The Chemistry of Interstellar Space
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SCIENCE
=
Science is not reality,
WHAT IS GOING ON?
Science tries to give a
AND WHY?
DESCRIPTION of the reality
The Chemistry of Interstellar Space
D A Adriaens / F Goumans
(ex-)UCL Chemistry Department
•Astronomy
•History
•Star Cycle
•Chemistry
•Nuclear Reactions
•Spectra
•Molecules in the universe
The Orion Nebula
Stonehenge
•2900 BC  2000 BC
•Astronomical calendar
or
•Religious Temple
•Midsummer and Midwinter
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History
• Greek
– Aristarchus (310-230BC): sun in centre of the universe
– Aristotle (384-322BC): earth in centre of the universe
– Ptolemy (90-168AD): earth in centre of the universe
• China
– 1054: Supernova
History
• 16-17th century: Age of Enlightenment
–
–
–
–
–
Copernicus (1473-1543) (heliocentrism, circular orbits)
Galileo Galilei (1564-1642) (4 moons of Jupiter)
Johannes Kepler (1571-1630) (3 laws of Kepler, ellipses)
Newton (1643-1727) (3 laws of motion)
…
• Revolution in Sciences
History
• Now ?
– Albert Einstein (1879-1955)
– Stephen Hawking (1942-…)
> relativity and advanced cosmology
“A brief history of time”
Star Cycle
Only via STATIC observations, not dynamic
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Star Cycle
Star Cycle
Only via STATIC observations, not dynamic
Nuclear reactions in a star
•Small stars: H  He
•Medium stars: H  He  C
•Massive stars: H  He  C  O  Ne, Na, Mg, S, Si, Ca, Fe, Ni, Cr, Cu, …
Basis for life
“We are all made of stars”
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Nuclear reactions: proton-proton
Nuclear reactions in a star
1H
1H
2H
+
1H
+
e-
+
1H
3He
3He
3He
+
+g
3He

+ ne
2H
+
+
3He

4He
+
4He

[7Be]
+
7Li
1H
+
[7Be]

