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

9. MOLECULAR ROTATION
“radio” emissions
DE = hn

14. MOLECULAR VIBRATIONS
Infrared absorption

18. Cosmic rays produce ions

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

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

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

23. 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

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

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

30. (diffusion)

31. 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 ??????????

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

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

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

35. 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.

36. PROBABILISTIC MASTER EQUATION

37. 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)