ASTROPHYSICAL MODELLING AND SIMULATION

1. ASTROPHYSICAL MODELLING AND SIMULATION Eric Herbst Departments of Physics, Chemistry, and Astronomy The Ohio State University

2. Hot core

4. Some Important Molecules with Rotational Spectra to be Studied • Glycine above 100 GHz • Deuterated isotopomers; e.g., CH2DOD • Most molecules in THz region.

5. Successes for quiescent cores: • Reproduces 80% of abundances including ions, radicals, isomers • Predicts strong deuterium fractionation

6. STANDARD NETWORKS • New Standard Model (nsm): designed for low temperature but useable through 300 K • nsm.2003: includes new rapid neutral-neutral rx at low temp. according to European network • UMIST: rate99, rate95: all temperatures but must use care

7. some radical-stable reactions

9. FORMATION OF GASEOUS WATER H2 + COSMIC RAYS  H2+ + e Elemental abundances: C,O,N = 10(-4); C<O Elemental abundances: C,O,N = 10(-4); C<O H2+ + H2 H3+ + H H3+ + O  OH+ + H2 OHn+ + H2  OHn+1+ + H H3O+ + e  H2O + H; OH + 2H, etc

10. FORMATION OF O2 ,N2 CO OH + O  O2 + H OH + N  NO + H NO + N  N2 + O CH + O  CO + H CO, N2 + He+ C+, N+ +… Precursor to ammonia, hydrocarbons

11. Overall and particular agreement: pure gas-phase (nsm) TMC-1 nsm.2003

12. “Primary” Fractionation Reaction (i) H3+ + HD H2D+ + H2 + 232 K (ii) H2D+ + CO DCO+ + H2 (iii) H2D+ + e D + 2H, etc Heavy depletion/low ionization  severe fractionation

13. Classes of Poorly Understood Gas-Phase Reactions • Ion-molecule formation of saturated molecules in hot cores • Deuterium fractionation reactions • Atom/Radical--neutral reactions • Radiative association reactions • Dissociative recombination reactions

14. Hot Core Chemistry • Methanol and formaldehyde are formed on cold grains by hydrogenation of CO • Rising temperatures put them into the gas phase • A number of postulated reactions produce ethanol, methyl formate, dimethyl ether, etc. but laboratory work is by and large lacking. • Ex: CH3OH2+ + H2CO -> H2COOCH3+ + H2 • ?????

15. Fractionation Reactions (10 K) • H3+ + HD  H2D+ + H2 • H2D+ + HD  HD2+ + H2 • HD2+ + HD  D3+ + H2 ??? HD/D2 = 3(-5)  D3+ » H3+ under high density conditions

16. Atom/Radical-Neutral Reactions Neutrals: unsaturated hydrocarbons Radicals: C, CN, CCH 1) Inverse T dependence 2) Large rate coefficients by 10-50 K: k 10(-10) cm3 s-1 How true is this generally???? No US capability for low T work???

17. Critical Neutral-Neutral Rx • O Atom Reactions: • O + c/l-CnHm ???? • CN Radical Reactions: • CN + N  C + N2 • CN + O  CO + N (< 300 K) • C Atom Reactions: • C + Cn  ?????

18. Radiative Association Rx • A+ + B  AB+ + hn • must be measured at low density or three-body channel dominates • rate enhanced by (1) low T, (2) large size of reactants, (3) large bond energy. • mostly theory and three-body analogs • Ex: CH3+ + H2O  CH3OH2+ + hn only known gas-phase synthesis • Role of competitive channels unclear • What happens for larger systems???

19. Radiative Association Rx- II • A + B  AB+ hn • Critical reactions: • C + C2n-1  C2n

20. Dissociative Recombination What are the products?? • H3O+ + e  H2O + H % Method Reference

21. Dissociative Recombination II HCNH+ + e  HCN + H HNC + H CN + 2H CH3OHD+ + e  CH3OH + D ??? CH2DOH2+ + e  CH2DOH + H ??

22. 106 sites

23. TYPES OF SURFACE REACTIONS REACTANTS: MAINLY MOBILE ATOMS AND RADICALS A + B AB association H + H H2 H + X XH (X = O, C, N, CO, etc.) WHICH CONVERTS O  OH  H2O C CH  CH2  CH3  CH4 N  NH  NH2  NH3 CO  HCO  H2CO  H3CO  CH3OH X + Y XY ??????????

24. Some Surface Reactions Studied in Detail* * Allows estimation of reaction rate on dust

25. SIMULATION OF SURFACE CHEMISTRY • Rate equations • Modified rate equations (semi-empirical) • Monte Carlo • Direct Master equation (Biham et al.; Green et al.; Stantcheva et al.)