Astrochemistry Les Houches Lectures September 2005 Lecture 1

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AstrochemistryLes Houches LecturesSeptember
2005Lecture 1

• T J Millar
• School of Physics and Astronomy
• University of Manchester
• PO Box88, Manchester M60 1QD

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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.
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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
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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

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Interstellar Gas

• Giant Molecular Clouds (GMCs)
• T 10-50 K, n 1011 - 1013 m-3, ltngt 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

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Interstellar Gas
Gas and star formation in the Eagle Nebula
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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
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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 gt 10-6 Solid
H2O, CO gaseous H2O, CO
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Interstellar Organic Molecules
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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
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Chemical Kinetics
A B ? C D k ltsvgt 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
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First-order kinetics
A h? ? C D ß (units s-1)
Loss of A (and B) per unit volume per second
is dn(A)/dt - ßn(A) m-3 s-1 where ß
photodissociation rate of A Aside The number,
more accurately, flux of UV photons or cosmic-ray
particles, is contained within ß or ? -
First-order kinetics rate of formation and loss
proportional to the concentration of one reactant
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General case

• dn(Xj)/dt S klmXlXm S ßnXn
• - XjS kjlXl S ßj m-3 s-1
• or dX/dt FX LXX
• 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 LXXss so that Xss FX/LX
• Need to solve a system of non-linear algebraic
  equations
• - solve using Newton-Raphson methods

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Time scales
dX/dt FX LXX 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 Xss1
e-t/tc where tc 1/LX Note As t ? 8, X ?
Xss When t tc, X,tc 0.63Xss, so most
molecular evolution occurs within a few times tc
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One-body reactions
Photodissociation/photoionisation Unshielded
photorates in ISM ß0 10-10 s-1 Within
interstellar clouds, characterise extinction of
UV photons by the visual extinction, AV, measured
in magnitudes, so that ß ß0exp(-bAV) where b
is a constant ( 1- 3) and differs for different
molecules
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Cosmic Ray Ionisation
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 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
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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)
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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
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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 h? Photon
emission more efficient as size of complex grows,
therefore can be important in synthesising large
molecular ions CH3 H2 ? CH5 h ? k(T) 1.3
10-13(T/300)-1 cm3 s-1 CH3 HCN ? CH3CNH h
? k(T) 9.0 10-9(T/300)-0.5 cm3 s-1