ISM & Astrochemistry
Lecture 2
Protoplanetary Nebula
The evolutionary stage between evolved stars and planetary
nebula
CRL 618 – many organic molecules
Including the only extra-solar system
detection of benzene, C6H6
Time scale of chemistry and
evolution of this object is 600-1000
years
Molecule formation in shocks
Supersonic shock waves: Sound speed ~ 1 km s-1
Shocks compress and heat the gas
Hydrodynamic (J-type) shocks: immediately post-shock, density
jumps by 4-6, gas temperature ~ 3000(VS/10 km s-1)2
Gas cools quickly (~ few tens, hundred years) and increases its
density further as it cools – path lengths are small.
MHD (C-type) shocks: shock front is preceded by a magnetic
precursor, gas density and temperature change continuously, ions
and neutrals move at different velocities – path lengths are large
Importance for chemistry: Endothermic neutral-neutral
reactions can occur.
Interstellar and Circumstellar Molecules
H2
H3+
CH3
CH4
CH3OH
CH3NH2
HCOOCH3
(CH3)2O
(CH3)2CO
CO
CH2
NH3
CH2NH
CH3SH
CH3CCH
CH3C3N
C2H5OH
CH3C5N
CS
NH2
H3O+
H2CCC
C2H4
CH3CHO
HC6H
C2H5CN
CH3CH2CHO
CN
H2O
H2CO
c-C3H2
CH3CN
c-CH2OCH2
C7H
CH3C4H
(CH2OH)2
C2
H2S
H2CS
CH2CN
CH3NC
CH2CHCN
HOCH2CHO
C8H
HCOOC2H5
CH
CCH
c-C3H
NH2CN
CH2CHO
HC5N
CH3COOH
HC7N
HC9N
CH+
HCN
l-C3H
CH2CO
NH2CHO
C6H
H2CCCHCN
CH3CONH2
CH3C6H
HF
HNC
C2H2
HCOOH
HC3NH+
CH2CHOH
H2C6
CH3CHCH2
C6H6
CF+
HCO
HCNH+
C4H
H2CCCC
C6H-
CH2CHCHO
C8H-
C3H7CN
SiO
HCO+
H2CN
HC3N
C5H
SiS
HOC+
HCCN
HCCNC
HC4H
SiC
N2H+
HNCO
HNCCC
HC4N
SiN
HNO
HOCN
H2COH+
c-C3H2O
NH
HCS+
HCNO
C4H-
CH2CNH
NO
C3
HNCS
SiH4
C5N-
SO
C2O
HSCN
C5
C5N
SO+
C2S
C3N
SiC4
CP
SO2
C3O
CNCHO
PO
N2O
C3S
PN
CO2
C3N-
HCl
H2O+
HCO2+
KCl
H2Cl+
CNCHO
AlCl
OCS
C-SiC2
OH
MgNC
AlF
AlNC
AlOH
NaCl
OH+
MgCN
SiNC
CCP
HCP
FeO
SH
NaCN
CO+
O2
N2
CN-
SiCN
NH2CH2CN
HC11N
C2H5OCH3
One-body reactions
Photodissociation/photoionisation:
Unshielded photorates in ISM: β0 = 10-10 s-1
Within interstellar clouds, characterise extinction of UV
photons by the visual extinction, AV, measured in magnitudes,
so that:
β = β0exp(-bAV)
where b is a constant (~ 1- 3) and differs for different
molecules
Cosmic Ray Ionisation
H2 + crp → H2 +
+
e-
H2+ + H2 → H3+ + H
He + crp → He+ + e-
H3+: P.A.(H2) very low
Proton transfer reactions very
efficient
Key to synthesising molecules
He+: I.P.(He) very large
Breaks bonds in reaction
Key to destruction of molecules
He+ + H2 → products
exothermic but
unreactive
IS Chemistry efficient because
He+ does not react with H2
Two-body reactions
Ion-neutral reactions:
Neutral-neutral reactions:
Ion-electron dissociative recombination
(molecular ions)
Ion-electron radiative recombination
(atomic ions)
Radiative association
Three-body reactions (only if density is very large,
1013 cm-3)
Formation of Molecules
Ion-neutral reactions:
Activation energy barriers
rare if exothermic
Temperature independent (or
inversely dependent on T)
Neutral-neutral reactions:
Often have activation energy
barriers
Often rate coefficient is
proportional to temperature
Formation of Molecules
Ion-electron dissociative
recombination reactions:
Fast, multiple products, inverse
T dependence
Atomic ion-electron radiative
recombination recombination:
Neutral complex stabilises by
emission of a photon, about
1000 times slower than DR
rate coefficients
Radiative association:
A+ + B → AB+ + hν
Photon emission more efficient as