8Be

+

1H
+
e-

1H
8Be

4He

4He
+
4He
+
+
8B
e+
+
e+
+
+g
4He
+g
+ ne
4He
+ ne
1H
+ ne
7Li

+ ne
e+
1H
[7Be]
8B

2H
+
1H
4He
+ 4He  [8Be]
4He
+ [8Be]  12C + g
4He
+ 12C  16O + g
12C
+ 1H  [13N] + g
[13N]  13C + e+ + ne
13C
+ 1H  14N + g
14N
+ 1H  [15O] + g
[15O]  15N + e+ + ne
15N
+ 1H  12C + 4He
15N
+ 1H  16O + g
16O
+ 1H  [17F] + g
[17F]  17O + e+ + ne
17O
+ 1H  14N + 4He
Spectra: The sun
Why is our sun yellow?
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Spectra
Spectra: solar spectrum
Absorption and emission
Some atomic spectra
H
Na
He
Mg
Li
Al
Be
Si
B
P
C
S
N
Cl
O
Ar
F
K
Ne
Hg
An owl’s view on the Universe
Infrared spectra
Molecular spectra: more complicated
Seeing in the Infrared
The Orion Constellation
An owl’s view on the Universe
Molecules in the Interstellar Matter
• clouds of dust
• these regions are
nurseries for stars
• rich in complex
molecules
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Molecules in the ISM
2 atoms
3 atoms
4 atoms
5 atoms
6 atoms
> 6 atoms
H2
H2 O
H2 S
HCN
HNC
CO2
SO2
MgCN
MgNC
NaCN
N2 O
NH2
OCS
HCO
C3
C2H
HCO+
HOC+
N2 H+
HNO
HCS+
H3 +
C2O
C2S
SiC2
H2 D+
CH2
NH3
H2CO
H2CS
C2H2
HNCO
HNCS
H 3 O+
HOCO+
C3S
H2CN
cyclic-C3H
linear-C3H
HCCN
H2CO+
C2CN
C3O
HCNH+
CH2D+
CH4
SiH4
CH2NH
NH2CN
CH2CO
HCOOH
HC2CN
HCCNC
cyclic-C3H2
linear-C3H2
CH2CN
C4H
C4Si
C5
HNCCC
CH3OH
CH3SH
C2H4
CH3CN
CH3NC
HC2CHO
NH2CHO
C4H2
C5H
C5O
CH3C2H
CH3CHO
CH3NH2
CH2CHCN
HC4CN
C6H
CH3COOH
HCOOCH3
CH3C2CN
(CH3)2O
CO
CSi
CP
CS
NO
NS
SO
HCl
NaCl
KCl
AlCl
AlF
PN
SiN
SiO
SiS
NH
OH
C2
CN
HF
CO+
SO+
CH
CH+
• > 150 species
• from H2 to HC11N
C2H5OH
C2H5CN
CH3C4H
HC6CN
(CH3)2CO
CH3C4N?
NH2CH2COOH
HC8CN
HC10CN
Role of molecules
• Stars form in gas clouds
collapsing under their own
weight
• Must dissipate heat formed
in process
• Heat radiated away by
molecules
Spectroscopy of stars
• Region of spectrum characteristic to motion
Radio
Wavelength
Rotation
Microwave
Infrared
Vibration
Visible/UV
Electronic excitation
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Formation of molecules
• Conditions very harsh in the ISM
– Extremely low pressure (10-15 mbar): few collisions
– Extremely low temperature (10-20K): barrierless
Energy
A
+ B
Atoms
A
B
Molecule
• Very low chance of reactive encounters
Formation of molecules
• For several molecules ‘too high’ abundances for
gas-phase reactions only
H2 Formation problem
Interstellar Ices
H + H  H2*  H2 + hv
Gas phase alone cannot
account for observed
abundances
H + H  H2*
H2* + M  H2 + M
DUST GRAINS
Dust grains
• Gas clouds contain
dust particles
• Molecules freeze
out as “ices” (~10K)
• Ices grow by reactions at grain
(H, N, O, CO yielding H2O, NH3, CO2, CH3OH)
Dust grains
• ~1% of the mass of the ISM
• up to 10mm in size
• carbonaceous and silicate material
• bare or covered by ices
– H2O, CO, CO2, CH3OH, NH3
• amorphous
• fluffy, open structure (porous)
Formation of molecules
• Heterogenous reaction at the dust particle’s surface
– H2 formed by such a reaction
• 2 mechanisms:
– 2 H meet on surface (Langmuir-Hinshelwood)
– 1 gaseous H meets an H on surface (Eley-Rideal)
• Other molecules formed on surfaces:
– H2O, CO, CO2, CH3OH and NH3 (yielding ices)
Ice formation
• Mechanisms of formation unknown
• We ‘assume’ some reaction model
CH3OH formation
COa + Ha  HCOa
HCOa + Ha  CH2Oa
CH2Oa + Ha  CH3Oa
CH3Oa + Ha  CH3OHa
H2O formation
Oa + H2,a  OHa + H(g)
OHa + H2,a  H2Oa + H(g)
Experiments
• Experimentalists mimic the interstellar medium
– low T and p
– use model surfaces
– observe ad/desorption and reactions:
Adsorption
Desorption
Reaction
Diffusion
Simulating the ISM
What do experimentalists do?
• Surface infrared spectroscopy
– which molecules are adsorbed?
• Temperature-programmed desorption
– which molecules adsorbs most strongly?
Theory
• Theoretical chemists calculate
reaction mechanisms
– Explain/clarify experiments
– Experimentally inaccessible data
Energy
Data
A
+ B
Atoms
A
B
Molecule
• Theoretical physicists
model star formation
Summary
• Chemistry important in the evolution of the universe
• Molecules play a crucial role in star formation
– loss of heat during collapse
• (Some) molecules formed on dust grain surfaces
• Chemists are investigating reactions in the lab
• Theoreticians are computing reactions
 Understanding star formation and the universe
Acknowledgements
Contact me via:
drdadriaens@googlemail.com
http://www.ucl.ac.uk/chemsea
Thanks to:
•Dr Fedor Goumans
•Dr Wendy Brown
•Rosie Coates
•Imperial College
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