ERIC HERBST DEPARTMENTS OF PHYSICS, CHEMISTRY AND ASTRONOMY THE OHIO STATE UNIVERSITY

The Role of Granular Chemistry in Interstellar Clouds ERIC HERBST DEPARTMENTS OF PHYSICS, CHEMISTRY AND ASTRONOMY THE OHIO STATE UNIVERSITY

Should we model surface chemistry? • PRO: obviously important, new experiments being undertaken, new theoretical approaches • CON: “last refuge of a scoundrel”; too complex to do anything but throw the stuff together in the lab

Dust Particles Size range 5-250 nm before coagulation Also spectroscopic evidence for aromatic species (PAH’s)

H2O CO2 CO CH3OH H2CO XCN HCOOH CH4 NH3 Major Ice Mantle Species

T(gr) = 10-20 K Gould & Salpeter 1963 Occurs even in diffuse clouds

TYPES OF SURFACE REACTIONS: Low T 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 Tielens & Hagen 1982 Charnley et al. 1997 X + Y XY (CO + O  CO2) Hasegawa et al. 1991

Gas-grain model for cold dense pre-stellar cores 4000+ gas-phase reactions accretion Non-thermal desorption Inefficient! B A Ices develop (Spitzer) Garrod et al. 2006

SOME NEW GAS-PHASE RESULTS OF GAS-GRAIN MODEL FOR TMC1-CP New: desorption following exothermic reactions Garrod et al. 2006

Hot Core Chemistry: A New Phase? 100-300 K desorption Saturated organic molecules such as ethers, alcohols 10 K Surface chemistry (H-poor) Cold phase +accretion + surface chemistry (H-rich) Includes cosmic-ray-induced photon bombardment

Gas-grain model for warming sources accretion hn Evaporation efficient! B A Ices lost Garrod & Herbst 2006

Warm Surface Chemistry • HCO + CH3O  HCOOCH3 • HCO + OH  HCOOH • CH3 + CH3O  CH3OCH3 Allen & Robinson 1977

Some Results for Standard Warm-up GAS Dust

ED Eb “physisorption” (diffusion) “flat” surface

Modelling Diffusive Surface Chemistry I Consider a species A that hops to the next site to find and react with a species B: Now consider a number of moving species A: Flawed approach must be modified, as we shall see later.

Desorption & Diffusion for heavies at 10 K Non-thermal desorption: cosmic rays, exothermic reactions, etc. (Williams)

Problems with Rate Equations • a) inaccurate treatment of random walk; problem known as “back diffusion” (Krug); factor of approximately three. • b) overestimate of rate in “accretion limit” where average number of reactive particles less than unity and discreteness and fluctuations important (Tielens); small particle problem, potentially major but depends on rates of desorption and diffusion; the faster the rates, the greater the potential problem • Classes of solutions: modified rate approach (Caselli;b), continuum diffusion equations (Krug; a); stochastic methods (macroscopic & microscopic {a} & {b}) • Methods important in cellular biochemistry!!!!

MACROSCOPIC STOCHASTIC METHODS Idea is to follow probabilities that a certain number of atoms/molecules of a species are present on a particular grain as a function of time. MONTE CARLO METHOD: use random numbers (Charnley) MASTER EQUATION METHOD: propagate probabilities forward via differential equations (Biham et al.; Green et al.; Stantcheva et al.)

State of the Art • Can use master equation method to run moderate-size surface chemistry network along with large gas-phase network for cold cores. • Some differences with modified rate equation results depending on energy parameters & dust sizes. • New approximations coming soon: moment method of Biham et al.

CTRW (MICROSCOPIC) APPROACHQiang, Cuppen & Herbst (2005) • A Monte Carlo approach in which the actual positions of individual species on a lattice are followed with time. A B C

Irregularities on rough surface Binding energy affected by environment

CTRW Studies • H2 formation on amorphous and rough grains of silicates and carbon in diffuse clouds. Roughness helps to increase temperature range of high efficiency. Generalization of two-site approach of Cazaux & Tielens. • H2 formation on flat and rough small grains exposed to stochastic heating by photons • Formation of ice mantles (in progress) • Desorption of mantles by cosmic ray attack • Inclusion of hot-atom & Eley-Rideal mechanisms • Tabulation of rate coefficients for rough grains

Summary • Gas-grain models of the chemistry in assorted interstellar sources are useful and preferable to gas-phase codes despite simplifications. • Rate equations for flat surfaces can be unreliable if the number of reactive species per grain is much less than unity on average. • Of the basic stochastic methods used for flat grains, the master equation approach can be easily integrated into medium-scale gas-grain models. • For rough or amorphous surfaces, a more advanced stochastic treatment (e.g. CTRW) is useful.

OSU ASTROCHEMISTRY GROUP • Herma Cuppen (postdoc); surface science • Valentine Wakelam (postdoc); uncertainties, sensitivities, bistability • Rob Garrod (postdoc); gas-grain code • Atsuko Maeda (postdoc); spectroscopy • Qiang Chang (grad student); surface science • Donghui Quan (grad student); gas-phase models

ERIC HERBST DEPARTMENTS OF PHYSICS, CHEMISTRY AND ASTRONOMY THE OHIO STATE UNIVERSITY