Chemistry in Interstellar Space
ERIC HERBST
DEPARTMENTS OF PHYSICS,
CHEMISTRY AND ASTRONOMY
THE OHIO STATE UNIVERSITY
dense (giant) molecular clouds
organic molecules
H
core
4 -3
n = 10 cm
T = 10 K
2
PDR’s
embedded
stars
hot
ionized
gas
HII region
protoplanetary disk
studied in millimeter-wave and IR
MOLECULAR ROTATION
“radio” emissions
DE = hn
GAS PHASE INTERSTELLAR/CIRCUMSTELLAR MOLECULES - HIGH RESOLUTION (9/02)
_____________________________________________________________________________________________
H2
KCl
HNC
C3S
C5
C6H
HC4CN
HCO
NH3
CH3
H3O+
CH
AlCl
CH4
CH3OH
AlF
HCO+
H2CO
SiH4
CH3SH
NH
PN
HOC+
H2CS
CH2NH
C2H4
OH
SiN
HN2+
HCCH
H2C3(lin)
CH3CN
C2
SiO
HNO
HCNH+
c-C3H2
CH3NC
C7H, C6H2
C8H
HCOOCH3
CH3COOH
CH3C2CN
H2C6(lin)
C6H2
H2COHCHO
C2H5OH
(CH3)2O
CH+
CN
SiS
HCS+
H2CN
CH2CN
HC2CHO
C2H5CN
CO
CO+
SO+
C3
C2O
CO2
C2S
C3H(lin)
c-C3H
NH2CN
CH3C4H
CH2CO
NH2CHO
HC3NH+
HCCN
HCOOH
C4H2
H2C4(lin)
HNCO
SiC3
HOCO+
C4H
HNCS
C2CN
C3O
NaCN
HCCNC
HNCCC
C4Si
H2COH+
CSi
+
CP
H3
CS
HF
NO
CH2
NH2
SiC2
SiCN
SO2
NS
SO
HCl
NaCl
H2O
H2S
C2H
HCN
OCS
MgNC
MgCN
N2O
HC2CN
C5H
C5N
CH3NH2
CH2CHOH
CH3CCH
CH3CHO
CH2CHCN
c-CH2OCH2
c-CH2SCH2
HC6CN
(CH2OH)2
(CH3)2CO
CH3C4CN?
NH2CH2COOH?
HC8CN
c-C6H6
HC10CN
+ ISOTOPOMERS
MOLECULAR VIBRATIONS
Infrared absorption
POTENTIAL ENERGY OF REACTION
activation energy
typical neutral reactions
radical-radical reactions
A+B
ion-molecule reactions
k(T) = A(T) exp(-Ea /kT)
C+ D
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
ORGANIC SYNTHESIS CONT.
SOME SYNTHETIC REAC TION CLASSES:
A. CARBON INSERTION
C+ + CH 4 -----> C2H3+ + H
------> C2H2+ + H2
B. CONDENSATION
C2H2+ + C2H2 -----> C4H3+ + H
C. ATOM IC INSERTION
N + C3H3+
-----> HC3NH+ + H
D. RADIATIVE ASSOCIATION
CH3+ + H2O -----> CH3OH2+ + hn
E. NEUTRAL-NEUTRAL
C + C2H2

C3H + H
NEUTRAL-NEUTRAL RX (CONT)
CN + C2H2  HCCCN + H
CCH + C2H2  C4H2 + H
CCH + HCN  HCCCN + H
YES
YES
NO
O + CCH  CO + CH
k = 1.2 10(-11) cm3 s-1
MAYBE (Ea = 250K?)
CURRENT GAS-PHASE MODEL NETWORKS
4,000 reactions; 10-20% "studied";
400 species through 13 atoms in size
elements: H, He, N, O, C, S, Si, Fe, Na, Mg, P, Cl
elemental abundances: “low metal”
photodestruction: external, internal (via cosmic rays)
Latest network – osu.2003 – contains over 300 rapid
neutral-neutral reactions. Rate coefficients estimated by
Ian Smith and others.
GAS-PHASE MODELS
A+ + B  C+ + D
k1
C+ + D  PRODUCTS
k2
d[C+]/dt = k1[A+][B] – k2[C+][D]
Constraints: initial concentrations, elemental
abundances, density, charge neutrality
Steady-state solution: d[C+]/dt = 0
exists for constant density but takes very long
(107 yr) to be achieved.
GAS-PHASE HOMOGENEOUS MODELS OF QUIESCENT CORES
(one phase, constant physical conditions)
"SUCCESSES"
+
1. IONS ( H3 , HCO +, HC3NH+)
2. METASTABLES (HNC)
HCNH + + e ----> HCN + H; HNC + H
3. ISOTOPIC FRACTIONATION

H3+ + HD <====> H2D+ + H2
4. UNSATURATED MOLECULES
A+ + H2 -------> No Reaction
5. ORDER-OF-MAGNITUDE AGREE MENT WITH PERHAPS
80% OF MOLECULES
(diffusion)
TYPES OF SURFACE REACTIONS
REACTANTS: MAINLY MOBILE
ATOMS AND RADICALS
A +
B 
H +
AB
H  H2
association
X  XH (X = O, C, N, CO,
etc.)
WHICH CONVERTS
H +
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
dNH/dt = kaccnH - kevapNH - KH-HNHNH
- kcrdNH
Only accurate if there are lots of reactive species on
every dust particle.
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
dNH/dt = kaccnH - kevapNH - KH-HNHNH
replaced when NH << 1 by a series of
coupled equations for Pn(H):
<NH> =
 n Pn(H)
dP0(H)/dt = ……….
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)