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Interstellar chemistry
Liv Hornekær
Overall structure of the ISM
Overview picture
21 cm
2.6 mm
21 cm observations
Leiden-Dwingeloo/Argentina/Bonn
H atom
J=L+S F=I+J
La: 121.6 nm, 10.2 eV
From spectroscopy of the solar photosphere.
Relative abundance of the elements: A(X) = 106 (N
X/N
H)
2.6 mm
CO
J=0->1
CO
Overview of interstellar molecules
Molecules with Two Atoms
AlF AlCl C2 CH CH+ CN CO CO+ CP CS CSi HCl H2 KCl
NH NO NS NaCl OH PN SO SO+ SiN SiO SiS HF SH FeO N2
Molecules with Three Atoms
C3 C2H C2O C2S CH2 HCN HCO HCO+ HCS+ HOC+
H2O H2S HNC HNO MgCN MgNC N2H+ N20 NaCN OCS SO2 SiC2 CO2 NH2
H3+ AlNC
Molecules with Four Atoms
C3H C3H C3N C3O C3S C2H2 CH2D+ HCCN HCNH+ HNCO HNCS HOCO+
H2CO H2CN H2CS H3O+ SiC3 NH3
Molecules with Five Atoms
C5 C4H C4Si l-C3H2 c-C3H2 CH2CN CH4 HC3N HC2NC HCOOH H2CHN
H2C20 H2NCN HNC3 SiH4 H2COH+
Molecules with Six Atoms
C5H C5O C2H4 CH3CN CH3NC CH3OH CH3SH HC3NH+ HC2CHO HCONH2
H2C4 C5N
Molecules with Seven Atoms
C6H CH2CHCN CH3C2H HC5N HCOCH3 NH2CH3 C2H4O CH2CHOH
Molecules with Eight Atoms
CH3C3N HCOOCH3 CH3COOH C7H H2C6 CH2OHCHO CH2CHCHO
Molecules with Nine Atoms
CH3C4H CH3CH2CN (CH3)20 CH3CH20H HC7N C8H
Molecules with Ten Atoms
CH3C5N (CH3)2CO NH2CH2COOH CH3CH2CHO
Molecules with Eleven Atoms
HC9N
Molecules with Thirteen Atoms
HC11N
Chemical reactions in general
Reaction rate: k=Ae-Ea/kT
Stabilizing reaction products
Stabilizing reaction products
Radiation stabilization
Collisional stabilization:
AB*+ M AB + M
Chemical reactions in general
Reaction rate: k=Ae-Ea/kT
Reactions including radicals: Ea small
Reactions including ions: Ea ~ 0
Ion-molecule reactions
A+ + B -> C
+ + D
Typical rate: k=10-9
cm3/s
Rate equation:
= -k(T)n(A)n(B)
dn(A)
dt
Chemical models
~1000 gas phase reactions
~1-2 chemical reactions on dust grain surfaces.
A few models include ~40 surface reactions.
Reaction rates from:
1) Experimental measurements
2) Extrapolation of experimental measurements at high
temperature to interstellar temperatures
3) Theoretical calculations
4) Guess – or trial and error (Fitting the model to observations)
Experimental measurements
Ian Sims
CN + C2H6
Reaction rate: k=Ae-Ea/kT
Experimental measurements
Ian Sims
CN + C2H6
Reaction rate: k=Ae-Ea/kT
x
Carbon chemistry
• I.P. of C: 11.26 eV < 13.6 eV => majority of carbon as
C+
• C+ + H
2 → CH
2
+ + hν possible at low T (starting
reaction)
• CH2
+ => fast ion-molecule chemistry giving: CH, C
2,
…
• C+ + H
2 → CH
+ + H: endothermic by 0.4 eV
Average Interstellar Radiation
field
Sum of CMB (radio/FIR), thermal emission from dust (IR), cool stars + OB stars
(VIS, UV), hot ionized medium (FUV and X-ray)
Average Interstellar Radiation
field
Carbon chemistry
• I.P. of C: 11.26 eV < 13.6 eV => majority of carbon as
C+
• C+ + H
2 → CH
2
+ + hν possible at low T (starting
reaction)
• CH2
+ => fast ion-molecule chemistry giving: CH, C
