ERIC HERBST DEPARTMENTS OF PHYSICS, CHEMISTRY AND ASTRONOMY THE OHIO STATE UNIVERSITY Gas and Dust (Interstellar) Astrochemistry

Efficient Low T Gas- PhaseReactions 1.Ion-molecule reactions 2.Radiative association reactions 3.Dissociative recombination reactions 4.Radical-radical reactions 5.Radical-stable reactions E a = 0 In areas of star formation, reactions with barriers occur. Exothermic

Dissociative recombination: product branching fractions such as N 2 H + + e  NH + N (Geppert et al.); determination of H e rate!! (McCall et al.) Radical-stable reactions: Reactions of C, CN, CCH with hydrocarbons at low temperatures (Rowe, Smith, Sims) Important New Studies

Chemistry/history imperfect; heterogeneous New uncertainty analyses

Pre-stellar Cores Salient features: cold, collapsing. With some heavy species depleted towards center; extreme deuterium fractionation. Fit most simply with gas-phase + accretion models including shell structure (Roberts et al. 2004); hydrodynamic models coming on line.

PDR Models Multi-slab gas-phase models with careful radiative transfer; useful for an assortment of diffuse & dense heterogeneous objects, especially close to stars (  Persei,  Orion bar, Horsehead nebula) Diffuse sources: H 3 + problem (Le Petit et al.) Dense sources: Cannot quite reproduce abundance of large molecules detected recently (Teyssier et al.) Protoplanetary disks

(diffusion) “physisorption” E diff E des

X + Y  XY (CO + O  CO2) ?????????? D atoms react in same manner as H atoms 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 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.)

MODELLING DIFFUSIVE SURFACE CHEMISTRY Rate Equations The rate coefficient is obtained by Method accurate if N>1Biham et al. 2001

Master Equation F=fS (atoms per grain per second) A=a/S (fraction of grain surface per second) n=0,1,2... First combined gas-grain model with master equation published this year.

More Detail on H 2 Formation Is the master equation approach exactly right? NO Subtle problem is that back diffusion to sites already occupied is ignored. Hence, efficiency of H 2 formation may be too high. New Monte Carlo method can treat amorphous and irregular grains!

Rough Olivine also shows enhanced efficiency at higher T

Protostellar Cores Much more complex than pre-stellar case; star formation, winds, and shocks well advanced. Heat from star formation evaporates grain mantles close to star. Strong deuterium fractionation for methanol thought to arise from grain processes in previous cold era involving D atoms.

HOT MOLECULAR CORES Hot cores are regions of warm, quiescent gas near high-mass star-forming regions. Temperatures are K and densities are typically 10 7 cm -3. They are associated with a variety of saturated gas- phase organic molecules: methanol, ethanol, acetaldehyde, methyl formate, acetic acid, glycolaldehyde, ethylene oxide, dimethyl ether, and possibly diethyl ether, glycine, and ethyl methyl ether.

Hot Molecular Cores, cont. The standard chemical model produces complex species in the gas following desorption of methanol as grains begin to heat up as a result of star formation. Much of the gas-phase chemistry has not been studied in the laboratory.

Ab Initio Calculations

TWO EXPERIMENTS 1) SIFT AT HANSCOM AF BASE dominant product cluster ion (high density) 2) ICR AT WATERLOO, CANADA dominant product CH 3 OCH 2 + (low density) CONCLUSION: no major channel to produce protonated methyl formate. New approaches needed for hot core chemistry?