Meijer - Astrochemistry Flashcards
Describe what was happening at 10^-42 s after the Big Bang
- Size = 10^-33 m
- Temp = 10^32 K
- Elementary forces (Gravity, Weak nuclear force, Strong nuclear force, Electrostatic force) are equal
Describe what was happening at 10^-35 s after the Big Bang
- Temp = 10^27 K
- Strong nuclear force separates
- Universe is a sea of quarks
- Inflation until 10^-32 S
Describe what was happening at 10^-12 s after the Big Bang
- Size = 2 lightyears
- Temp = 10^15 K
- Universe as we know it
- Weak Nuclear force separates
- Still too hot for protons/neutrons
Describe what was happening at 10^-6 s after the Big Bang
- Size = Solar System
- Temp = 10^13 K
- Protons and Neutrons
- Protons/Neutrons interconvert
- Photons convert into electron/positron pairs
- 10^10 photons for every proton/neutron
- Proton/neutron ratio approx. 1
Describe what was happening at 1 s after the Big Bang
- Size = 4 lightyears
- Temp = 10^10 K
- Universe transparent to neutrons; conversion of neutrons into protons
- Annihilation quicker than production
Describe what was happening at 100 s after the Big Bang
Deuterium starts to form
Describe what was happening at 180 s after the Big Bang
- 50 lightyears
- 10^9 K
- Nucleosynthesis stops
- Temp and pressure too low
- all matter exists as ions
Describe what was happening at 3 x10^5 years after the Big Bang
- Formation of atoms
- Decoupling of Matter-Radiation
- Universe becomes transparent
- Cosmic Background Radiation: Decoupling matter/radiation in thermal equilibrium
- Distribution of photon energies
- Distribution described by black body radiation
What is the Doppler effect?
EM waves contract when moving towards observer
EM waves expand when moving away from observer
Define Flux
The energy per second through a surface emitted by a black body radiator
What is the “Proof” of the big bang theory
- Big bang nucleosynthesis: Abundances and distribution of elements
- Cosmic Microwave Background Radiation: Uniform and described by black body radiation
- Expansion: red shift
What considerations need to be made when making spectroscopic measurements
- Light source is star
- Observe perpendicular to the galactic plane
- Need object to be observed with no surrounding objects
- Need line of sight
Factors that add/remove intensity from photon flux
- Stimulated Absorption
- Stimulated Emission
- Spontaneous Emission
- Elastic Scattering (Rayleigh)
- Inelastic scattering (Raman)
- -> only really occurs when particle is <= wavelength of light
Issues which occur in astro spectroscopy
- Line broadening
- Lifetime broadening
- Pressure broadening
- Line of sight
- Doppler effect
- Line shift/broadening
- Resolution
–> Red shift can be v. large (ca. 900 nm)
Jeans mass
The mass for a cloud to collapse under gravity
Low mass stars: M <= Msun
- Stops at He burning
- Core contracts
- Shell expands
- Star turns into white dwarf into black dwarf
High mass stars: M >= 20 Msun
- alpha capture - Oxygen most abundant element (except H/He)
- C/O burning
- Elements upto 40Ca
- Si burning –> Fe –> Beyond Fe becomes endothermic
- Odd/Even abundances
High mass stars M >= Msun (Red (Super) Giants)
- After O/Si burning, further collapse
- Core: Neutron star
- (Super) Nova
- Heavy elements
Star classification
White Dwarf - Low Luminosity, Low Temp
Blue Giants - High Luminosity, Low Temp
Red Giants - Medium Luminosity, High Temp
Red supergiants - High Luminosity, High Temp
Why can CO be generally detected more easily than H2 conventionally
CO has dipole moment –> Pure rotational spectrum
H2 does not, should not have a rotational spectrum
H2 does have a quadrupole moment but lifetime for transition is approx. 100 years so is very slow and is very difficult to detect.
Triplet:Singlet H ratio is 3:1 statistically
Lifetime of triplet H is 1 mil years - spin-orbit coupling of H is very weak; time is reduced in dense environments to ca. 500 years
- Lots of H in space so can actually detect
- CO acts as a marker to H2
What is star formation governed by?
