Introduction to Astrochemistry

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Introduction to Astrochemistry
School of Physics and Astronomy FACULTY OF
MATHEMATICS PHYSICAL SCIENCES
Paola Caselli
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Outline

• Astrochemical processes
• The formation of H2
• H3 formation
• The chemistry initiated by H3
• Formation and destruction of CO
• Nitrogen chemistry
• Deuterium fractionation
• Surface chemistry
• Examples pre-stellar cores, protostellar
  envelopes, outflows, hot cores, protoplanetary
  disks

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Interstellar Molecules
Known Interstellar Molecules (Total 151 as of
today)
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How do molecules form in the interstellar medium
?
The most elementary chemical reaction is the
association of A and B to form a molecule AB with
internal energy
A B ? AB
The molecule AB must loose the internal energy.
In the Earth atmosphere, where the number of
particles per cubic centimeter (cc) is very large
(1019), the molecule looses its energy via
three-body reactions
AB M ? AB
But this is not an efficient process in
interstellar clouds (FAB10-36n3cm-3s-1), where
the number of particles per cc ranges between a
few hundred and 107.
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1. The formation of H2
The reaction that starts the chemistry in the
interstellar medium is the one between two
hydrogen atoms to form molecular hydrogen
H H ? H2 This reaction happens on the
surface of dust grains.
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1. The formation of H2
The H2 formation rate (cm-3 s-1) is given by
(Gould Salpeter 1963 Hollenbach Salpeter
1970 Jura 1974 Pirronello et al. 1999 Cazaux
Tielens 2002 Habart et al. 2003 Bergin et al.
2004 Cuppen Herbst 2005)
nH? gas number density vH? H atoms speed in
gas-phase A ? grain cross sectional area ng? dust
grain number density SH? sticking probability ? ?
surface reaction probability
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Once H2 is formed, the fun starts
H2 is the key to the whole of interstellar
chemistry. Some important species that might
react with H2 are C, C, O, N To decide whether
a certain reaction is chemically favored, we need
to examine internal energy changes.
H2 4.48
CH 3.47
OH 4.39
CH 4.09
OH 5.10
Question Can the following reactions proceed in
the cold interstellar medium?
O H2 ? OH H ?? O H2 ? OH H ??
C H2 ? CH H ?? C H2 ? CH H ??
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Once H2 is formed, the fun starts
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Once H2 is formed, the fun starts
H2 4.48
CH 3.47
OH 4.39
CH 4.09
OH 5.10
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Some technical details Ion-Neutral reactions
A B ? C D
Exothermic ion-molecule reactions do not possess
activation energy because of the strong
long-range attractive force(Herbst Klemperer
1973 Anicich Huntress 1986)
R
V(R) - ? e2/2R4
kLANGEVIN 2 ?e(?/?)1/2 ?
10-9 cm3 s-1 independent on T
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Some techincal details Neutral-Neutral reactions
A BC ? AB C
Energy to break the bond of the reactant.
1 eV for endothermic reactions E ?
0.1-1 eV for exothermic reactions
kb T lt 0.01 eV in molecular clouds
Energy released by the formation of the new bond.
Duley Williams 1984, Interstellar Chemistry
Bettens et al. 1995, ApJ
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2. Cosmic-ray ionization of H2
After the formation of molecular hydrogen, cosmic
rays ionize H2 initiating fast routes towards the
formation of complex molecules in dark clouds
H2 c.r. ? H2 e- c.r.
Once H2 is formed (in small percentages), it
very quickly reacts with the abundant H2
molecules to form H3, the most important
molecular ion in interstellar chemistry
H2 H2 ? H3 H
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The cosmic-ray ionization rate, ?
(Tielens 2005, The Physics and Chemistry of the
Interstellar Medium)
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3. The chemistry initiated by H3
Once H3 is formed, a cascade of reactions
greatly enhance the chemical complexity of the
ISM. In fact, H3 can easily donate a proton
and allow larger molecules to build.
Example ? OXYGEN CHEMISTRY (the formation of
water in the ISM)
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3. The chemistry initiated by H3
CARBON CHEMISTRY (the formation of hydrocarbons)
The formation of more complicated species from
neutral atomic carbon begins with a sequence very
similar to that which starts the oxygen chemistry
CH
A. Proton transfer from H3 to a neutral atom B.
Hydrogen abstraction reactions terminating in a
molecular ion that does not react with
H2 C. Dissociative recombination with electrons.
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4. Formation and destructio1n of CO
a C H3O ? HCO H2 b O CH3 ? HCO
H2 c HCO e ? CO H is the most important
source of CO.
CO is very stable and difficult to remove. It
reacts with H3 d H3 CO ? HCO H2 but
reaction c immediately reform CO.
The main mechanisms for removing CO are e
He CO ? He C O f h? CO ? C O
Some of C react with OH and H2O (but not with
H2) g C OH ? CO H h CO H2 ?
HCO H i C H2O ? HCO H
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The timescale to form CO
Assume dark region where all H is in H2 and all
atoms more massive than He are in neutral atomic
form.
The timescale on which almost all carbon becomes
contained in CO (nO gt nC) is at least equal to
the timescale for one hydrogen molecule to be
ionized for every C nC/? n(H2) 2 nC/? nH
For ? 6?10-17 s-1 and nC/nH 10-4, the above
expression gives a value of 105 yr.
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5. Nitrogen Chemistry
Nitrogen chemistry differs from that of oxygen
and carbon N H3 ? NH H2 The
N-chemistry starts with a neutral- neutral
reaction (e.g.) CH N ? CN H
?

