Astrochemistry University of Helsinki, December 2006 Lecture 4 T J Millar, School of Mathematics and...
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Transcript of Astrochemistry University of Helsinki, December 2006 Lecture 4 T J Millar, School of Mathematics and...
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