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CHM4023 > Meijer - Astrochemistry > Flashcards

Meijer - Astrochemistry Flashcards

1
Q

Describe what was happening at 10^-42 s after the Big Bang

A
  • Size = 10^-33 m
  • Temp = 10^32 K
  • Elementary forces (Gravity, Weak nuclear force, Strong nuclear force, Electrostatic force) are equal
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2
Q

Describe what was happening at 10^-35 s after the Big Bang

A
  • Temp = 10^27 K
  • Strong nuclear force separates
  • Universe is a sea of quarks
  • Inflation until 10^-32 S
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3
Q

Describe what was happening at 10^-12 s after the Big Bang

A
  • Size = 2 lightyears
  • Temp = 10^15 K
  • Universe as we know it
  • Weak Nuclear force separates
  • Still too hot for protons/neutrons
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4
Q

Describe what was happening at 10^-6 s after the Big Bang

A
  • 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
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5
Q

Describe what was happening at 1 s after the Big Bang

A
  • Size = 4 lightyears
  • Temp = 10^10 K
  • Universe transparent to neutrons; conversion of neutrons into protons
  • Annihilation quicker than production
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6
Q

Describe what was happening at 100 s after the Big Bang

A

Deuterium starts to form

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7
Q

Describe what was happening at 180 s after the Big Bang

A
  • 50 lightyears
  • 10^9 K
  • Nucleosynthesis stops
  • Temp and pressure too low
  • all matter exists as ions
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8
Q

Describe what was happening at 3 x10^5 years after the Big Bang

A
  • 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
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9
Q

What is the Doppler effect?

A

EM waves contract when moving towards observer

EM waves expand when moving away from observer

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10
Q

Define Flux

A

The energy per second through a surface emitted by a black body radiator

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11
Q

What is the “Proof” of the big bang theory

A
  • Big bang nucleosynthesis: Abundances and distribution of elements
  • Cosmic Microwave Background Radiation: Uniform and described by black body radiation
  • Expansion: red shift
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12
Q

What considerations need to be made when making spectroscopic measurements

A
  • Light source is star
  • Observe perpendicular to the galactic plane
  • Need object to be observed with no surrounding objects
  • Need line of sight
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13
Q

Factors that add/remove intensity from photon flux

A
  • Stimulated Absorption
  • Stimulated Emission
  • Spontaneous Emission
  • Elastic Scattering (Rayleigh)
  • Inelastic scattering (Raman)
  • -> only really occurs when particle is <= wavelength of light
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14
Q

Issues which occur in astro spectroscopy

A
  • Line broadening
  • Lifetime broadening
  • Pressure broadening
  • Line of sight
  • Doppler effect
  • Line shift/broadening
  • Resolution

–> Red shift can be v. large (ca. 900 nm)

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15
Q

Jeans mass

A

The mass for a cloud to collapse under gravity

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16
Q

Low mass stars: M <= Msun

A
  • Stops at He burning
  • Core contracts
  • Shell expands
  • Star turns into white dwarf into black dwarf
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17
Q

High mass stars: M >= 20 Msun

A
  • 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
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18
Q

High mass stars M >= Msun (Red (Super) Giants)

A
  • After O/Si burning, further collapse
  • Core: Neutron star
  • (Super) Nova
  • Heavy elements
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19
Q

Star classification

A

White Dwarf - Low Luminosity, Low Temp
Blue Giants - High Luminosity, Low Temp
Red Giants - Medium Luminosity, High Temp
Red supergiants - High Luminosity, High Temp

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20
Q

Why can CO be generally detected more easily than H2 conventionally

A

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

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21
Q

What is star formation governed by?

A
  • Gravitational pull; Heat & Pressure
  • Heat pressure overcome by molecules
  • Once star ignites, formation of planets starts
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22
Q

Issues when forming stars

A
  • Abundances
  • Mixing (density)
  • Temperature
  • Dissipation
  • Cosmic Rays
  • Shock waves
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23
Q

Fractionation

A

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
24
Q

What are cosmic rays

A

Sharp changes in pressure or EM fields –> hydrodynamics

25
Q

What are the 4 regions of the ISM

A

Termination Shock
Heliosheath
Heliopause
Bow shock

26
Q

Define the ISM

A

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

27
Q

Explain the termination shock

A
  • 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
28
Q

Why is the ISM important in astrochemistry

A
  • 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
29
Q

What types of reactions occur in the ISM?

A

Gas-Phase
Gas-Surface
- Dominated by H, O, N, C - He+ is very reactive and ionises most species, driving reactions
Surface reactions

30
Q

Energy sources in the ISM

A
  • Cosmic rays: shocks/collisions (heating)
  • Light: Photoionisation & heating
  • X-Rays: Give reactive species
31
Q

How does the formation of H2 occur in the ISM?

A

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

32
Q

Explain Processing to get simple sugars

A 
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
33
Q

What are the 4 interstellar mediumenvironments?

A

Diffuse Clouds
Dense Clouds
Circumstellar Disk
Photon-Dominated Region

34
Q

Diffuse Clouds

A
  • n = 1-100 cm^-3
  • T = 100 K
  • Bare grains: Temp is high enough to prevent adsorption onto surface
  • Have atoms, not molecules
35
Q

Dense Clouds

A
  • 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
36
Q

Circumstellar Disk

A
  • 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
37
Q

Photon-Dominated Region

A
  • High T
  • Radiation
  • High radiation, bonds dissociate, high magnetic field
  • electrons changing direction in field will release hv
  • Detect using a Free-Electron Laser
38
Q

Explain the Hard Spheres Model

A

Collision = Reaction

If particle centres are < d apart then reaction works in 3D

39
Q

How to obtain rate constants

A

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

40
Q

Interstellar Dust Grains

A
  • 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
41
Q

Dust grain composition

A

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

42
Q

Determining ice grain structure by meteorites

A

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

43
Q

Carbon structure of dust grains

A
  • Random covalent network
  • Amorphous, full of defects
  • Defects can give reaction centres on surface
44
Q

Hot Atom Model

A

“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
45
Q

Ices construction

A 
- 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
46
Q

Temperature Programmed Desorption Experiment

A

(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

47
Q

Define the rest frequency

A

The frequency measured corrected for the Doppler effect

48
Q

What must be present to describe the chemical model of an interstellar cloud?

A

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

49
Q

How to determine the Chemical model of interstellar clouds

A
  • Astronomical observations
  • Lab-based Astrochemistry
  • Computational astrochemistry
  • Quantum Chemical Modelling
50
Q

Factors for formation of life

A
  • 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
51
Q

Earth life timeline

A
  • 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
52
Q

RNA world timeline

A
  • Pre-biotic soup
  • Stereoregular mononucleotides
  • -> Against entropy, needs energy favourability
  • -> Stereospecific polymerisation
53
Q

Role of solvent

A

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?

54
Q

Extra-terrestrial life?

A
  • Panspermia
  • ET amino acids –? polarised light in dust clouds destroy specific enantiomers
    Atmospheric entry?
    Timeline?
55
Q

Urey-Miller experiment

A

Get: Ala, Formic acid, Glycene, Glycolic aldehyde, Lactic acid in a racemic mixture
BUT
Very low yield and any O2 will kill reaction

56
Q

Urey-Miller experiment problems

A

Phosphate esters - need energy storage and release
Thymine not formed
Ribose
Amino acid polymerisation not entropically favourable
Chirality –> life elsewhere opposite chemistry??

CHM4023 (5 decks)

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