The Chemistry of Stellar and Planetary
Formation
Eric Herbst
Departments of Physics, Astronomy,
and Chemistry
The Ohio State University
The Center of the Milky Way
100,000 lt yr
Andromeda: a “nearby” spiral galaxy
The Eagle Nebula: active star forming region in our
galaxy
The Horsehead Nebula (also in our galaxy – Orion)
Dense Interstellar Cold Core
10 K
10(4) cm-3
H2
dominant
sites of star
formation
0.5 lt yr
500 lt yr away
Extinction consistent
with size distribution
from 5-250 nm.
Dust constitutes 1%
of mass in a cloud.
IR spectral studies
yield information
about cores of
dust particles
(silicates, carbon)
as well as icy mantles
Dominant Mantle Species: water, CO2, CO, CH3OH
Work-horse method for gas-phase molecules
Must compare with results of laboratory spectra
The soon-to-be
Herschel Space
Observatory
Cold Core
Low-mass Star
Formation
adiabatic collapse
Protostar
T = 10 K
n = 104 cm-3
Molecule factory
Star + Disk
hot core
100 K
Via high-resolution gas-phase analytical spectroscopy:
133 neutral molecules
(February 2008)
18 molecular ions (main isotopes)
14 positive
4 negative
H
C, N, O
S, Si, P, K, Na, Mg, Al, F
Most in our own
galaxy only.
Gaseous interstellar molecules (151)
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
MgCN
H2CS
CH2NH
C2H4
CH3CHO
HC6H
C2H5CN
CH3CH2CHO
OH
SiO
H2O
MgNC
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
H2CCHO
H2CCHOH
H2C6
CH2CHCH3
HC8CN
CO
FeO
HCO
SiNC
HNCS
CH2CO
C5H
CH2CHCHO
CS
SiC
c-SiC2
CCP
HCCN
HCOOH
C5N
C2H6
CH3C6H
CP
MgCN
C2CN
C4H
HC4N
NO
MgNC
C3O
HC2CN
C5S(?)
N = 12
NS
AlNC
C3S
HC2NC
HC4H
C6H6
SO
HCP
H3+
c-SiC3
C4Si
CH2CNH
HCl
CH+
C3
HCO+
C3N-
HNCCC
HC2CHO
NaCl
CO+
C2O
HOC+
H3O+
CNCHO
c-C3H2O
KCl
SO+
CO2
N2H+
HCNH+
H2COH+
N2(?)
CF+
HCS+
HOCO+
C4H-
N = 13
HC10CN
HC3NH+
C6H-
C8H-
The Chemistry of Cold Cores
• Do chemical reactions take place under
low density and low temperature
conditions?
• Collision interval = 1 day to 1 millenium
k(T) = A(T)exp(-Ea/RT)
• How can we convert atoms into
molecules?
• H+H H+H
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
T = 10-20 K
Gould & Salpeter
FORMATION OF GASEOUS
WATER IN COLD CORES
H2 + COSMIC RAYS  H2+ + e
H2+ + H2  H3+ + H
H3+ + O  OH+ + H2
OHn+ + H2  OHn+1+ + H
H3O+ + e  H2O + H; OH + 2H, etc
+ longer pathways to unsaturated organic species……
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.)
What is a model?
• Simulates chemistry in the gas and on surfaces
• 6000 gas-phase reactions; 200 surface reactions
• Physical conditions can be homogeneous and timeindependent, or can be heterogeneous and/or time
dependent
• Molecular concentrations can be calculated. Comparison
with observation yields physical conditions and history of
object.
• Cold cores (gas + ices) fit best at age of 105 yr (80% of
molecules fit to within observational error).
• Can even simulate what ices look like!
Development of
ice mantle in
cold interstellar
core
Cuppen &
Herbst, 2007
Hot Core Chemistry
100-300 K
evaporation
10 K
Cold phase
+accretion +
surface
chemistry (Hrich)
Surface
chemistry
involving
heavy
radicals
(photochemistry)
Saturated organic
molecules such
as ethers,
alcohols
Garrod &
Herbst (2006)
ORGANIC MOLECULES
PREDICTED IN HOT CORES
• 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)
Protoplanetary Disk
Cosmic rays
UV
X-ray
midplane
UV
500 AU
T Tauri star – 106
yr old
0.01-0.1 M0
Keplerian rotation
ALMA: the future…….
http://www.physics.ohio-state.edu/~eric/
Vertical Distribution
70
T [K]
density
50
40
30
densiy [cm-3]
60
109 9
10 cm-3
temperature
108108
10710
7
10
106
6
105
105
20
0
20
40
60
80
R = 105 AU
photodissociation
100
Z(AU)
accretion
Too detailed for observers
Icy
Molecular
Layer Layer
PDR
HOT CORE IN
ORION
Molecular inventory contains
gaseous saturated (H-rich)
“normal” molecules, not detected
in colder regions. Ice mantles no
longer exist.
Negative Ions in Clouds
• Herbst (1982) considered the possible
abundance of anions in cold regions of the
ISM based on radiative attachment
mechanism:
• A + e → A- + hn
• and estimated their maximum abundance
to be 1% of the neutral counterparts.