2,
…
• C+ + H
2 → CH
+ + H: endothermic by 0.4 eV
Oxygen chemistry
I.P. of O: 13.618 > 13.598 eV => majority of oxygen as O
• Ionization by cosmic rays
• H2 or H + C.R. → H
2
+ or H
+ + C.R. + e
• H2
+ + H
2 → H
3
+ + H (fast)
• H+ or H
3
+ reacts with oxygen:
• H+ + O ↔ H + O
+ , O
+ + H
2 → OH
+ + H
• H3
+ + O → OH
+ + H
2
• When OH+ is formed fast ion-molecule reactions give OH, H
2O and
CO
• OH abundace is proportional to the ionization rate by cosmic
radiation: ζCR
=> observed OH abundance used to determine ζCR
Overview of interstellar molecules
Molecules with Two Atoms
AlF AlCl C2 CH CH+ CN CO CO+ CP CS CSi HCl H2 KCl
NH NO NS NaCl OH PN SO SO+ SiN SiO SiS HF SH FeO N2
Molecules with Three Atoms
C3 C2H C2O C2S CH2 HCN HCO HCO+ HCS+ HOC+
H2O H2S HNC HNO MgCN MgNC N2H+ N20 NaCN OCS SO2 SiC2 CO2 NH2
H3+ AlNC
Molecules with Four Atoms
C3H C3H C3N C3O C3S C2H2 CH2D+ HCCN HCNH+ HNCO HNCS HOCO+
H2CO H2CN H2CS H3O+ SiC3 NH3
Molecules with Five Atoms
C5 C4H C4Si l-C3H2 c-C3H2 CH2CN CH4 HC3N HC2NC HCOOH H2CHN
H2C20 H2NCN HNC3 SiH4 H2COH+
Molecules with Six Atoms
C5H C5O C2H4 CH3CN CH3NC CH3OH CH3SH HC3NH+ HC2CHO HCONH2
H2C4 C5N
Molecules with Seven Atoms
C6H CH2CHCN CH3C2H HC5N HCOCH3 NH2CH3 C2H4O CH2CHOH
H2 formation
No dipole allowed transitions
No radiation stabilization
H
3-body collisions – ok at high density
Diffuse/Dense cloud ISM densities => ~No 3-body collisions
Coalsack nebula in the
Southern Cross
H2 absorption bands in diffuse
clouds
HD110432 behind the Coalsack Nebula
ISM and star formation
H2
H2
H2 is an important cooling agent and
key to developing chemical complexity in ISM
- But how is H2 formed under ISM conditions?
202 nm
202 nm
Destruction mechanisms for H2
Lyman transition
Werner
transition
+ Cosmic
radiation
H2 formation rate from
observations
Typical diffuse cloud rate: k~3 10-17
n(H) cm3s-1
H2 formation
No dipole allowed transitions
No radiation stabilization
H
3-body collisions – ok at high density
Diffuse/Dense cloud ISM densities => ~No 3-body collisions
H2 formation in the gas phase
Radiative association (slow):
H + e- H
- + hn
Associative detachment:
H + H- H
2 + e
-
Typical diffuse cloud rate: k~10-21
n(H) cm3s-1
H2 formation rate: k~3 10
-17 n(H) cm
3s-1
Surface reactions
H
Surface reactions
Surface reactions
Objects: Dark nebulae
Molecules and dust grains
Depletion
D(X) = log(NX/N
H)-log(N
X/N
H)ISM
Relative depletion: d(X) = 1-10D(X)
=1-(NX/N
H) / (N
X/N
H)ISM
Measured in UV => diffuse and intercloud medium.
Absorption spectrum
H2O
H20
CH
CH
CO
Silicates
(Mg2SiO4 ,
Fe2SiO4)
Silicates
MgSiO3 (enstatite): Si-O stretch 9.7 mm O-Si-O bend 19.0 mm
Mg2SiO4 (fosterite): Si-O stretch 10.0 mm O-Si-O bend 19.5 mm
FeSiO3 (ferrosilite): Si-O stretch 9.5 mm O-Si-O bend 20.0 mm
SiC (Silicon carbide): Si-C stretch 11.2 mm
Carbon
Hydrogenated amorphous carbon: C-H stretch: 3.4 mm
Observed in the diffuse ISM
Measured towards Sgr A.
