Chemistry in Interstellar Space

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Presentation transcript:

Chemistry in Interstellar Space ERIC HERBST DEPARTMENTS OF PHYSICS, CHEMISTRY AND ASTRONOMY THE OHIO STATE UNIVERSITY

MOLECULAR ROTATION “radio” emissions DE = hn

MOLECULAR VIBRATIONS Infrared absorption

Cosmic rays produce ions

Radical-Neutral Reactions Radicals: C, CN, CCH 1) Inverse T dependence 2) Large rate coefficients by 10-50 K: k ~ 10(-10) cm3 s-1

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

FORMATION OF HYDROCARBONS H3+ + C  CH+ + H2 CHn+ + H2  CHn+1+ + H; n=1,2 CH3+ + H2  CH5+ + hn CH5+ + e  CH4 + H (5%)  CH3 + 2H (70%) CH5+ + CO  CH4 + HCO+

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

NEUTRAL-NEUTRAL RX (CONT) CN + C2H2  HCCCN + H YES CCH + C2H2  C4H2 + H YES CCH + HCN  HCCCN + H NO O + CCH  CO + CH k = 1.2 10(-11) cm3 s-1 MAYBE (Ea = 250K?)

Latest network – osu.2003 – contains over 300 rapid neutral-neutral reactions. Rate coefficients estimated by Ian Smith and others.

(diffusion)

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 ??????????

MODELLING DIFFUSIVE SURFACE CHEMISTRY Rate Equations - kcrdNH Only accurate if there are lots of reactive species on every dust particle.

(COLD CLOUDS; silicate grains) GRAIN MANTLE GROWTH (COLD CLOUDS; silicate grains)

% Agreement in TMC-1 Gas-phase species Roberts & Herbst 2002

Other Approaches Monte Carlo method Modified rate method (semi-empirical) Probabilistic master equation Second method changes rate coefficients so that fractional abundances do not exist. Last method follows probabilities for specific numbers of species; easily coupled with rate equations for the gas phase but computationally intensive.

PROBABILISTIC MASTER EQUATION

Some Outstanding Astrochemical Problems How to make gas-phase models more robust How to construct gas-grain models and predict mantle abundances accurately How to model the chemistry of star- and planet-forming regions (heterogeneity and time dependence)