1. Astrochemistry Les Houches Lectures September 2005 Lecture 1
T J Millar
School of Physics and Astronomy
University of Manchester
PO Box88, Manchester M60 1QD

2. Astrochemistry
Astrochemistry is the study of the synthesis of molecules in space and their use in determining the properties of Interstellar Matter, the material between the stars.
An IR image of the B68 dark cloud taken with the VLT.

3. Interstellar Matter Comprises Gas and Dust
Dust absorbs and scatters (extinguishes) starlight
Top row – optical images of B68
Bottom row – IR images of B68
Dust extinction is less efficient at longer wavelengths

4. Interstellar Gas Dark Clouds - T ~ 10 K, n ~ 1010 - 1012 m-3
Not penetrated by optical and UV photons. Little ionisation. Material is mostly molecular, dominant species is H2. Over 60 molecules detected, mostly via radio astronomy.
Masses 1 – 500 solar masses, size ~ 1-5 pc
Typically can form 1 or a couple of low-mass (solar mass) stars.
Example – B68

5. Interstellar Gas Giant Molecular Clouds (GMCs)
T ~ K, n ~ m-3, <n> ~ m-3
Material is mostly molecular. About 100 molecules detected. Most massive objects in the Galaxy.
Masses ~ 1 million solar masses, size ~ 50 pc
Typically can form thousands of low-mass stars and several high-mass stars.
Example – Orion Molecular Cloud, Sagittarius,
Eagle Nebula

6. Interstellar Gas
Gas and star formation in the Eagle Nebula

7. Interstellar Dust The interstellar extinction curve
absorption plus scattering
UV extinction implies small (100 nm) grains
Vis. Extinction implies normal (1000 nm) grains
n(a)da ~ a-3.5da
Silicates plus carbonaceous grains
Mass dust/Mass gas ~ 0.01
Dense gas – larger grains with icy mantles
Normal – nd/n ~ 10-12
The interstellar extinction curve

8. 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

9. Interstellar Organic Molecules
CH+
HCN
H2CO
HC3N
CH3OH
HC5N
HCOOCH3
HC7N CS
HNC
H2CS
HOCHO
CH3CN
CH3CCH
CH3C3N
HC9N CO
HCO
H2CN
CH2NH
CH3NC
CH3NH2
CH3COOH
HC11N CN
OCS
HNCO
CH2CO
CH3SH
CH3CHO
CH2OHCHO
C2H5CN C2
CH2
HNCS
NH2CN
NH2CHO
CH2CHCN
H2C6
CH3C4H CH
C2H
C3H
C4H
C5H
C6H
CH3C5N
CO+ C3
c-C3H
c-C3H2
H2C4
c-C2H4O
CH3OCH3
CF+
CO2
C3N
H2C3
HC3NH+
CH2CHOH
C2H5OH
C2O
C3O
CH2CN
CH3COCH3
C2S
C3S
HCCNC
OHCH2CH2OH
HCO+
CH3
HNCCC
NH2CH2COOH?
HOC+
C2H2
CH4
HCS+
HOCO+
H2COH+
HCNH+

10. ND3 in Interstellar Clouds
Submillimetre detection of ND3 by Lis et al., Astrophysical Journal, 571, L55 (2002)
ND3/NH3 = , compared with (D/H)3 ~

11. Chemical Kinetics A + B → C + D k = <σv> m3 s-1
Loss of A (and B) per unit volume per second is:
dn(A)/dt = - kn(A)n(B) m-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) m-3 s-1
- Second-order kinetics – rate of formation and loss proportional to the concentration of two reactants

12. 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) m-3 s-1
where β = photodissociation rate of A
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

13. 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

14. 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

15. One-body reactions Unshielded photorates in ISM: β0 = 10-10 s-1
Photodissociation/photoionisation:
Unshielded photorates in ISM: β0 = 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

16. Cosmic Ray Ionisation H3+: P.A.(H2) very low H2 + crp → H2+ + e-
Proton transfer reactions very efficient
Key to synthesising molecules
He+: I.P.(He) very large
Breaks bonds in reaction
Key to destruction of molecules
IS Chemistry efficient because He+ does not react with H2
H2 + crp → H2+ + e-
H2+ + H2 → H3+ + H
He + crp → He+ + e-
He+ + H2 → products
exothermic but unreactive

17. 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)

18. 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

19. 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) = (T/300)-1 cm3 s-1
CH3+ + HCN → CH3CNH+ + h ν
k(T) = (T/300)-0.5 cm3 s-1