1. 17/Nov/20091 SUNY Stony Brook Astrochemistry Lecture Astrochemistry Adwin Boogert NASA Herschel Science Center, Caltech, Pasadena, CA

2. 17/Nov/20092 SUNY Stony Brook Astrochemistry Lecture Contents What is Astrochemistry? Chemical Reactions in Space Gas Phase neutral and ion reactions Grain surface chemistry Tunneling Mantle growth Ice formation threshold Ice processing Laboratory simulations Thermal processing Energetic processing Observing Interstellar Molecules Gas Phase IR versus radio observations Detected Species

3. 17/Nov/20093 SUNY Stony Brook Astrochemistry Lecture Contents Observing Interstellar Molecules Solid State Band profiles Polar versus apolar ices; Sublimation Amorphous versus Crystalline ices; Time scales Grain size/shape effects Column densities Molecular Evolution: Dense Clouds Low and High Mass Young Stars Hot Cores+Disks Stars Stellar Death Diffuse Clouds Astrobiology Future: Herschel, ALMA, JWST

4. 17/Nov/20094 SUNY Stony Brook Astrochemistry Lecture Reading Material covered in this lecture is described at a similar level in “Complex Organic Interstellar Molecules”, E. Herbst and E. F. van Dishoeck, ARA&A 2009, 47, 427-480. No need to read sections 2, 3.3, 5.2, 5.3, 6.4-6.6. For the interested: More advanced astrochemistry chapters in “The Physics and Chemistry of the Interstellar Medium”, A. G. G. M. Tielens, ISBN 0521826349. Cambridge, UK: Cambridge University Press, 2005. Astrobiology: “An Introduction to Astrobiology”, eds. I. Gilmour and M. A. Sephton, ISBN 0521546214. Cambridge, UK: Cambridge University Press, 2003, 2004.

5. 17/Nov/20095 SUNY Stony Brook Astrochemistry Lecture What is Astrochemistry? Astrochemistry studies molecules anywhere in the universe: how are they formed? how are they destroyed? how complex can they get ? how does molecular composition vary from place to place? use them as tracer of physical conditions (temperature, density)? how are molecules in space related to life as we know it (astrobiology)?

6. 17/Nov/20096 SUNY Stony Brook Astrochemistry Lecture Chemical Reactions in Space Key factors in interstellar chemistry: Abundance H factor 1000 larger than any other (reactive) elements Away from very strong UV fields: H,N,C,O atoms 'locked up' in H 2, N 2, CO. Left over atoms determine chemical environment: Reducing environment if H>O Oxidizing environment if H<O Cosmic Abundances H 0.9 H 2 He 0.1 inert O 7e-4 CO C 3e-4 CO N 1e-4 N 2 Ne 8e-5 inert Si 3e-5 dust Mg 3e-5 dust S 2e-5 Fe 4e-6 dust

7. 17/Nov/20097 SUNY Stony Brook Astrochemistry Lecture Chemical Reactions in Space More key factors in interstellar chemistry: Densities atoms and molecules in interstellar medium extremely low: 1-10 5 particles/cm 3. Compare: earth atmosphere 10 19 ultra-high vacuum 10 8 Therefore chemistry quite unusual compared to earth standards. Rare earth species (discussed in a few slides) are abundant in the ISM: HCO + [formyl ion] H 3 + [protonated dihydrogen] Types of chemistry: Gas phase chemistry Grain surface chemistry (freeze out <100 K) Energetic processing ices

8. 17/Nov/20098 SUNY Stony Brook Astrochemistry Lecture Gas Phase Chemical Networks Despite extreme vacuum conditions, long time scales allow for complex gas phase chemistry. Ion-neutral reactions orders of magnitude faster than neutral-neutral. Species with ionization potential <13.6 eV likely photo-ionized (C  C + ) Cosmic rays also important ionization sources

9. 17/Nov/20099 SUNY Stony Brook Astrochemistry Lecture Some Key Gas Phase Reactions H 3 + : (recently discovered, see http://h3plus.uiuc.edu) H 2 + CR  H 2 + + e - H 2 + + H 2  H 3 + + H HCO + : H 3 + + CO  HCO + + H 2 H 2 O: O + H +  O + + H O + + H 2  OH + + H OH + + H 2  H 2 O + + H H 2 O + + H 2  H 3 O + + H H 3 O + + e -  H 2 O + H Collides and excites H 2, source of UV in dense clouds

