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/