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

Hot core

Some Important Molecules with Rotational Spectra to be Studied Glycine above 100 GHz Deuterated isotopomers; e.g., CH 2 DOD Most molecules in THz region.

Successes for quiescent cores: (1)Reproduces 80% of abundances including ions, radicals, isomers (2)Predicts strong deuterium fractionation

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

some radical-stable reactions

FORMATION OF GASEOUS WATER H 2 + COSMIC RAYS  H e Elemental abundances: C,O,N = 10(-4); C<O H H 2  H H H O  OH + + H 2 OH n + + H 2  OH n H H 3 O + + e  H 2 O + H; OH + 2H, etc

FORMATION OF O 2,N 2 CO OH + O  O 2 + H OH + N  NO + H NO + N  N 2 + O CO, N 2 + He +  C +, N + +… Precursor to ammonia, hydrocarbons CH + O  CO + H

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

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

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

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: CH 3 OH H 2 CO -> H 2 COOCH H 2 ?????

Fractionation Reactions (10 K) H HD  H 2 D + + H 2 H 2 D + + HD  HD H 2 HD HD  D H 2 HD/D 2 = 3(-5)  D 3 + » H 3 + under high density conditions ???

Atom/Radical-Neutral Reactions Radicals: C, CN, CCH 1) Inverse T dependence 2) Large rate coefficients by K: k  10(-10) cm 3 s -1 Neutrals: unsaturated hydrocarbons How true is this generally???? No US capability for low T work???

Critical Neutral-Neutral Rx O Atom Reactions: O + c/l-C n H m  ???? CN Radical Reactions: CN + N  C + N 2 CN + O  CO + N (< 300 K) C Atom Reactions: C + C n  ?????

Radiative Association Rx A + + B  AB + + h 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: CH H 2 O  CH 3 OH h  only known gas-phase synthesis Role of competitive channels unclear What happens for larger systems???

Radiative Association Rx- II A + B  AB + h Critical reactions: C + C 2n-1  C 2n

Dissociative Recombination H 3 O + + e  H 2 O + H % Method Reference 5FA Williams et al ± 1Storage ring Jensen et al ± 5Storage ring Neau et al ± 8Storage ring VC et al What are the products??

CH 3 OHD + + e  CH 3 OH + D ??? Dissociative Recombination II HCNH + + e  HCN + H HNC + H CN + 2H CH 2 DOH e  CH 2 DOH + H ??

10 6 sites

TYPES OF SURFACE REACTIONS REACTANTS: MAINLY MOBILE ATOMS AND RADICALS A + B  AB association H + H  H 2 H + X  XH (X = O, C, N, CO, etc.) WHICH CONVERTS O  OH  H 2 O C  CH  CH 2  CH 3  CH 4 N  NH  NH 2  NH 3 CO  HCO  H 2 CO  H 3 CO  CH 3 OH X + Y  XY ??????????

Some Surface Reactions Studied in Detail* SystemSurfaceReference H + H/D Olivine, amorphous C, amorphous ice Vidali and co- workers CO + OIce mantles Vidali and co- workers CO + O & O + O 2 Matrix isolation Grim & d’Hendecourt * Allows estimation of reaction rate on dust

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.)