10. 17/Nov/200910 SUNY Stony Brook Astrochemistry Lecture More realistic grain: Many molecules (H 2, H 2 O) much more easily formed on grain surfaces. Freeze out <100 K. Interstellar ‘ice’ or ‘dirty ice’: any frozen volatile, e.g. H 2 O, H 2 O mixtures, pure CO. Grain Surface Chemistry

11. 17/Nov/200911 SUNY Stony Brook Astrochemistry Lecture Grain Surface Chemistry “Grain surfaces are the watering holes of astrochemistry where species come to meet and mate.” (Tielens 2005) Species accreted from gas are chemisorbed or physisorbed on grains, allowing for much longer time to find reaction partner than in gas phase Species move fast over surface, meeting partners many times, allowing for tunneling through activation barriers. e.g. H atom has 50% probability of tunneling through 3400 K barrier. At molecular cloud densities (10 4 -10 5 cm -3 ) it takes a few days for an atom to stick to a grain and 5*10 5 yrs for all gas to deplete on grains, much less than cloud lifetime.

12. 17/Nov/200912 SUNY Stony Brook Astrochemistry Lecture Ice Mantle Growth H 2 O formed by grain surface reactions, CO formed in gas and inertly condenses on grains. Grain mantle thickness: Mass growth rate: dm/dt=S*  *a 2 *n* * Radius growth rate: da/dt=(dm/dt)/(4*  *a 2 *  )  da/dt=S*n* * /(4*  ) Mantle thickness independent of grain radius Dense clouds can have mantles as thick as 0.1 um, and in deeply embedded protostars even more. Mantle thicker than most grain cores according to MRN grain size distribution n(a)~a -3.5, a min =0.005 μm, a max =0.25 μm

13. 17/Nov/200913 SUNY Stony Brook Astrochemistry Lecture Ice Mantle Growth Due to grain temperature and interstellar radiation field ices form only if visual extinction (A V ) large enough: the ice formation threshold Taurus cloud: H 2 O ices absent below visual extinction A V ~3 and CO ices below A V ~7. Difference due to lower T sub of CO. Variation between clouds due to different temperature/radiation field Column Density  CO H2OH2O Extinction (A V ) 

14. 17/Nov/200914 SUNY Stony Brook Astrochemistry Lecture Chemical processes occurring in space can be simulated in laboratory at low T (≥10 K) and low pressure. Thin films of ice condensed on a surface and absorption or reflection spectrum taken. Temperature and irradiation by UV light or energetic particles of ice sample can be controlled. Astrophysical laboratories: Leiden, Catania, NASA Ames/Goddard, Paris Gerakines et al. A&A 357, 793 (2000) Simulating Interstellar Ices

15. 17/Nov/200915 SUNY Stony Brook Astrochemistry Lecture Thermal Processing of Ices New molecules easily produced by heating acid/base mixtures. Example shown H 2 O/NH 3 /HNCO=120/10/1 at 15, 52, 122 K NH 3 +HNCO -->NH 4 + + OCN - NH 4 + and OCN - have spectral characteristics that fit interstellar 4.62 and 6.85 μm bands. Relative intensities not in agreement with observations, however, when requiring charge balance; further study needed. Van Broekhuizen et al., A&A 415, 425 (2004)

16. 17/Nov/200916 SUNY Stony Brook Astrochemistry Lecture Energetic Processing of Ices Chemical processing of ices by UV photons and cosmic rays can be simulated Top figure shows H 2 O/CO/NH 3 ice mixture after photo-processing with hard UV photons Bottom figure shows similar spectra compared to a YSO. Heating after irradiation can explain the 6.85 μm band. Long exposure to photons or particles can form very complex molecules, incl. Amino acids and PAHs. Relevance to interstellar medium is hard to prove. See slides on diffuse medium 415, 425-436 (2004)

17. 17/Nov/200917 SUNY Stony Brook Astrochemistry Lecture Observing Gas Phase Molecules symmetric stretch v1bend v2 asymmetric stretch v1 rotation axis A rotation axis C rotation axis B H 2 O vibration modes H 2 O rotation modes Molecules detected (mostly) by vibrational and rotational transitions, at infrared and radio wavelengths. Electronic transitions occur at X-ray/UV wavelengths →extinction-limited

