Astrochemistry University of Helsinki, December 2006 Lecture 1

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AstrochemistryUniversity of Helsinki, December
2006Lecture 1

• T J Millar, School of Mathematics and Physics
• Queens University Belfast,Belfast BT7 1NN,
  Northern Ireland

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Interstellar Matter

• 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.

• 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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Diffuse Interstellar Clouds
Temperature 80-100K Density 102 cm-3 Slab-like,
thickness 1019 cm Clouds permeated by UV
radiation - with photon energies less than
IP(H) Carbon is photoionised f(e-) 10-4 Cloud
mostly atomic f(H2) lt 0.3 Few simple diatomics
CO, OH, CH, CN, CH f(M) 10-6-10-8
The Pleiades
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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 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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Low Mass Star Formation

• Dark cloud (time scales ?)
• Centrally Condensed Dense Core
• Protostellar Disk Envelope
• Protostellar Disk Outflow Envelope
• Star Planetary System

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Protoplanetary Disks
Observed directly around low-mass protostars
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Protoplanetary Disks
Thin accretion disks from which protostar
forms Inflow from large radii (100 AU) onto
central protostar Temperature of outer disk is
cold (10 K) n(H2) 1016 1021 m-3 Molecular gas
is frozen on to dust grains in outer
disk Temperature of inner disk is 100 K at 10
AU, 1000 K at 1 AU Ices evaporate in inner disk
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PPD Schematic
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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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Star-Forming Hot Cores
Density 106 - 108 cm-3 Temperature 100-300
K Very small UV field Small saturated molecules
NH3, H2O, H2S, CH4 Large saturated molecules
CH3OH, C2H5OH, CH3OCH3 Large deuterium
fractionation Few molecular ions - low ionisation
? f(CH3OH) 10-6
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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. Importance for
chemistry Endothermic neutral-neutral reactions
can occur.
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Evolved carbon-rich stars

• IRC10216 (CW Leo)
• Brightest object in the sky at 2 microns
  optically invisible
• Has an extended ( 1 lt yr) circumstellar
  envelope expanding at a velocity of 15 km s-1
• Very rich carbon chemistry about 60
  molecules detected, mostly linear hydrocarbons
• LTE chemistry near photosphere makes simple
  molecules, CO, N2, HCN, C2H2
• Carbonaceous dust (and PAHs) made in this type
  of object

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Protoplanetary Nebula
The evolutionary stage between AGB 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

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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 Abundances
H 1.0 (D 1.6e-5) He 0.1 C 0.00073 N 0.00002 O
0.00018 S lt1e-6 Mg, Si, Fe, lt 1e-9
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Interstellar Organic Molecules
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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