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Chemistry Databases and Reaction Networks for Stellar Atmospheres

Chemistry Databases and Reaction Networks for Stellar Atmospheres. Inga Kamp & Sven Wedemeyer-Böhm. CO in the Sun as a motivation Chemical networks: various approaches & solvers Implementation in CO 5 BOLD Rate quality and completeness of the network

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Chemistry Databases and Reaction Networks for Stellar Atmospheres

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  1. Chemistry Databases and Reaction Networks for Stellar Atmospheres Inga Kamp & Sven Wedemeyer-Böhm • CO in the Sun as a motivation • Chemical networks: various approaches & solvers • Implementation in CO5BOLD • Rate quality and completeness of the network • Prospects for larger networks and different species

  2. Collaborators: Sven Wedemeyer-Böhm (KIS, Freiburg) Bernd Freytag (Los Alamos) Matthias Steffen (AIP, Potsdam) Jo Bruls (KIS, Freiburg) Oskar Steiner (KIS, Freiburg) Werner Schaffenberger (Graz)

  3. CO observations in the Sun CO (Dv = 1) fundamental and (Dv =2) first overtone bands suggest that the temperature decreases monotonically outwards - no temperature minimum Solution: inhomogeneous atmosphere with coexisting hot and cool areas Cool areas maybe caused by a runaway process: CO formation and subsequent enhanced CO cooling lead to a “cooling catastrophe” [Ayres & Testerman 1981]

  4. Chemical Networks Three different approaches: Instantaneous Chemical Equilibrium (ICE) Chemical Equilibrium (CE) Time dependent chemistry with advection (TD) The chemistry depends on local quantities such as T, n and the solution is calculated for t=∞ (stationary solution) The chemistry depends on local quantities such as T, n and the solution is advanced over t of the hydro timestep The chemistry depends on local quantities such as T, n; the solution of the previous timestep is advected according to the hydrodynamical flow before the chemistry solution is advanced over t of the hydro timestep

  5. Two methods: Equilibrium Constants Rate Coefficients fictious partial pressure for each atom(!) pre-tabulated equilibrium constants and particle density for each species(!) parametrized rate coefficients

  6. Three solvers: Dvode Newton-Raphson Neural Networks Initial value ODE solver for stiff systems with adjustable stepsize h 5th order BDF (Gear) Iterative solution of a non-linear system of equations Approximation of a set of non-linear continous functions with Nh neurons

  7. Three solvers: Dvode Newton-Raphson Neural Networks Initial value ODE solver for stiff systems with adjustable stepsize h 5th order BDF (Gear) Iterative solution of a non-linear system of equations T Pi fictiouspartial pressure n(H) n(e-) [Asensio Ramos & Socas-Navarro 2005]

  8. The CO5BOLD Chemical Network Operator splitting: 1) Continuity equation (advection) 2) Rate equation (chemistry) Chemistry is the limiting factor in computing time --> networks have to be small to be feasible tn-1 tn tn tn+1 tn+1 CO CO advection chemistry chemistry advection [Wedemeyer-Böhm, Kamp, Freytag, Bruls 2004]

  9. The CO5BOLD Chemical Network 8 species: H, C, O, M H2, CO, CH, OH 27 reaction rates [Wedemeyer-Böhm, Kamp, Freytag, Bruls 2004] Neutral-neutral reactions: Rij = A (T/300)B exp(-C/T) ninj Three-body reactions: Rij = A (T/300)B ninjn(M)

  10. The CO5BOLD Chemical Network 8 species: H, C, O, M H2, CO, CH, OH 27 reaction rates M M M [Wedemeyer-Böhm, Kamp, Freytag, Bruls 2004] M M M Neutral-neutral reactions: Rij = A (T/300)B exp(-C/T) ninj Three-body reactions: Rij = A (T/300)B ninjn(M)

  11. The CO5BOLD Chemical Network O + CH Rij(300K) = 2.25 10-11 C + OH branching ratios CO + H Rij(300K) = 1.81 10-11 CO + H C + O + H Souces for reaction rates: critical evaluation of the literature UMIST (Le Teuff et al. 2000) Konnov’s combustion database (Konnov 2000) Baulch et al. (1972, 1976) Westley (1980) Ayres & Rabin (1996) 5000 K range

  12. The CO5BOLD Chemical Network Ayres & Rabin derived rate from detailed balance between H+CO and C+OH (5000K) UMIST is based on Westley (1980), but differs by a factor 5! We use original rate by Westley (1980) combustion data 5000 K range

  13. The CO5BOLD Chemical Network Difference of CO number density in the (T,n) parameter range of the solar atmosphere Parameter study for extended network: H, C, O, M, H2, CO, CH, OH and 27 reaction rates vs. H, C, O, M, H2, CO, CH, OH, N, NH, N2, NO, CN and 58reaction rates result after ∆t = 0.1 s [Asensio Ramos et al. 2003]

  14. The CO5BOLD Chemical Network Average CO number density over height: At heights above ~600 km, CE and ICE are no longer good approximations for the chemistry; TD becomes important

  15. The CO5BOLD Chemical Network TD/UICE no difference TD/CE CE/UICE

  16. Outlook • Add more species, OH and CH might be interesting for the Sun --> networks have to be tested and have to stay small. • Use a solver that allows better optimization --> Heidelberg group (DAESOL, Bauer et al. 1997) • More laboratory measurements!!!! Many rates are still guesses or vast extrapolation. • Get better reaction rate databases (UMIST mostly for interstellar and circumstellar physics, Konnov’s database not well documented and maintanance unclear, database of equilibrium constants not publicly available).

  17. Thank you!

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