- Gravitational pull; Heat & Pressure
- Heat pressure overcome by molecules
- Once star ignites, formation of planets starts
Issues when forming stars
- Abundances
- Mixing (density)
- Temperature
- Dissipation
- Cosmic Rays
- Shock waves
Fractionation
H2 + D HD + H
- Electronically no change
- Zero point energy of H2 higher than for HD
- Very slightly exothermic - Equilibrium to the right
- Any collision between H2 & D likely to produce HD
- More likely to detect HD
- -> Need to get rid of reaction heat; product may not be stable
What are cosmic rays
Sharp changes in pressure or EM fields –> hydrodynamics
What are the 4 regions of the ISM
Termination Shock
Heliosheath
Heliopause
Bow shock
Define the ISM
Area between stars
Volume of where star has an effect
At some point (Termination shock) the solar wind will stop due to lack of energy
Explain the termination shock
- Speed of solar wind below speed of sound
- Keeps moving but pressure of solar wind is in equilibrium with pressure of particles
- Beyond heliopause there are unknown effects
Why is the ISM important in astrochemistry
- Detect molecules - figure out what it is and how it got there
- Indication of how things develop
- Detection of a molecule
- Determination of abundances
- Physical conditins
- Optical extinction
- Chemical Network
- Kinetics
- -> May need to do sensitivity analysis to determine which rates of reactions are important
What types of reactions occur in the ISM?
Gas-Phase
Gas-Surface
- Dominated by H, O, N, C - He+ is very reactive and ionises most species, driving reactions
Surface reactions
Energy sources in the ISM
- Cosmic rays: shocks/collisions (heating)
- Light: Photoionisation & heating
- X-Rays: Give reactive species
How does the formation of H2 occur in the ISM?
Gas phase are most common reactions but due to low density, only 2-body reactions occur
H + H –> H2 + hv
The reaction needs to release energy
To efficiently release hv, a dipole is needed
Quadrupole has a very long lifetime so cannot dissipate energy this way
Gas reaction does not occur
Use a surface to dissipate energy
Surface acts as third body and concentration medium
Surface acts as a heterogeneous catalyst
Explain Processing to get simple sugars
Particle interacts with a surface May lead to further reactions Get secondary reactions Mixture of material on surface Bombard with radiation Colour change "Yellow Goo" Complex chemistry Simple sugars
What are the 4 interstellar mediumenvironments?
Diffuse Clouds
Dense Clouds
Circumstellar Disk
Photon-Dominated Region
Diffuse Clouds
- n = 1-100 cm^-3
- T = 100 K
- Bare grains: Temp is high enough to prevent adsorption onto surface
- Have atoms, not molecules
Dense Clouds
- n = 10^6 cm^-3
- T = 10 K
- Ices
- New star formation
- Star light gets filtered quickly
- Far UV and X-ray filtered quickly
- Molecules can survive
- Density leads to collisions
- Star formation
Circumstellar Disk
- Depends on age and radiation field
- Young star; lots of UV
- Older star; molecules survive better
- Atoms only
- Scattering
- Molecules survive at high T, water survives at 3000 K
- Lots of dust, scatter light, can detect scattered light
Photon-Dominated Region
- High T
- Radiation
- High radiation, bonds dissociate, high magnetic field
- electrons changing direction in field will release hv
- Detect using a Free-Electron Laser
Explain the Hard Spheres Model
Collision = Reaction
If particle centres are < d apart then reaction works in 3D
How to obtain rate constants
Calculations: Classical trajectories, Quantum Dynamics
Measurements: CRESU
- uses gas escaping from small holes to get supersonic expansion
- get particles through collimator which are rotationally and vibrationally cold but translationally hot
Interstellar Dust Grains
- Visual extinction; scatter photons as they are approx. the size of the wavelength of a photon
- Visual polarisation: Charged so have preferential orientation in a magnetic field –> different scattering due to orientation and shape
- Nebulosity - cannot see past it leading to difficult observation