tN2 106 yr
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N2 vs. CO
Chemistry in Photodissociation Regions
(PDRs) Sternberg Dalgarno 1995
The Orion Bar
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Chemical Evolution?
Suzuki et al. 1992
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Chemical Evolution?
Dust has to be taken into account! Freeze-out vs.
free-fall
Walmsley 1991 van Dishoeck et al. 1993
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Evidences of freeze-out solid features
Spitzer
from van Dishoeck et al. 2003
Pontoppidan et al. 2007
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Evidences of freeze-out the missing CO
C17O(1-0) emission (Caselli et al. 1999)
CO hole
Dust grain
Molecules freeze out onto dust grains in the
center of pre-stellar cores ?
dust peak
0.05 ly
Dust emission in a pre-stellar core (Ward-Thompson
et al. 1999)
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Evidences of freeze-out deuterium fractionation

N2D(2-1)
N2H(1-0)
D-fractionation increases towards the core
center (0.2 Caselli et al. 2002 Crapsi et al.
2004, 2005)
Dust emission in the pre-stellar core L1544
(Ward-Thompson et al. 1999)
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Evidences of freeze-out deuterium fractionation
6. Deuterium fractionation
H2D / H3 (and D/H) increases (i) in cold gas
H3 HD ? H2D H2 230 K
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Evidences of freeze-out deuterium fractionation
H2D in L1544
Vastel et al. 2006
o-H2D CSO
Caselli et al. 2003, 2008
N2H(1-0) IRAM
N2D(2-1) IRAM
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Evidences of freeze-out deuterium fractionation
D-fractionation and ion fraction
g-
e-
PAH-, PAH
Wootten et al. 1979 Guelin et al. 1982 Bergin et
al. 1998 Caselli et al. 1998 Dalgarno 2006
H3
neutrals
?/n(H2)
HCO, N2H
HD
H2
e-
neutrals
Uncertainties PAHs, PAH-s neutrals (O)
orthopara H2
DCO, N2D
H2D
1/3
g-
PAH-
PAH
HD
H2
e-
neutrals
DCO, N2D
D2H
2/3
g-
PAH-
PAH
HD
H2
e-
neutrals
DCO, N2D
D3
g-
PAHs
PAH-
PAH
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What happens after a protostar is born?
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What happens after a protostar is born?
Large abundances of multiply deuterated species
in (Class 0) protostellar envelopes (Ceccarelli
et al. 1998 Parise et al. 2002, 2004, 2006 van
der Tak et al. 2002 Vastel et al. 2003)
D2CO/H2CO 0.1 CHD2OH/CH3OH 0.02 D2S/H2S
0.02 ND3/NH3 0.001 CD3OH/CH3OH 0.02
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What happens after a protostar is born?
Complex organic molecules in hot cores and hot
corinos (e.g. Wright et al. 1996 Cazaux et al.
2003 Bottinelli et al. 2004,2008 Kuan et al.
2004)
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What happens after a protostar is born?
Strong H2O, SiO, CH3OH, NH3, emission (e.g.
Bachiller 1996) and complex molecules (C2H5OH,
HCOOCH3 Arce et al. 2008) along outflows.
Jørgensen et al. 2004
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What happens after a protostar is born?

• dust heating, X-rays nearby protostars (mantle
  processing and evaporation)
• dust (mantles and cores) sputtering
  vaporization along protostellar outflows

