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Astrochemistry University of Helsinki, December 2006 Lecture 4

T J Millar, School of Mathematics and Physics Queen’s University Belfast,Belfast BT7 1NN, Northern Ireland. Astrochemistry University of Helsinki, December 2006 Lecture 4. Stellar Evolution. Chemical Structure of AGB CSE. Photochemistry in CSEs.

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Astrochemistry University of Helsinki, December 2006 Lecture 4

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  1. T J Millar, School of Mathematics and Physics Queen’s University Belfast,Belfast BT7 1NN, Northern Ireland AstrochemistryUniversity of Helsinki, December 2006Lecture 4

  2. Stellar Evolution

  3. Chemical Structure of AGB CSE

  4. 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

  5. Photochemistry in CSEs Shell 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)

  6. 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 IRC+10216 (Millar, Herbst & Bettens 2000)

  7. Cyanopolyyne formation Shell distributions – rapid formation of hydrocarbons IRC+10216 Neutral chemistry important in forming cyanopolyynes and other molecules (Guelin et al. 2000) (Millar, Herbst & Bettens 2000)

  8. Photochemistry in CSEs Shell distributions – the creation of anions IRC+10216 Formation by electron attachment. Destruction by photodetachment, collisions with cations Anion/Neutral ratios ~ 0.01-0.1

  9. Metal species in IRC+10216 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

  10. 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) IRC+10216

  11. Anions in Dark Clouds TMC-1 Formation by electron attachment. Destruction by collisions with cations Anion/Neutral ~ 0.01-0.1

  12. Anions in PDRs Horsehead Nebula Formation by electron attachment. Destruction by photons and by collisions with cations Anion/neutral ratio ~0.1

  13. Enhancement Factors The electron fraction can be probed by observation of DCO+/HCO+ and N2D+/N2H+

  14. Enhancement Factors – Depleted Cores

  15. Electron Fraction f(e) ~ 10-7 – 10-8 in dark clouds

  16. IRAS 16293-2422 N2D+ 3-2 D2CO 5-4 13CS 5-4 OCS 9-8

  17. 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

  18. Astrophysical Chemistry Group (Royal Society of Chemistry/Royal Astronomical Society) Dust, Gas and Chemistry in SpaceBelfast, January 4-5 2007 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

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