C60
+
C60 and C70 detected in
protoplanetary nebula Tc 1
C60
C70
Cami et al., Science 329, 1180, Sept. 2010
A few % of C
Diamond
ISO spectra of two pre-main-sequence stars
Lower curves are laboratory absorption
spectra for diamond nano-crystals
Aromatic hydrocarbon related
features
6.8 -10.8 microns
Aromatic hydrocarbon related
features
3.3 mm: C-H stretch
6.2 mm: C-C stretch
7.7 mm: C-C stretch
8.7 mm: C-H in plane bend
11.2 mm: C-H out of plane bend
6.8 -10.8 microns
PAH’er
Polycykliske Aromatiske Kulbrinter
Grænsen mellem molekyle og nano-partikel
Benzene Pyrene
Emission fra PAH’er
teori – 850 K PAH’er
Gennemsnitlig interstellar emission, => kun ~5% af C i PAH’er
PAH’er
Carbon
Interstellar carbon
Ice
Dust enshrouded protostar
H2O: O-H stretch: 3.05 mm
CO: C-O stretch: 4.67 mm
CH3OH: O-H stretch: 3.05 mm
H-O-H bend: 6.0 mm
C-H stretch: 3.53 mm
Water ice - morphology
Spectral lines – vibrational bands modified by local environment
Hindered rotation
Rotationel splitting
CO ice
Mixed ice
High shielding, low temperature: Even very volatile
molecules (e.g. CO) condenses out on dust grains.
Observations of 4.67 mm absorption in C-O
stretch, shows that CO and water are not mixed
Different molecules condenses at different temperatures
1. Large grains:
~100 nm
2. Surface structure or
smaller grains ?
4. PAHs or other
very small grains (VSG) < 10nm
3. Small carbon grains
< 20 nm
1 2 3
4
Greenberg 1996
Average interstellar extinction
curve
Dust grain sizes
Large grains: 20 nm - 1 mm
Silicate or carbon
Separate populations ? / composite grains ? /
Grains with carbon mantles ?
Very small grains: 1-20 nm,
Carbon
PAHs
Pre-solar dust grains in meteorites
Onion-like graphite particles
Silicates: Olivines (Mg2SiO
4, Fe
2SiO
4)
Atoms and molecules on surfaces
1
2a
2b Sticking
Sticking + hot atom
Scattering
Sticking:
S = Probability of 2a+2b
Sticking => adsorbed
Possible mechanisms for H2 formation
on surfaces
Desorption
Langmuir-Hinshelwood:
Adsorption
Diffusion
Recombination
Desorption
Eley-Rideal:
H(ads) + H(gas) H2(gas)
Hot Atom:
Strong/Weak
Adsorption Diffusion
Recombination
The effect on the ISM of H2 formation
v
Kinetic energy ?
n = 0
J = 0
Molecular excitation ?
Ereleased
~ 4.5 eV (50.000 oC)
=>
Dust grain heating ?
The effect on the ISM of H2
formation
Flores
Dashed line: No energy branching into kinetic energy
Curve 1: 0.5 eV in kinetic energy
Curve 2: 1.5 eV in kinetic energy
Curve 3: 2.25 eV in kinetic energy
Bringing the Interstellar medium to
a laboratory near you
Re-creating interstellare conditions ?
Interstellar pressure:
P= 10-13
atm
Interstellar temperatures:
T = 6-1000 K
Relevant surfaces:
Ice, graphite, PAHs, amorphous carbon, silicates
Surfaces of interstellar relevance
Water ice Atomic deposition
shield Cu
Graphite
T
H2 formation at high temperatures
– bare grains
Orion nebula – Orion bar
H on graphite
Eva Rauls
Neumann et al. Appl. Phys. A 55, 489 (1992)
Jeloica & Sidis, Chem. Phys. Lett. 300, 157 (1999)
Sha et al, Surface Science 496, 318 (2002)
H-Dimers on graphite
Dimer A
Dimer B
103 x 114 Å2
Vt = 884 mV, I
t = 0.16 nA
Dimers: Theori vs. Experiment
Vt = 884 mV, I
t = 0.16 nA
Vt=0.9 V, LDOS=1x10
-6 (eV)
-1 Å
-3
e. f.
Ortho dimer - Dimer A Para dimer - Dimer B
Pair formation
Hornekær et al. Phys. Rev. Lett. 97, 186102 (2006)
n=1 => First order
desorption
490 K => 1.4 eV
580 K => 1.6 eV
dQ
dt = -k0 e
- / T Qn kB EB
Zecho et al, J. Chem. Phys. 117, 8486 (2002)
H2 formation on graphite - TDS
H2 formation
Hornekær et al. Phys. Rev. Lett. 96, 156104 (2006)
Ortho Meta Para
Eley Rideal - Abstraction
Sha et al. (2002)
Zecho et al. (2002)
Matinazzo & Tantardini (2006)
Morisset et al. (2004)
Jeloaica & Sidis (2001)
Meijer et al. (2001)
Bachellerie et al. (2007)
Thomas et al. (2008)
H2 formation at lower temperatures
- ice covered surfaces
Water ice - morphology
Thermal Desorption Spectroscopy
HD from amorphous
water ice
D2 from graphite
Thermal Desorption Spectroscopy
HD from amorphous
water ice
D2 from graphite
Kinetic energy of formed H2
Laser desorption –
Time-of-flight
H D
QMS
ASW
Laser
Kinetic energy of D2 formed on
graphite
0 1.0 2.0 3.0 4.00
0.002
0.004
0.006
0.008
0.01
0.012
0.014
0.016
Translational Energy (eV)
D(E
)
~1.3 eV i translation
S. Baouche et al, J. Chem. Phys. 2006
Kinetic energy distribution
Baouche et al., J. Chem. Phys. 125, 084712 (2006 )
Kinetic energy of HD formed
on porous water ice
HD
D2
Solid line:
