•Chemistry in Protoplanetary
Disks
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
CLASSIFICATIONS OF PROTOSTARS
m
m
m
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
“Primary” Fractionation Reaction
(i) H3+ + HD
H2D+ + H2
+ 232 K
H2D+/H3+ >> HD/H2
Signature of depletion!!!
Other
deuterated
species
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.
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
(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
??????????
H + HX  H2 + X abstraction
Diffusive Surface Chemistry
Rate equations (non-discrete)
 Modified rate equations
 Stochastic approaches

a) Monte Carlo

b) Master equation

Some Star-forming Regions
quiescent cores (TMC-1; gas-grain)
 pre-stellar cores (L1544; gas-grain)
 low mass protostars (IRAS 16293-2422)
 protoplanetary disks (DM Tau; gas +
accret./desorp.)
 hot cores (Orion KL; gas-grain)

Protoplanetary Disk (Proplyd)
Column density
Cosmic rays
UV
X-ray
midplane
UV
500 AU
T Tauri star – 106
yr old
0.01-0.1 M0
Keplerian rotation
Temperature and Density Distribution
(D’Alessio et al. 1998, 1999)
A. MIDPLANE
Radius (AU)
n(cm-3)
T (K)
1
10(14)
600
10
10(12)
50
100
10(9)
20
400
10(7)
10
Heavy species
condensed
onto grains
Calculated Major Icy Species at 30 AU
from Star (Aikawa and Herbst 1999)





Cloud  Disk  migration inwards
can be compared with solid cometary
material in solar system
H2O**, CO, NH3, CO2, HCN ices
Deuterated species HDO, DCN
Deuterium fractionation in reasonable
agreement with several comets
Gaseous Molecular
Abundances
● Molecular Line Survey at IRAM30m telescope (Dutrey et al. 1
Abundance relative to H2
Species
CO
HCN
CN
CS
H2CO
HCO+
C 2H
Disk(DMTau)
1.4 × 10-5
5.5 × 10 -10
3.2 × 10 -9
3.3 × 10 -10
2.0 × 10 -10
7.4 × 10 -10
1.1 × 10 -8
Cloud(TMC1)
7 × 10 -5
2 × 10 -8
3 × 10 -8
1 × 10 -8
2 × 10 -8
8 × 10 -9
8 × 10 -8
Gaseous molecular abundances in disks
are different from those in clouds, typically lower,
but these are averages in outer disk.
Chemical Models of Gas in
Outer Disk
Performed by 3-4 different groups
Chemistry dependent on physical model chosen
and on radial and vertical distances
Models start with dense cloud abundances and
run for 10(6) yr at fixed physical conditions and
with various sources of radiation through an
inhomogeneous region.
Collaborators: Aikawa, van Dishoeck, van Zadelhof
Details of Models
Divide outer disk into elements of space; each with
density and temperature
Vertical Distribution
Ｒ＝１０５ＡＵ
109 9
10 cm-3
temperature
8 8
1010
T [K]
density
7 7
1010
6
10
6
10
105
105
0
20
40
60
80
densiy [cm-3]
70
70 K
60
60
50
50
40
40
30
30
20
20
R = 105 AU
photodissociation
100
Z(AU)
accretion
Too detailed for observers
Icy
Molecular
Layer Layer
PDR
Column Density
2D rad. trans
previous 1Dcalc.
Still, too detailed for most observations!
RESULTS – COLUMN DENSITIES (cm-2)
Species
Theory
DM Tau
H2
CO
CN
CS
CCH
HCN
HNC
HCO+
H2CO
HDO
4(21)
6 (17)
2 (13)
1(12)
8(12)
2(12)
2(12)
5(12)
4(12)
6(11)
4(21)
6(16)
1(13)
1(12)
4(13)
2(12)
9(11)
1(12)
1(12)
LkCa15
2(18)
2(13)
2(13)
1(15)
<5(12)
2(13)
4(14)
Aikawa et al. (2002); physical model of D’Alessio;
theoretical results at 373 AU and 10(6) yr
Line Profile from Model Disk
Line profiles are calculated using non-LTE 2D radiation code.
Comparison with LkCa15 (JCMT)
(Rout=400AU, incl.=60o)
CO J=6-5
CO 3-2
CO 2-1
HCO+ 4-3
HCN 4-3
CN 3-2