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7. Surface Chemistry
thermal hopping
quantum tunneling
106 sites
Tielens Hagen (1982) Tielens Allamandola
(1987) Hasegawa et al. (1992) Tielens
1993 Cazaux Tielens (2002) Cuppen Herbst
(2005) Cazaux et al. (2008) Garrod (2008)
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7. Surface Chemistry
REACTANTS MAINLY MOBILE ATOMS AND
RADICALS A B ? AB
association H H ? H2   H X
? XH (X O, C, N, CO, etc.)
Accretion
  WHICH CONVERTS   O ? OH ? H2O   C ? CH
? CH2 ? CH3 ? CH4   N ? NH ? NH2 ? NH3   CO ?
HCO ? H2CO ? H3CO ? CH3OH
?10/Tk1/2 n(H2) days
DiffusionReaction
tqt(H) ?10-5-10-3 s
Watson Salpeter 1972 Allen Robinson 1977
Pickes Williams 1977 dHendecourt et al.
1985 Hasegawa et al. 1992 Caselli et al. 1993
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What happens in protoplanetary disks?
Aikawa Herbst 1999 Markwick Charnley 2003
Aikawa Nomura 2006 Bergin et al. 2007 Dutrey
et al. 2007 Meijerink, Poelman et al. 2008
Semenov et al. 2008
Henning Semenov 2008
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What happens in protoplanetary disks?
Chemical Structure of PPDs
surface
UV, X-rays
intermediate
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midplane
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UV, c.r.
Surface layer n104-5cm-3, Tgt50K ?Photochemistry
(high CN/HCN) Intermediate n106-7cm-3,
20ltTlt40K ?Dense cloud chemistry (freeze-out,
D-fractionation) Midplane ngt107cm-3,
Tlt20K ?Freeze-out (are parent-cloud species
preserved?)
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What happens in protoplanetary disks?
DCO (van Dishoeck et al. 2003 Guilloteau et al.
2006) and H2D (Ceccarelli et al. 2004) detected.
DCO/HCO and DCN/HCN 0.05 in TW Hydrae
DCO/HCO0.05 in L1544
QI, WILNER, AIKAWA, BLAKE, HOGERHEIJDE 2008
HCO(3-2)
DCO(3-2) first image!
HCN(3-2)
ALMAis needed !
DCN(3-2) first image!
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Links to the Solar System ?
HDO IN THE DISK OF DM Tau HDO/H2O0.01
SOURCE HDO/H2O
Class 0 protostars (HC) 0.03
Protoplanetary disks 0.01?
Comets 3.0x10-4
Carbonaceous chondrites 1.5?10-4
Oceans 1.6?10-4
Ground transition at 464 GHz with JCMT
(Ceccarelli et al. 2005)
Herschel is needed !
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Links to the Solar System ?
The assemblage of planets
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Links to the Solar System ?
Chondrites interstellar ovens?
cement
condrule
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Links to the Solar System ?

• Cement between chondrules
• Consists of tiny particles ( interstellar dust)
• Often contains water and carbon
• Often contains hydrous minerals resulting from
  ancient interaction of liquid water and primary
  minerals.

Must have been liquid water in planetesimals!
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Links to the Solar System ?
Carbonaceous chondrites contain a substantial
amount of C, up to 3 by weight.
70 amino acids have been identified in
carbonaceous chondrites 8 of these are found in
terrestrial proteins (Botta Bada 2002, Survey
Geophys.)
L-Alanine
L-Aspartic Acid
L-Glutamine
Glycine
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Exoplanets
Brown Dwarf 2M1207 and its planetary companion
(14 MJ Chauvin et al. 2005).
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Exoplanets
Initial studies of hot Jupiters atmospheres
Richardson et al. 2007 (Spitzer) de Mooij et
al. 2009 (WHTUKIRT) Sing Lopez-Morales 2009
(MagellanVLT)
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Exoplanets
Darwin TPF will detect Biomarkers
O3
H2O vapor.
Spectroscopic Chemical Analysis of Atmophere.
Methane Disequilibrium chemicals. CH4 O2 --gt
CO2 H2O
Courtesy Prof. G.W. Marcy, University of
California, Berkeley
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Exoplanets
NASA's Kepler spacecraft, scheduled to launch in
March on a journey to search for other Earths,
has arrived in Cape Canaveral, FL
For four years, Kepler will monitor 100,000 stars
in our Galaxy,looking for (Earthlike) planetary
transits.
http//planetquest.jpl.nasa.gov/news/keplerArrival
.cfm
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Summary
Prestellar cores CN, N2H, NH3, N2D, DCO,
o-H2D Ion-molecule reactions, freeze-out,
deuterium fractionation, surface chemistry
Outflows H2O, CH3OH, NH3, SiO, S-bearing
species Grain sputtering, grain-grain collisions,
neutral-neutral reactions
Hot Cores CH3CN, HCOOCH3, complex saturated
molecules Grain mantle evaporation,
neutral-neutral reactions, surface chemistry
PP Disks CO, CN, HCN,N2H,HCO, DCO,
o-H2D Ion-molecule reactions, freeze-out,
D-fractionation, surface chemistry,
photochemistry, X-rays, dust coagulation
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Main Uncertainties

• Cosmic-ray ionization rate
• Elemental abundance in dark clouds (e.g. metals)
• Oxygen chemistry (? Herschel)
• PAHs abundance
• Surface chemistry and gas phase high-T chemistry
• H2 ortho-to-para ratio

Constant need of interaction with real chemists
(theory lab, gas-phasesolid state), who
provide rate coefficients, collisional rates,
transition frequencies