size of complex grows, therefore
can be important in synthesising
large molecular ions
CH3+ + H2 → CH5+ + h ν
k(T) = 1.3 10-13(T/300)-1 cm3 s-1
CH3+ + HCN → CH3CNH+ + h ν
k(T) = 9.0 10-9(T/300)-0.5 cm3 s-1
Chemical Kinetics
A+B→C+D
k = <σv> cm3 s-1
Loss of A (and B) per unit volume per second is:
dn(A)/dt = - kn(A)n(B)
cm-3 s-1
where n(A) = no. of molecules of A per unit volume
Formation of C (and D) per unit volume per second is:
dn(C)/dt = + kn(A)n(B)
cm-3 s-1
- Second-order kinetics – rate of formation and loss
proportional to the concentration of two reactants
First-order kinetics
A + hν → C + D
β (units s-1)
Loss of A (and B) per unit volume per second is:
dn(A)/dt = - βn(A)
cm-3 s-1
where β = photodissociation rate of A (s-1)
Aside: The number, more accurately, flux of UV photons or
cosmic-ray particles, is contained within β or ς
- First-order kinetics – rate of formation and loss proportional
to the concentration of one reactant
General case
dn(Xj)/dt = Σ klm[Xl][Xm] + Σ βn[Xn]
- [Xj]{Σ kjl[Xl] + Σ βj}
m-3 s-1
or d[X]/dt = FX – LX[X]
Need to solve a system of first-order, non-linear ODEs
- solve using GEAR techniques
-Steady-state approximation – rate of formation = rate of loss
FX = LX[X]ss so that [X]ss = FX/LX
Need to solve a system of non-linear algebraic equations
- solve using Newton-Raphson methods
Time scales
d[X]/dt = FX – LX[X]
For simplicity, assume FX and LX are constants and [X] = 0 at
t =0 (initial condition)
Solution is:
[X,t] = (FX/LX){1 – e-Lxt}
[X,t] = [X]ss{1 – e-t/tc}
where tc = 1/LX
Note: As t → ∞, [X] → [X]ss
When t = tc, [X,tc] = 0.63[X]ss, so most molecular evolution
occurs within a few times tc
Grain Surface Time-scales
Collision time:
tc = [vH(πr2nd)]-1 ~ 109/n(cm-3) years
Thermal hopping time:
th = ν0-1exp(Eb/kT)
Tunnelling time:
tt = v0-1exp[(4πa/h)(2mEb)1/2]
Thermal desorption time:
tev = ν0-1exp(ED/kT)
Here Eb ~ 0.3ED, so hopping time < desorption time
For H at 10K, ED = 300K, tt ~ 2 10-11 s, th ~ 7 10-9 s
Tunnelling time < hopping time only for lightest species (H, D)
For O, ED ~ 800K, th ~ 0.025 s.
For S, ED ~ 1100K, th ~ 250 s, tt ~ 2 weeks
Heavy atoms are immobile
compared to H atoms
Formation of H2
Gas phase association of H atoms far too slow, k ~ 10-30 cm3 s-1
Gas and dust well-mixed
In low-density gas, H atoms
chemisorb and fill all binding
sites (106) per grain
Subsequently, H atoms physisorb
Surface mobility of these H
atoms is large, even at 10 K.
H atoms scans surface until
it finds another atom with
which it combines to form H2
Formation of Molecular Hydrogen
Gas-Phase formation:
H + H → H2 + hν
very slow, insignificant in ISM
Grain surface formation:
Langmuir-Hinshelwood
(surface diffusion)
Eley-Rideal
(direct hit)
Grain Surface Chemistry
Zero-order approximation:
Since H atoms are much more mobile than heavy
atoms, hydrogenation dominates if n(H) > Σn(X), X = O, C, N
Zero-order prediction:
Ices should be dominated by the hydrogenation of the
most abundant species which can accrete from the gas-phase
Accretion time-scale:
tac(X) = (SXvXσnd)-1, where SX is the sticking
coefficient ~ 1 at 10K
tac (yrs) ~ 109/n(cm-3) ~ 104 – 105 yrs in a dark cloud
Interstellar Ices
Mostly water ice
Substantial components:
- CO, CO2, CH3OH
Minor components:
- HCOOH, CH4, H2CO
Ices are layered
- CO in polar and non-polar
ices
Sensitive to f > 10-6
Solid H2O, CO ~ gaseous H2O, CO