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The Chemistry in Interstellar Clouds
Eric Herbst
Departments of Physics, Astronomy, and Chemistry
The Ohio State University
Andromeda: a “nearby” spiral galaxy
100,000 lt yr
Molecules seen at long
wavelengths
10 K
10(4) cm-3
H2 dominant
sites of star formation
Cold Dense Interstellar Cloud
Dust particles block out light
Dust constitutes 1%of mass in a cloud.
IR spectral studies yield informationabout molecules in the gas and on dust particles but the technique is difficult.
AN INTERSTELLAR GRAIN
0.1
silicates & carbonaceous material
ices
+ small grains and PAH’s
Water, CO, CO2
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
E = h
Unlike vibrations, rotations occur only in the gas.
The Case of TMC-1CO J=10
133 neutral molecules (September 2008)
18 molecular ions 14 positive4 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.
Gaseous Interstellar Molecules
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 CH3C6H
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
HClCH+
C3 HOC+ C3N- HNCCC HC2CHO
NaClCO+
C2O N2H+ H3O+ CNCHO c-C3H2O N = 13
KClSO+
CO2 HCS+ HCNH
+H2COH+ HC10CN
N2(?) CF+
HCNO
HOCO+ C4H- 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
The higher the temperature, the faster the reaction.
Activation energy
In Cold Interstellar Clouds
Must be all downhill at low temperatures!
Cosmic rays produce ions
A + B
C + D
activation energy
POTENTIAL ENERGY OF REACTION
typical neutral reactions
radical-radical reactions
ion-molecule reactions
k(T) = A(T) exp(-E /kT)a
H
H
H2
Formation of Hydrogen
dust particle
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
+ + HH3
+ + O OH+ + H2
OHn+ + H2 OHn+1
+ + HH3O+ + 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 + B AB associationH + H H2
H + X XH (X = O, C, N, CO,
etc.) WHICH CONVERTS
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
OHH
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
Protostar
Star + Disk
T = 10 K n = 104 cm-3
adiabatic collapse
hot core
100 K
Low-mass Star Formation
Molecule factory
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/