AstrochemistryLes Houches Lectures

September 2005Lecture 1

T J MillarSchool of Physics and Astronomy

University of ManchesterPO Box88, Manchester M60 1QD

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.

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

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 pcTypically can form 1 or a couple of low-mass (solar mass) stars.Example – B68

Interstellar Gas

• Giant Molecular Clouds (GMCs)T ~ 10-50 K, n ~ 1011 - 1013 m-3, <n> ~ 6 108 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

Interstellar Gas

Gas and star formation in the Eagle Nebula

Interstellar Dust• Interstellar extinction- 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

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

Interstellar Organic MoleculesCH+ 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+

ND3 in Interstellar Clouds

Submillimetre detection of ND3 by Lis et al., Astrophysical Journal, 571, L55 (2002)

ND3/NH3 = 8 10-4, compared with (D/H)3 ~ 3 10-15

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

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

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

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-

He+ + H2 → products

exothermic but unreactive

H3+: P.A.(H2) very low

Proton transfer reactions very efficientKey to synthesising molecules

He+: I.P.(He) very largeBreaks bonds in

reactionKey to destruction of molecules

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)

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