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