Astrochemistry University of Helsinki, December 2006 Lecture 1 T J Millar, School of Mathematics and Physics Queen’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 Clouds Temperature: K Density: 10 2 cm -3 Slab-like, thickness ~ cm Clouds permeated by UV radiation - with photon energies less than IP(H) Carbon is photoionised f(e - ) ~ Cloud mostly atomic f(H 2 ) < 0.3 Few simple diatomics – CO, OH, CH, CN, CH + f(M) ~ The Pleiades

Interstellar Gas Dark Clouds - T ~ 10 K, n ~ m -3 Not penetrated by optical and UV photons. Little ionisation. Material is mostly molecular, dominant species is H 2. 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

Interstellar Ices Mostly water ice Substantial components: - CO, CO 2, CH 3 OH Minor components: - HCOOH, CH 4, H 2 CO Ices are layered - CO in polar and non-polar ices Sensitive to f > Solid H 2 O, CO ~ gaseous H 2 O, CO

Low Mass Star Formation Dark cloud (time scales ?) Centrally Condensed Dense Core Protostellar Disk + Envelope Protostellar Disk + Outflow + Envelope Star + Planetary System

Protoplanetary Disks Observed 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(H 2 ) ~ – 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 ~ K, n ~ m -3, ~ 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 Cores Density: cm -3 Temperature: K Very small UV field Small saturated molecules: NH 3, H 2 O, H 2 S, CH 4 Large saturated molecules: CH 3 OH, C 2 H 5 OH, CH 3 OCH 3 Large deuterium fractionation Few molecular ions - low ionisation ? f(CH 3 OH) ~ 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(V S /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 stars IRC (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, N 2, HCN, C 2 H 2 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, C 6 H 6 Time scale of chemistry and evolution of this object is 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 -3.5 da -Silicates plus carbonaceous grains -Mass dust/Mass gas ~ Dense gas – larger grains with icy mantles -Normal – n d /n ~ The interstellar extinction curve

Interstellar Abundances H1.0 (D1.6e-5) He0.1 C N O S<1e-6 Mg, Si, Fe,< 1e-9

Interstellar Organic Molecules

One-body reactions Photodissociation/photoionisation: Unshielded photorates in ISM: β 0 = s -1 Within interstellar clouds, characterise extinction of UV photons by the visual extinction, A V, measured in magnitudes, so that: β = β 0 exp(-bA V ) where b is a constant (~ 1- 3) and differs for different molecules

Cosmic Ray Ionisation H 2 + crp → H e - H H 2 → H H He + crp → He + + e - He + + H 2 → products exothermic but unreactive H 3 + : P.A.(H 2 ) 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 H 2