The Chemistry of Interstellar Space

3. The Chemistry of Interstellar Space What did you just see? SCIENCE = WHAT IS GOING ON? AND WHY? Science is not reality, Science tries to give a DESCRIPTION of the reality

4. D A Adriaens / F Goumans (ex-)UCL Chemistry Department The Chemistry of Interstellar Space

5. Astronomy • History • Star Cycle • Chemistry • Nuclear Reactions • Spectra • Molecules in the universe The Orion Nebula

6. Stonehenge • 2900 BC  2000 BC • Astronomical calendar • or • Religious Temple • Midsummer and Midwinter Go back to main

7. Greek Aristarchus (310-230BC): sun in centre of the universe Aristotle (384-322BC): earth in centre of the universe Ptolemy (90-168AD): earth in centre of the universe China 1054: Supernova History

8. 16-17th century: Age of Enlightenment Copernicus (1473-1543) (heliocentrism, circular orbits) Galileo Galilei (1564-1642) (4 moons of Jupiter) Johannes Kepler (1571-1630) (3 laws of Kepler, ellipses) Newton (1643-1727) (3 laws of motion) … Revolution in Sciences History

9. Now ? Albert Einstein (1879-1955) Stephen Hawking (1942-…) History > relativity and advanced cosmology “A brief history of time”

10. Star Cycle Only via STATIC observations, not dynamic Go back to main

11. Star Cycle

12. Star Cycle Only via STATIC observations, not dynamic

13. Nuclear reactions in a star • Small stars: H  He • Medium stars: H  He  C • Massive stars: H  He  C  O  Ne, Na, Mg, S, Si, Ca, Fe, Ni, Cr, Cu, … Basis for life “We are all made of stars” Go back to main

14. Nuclear reactions: proton-proton

15. Nuclear reactions in a star 4He + 4He  [8Be] 4He + [8Be]  12C + g 4He + 12C  16O + g 12C + 1H  [13N] + g [13N]  13C + e+ + ne 13C + 1H  14N + g 14N + 1H  [15O] + g [15O]  15N + e+ + ne 15N + 1H  12C + 4He 15N + 1H  16O + g 16O + 1H  [17F] + g [17F]  17O + e+ + ne 17O + 1H  14N + 4He 1H + 1H  2H + e+ + ne 1H + e- + 1H  2H + ne 2H + 1H  3He + g 3He + 1H  4He + e+ + ne 3He + 3He  4He + 1H + 1H 3He + 4He  [7Be] + g [7Be] + e-  7Li + ne 7Li + 1H  4He + 4He [7Be] + 1H  8B + g 8B  8Be + e+ + ne 8Be  4He + 4He

16. Spectra: The sun Why is our sun yellow? Go back to main

17. Spectra

18. Spectra: solar spectrum

19. Absorption and emission

20. Some atomic spectra H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Hg

21. An owl’s view on the Universe

22. Infrared spectra Molecular spectra: more complicated

23. Seeing in the Infrared The Orion Constellation

24. An owl’s view on the Universe

25. Molecules in the Interstellar Matter • clouds of dust • these regions are nurseries for stars • rich in complex molecules Go back to main

26. Molecules in the ISM • > 150 species • from H2 to HC11N

27. Role of molecules • Stars form in gas clouds collapsing under their own weight • Must dissipate heat formed in process • Heat radiated away by molecules

28. Radio Microwave Wavelength Infrared Visible/UV Spectroscopy of stars • Region of spectrum characteristic to motion • Rotation • Vibration • Electronic excitation Go back to main

29. Formation of molecules • Conditions very harsh in the ISM • Extremely low pressure (10-15 mbar): few collisions • Extremely low temperature (10-20K): barrierless • Very low chance of reactive encounters

30. Formation of molecules • For several molecules ‘too high’ abundances for gas-phase reactions only Interstellar Ices Gas phase alone cannot account for observed abundances H2 Formation problem H + H  H2*  H2 + hv H + H  H2* H2*+ M  H2 + M • DUST GRAINS

31. Dust grains • Gas clouds contain dust particles • Molecules freeze out as “ices” (~10K) • Ices grow by reactions at grain • (H, N, O, CO yielding H2O, NH3, CO2, CH3OH)

32. Dust grains • ~1% of the mass of the ISM • up to 10mm in size • carbonaceous and silicate material • bare or covered by ices • H2O, CO, CO2, CH3OH, NH3 • amorphous • fluffy, open structure (porous)

33. Formation of molecules • Heterogenous reaction at the dust particle’s surface • H2 formed by such a reaction • 2 mechanisms: • 2 H meet on surface (Langmuir-Hinshelwood) • 1 gaseous H meets an H on surface (Eley-Rideal) • Other molecules formed on surfaces: • H2O, CO, CO2, CH3OH and NH3 (yielding ices)

34. Ice formation • Mechanisms of formation unknown • We ‘assume’ some reaction model CH3OH formation COa + Ha  HCOa HCOa + Ha CH2Oa CH2Oa + Ha CH3Oa CH3Oa + Ha  CH3OHa H2O formation Oa + H2,a OHa + H(g) OHa + H2,a H2Oa + H(g)

35. Adsorption Desorption Reaction Diffusion Experiments • Experimentalists mimic the interstellar medium • low T and p • use model surfaces • observe ad/desorption and reactions:

36. Simulating the ISM

37. What do experimentalists do? • Surface infrared spectroscopy • which molecules are adsorbed? • Temperature-programmed desorption • which molecules adsorbs most strongly?

38. Data • Theoretical physicists model star formation Theory • Theoretical chemists calculate reaction mechanisms • Explain/clarify experiments • Experimentally inaccessible data

39. Summary • Chemistry important in the evolution of the universe • Molecules play a crucial role in star formation • loss of heat during collapse • (Some) molecules formed on dust grain surfaces • Chemists are investigating reactions in the lab • Theoreticians are computing reactions • Understanding star formation and the universe

40. Acknowledgements Contact me via: drdadriaens@googlemail.com http://www.ucl.ac.uk/chemsea • Thanks to: • Dr Fedor Goumans • Dr Wendy Brown • Rosie Coates • Imperial College Go back to main