AstrochemistryUniversity of Helsinki, December 2006

Lecture 4

T J Millar, School of Mathematics and PhysicsQueen’s University Belfast,Belfast BT7 1NN, Northern Ireland

Stellar Evolution

Chemical Structure of AGB CSE

Photochemistry in CSEsDestruction of Parents by IS UV Radiation Field

Self-Shielding

H2 – very reactive, daughter (H atoms) unreactive

CO – very unreactive, daughters (C, C+) very reactive

Cosmic ray ionisation

f(H3

+) varies as r2

N(H3

+) ~ 1012 cm-2 for CRI rate of 10-17 s-1, an order of magnitude less than

that detected in the interstellar medium

Photodissociation and photoionisation

Acetylene is the species which determines the complexity of the hydrocarbon chemistry

Photochemistry in CSEsShell distributions – the photodestruction of acetylene

IRC+10216

Acetylene has a relatively large photoionisation cross-section

Ions and radicals form in outer CSE where both density and UV field are relatively large

(Millar, Herbst & Bettens 2000)

Hydrocarbon formationShell distributions – rapid formation of hydrocarbons

IRC+10216C

2H and C

2H

2+ are both

very reactive with acetylene and derivative species

Peak abundances occur at slightly larger radii as size increases

Degradation of grains may give inverse behaviour

(Millar, Herbst & Bettens 2000)

Cyanopolyyne formationShell distributions – rapid formation of hydrocarbons

IRC+10216

Neutral chemistry important in forming cyanopolyynes and other molecules

(Millar, Herbst & Bettens 2000)

(Guelin et al. 2000)

Photochemistry in CSEsShell distributions – the creation of anions

IRC+10216

Formation by electron attachment.

Destruction by photodetachment, collisions with cations

Anion/Neutral ratios ~ 0.01-0.1

Metal species in IRC+10216LTE calculations predict metal halides (Tsuji)

NaCl, KCl, AlCl, AlF – observed centrally peaked, spatial scales few arcsec

fractional abundance (AlF, AlCl/H2) ~ 1e-7 for 5 arcsec source (Highberger et al 2001, Guelin et al. 1997) – large fraction of metal NOT in dust

MgCN, MgNC, AlNC, SiCN, SiNC – shell distribution, spatial scales ~15 arcsec

Metal compound formationMgNC formation – N(MgNC) ~ 2e13 cm-2 ~observed

IRC+10216Are outer shell species formed in shock chemistry and transported outward, or by photochemistry ?

MgNC formed by radiative association of Mg+ with cyanopolyynes

(Dunbar & Petrie 2002)

Anions in Dark Clouds

TMC-1

Formation by electron attachment.

Destruction by collisions with cations

Anion/Neutral ~ 0.01-0.1

Anions in PDRs

Horsehead Nebula

Formation by electron attachment.

Destruction by photons and by collisions with cations

Anion/neutral ratio ~0.1

S1T = k 1fk 1r+αf e Σk Mf M

[ HD ][H2 ]

Enhancement Factors

The electron fraction can be probed by observation of DCO+/HCO+ and N

2D+/N

2H+

Enhancement Factors – Depleted Cores

Electron Fraction

f(e) ~ 10-7 – 10-8 in dark clouds

IRAS 16293-2422

OCS 9-813CS 5-4

N2D+ 3-2 D2CO 5-4

The FutureHerschel Space Observatory 3.5m, 80-670 microns, launch 2008, 3 year lifetime

ALMA– 64 12m radio telescopes at 5000m altitude in Atacama Desert, Chile

Dust, Gas and Chemistry in SpaceBelfast, January 4-5 2007

Astrophysical Chemistry Group(Royal Society of Chemistry/Royal Astronomical Society)

Invited Speakers:

Eric Herbst (OSU)

Liv Hornekaer (Aarhus)

Martin McCoustra (Heriot-Watt)

Klaus Pontoppidan (Caltech)

Student travel bursaries available

Web-site: www.astrochemistry.org.uk