Astrochemistry University of Helsinki, December 2006 Lecture 1 T J Millar, School of Mathematics and...
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![Page 1: Astrochemistry University of Helsinki, December 2006 Lecture 1 T J Millar, School of Mathematics and Physics Queen’s University Belfast,Belfast BT7 1NN,](https://reader036.fdocuments.us/reader036/viewer/2022062517/56649f215503460f94c39e2f/html5/thumbnails/1.jpg)
AstrochemistryUniversity of Helsinki, December 2006
Lecture 1
T J Millar, School of Mathematics and PhysicsQueen’s University Belfast,Belfast BT7 1NN, Northern
Ireland
![Page 2: Astrochemistry University of Helsinki, December 2006 Lecture 1 T J Millar, School of Mathematics and Physics Queen’s University Belfast,Belfast BT7 1NN,](https://reader036.fdocuments.us/reader036/viewer/2022062517/56649f215503460f94c39e2f/html5/thumbnails/2.jpg)
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
– 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.
![Page 3: Astrochemistry University of Helsinki, December 2006 Lecture 1 T J Millar, School of Mathematics and Physics Queen’s University Belfast,Belfast BT7 1NN,](https://reader036.fdocuments.us/reader036/viewer/2022062517/56649f215503460f94c39e2f/html5/thumbnails/3.jpg)
Diffuse Interstellar CloudsTemperature: 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) < 0.3
Few simple diatomics – CO, OH, CH, CN, CH+
f(M) ~ 10-6-10-8 The Pleiades
![Page 4: Astrochemistry University of Helsinki, December 2006 Lecture 1 T J Millar, School of Mathematics and Physics Queen’s University Belfast,Belfast BT7 1NN,](https://reader036.fdocuments.us/reader036/viewer/2022062517/56649f215503460f94c39e2f/html5/thumbnails/4.jpg)
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
![Page 5: Astrochemistry University of Helsinki, December 2006 Lecture 1 T J Millar, School of Mathematics and Physics Queen’s University Belfast,Belfast BT7 1NN,](https://reader036.fdocuments.us/reader036/viewer/2022062517/56649f215503460f94c39e2f/html5/thumbnails/5.jpg)
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
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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 DisksObserved directly around low-mass protostars
![Page 8: Astrochemistry University of Helsinki, December 2006 Lecture 1 T J Millar, School of Mathematics and Physics Queen’s University Belfast,Belfast BT7 1NN,](https://reader036.fdocuments.us/reader036/viewer/2022062517/56649f215503460f94c39e2f/html5/thumbnails/8.jpg)
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, <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
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Interstellar Gas
Gas and star formation in the Eagle Nebula
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Star-Forming Hot CoresDensity: 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 starsIRC+10216 (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 0.1C 0.00073N 0.00002O 0.00018S <1e-6Mg, Si, Fe, < 1e-9
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Interstellar Organic Molecules
H2COH+HOCO+HCS+
CH4C2H2HOC+
HCNH+
NH2CH2COOH?HNCCCCH3HCO+
OHCH2CH2OHHCCNCC3SC2S
CH3COCH3C6H-CH2CNC3OC2O
C2H5OHCH2CHOHHC3NH+H2C3C3NCO2CF+
CH3OCH3c-C2H4OH2C4c-C3H2c-C3HC3CO+
CH3C5NC6HC5HC4HC3HC2HCH
CH3C4HH2C6CH2CHCNNH2CHONH2CNHNCSCH2C2
C2H5CNCH2OHCHOCH3CHOCH3SHCH2COHNCOOCSCN
HC11NCH3COOHCH3NH2CH3NCCH2NHH2CNHCOCO
HC9NCH3C3NCH3CCHCH3CNHOCHOH2CSHNCCS
HC7NHCOOCH3HC5NCH3OHHC3NH2COHCNCH+
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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
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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 lowProton transfer reactions
very efficientKey to synthesising molecules
He+: I.P.(He) very largeBreaks bonds in reaction
Key to destruction of molecules
IS Chemistry efficient because He+ does not react with H2