ASTROPHYSICAL
MODELLING AND
SIMULATION
Eric Herbst
Departments of Physics,
Chemistry, and Astronomy
The Ohio State University
dense (giant) molecular clouds
organic molecules
Hot core
core
4 -3
n = 10 cm
T = 10 K
H
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
Some Important Molecules
with Rotational Spectra to be
Studied
Glycine above 100 GHz
Deuterated isotopomers; e.g., CH2DOD
Most molecules in THz region.
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)
Successes for quiescent cores:
(1)Reproduces 80% of abundances
including ions, radicals, isomers
(2)Predicts strong deuterium fractionation
STANDARD NETWORKS
New Standard Model (nsm): designed
for low temperature but useable through
300 K
nsm.2003: includes new rapid neutralneutral rx at low temp. according to
European network
UMIST: rate99, rate95: all temperatures
but must use care
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
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 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
Overall and particular agreement:
pure gas-phase (nsm)
TMC-1
nsm.2003
“Primary” Fractionation Reaction
(i) H3+ + HD
(ii) H2D+ + CO
(iii) H2D+ + e
H2D+ + H2 + 232 K
DCO+ + H2
D + 2H, etc
Heavy depletion/low ionization 
severe fractionation
Classes of Poorly Understood
Gas-Phase Reactions
Ion-molecule formation of saturated
molecules in hot cores
Deuterium fractionation reactions
Atom/Radical--neutral reactions
Radiative association reactions
Dissociative recombination reactions
Hot Core Chemistry
Methanol and formaldehyde are formed on
cold grains by hydrogenation of CO
Rising temperatures put them into the gas
phase
A number of postulated reactions produce
ethanol, methyl formate, dimethyl ether, etc.
but laboratory work is by and large lacking.
Ex: CH3OH2+ + H2CO -> H2COOCH3+ + H2
?????
Fractionation Reactions (10 K)
H3+ + HD  H2D+ + H2
H2D+ + HD  HD2+ + H2
HD2+ + HD  D3+ + H2
HD/D2 = 3(-5)  D3+ » H3+
under high density conditions
???
Atom/Radical-Neutral
Reactions
Radicals: C, CN, CCH
Neutrals: unsaturated
hydrocarbons
1) Inverse T dependence
2) Large rate coefficients by
10-50 K: k  10(-10) cm3 s-1
How true is this generally???? No US
capability for low T work???
Critical Neutral-Neutral Rx
O Atom Reactions:
O + c/l-CnHm  ????
CN Radical Reactions:
CN + N  C + N2
CN + O  CO + N (< 300 K)
C Atom Reactions:
C + Cn  ?????
Radiative Association Rx
A+ + B  AB+ + hn
must be measured at low density or threebody channel dominates
rate enhanced by (1) low T, (2) large size of
reactants, (3) large bond energy.
mostly theory and three-body analogs
Ex: CH3+ + H2O  CH3OH2+ + hn only
known gas-phase synthesis
Role of competitive channels unclear
What happens for larger systems???
Radiative Association Rx- II
A + B  AB + hn
Critical reactions:
C + C2n-1  C2n
Dissociative Recombination
H3O+
%
What are the products??
+ e  H 2O + H
Method
Reference
5
FA
25 ± 1
Storage
ring
Storage
ring
Storage
ring
18 ± 5
33 ± 8
Williams et
al 1996
Jensen et
al. 2000
Neau et al.
2000
VC et al.
1997
Dissociative Recombination II
HCNH+ + e  HCN + H
HNC + H
CN + 2H
CH3OHD+ + e  CH3OH + D ???
CH2DOH2+ + e  CH2DOH + H ??
106 sites
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
X + Y  XY ??????????
Some Surface Reactions
Studied in Detail*
System
Surface
Reference
H + H/D
Olivine,
amorphous C,
amorphous ice
Vidali and coworkers
CO + O
Ice mantles
Vidali and coworkers
CO + O &
O + O2
Matrix isolation Grim &
d’Hendecourt
* Allows estimation of reaction rate on dust
SIMULATION OF SURFACE
CHEMISTRY
Rate equations
Modified rate equations (semi-empirical)
Monte Carlo
Direct Master equation (Biham et al.;
Green et al.; Stantcheva et al.)