AstrochemistryUniversity of Helsinki, December 2006
Lecture 1
T J Millar, School of Mathematics and PhysicsQueen’s University Belfast,Belfast BT7 1NN, Northern
Ireland
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.
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
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 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
Low Mass Star Formation• Dark cloud (time
scales ?)• Centrally
Condensed Dense Core
• Protostellar Disk + Envelope
• Protostellar Disk + Outflow + Envelope
• Star + Planetary System
Protoplanetary DisksObserved directly around low-mass protostars
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
PPD Schematic
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
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
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.
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
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
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 Abundances
H 1.0(D 1.6e-5)He 0.1C 0.00073N 0.00002O 0.00018S <1e-6Mg, Si, Fe, < 1e-9
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+
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 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