Astrochemistry University of Helsinki, December 2006 Lecture 4

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AstrochemistryUniversity of Helsinki, December
2006Lecture 4

• T J Millar, School of Mathematics and Physics
• Queens University Belfast,Belfast BT7 1NN,
  Northern Ireland

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Stellar Evolution
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Chemical Structure of AGB CSE
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Photochemistry in CSEs
Destruction 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
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Photochemistry in CSEs
Shell distributions the photodestruction of
acetylene
IRC10216
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)
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Hydrocarbon formation
Shell distributions rapid formation of
hydrocarbons
C2H and C2H2 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
IRC10216
(Millar, Herbst Bettens 2000)
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Cyanopolyyne formation
Shell distributions rapid formation of
hydrocarbons
IRC10216
Neutral chemistry important in forming
cyanopolyynes and other molecules
(Guelin et al. 2000)
(Millar, Herbst Bettens 2000)
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Photochemistry in CSEs
Shell distributions the creation of anions
IRC10216
Formation by electron attachment. Destruction by
photodetachment, collisions with
cations Anion/Neutral ratios 0.01-0.1
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Metal species in IRC10216
LTE 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
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Metal compound formation
MgNC formation N(MgNC) 2e13 cm-2 observed
Are 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)
IRC10216
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Anions in Dark Clouds
TMC-1
Formation by electron attachment. Destruction by
collisions with cations Anion/Neutral 0.01-0.1
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Anions in PDRs
Horsehead Nebula
Formation by electron attachment. Destruction by
photons and by collisions with cations Anion/neutr
al ratio 0.1
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Enhancement Factors
The electron fraction can be probed by
observation of DCO/HCO and N2D/N2H
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Enhancement Factors Depleted Cores
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Electron Fraction
f(e) 10-7 10-8 in dark clouds
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IRAS 16293-2422
N2D 3-2
D2CO 5-4

OCS 9-8
13CS 5-4
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The Future
Herschel Space Observatory 3.5m, 80-670 microns,
launch 2008, 3 year lifetime
ALMA 64 12m radio telescopes at 5000m altitude
in Atacama Desert, Chile
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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