions

• Astrochemistry studies the chemical processes and reactions occurring in space, particularly in the interstellar medium and within stars and planets
• Interstellar medium (ISM) consists of gas and dust between stars, providing the raw materials for star formation
• Molecular clouds are dense regions within the ISM where star formation occurs, composed primarily of molecular hydrogen ($H_2$) and other molecules
• Protostar forms when a portion of a molecular cloud collapses under its own gravity, marking the early stage of star formation
• Accretion disk is a flattened disk of gas and dust surrounding a protostar, from which planets can form
• Astrochemical models simulate the complex chemical reactions and processes occurring in space, helping to understand the observed abundances of molecules
• Spectroscopy is the primary observational technique used in astrochemistry, analyzing the absorption and emission of light by atoms and molecules

The Interstellar Medium

• Interstellar medium (ISM) is the matter that exists between stars, consisting of gas (99%) and dust (1%)
• Composition of the ISM includes hydrogen (70%), helium (28%), and heavier elements (2%)
• Gas in the ISM exists in various forms: atomic, molecular, and ionized
• Atomic hydrogen (HI) is found in regions with low densities and high temperatures
• Molecular hydrogen ($H_2$) is the most abundant molecule in the ISM, found in dense, cold regions called molecular clouds
• Dust grains in the ISM are composed of silicates, graphite, and ices, playing a crucial role in interstellar chemistry
• Interstellar radiation field (ISRF) is the combined radiation from stars, affecting the chemistry and heating of the ISM
• Cosmic rays, high-energy charged particles, ionize and heat the ISM, driving chemical reactions

Stages of Star Formation

• Star formation begins with the gravitational collapse of a dense, cold molecular cloud
• Protostellar phase occurs when a dense core within the molecular cloud collapses, forming a protostar surrounded by an accretion disk
• Protostar continues to accrete matter from the surrounding disk, increasing in mass and temperature
• T Tauri phase is characterized by a pre-main-sequence star with strong stellar winds and bipolar outflows, clearing away surrounding material
• Main sequence phase begins when the star reaches sufficient temperature and pressure to initiate hydrogen fusion in its core, achieving hydrostatic equilibrium
• Stellar feedback, such as radiation and winds, can trigger or suppress further star formation in the surrounding region

Molecular Clouds and Their Composition

• Molecular clouds are dense, cold regions within the ISM, serving as the birthplaces of stars
• Composition of molecular clouds is primarily molecular hydrogen ($H_2$), with a small fraction of other molecules and dust
• Density of molecular clouds ranges from $10^2$ to $10^6$ particles per $cm^3$, much higher than the average density of the ISM
• Temperature in molecular clouds is typically around 10-20 K, allowing for the formation of complex molecules
• Common molecules found in molecular clouds include carbon monoxide (CO), water ($H_2O$), ammonia ($NH_3$), and methanol ($CH_3OH$)
• Dust grains in molecular clouds provide surfaces for chemical reactions and shield the interior from UV radiation
• Giant Molecular Clouds (GMCs) are the largest and most massive molecular clouds, with masses up to several million solar masses and sizes of 100 light-years

Chemical Reactions in Space

• Chemical reactions in space occur in both the gas phase and on the surfaces of dust grains
• Gas-phase reactions include ion-molecule reactions, neutral-neutral reactions, and photodissociation
• Dust grain surface reactions involve the adsorption of atoms and molecules onto grain surfaces, where they can react and form new molecules
• Cosmic rays and UV radiation drive many chemical reactions by ionizing and dissociating molecules
• Formation of $H_2$ occurs primarily on dust grain surfaces, as two hydrogen atoms combine to form a molecule
• Complex organic molecules (COMs) are formed through a series of chemical reactions, often involving the addition of functional groups to simpler molecules
• Deuterium fractionation is the enrichment of deuterium in molecules, occurring in cold, dense regions where deuterated species are more stable

Observational Techniques

• Radio telescopes are used to observe molecular line emissions, as many molecules have rotational transitions in the radio part of the spectrum
• Millimeter and submillimeter wavelengths are particularly useful for studying the cold, dense gas in molecular clouds
• Infrared telescopes can detect the emission from dust grains and the vibrational transitions of molecules
• Space-based telescopes (Herschel, Spitzer) provide high-resolution observations without the interference of Earth's atmosphere
• Interferometry techniques (ALMA, VLA) combine multiple telescopes to achieve high angular resolution, enabling detailed studies of star-forming regions
• Spectral line surveys provide a comprehensive inventory of the molecules present in a source, helping to constrain chemical models

Astrochemical Models and Simulations

• Astrochemical models simulate the complex chemical processes occurring in space, considering gas-phase and grain-surface reactions
• Rate equations are used to calculate the time-dependent abundances of species based on the rates of formation and destruction
• Chemical networks include thousands of reactions and species, requiring computational methods to solve
• Radiative transfer models simulate the propagation of radiation through the ISM, accounting for absorption, emission, and scattering
• Hydrodynamic simulations couple the chemical evolution with the physical processes (gravity, turbulence, magnetic fields) governing the ISM
• Model predictions are compared with observations to constrain the physical and chemical conditions in star-forming regions

Implications for Planetary Formation

• Chemical composition of the ISM and molecular clouds determines the initial composition of protoplanetary disks
• Protoplanetary disks inherit the complex molecules formed in the preceding stages of star formation
• Chemical evolution in protoplanetary disks affects the composition of the planets that form from them
• Icy dust grains can agglomerate to form planetesimals, the building blocks of planets
• Delivery of water and organic molecules to planetary surfaces may have implications for the emergence of life
• Observations of protoplanetary disks (ALMA) provide insights into the chemical environment in which planets form
• Study of comets and meteorites provides a record of the chemical composition of the early Solar System