The Chemistry in Interstellar Clouds

1. The Chemistry in Interstellar Clouds Eric Herbst Departments of Physics, Astronomy, and Chemistry The Ohio State University

2. 100,000 lt yr Andromeda: a “nearby” spiral galaxy

3. Cold Dense Interstellar Cloud 10 K 10(4) cm-3 Molecules seen at long wavelengths H2 dominant sites of star formation Dust particles block out light

4. Dust constitutes 1% of mass in a cloud. IR spectral studies yield information about molecules in the gas and on dust particles but the technique is difficult.

6. Water, CO, CO2 + small grains and PAH’s Studied by infrared spectroscopy

7. The Eagle Nebula: active star forming region in our galaxy

8. The Horsehead Nebula (also in our galaxy)

10. Radio astronomy to study gaseous molecules

11. LMT (Large Millimeter Wave Telescope)

12. MOLECULAR ROTATION “radio” emissions DE = hn Unlike vibrations, rotations occur only in the gas.

13. The Case of TMC-1 CO J=10

14. Gaseous Interstellar Molecules 2-13 atoms 133 neutral molecules (September 2008) 18 molecular ions 14 positive 4 negative H C, N, O S, Si, P, K, Na, Mg, Al, F Spectra tell us what molecules are there, what concentrations they have, and what the physical conditions are.

15. 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!!!!!

16. Gaseous interstellar molecules (>150)

17. The Chemistry in Cold Interstellar Clouds Why is it so unusual? Atoms  Molecules in the gas and on dust particles

18. Chemical Reactions Activation energy The higher the temperature, the faster the reaction.

19. In Cold Interstellar Clouds Must be all downhill at low temperatures!

20. Cosmic rays produce ions

22. 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+ + H H3+ + O  OH+ + H2 OHn+ + H2  OHn+1+ + H H3O+ + e  H2O + H; OH + 2H, etc

24. 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.

25. TYPES OF SURFACE REACTIONS REACTANTS: MAINLY MOBILE ATOMS AND RADICALS A + B AB association H + 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

26. 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.)

27. 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.

28. Cold Core Low-mass Star Formation adiabatic collapse Protostar T = 10 K n = 104 cm-3 Molecule factory Star + Disk hot core 100 K

29. 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)

30. The Future Other New Telescopes

32. The soon-to-be Herschel Space Observatory

34. ALMA: the future…….

35. A starburst galaxy…… http://www.physics.ohio-state.edu/~eric/