Astrochemistry Flashcards
Luminosity Equation
(σT^4) x (4piR^2)
Wein’s Displacement Law
λ(max) = b/T
Doppler effect
Δλ/λ = v(source)/c
Parallax
p (arc seconds) = 1/d (parsecs)
Parsec to lighyears
pc = ly x 0.3066
Rydberg equation
ṽ = R(1/(n1)^2 - 1/(n2)^2)
Law of gravity
F = - Gm1m2/R^2
Gravitational energy
E = - Gm1m2/R
3 elements of the Big Bang theory
Big bang nucleosynthesis
Cosmic microwave background radiation
Expansion/inflation
Proof of big bang elements
BBN: abundance/distribution of the elements
CMBR: “uniform” and described via blackbody radiation
Expansion: red shift - light from galaxies shifted to longer wavelengths so getting further away
Factors that add/remove intensity from photon flux (5)
Stimulated adsorption Stimulated emission Spontaneous emission Elastic scattering (Rayleigh) Inelastic scattering (Raman)
Issues with molecular astronomy affects of spectra (4)
Line broadening: lifetime and pressure broadening
Line of sight
Doppler: shift and broadening
Resolution
Why is it difficult to detect H2?
No dipole moment, can only measure electronic part (no IR). Can detect CO (has dipole moment); assume where CO is H2 is also there
How do stars form?
Some particles held together via gravity. Mass becomes larger, then when it gets to a certain mass (Jean’s mass) it collapses in on itself. As cloud contracts it heats up, interacting and releasing photons which are released and which are not reabsorbed causing energy loss (temperature decreases) resulting in further contraction
What is a proto-start?
Early star - region closest to the star is free of dust due to radiation destroying everything. Further out less intense radiation, dust remains forming proto-planetary disk
Low mass star M < M(sun)
- stops at He burning
- core contracts
- shell expands
- star turns into white dwarf into black dwarf (carbon cinder)
High mass star M > 20 M(sun)
- alpha capture
- Carbon, oxygen burning
- Elements up to 40Ca
- silicon burning ultimately gives iron
- even elements more abundant
- after O and Si burning collapse into neutron star
- (super) nova –> heavy elements
Issues with molecules in space (6)
- abundances
- densities
- temperature
- dissipation
- cosmic rays
- shock waves
Molecular view of the universe (6)
- detection of molecules
- determination of abundances
- physical conditions
- optical extinction
- chemical network
- kinetics
Define ISM
region between stars - outside heliosphere
What are the four different environments in the ISM?
- diffuse clouds
- dense clouds
- circumstellar disk
- photon-dominated region
Describe a diffuse cloud
- n = 1 - 100 cm-3
- T = 100 K
- temperature too high for molecules to stick so bare grains
- radiation from stars easily penetrates so not many molecules
- get H2 and CO due to self-shielding
Describe a dense cloud
- n = 10^6 cm-3
- T = 10 K
- Ices
- star formation
- lower T means radiation can’t penetrate so less heating
- shielded from radiation means more molecules present
- densities are high so bright in IR
- can collapse under own mass (Jean’s mass)
Describe a circumstellar disk
- depends of age/radiation field - further away from star temperature drops - get closer to molecular cloud e.g. molecules and ices
- full of ices and dust - light scattering
Describe photon-dominated region
- extreme of circumstellar disk
- fast moving electrons - when the change direction the emit v. intense radiation
- mostly ions
Types of gas phase reactions in molecular clouds (7)
- Photoionization
- Photodissociation
- Cosmic ray ionisation
- Charge Exchange
- Exchange reaction
- Radiative association
- Recombination
What is the hard sphere model?
Molecules are treated as spheres that don’t interact and react when the spheres touch
Problems with hard-sphere model?
- In reality molecules do interact (e.g. attraction or repulsion)
- straight paths are very unlikely
- repulsive (positive) interaction will overestimate the cross-section and therefore reaction rate
- attractive (negative) interaction will underestimate the cross-section and there reaction rate
- molecules may not react if they collide (wrong orientation, insufficient energy etc.)
How to obtain reaction rate constants?
- Cross molecular beams (fixed energies) measure reaction probability at different temperatures
- Cresu:Rennes - run lots of simulations to reduce error
Effects of interstellar dust particles (10)
- Visual extinction
- Visual polarisation
- Nebulosity
- Extinction
- IR bands/emission
- Depletion
- Photo-electric effect
- Evaporation
- Increased abundance
- Reaction medium
Types of sticking
- Chemisorption
- Physisorption
Describe chemisorption
- 1 Angstrom
- ~ eV
- chemical bond forms, changing the electronic structure of substrate causing a barrier
- particle is immobile
Describe physisorption
- 3 Angstoms
- ~ 50 MeV
- van der Waals
- well depth determined by the polarizability of both the substrate and the incoming particle
- weak, high mobile
Why is the sticking probability dependent on the incoming energy of the particle?
For the particle to stick to the substrate it needs to dissipate its energy. It does this by exciting phonon modes in the substrate. If the molecule has too little energy there might not be a corresponding phonon excitation. If it can’t dissipate its energy it can’t stick
What are two examples of surface reaction models?
Eley-Rideal
Langmuir-Hinshelwood
What is the Eley-Rideal model?
- Particle becomes chemisorbed to surface; this happens to whole surface
- Different particle then interacts with stuck particle and the new molecule comes off
- depends on the relative binding energies of the particle to the surface and the incoming particle
- long residue time
What is the Lanmuir-Hinshelwood model?
- Particle becomes physisorbed onto surface and explores surface
- different particle also become physisorbed and explores surface
- weak binding highly mobile
- particles meet, react and de-adsorb
- residue times are short
- exothermisity determined by the bond formed are surface interaction is weak
- molecule comes off slowly as takes time to transfer rotational energy to translational
What information can you obtain from ices?
- temperature (e.g. if lots of water T< 100 K, lots of N2 or O2 then T< 20 K
What is the chemical model of an interstellar cloud? (8)
- Chemical composition
- Physical conditions
- Transport processes
- Photochemistry
- Reaction Rates
- Chemical networks
- Target species
- Propagate differential equations
Four theories to the emergence of life and define
- determinism - casual relationship between chemistry and physics with the emergence of life - inevitable
- contingency - random, luck
- anthropic principle - because we are here to question how life formed is must have been inevitable
- Kinetics vs. thermodynamics - want stability but not too stable (so it can evolve and repair)
What is the timeline in the RNA world?
- pre-biotic soup
- stereoregular mononucleotides
- RNA
- self-replicating RNA family
- Ribozymes (catalysis)
- proteic enzymes
- DNA
Role of water in emergence of life
- Liquid of a large temperature range
- polar
- Ice is less dense than water, prevents the water beneath from freezing, things can develop underneath ice
- high specific heat capacity so fluctuation in solar output won’t affect temperature
Describe Urey-Miller experiment
Chamber with water, methane, ammonia and hydrogen. Refluxed with electrodes to simulate lightening. Produced molecules like alanine, glycine and formic acid.
Problems with Urey-Miller experiment
- no way to get non-racemic mixture
- very low yield
- don’t know if the atmosphere was actually that reducing
