The Chemistry in Interstellar Clouds
Eric Herbst
Departments of Physics, Astronomy,
and Chemistry
The Ohio State University
100,000 lt yr
Andromeda: a “nearby” spiral galaxy
Cold Dense Interstellar Cloud
10 K
10(4) cm-3
H2
dominant
Molecules seen at long
wavelengths
sites of star
formation
Dust particles block out light
Dust constitutes 1%
of mass in a cloud.
IR spectral studies
yield information
about molecules in
the gas and on dust
particles but the
technique is difficult.
Water, CO,
CO2
+ small grains
and PAH’s
Studied by
infrared
spectroscopy
The Eagle Nebula: active star forming region in our
galaxy
The Horsehead Nebula (also in our galaxy)
Radio astronomy to study gaseous molecules
LMT (Large
Millimeter
Wave
Telescope)
MOLECULAR ROTATION
“radio” emissions
DE = hn
Unlike vibrations, rotations occur only in the gas.
The Case of TMC-1
CO J=10
Gaseous Interstellar Molecules
133 neutral molecules (September 2008)
18 molecular ions
14 positive
4 negative
H
C, N, O
S, Si, P, K, Na, Mg, Al, F
2-13 atoms
Spectra tell us what
molecules are there, what
concentrations they have,
and what the physical
conditions are.
Exotic Molecules
• Molecular ions – positive and negative
–
HCO+
C4H-
• Free radicals – odd number of electrons
–
•
•
•
•
C2H
Isomers – unusual structures HNC
Three-membered rings of carbon atoms
Hydrogen-poor molecules
BUT STILL MAINLY ORGANIC!!!!!
Gaseous interstellar molecules (>150)
N=2
N=3
N=4
N=5
N=6
N=7
N=8
N=9
N = 10
H2
AlCl
CH2
C2S
NH3
CH4
CH3OH
CH3NH2
HCOOCH3
(CH3)2O
(CH3)2CO
CH
PN
H2S
OCS
H2CO
SiH4
CH3SH
CH3CCH
CH3C2CN
C2H5OH
CH3C4CN
NH
SiN
NH2
CCP
H2CS
CH2NH
C2H4
CH3CHO
HC6H
C2H5CN
CH3CH2CHO
OH
SiO
H2O
SiNC
H2CN
C5
H2C4
c-CH2OCH2
C7H
CH3C4H
(CH2OH)2
O2(?)
SiS
HNO
NaCN
l-C3H
l-C3H2
CH3CN
CH2CHCN
HOCH2CHO
C8H
HF
PO
C2H
SO2
c-C3H
c-C3H2
CH3NC
HC4CN
CH3COOH
HC6CN
C2
SH
HCN
N2O
HCCH
H2CCN
NH2CHO
C6H
H2CCCHCN
CH3CONH2
N = 11
CN
AlF
HNC
SiCN
HNCO
H2NCN
CH2CNH
H2CCHOH
H2C6
CH2CHCH3
HC8CN
CO
FeO
HCO
HNCS
CH2CO
C5H
CH2CHCHO
CS
SiC
c-SiC2
HCCN
HCOOH
C5N
C2H6
CP
MgCN
C2CN
C4H
HC4N
NH2CH2CN
NO
MgNC
C3O
HC2CN
C5S(?)
N = 12
NS
AlNC
H3+
C3S
HC2NC
HC4H
C6H6
SO
HCP
HCO+
c-SiC3
C4Si
HCl
CH+
C3
HOC+
C3N-
HNCCC
HC2CHO
NaCl
CO+
C2O
N2H+
H3O+
CNCHO
c-C3H2O
KCl
SO+
CO2
HCS+
HCNH+
H2COH+
N2(?)
CF+
HCNO HOCO+
C4H-
CH3C6H
N = 13
HC10CN
HC3NH+
C6H-
C8H-
The Chemistry in Cold
Interstellar Clouds
Why is it so unusual?
Atoms  Molecules in the gas
and on dust particles
Chemical Reactions
Activation energy
The higher the temperature, the faster the reaction.
In Cold Interstellar Clouds
Must be all
downhill at low
temperatures!
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
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
GAS-PHASE MODELS
A+ + B  C+ + D
k1
C+ + E  PRODUCTS
k2
d[C+]/dt = k1[A+][B] – k2[C+][E]
Constraints: initial concentrations, elemental
abundances, density, charge neutrality
CURRENT GAS-PHASE MODEL NETWORKS
4,500 reactions; 400 species through 13 atoms
elements: H, He, N, O, C, S, Si, Fe, Na, Mg, P, Cl
Solved kinetically; yields concentrations of all
molecules as a function of time in clouds.
Best agreement with cold cloud gas at
10(5) – 10(6) yr; 80% of molecules
reproduced. Predicts new molecules.
TYPES OF SURFACE REACTIONS
REACTANTS: MAINLY MOBILE
ATOMS AND RADICALS
A +
H +
B 
H  H2
AB
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
Formation of Ices In Cold Cores
H
O
OH
H
H2O
Other ices formed: methane, ammonia, CO,
CO2, formaldehyde, methanol (all confirmed by
experiments at low temperature.)
Gas-Grain Models
• In cold cores, ice mantles build up as
chemistry proceeds both in the gas and on
surfaces.
• In hotter regions, grain mantles are
released into the gas and change the
chemistry to a saturated one.
Cold Core
Low-mass Star
Formation
adiabatic collapse
Protostar
T = 10 K
n = 104 cm-3
Molecule factory
Star + Disk
hot core
100 K
SOME ORGANIC MOLECULES IN
LATEST HOT CORE MODEL
• Dimethyl ether, methyl formate, formic
acid, glycolaldehyde, acetic acid, ethanol,
acetaldehyde, ketene, acetone, ethylene
glycol
• Methyl amine, urea, formamide,
acetamide, methoxyamine,
hydroxymethylamine
• Garrod, Widicus Weaver, & Herbst (2008)
The Future
Other New Telescopes
The soon-to-be
Herschel Space
Observatory
ALMA: the future…….
A starburst galaxy……
http://www.physics.ohio-state.edu/~eric/