•The Production of Complex
Molecules in Interstellar and
Circumstellar Sources
ERIC HERBST
DEPARTMENTS OF PHYSICS 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
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
Dust particles contain 1% of interstellar matter.
POTENTIAL ENERGY OF REACTION
activation energy
typical neutral reactions
radical-radical reactions
some radical-stable reactions
A+B
ion-molecule reactions
k(T) = A(T) exp(-Ea /kT)
C+ D
Cosmic rays produce
ions
Radical-Stable 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
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+ + h
E. NEUTRAL-NEUTRAL
C + C2H2

C3H + H
C3 + H2
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?)
O Reactions at Low
Temperature (cm3 s-1)
REACTION
NSM
O + CnH
(linear)
1.7(-11)
n=2
n>2
n even
NNM
LATEST
1.0(-10)
1.7(-11)
1.7(-11)
1(-10) x
exp(-250/T)
5(-11) x
(T/300)0.5
O + Cn
(linear)
n odd
UMIST
5(-11) x
exp(-900/T)
5(-11) x
(T/300)0.5
1.0(-10)
5(-11) x
exp(-900/T)
1.0(-10)
UNSATURATED SPECIES
HYDROGENATION WITH H2 DIFFICULT
BARE CLUSTERS PROMINENT
EXCEPTION: HOT CORES (NEAR NEW HIGH-MASS
STARS) WHERE GRAIN MANTLES EVAPORATE!!
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)
Quiescent cores:
(1)Reproduces 80% of abundances
including ions, radicals, isomers (105 yr);
(2) For longer times, use gas-grain model.
TOWARDS FULLERENES
Bettens, Herbst 1995,1996
Linear Chains
Spontaneous Isomerization
Monocyclic Rings
condensation
Tricyclic Rings
He+ conversion
GROWTH BY C+/C ADDITION
24
No. Carbon Atoms n
Fullerenes
48
IMPORTANT SYNTHETIC RX
C+ + CnHm  Cn+1Hm+ + h
Cn+1Hm+ + e   Cn+1Hm + h
C + CnHm  Cn+1Hm + h
RESULTS FOR FULLERENES
RESULTS VERY DEPENDENT ON REACTIVITY OF
LINEAR CLUSTERS AND UNSATURATED
HYDROCARBONS WITH O ATOMS, WHICH ARE
VERY ABUNDANT IN OXYGEN-RICH REGIONS.
Extended to synthesis of dust particles in
supernova remnants by Liu, Clayton, Dalgarno
WHAT ABOUT CARBON-RICH REGIONS?
IRC10216: An AGB (Old) Star
LTE
Dust and
PAH’s
Molecules and
dust here
C>O
N,T similar to
cloud
CO, C2H2,
HCN, H2
UV radiation
+ cosmic rays
PAH/DUST PRODUCTION
Occurs close to stellar photosphere (HOT; 900-1100 K)
a) Formation of benzyl radical A1- from acetylene
b) Reaction with acetylene to add CCH
A1- + C2H2  A1CCH + H
c) Radical formation
A1CCH + + H  A1-CCH + H2
d) Ring addition
A1CCH*
+ C2H2  A2-
Actual Distributions
GROWTH OF MOLECULES
Occurs via neutral and ionic (+ and -)
reactions. Modified network necessary to
account for acetylenic chemistry.
Photochemistry important in the
production of radicals such as CN
and CCH (Millar, Herbst, Bettens
2000)
C2H2 + h  CCH + H
GROWTH OF MOLECULES. II
CCH + C2nH2  C2n+2H2 + H
CN + C2nH2  HC2n+1N + H
C2nH reactions with hydrocarbons,
HCN(?), HNC(?) as well as
cyanoacetylenes.
GROWTH OF MOLECULES. III
C2H2+ + C2nHm  C2n+2Hm+1+ + H
e + Cn  Cn- + h
Cn- + Cm  Cn+m- + h
Model network through 23 C atoms: unsaturated
species prevail!! BUT…..
CYANOPOLYYNES in IRC
Radius 
DETECTABLE LARGE SPECIES
SPECIES
HC9N
HC15N
C8H
C10H
C8HC6H6
80-90 %
Cal. Col. Dens.
(cm-2)
5.8(13)
9.1(11)
1.1(14)
1.8(13)
2.7(13)
3.0(13)
Agreement
Measured
3(13)
5(12)
CRL618 (100X)
CRL618: A Protoplanetary
Nebula
Woods et al. 2002
Detection of
benzene
empty
250 K
Dense Thin Slab
(500 yr old)
Photons, X-rays from central star!
500 x normal ionization rate
SYNTHESIS OF BENZENE
C2H2+ + C2H2  C4H3+ + H
C4H3+ + C2H2  c-C6H5+ + h
c-C6H5+ + H2  c-C6H7+ + h
c-C6H7+ + e  c-C6H6 + H
SUMMARY
A. COMPLEX CARBONACEOUS SPECIES ARE
PRODUCED BOTH IN OXYGEN-RICH AND
CARBON-RICH OBJECTS
B. THE BEST SOURCE FOR DETECTING
SUCH MOLECULES MAY BE THE COMPLEX
PROTOPLANETARY NEBULA CRL 618.
ACCORDING TO MODEL, COLUMN
DENSITIES ORDERS OF MAGNITUDE
GREATER THAN IN IRC+10216!!!