ERIC HERBST

DEPARTMENTS OF PHYSICS, CHEMISTRY AND ASTRONOMY

THE OHIO STATE UNIVERSITY

Interstellar Chemistry: Triumphs & Shortcomings

Molecules seen in UV,visible, & IR absorption

Diffuse Matter

Molecules seen in IR absorption and radio emission

10 K

10(4) cm-3

H2 dominant

sites of star formation

Dense Interstellar Clouds

GAS PHASE INTERSTELLAR/CIRCUMSTELLAR MOLECULES - HIGH RESOLUTION (12/03) _____________________________________________________________________________________________ H2 KCl HNC NH3 C3S C5 C6H CH3 HC4CN CH AlCl HCO H3O+ CH4 CH3OH C7H, C6H2 C8H CH+ AlF HCO+ H2CO SiH4 CH3SH HCOOCH3 CH3COOH NH PN HOC+ H2CS CH2NH C2H4 CH3C2CN H2C6(lin) OH SiN HN2+ HCCH H2C3(lin) CH3CN C6H2 H2COHCHO C2 SiO HNO HCNH+ c-C3H2 CH3NC C2H5OH (CH3)2O

CN SiS HCS+ H2CN CH2CN HC2CHO C2H5CN CO CO+ C3 C3H(lin) NH2CN NH2CHO CH3C4H

CSi SO+ C2O c-C3H CH2CO HC3NH+ HC6CN CO2 C4H2 (CH2OH)2 CP H3

+ C2S HCCN HCOOH H2C4(lin) (CH3)2CO AlNC CS CH2 SiC2 HNCO C4H C5H CH3C4CN? HF SiCN SiC3 C5N NO NH2 SO2 HOCO+ HC2CN CH3NH2 NH2CH2COOH? NaCN CH2CHOH NS H2O OCS HNCS HCCNC CH3CCH HC8CN SO H2S MgNC C2CN HNCCC CH3CHO c-C6H6 HCl C2H MgCN C3O C4Si CH2CHCN HC10CN NaCl HCN N2O NaCN H2COH+ c-CH2OCH2 + ISOTOPOMERS c-CH2SCH2

AN INTERSTELLAR GRAIN

0.1

silicates & carbonaceous material

ices

+ small grains and PAH’s

H

H

H2

Formation of Hydrogen

dust particle

O

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

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

FORMATION OF HYDROCARBONS

H3+ + C CH+ + H2

CHn+ + H2 CHn+1

+ + H; n=1,2

CH3+ + H2 CH5

+ + h

CH5+ + e CH4

+ H (5%) CH3 + 2H (70%)

CH5+ + CO CH4

+ HCO+

FORMATION OF O2 ,N2 CO

OH + O O2 + H ****

OH + N NO + HNO + N N2 + O

CO, N2 + He+ C+, N+ +…

CH + O CO + H

ORGANIC SYNTHESIS CONT.

SOME SYNTHETIC REACTION CLASSES:

A. CARBON INSERTIONC+ + CH4 -----> C2H3+ + H

------> C2H2+ + H2B. CONDENSATION

C2H2+ + C2H2 -----> C4H3+ + H

C. ATOM IC INSERTION N + C3H3+ -----> HC3NH+ + H

D. RADIATIVE ASSOCIATIONCH3+ + H2O -----> CH3OH2+ + h

E. NEUTRAL-NEUTRAL

C + C2H2 C3H + H

RADICAL-NEUTRAL RX RADICAL-NEUTRAL RX (CONT)(CONT)

CN + C2H2 HCCCN + H

CCH + HCN HCCCN + H

YES

NO

CCH + C2H2 C4H2 + H YES

Barrier cannot be guessed!!

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

Latest network – osu.2003 – contains over 300 rapid neutral-neutral reactions. Rate coefficients estimated by Ian Smith and others for many of these. Verification needed!!

Solved kinetically; thermodynamics useless!

GAS-PHASE HOMOGENEOUS MODELS OF DENSE CLOUDS

"SUCCESSES"

1. IONS ( H3+, HCO+, HC3NH+)

2. ISOMERS (HNC) & RADICALS (OH) HCNH+ + e ----> HCN + H; HNC + H 3. ISOTOPIC FRACTIONATION H3+ + HD <====> H2D+ + H2 4. UNSATURATED MOLECULES A+ + H2 -------> No Reaction 5. ORDER-OF-MAGNITUDE AGREEMENT WITH PERHAPS 80% OF MOLECULES

Chemistry imperfect!!

SOME CRITICALLY NEEDED STUDIES

• 1) atom (O, C) – neutral reactions

• e.g. O + C4 CO + C3

• 2) unimolecular isomeric exchange

• e.g. HCN* HNC*

• 3) radiative association

• 4) recombination without dissociation?

• e.g. C60+ + e C60

SURFACE CHEMISTRY

• 1) Mechanisms (diffusive [Langmuir-Hinshelwood], Eley-Rideal, hot atom, impurity site)

• 2) Dependence on size, mantle, fluffy nature

• 3) Rate equations vs. stochastic treatments

• 4) non-thermal desorption (cosmic rays)

Some Outstanding Astrochemical Problems

• How to make gas-phase models more robust

• How to construct gas-grain models and predict mantle abundances accurately

• How to model the chemistry of star- and planet-forming regions (heterogeneity and time dependence)

WANTED! CHEMISTS

• Quantum chemists (potential surfaces)

• Dynamicists (reaction rates)

• Spectroscopists

• Surface scientists

Theorists & Experimentalists!!