45 K Maxwell-Boltzmann velocity dist.
L. Hornekær et al., Science, 302, 1943 (2003)
H2 formed on porous water ice
Surface structure
determines energy
partitioning
Porous surface:
Grain heating
Slow H2
Result
Surface structure determines energy partitioning
Energy distribution in H2 formation
on graphite
L1630 Orion bar
Latimer et al., CPL 455, 174 (2008)
Dust grain morphology
Bare grains – porous and non-porous
Ice covered grains – maybe compacted by
formation/processing
Orion nebula – Orion bar
Energy distribution in H2 formation
and PDR observations
Observation show overpopulation in v=4
Does not fit shock and UV fluorescence models
- WHY?
Photo Dissociation Regions:
gas temperatures: 100 – 1000K
Blue: PAH emission
Green: H2 vibrational
line emission (FAST)
Red: CO emission
(30 m Telescope,
IRAM)
Star at NW
L1630 Orion bar
Energy distribution in H2 formation
and PDR observations
Observation show overpopulation in v=4
Does not fit Shock and UV fluorescense models
- formation pumping?
Photo Dissociation Regions:
gas temperatures: 100 – 1000K
Blue: PAH emission
Green: H2 vibrational
line emission (FAST)
Red: CO emission
(30 m Telescope,
IRAM)
Star at NW
L1630 Orion bar
H2 formation in different ISM
environments?
Hgas
+Hgas
H2
H + H + H
2 Problematic under
diffuse cloud conditions
Fine under
Dense cloud and PDR conditions
Hornekær et al., Science (2003)
Hornekær et al., PRL (2006)
H + H + H
2 Alternative candidate
Rauls and Hornekær (2007)
Hornekær et al., PRL (2006)
Katz et al., ApJ (1999)
Cuppen et al., MNRAS (2005)
PAHs as an H2 catalyst
• Correlations between high H2
formation rates and PAH
emission observed in PDRs with
low UV flux
– Habart et al.*
• PAH cations considered for H2
formation
– Snow et al.^
– LePage et al.#
*Habart et al., A&A, 397, 623 (2003)
Habart et al. A&A, 414, 531 (2004)
^Snow et al. Nature 391, 259 (1998)
#LePage et al. Ap.J., 704, 274, (2009)
Here:
the role of neutral PAHs?
H-PAH interaction
Density Functional Theory (DFT) calculations
reveal low barrier routes to
H-PAH formation and PAH catalyzed H2 formation
Rauls and Hornekær, Astrophys. J. 679, 531 (2008)
60 meV
-1.4 eV
C24H13 C24H12 + H
H-PAH interaction
Rauls and Hornekær, Astrophys. J. 679, 531 (2008)
Density Functional Theory (DFT) calculations
reveal low barrier routes to
H-PAH formation and PAH catalyzed H2 formation
H-PAH formation
Superhydrogenated PAHs
• Evidence in IR emission
– C-H stretching mode
• 3.3 μm – aromatic
• 3.4 μm – aliphatic
• High UV flux (Orion bar)
– Limited excess hydrogen
• Low UV flux (IRAS 05341)
– Significant excess
hydrogen
– -CH3 or -H
M. P. Bernstein, et al., ApJ, 472, L127 (1996).
Reactions out of mass 300
σ = 0.6 ± 0.3 Å2
H-PAH formation
CDH groups
CD2 groups
Subsequent H irradiation
IR spectroscopy
Menella et al., Astrophys. J. Lett. 2012
Experiments reveal many different
pathways to H2 formation, which
together make H2 formation
efficient under the many different
physical and chemical conditions in
the ISM
Interstellar surface reactions
H O
C N
H2
ISM in the Milkyway
Overview picture
ISM and star formation
Eagle-Nebula -Pillars
Eagle-Nebula
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