- Particles contain elements which absorb in their own right so get absorption band differences
Dust grain composition
ICES - H2O/CO/CH4, can determine T as CH4 freezes out at a very low T and H2O freezes out at 100 K
SP3
SP2 - Sheets of C in graphitic formation –> large pi system
Fe/Mg/Si CORE - Mixed with oxygen
Determining ice grain structure by meteorites
Issue with heat through atmosphere - burns volatiles
Left with chondrites: partically C, partially GEMS (Glass Embedded Metal Silfides))
Particles are 300-400 nm –> Sintering in atmosphere makes them bigger
- Species left are “fluffy” with large surface area
- Molecules formed in the centre will lose internal excitation on going to the surface
–>Use Valence Electron Loss spectroscopy for dust grain composition
Carbon structure of dust grains
- Random covalent network
- Amorphous, full of defects
- Defects can give reaction centres on surface
Hot Atom Model
“mix of the two heterogeneous catalysis models”
- Species will interact with the surface without equilibrating with surface T
- Species bounce around on the surface until reaction
Ices construction
- Built up from H2O, CO, CO2, N2, O2 These freeze out at different temperatures H2O - 200 K CO - 60 K CO2 - 45 K O2 - 20 K --> can use spectroscopy to determine temp. If only H2O: T > 60 K If only O2: T < 20 K
Temperature Programmed Desorption Experiment
(1) Measure binding energy of molecule at surface
(2) Deposit material on surface at T
(3) Heat surface
(4) Measure temp. of surface
(5) “see” material coming off (mass spec.)
(6) Tells T to see material –> calculate binding energy
- -> Depends on crystal structure of material
- -> Can get multiple temperatures of desorption due to layers and defects trapping other species
Define the rest frequency
The frequency measured corrected for the Doppler effect
What must be present to describe the chemical model of an interstellar cloud?
Chemical Composition: H2O, Co, CH4, CO2 etc.
Physical compositions: Temp, Density, Mag field, UV etc
Transport Processes: Turbulance, solar wind, shock etc
Photochemistry: UV, Cosmic rays, radiating species
Rates: Ionisation, Cooling, Photon emission
Chemical Network: Specific product; Target Species
–> Propagate differential equations
How to determine the Chemical model of interstellar clouds
- Astronomical observations
- Lab-based Astrochemistry
- Computational astrochemistry
- Quantum Chemical Modelling
Factors for formation of life
- Determinism: Is life a definitive process? Is there one beginning or multiple “false-starts”?
- Contingency: Luck
- Anthropic principle: Can’t impose inevitability, consequence of portrayal of Darwinism
- Kinetics vs Thermodynamics: Essence of life needs to be stable but reactive enough to eventually change
- Panspermia: Bacteria from space landed and spawned on earth
Earth life timeline
- Ball of lava - 4.5 Gyr ago
- Rock - 3.9 Gyr ago
- Life - 3.5 Gyr ago
Life must have formed in 400,000,000 years –> fast on geologcial timescales
RNA world timeline
- Pre-biotic soup
- Stereoregular mononucleotides
- -> Against entropy, needs energy favourability
- -> Stereospecific polymerisation
Role of solvent
Water is critical to life on earth:
- Liquid over large T range: Consistent environment
- Polar: Dissolving; polar & non-polar environments –> driver for compartmentalism
- Density: Ice less dense than liquid water, allows life to continue below surface
- Large heat capacity: H-bonding, takes H2O a long time to equilibrate
Alternatives? ethane/Ethane?
Extra-terrestrial life?
- Panspermia
- ET amino acids –? polarised light in dust clouds destroy specific enantiomers
Atmospheric entry?
Timeline?
Urey-Miller experiment
Get: Ala, Formic acid, Glycene, Glycolic aldehyde, Lactic acid in a racemic mixture
BUT
Very low yield and any O2 will kill reaction
Urey-Miller experiment problems
Phosphate esters - need energy storage and release
Thymine not formed
Ribose
Amino acid polymerisation not entropically favourable
Chirality –> life elsewhere opposite chemistry??