18. 17/Nov/200918 SUNY Stony Brook Astrochemistry Lecture Observing Gas Phase Molecules Ro-vibrational transition rules lead to characteristic P and R branch spectrum, if there is permanent (e.g. CO) or induced (e.g. CH 4 ) dipole moment. N 2 and O 2 cannot be observed this way. Example CO fundamental (  J=1,  v=1): Pure rotational lines occur mostly in the far-IR/submm for species with permament dipole moments (e.g. CO, but not CH 4 ) Note that in solid state, no rotations allowed, leading to one broad vibrational spectrum 115 GHz 807 GHz 576 GHz 922 GHz 691 GHz 461 GHz 231 GHz 346 GHz

19. 17/Nov/200919 SUNY Stony Brook Astrochemistry Lecture Observing Gas Phase Molecules: Inventory 129 gas phase molecules currently detected in space (123 listed here) http://www.cv.nrao.edu/~awootten/allmols.html

20. 17/Nov/200920 SUNY Stony Brook Astrochemistry Lecture Observing Solid State Molecules H 2 O ice has many broad absorption bands: ● Symmetric stretch ● Asymmetric stretch ● Bending mode ● Libration mode ● Combination modes ● Lattice mode ● etc... Width, position and shape determined by solid state (dipole) interactions → band profile powerful diagnostic of ice environment and structure

21. 17/Nov/200921 SUNY Stony Brook Astrochemistry Lecture Ice Band Profiles Polar vs Apolar Ices Molecular dipole moment determines physical and spectral characteristics. Compare solid H 2 O and CO: Sublimation temperature much higher for H 2 O (90 K vs. 18 K in space) Bands much broader for H 2 O H 2 O/CO mixtures: distinct polar and apolar ices with different H 2 O/CO ratios that can spectroscopically be distinguished and sublimate at different T. Highly relevant for icy bodies (e.g. comets) as well, as dipole moment determines outgassing behaviour. 'Pockets' of apolar CO may result in sudden sublimation.

22. 17/Nov/200922 SUNY Stony Brook Astrochemistry Lecture CO band consists of 3 components, explained by laboratory simulations as originating from CO in 3 distinct mixtures: – 'polar' H 2 O:CO – 'apolar' CO 2 :CO – 'apolar' pure CO (Boogert, Hogerheijde & Blake, ApJ 568,761, 2002) Ice Band Profiles Polar vs Apolar Ices

23. 17/Nov/200923 SUNY Stony Brook Astrochemistry Lecture Ice Band Profiles Polar vs Apolar Ices Indeed, CO ice profiles vary dramatically in different lines of sight, as apolar component highly volatile. 'Older' YSOs have less apolar CO

24. 17/Nov/200924 SUNY Stony Brook Astrochemistry Lecture Ice Band Profiles Amorphous vs. Crystalline Interstellar H 2 O ices formed in amorphous phase, as evidenced by prominent 'blue' wing. Crystallization by protostellar heat. [long wavelength wing originates from scattering on large grains and NH 3 :H 2 O complexes] Crystallization temperature ~120 K in laboratory, but ~70 K in space due to longer time scales. [Time scale ~exp(E barrier /T) (~1 hour in lab, 10 5 yr in space). For same reason sublimation temperature in lab (~180 K) higher than in space (~90 K)]. Interstellar H 2 O ices formed in amorphous phase, as evidenced by prominent 'blue' wing. Crystallization by protostellar heat. [long wavelength wing originates from scattering on large grains and NH 3 :H 2 O complexes] Crystallization temperature ~120 K in laboratory, but ~70 K in space due to longer time scales. [Time scale ~exp(-T/E barrier ) (~1 hour in lab, 10 5 yr in space). For same reason sublimation temperature in lab (~180 K) higher than in space (~90 K)].

25. 17/Nov/200925 SUNY Stony Brook Astrochemistry Lecture Ice Band Profiles Grain Shape and Size Effects Laboratory and interstellar absorption spectra cannot always be directly compared: Scattering on large (micron sized) grains leads to 3 μm red wing (often observed) Surface modes in small grains may lead to large absorption profile variations: For ice refractive index m=n+ik, absorption cross section ellipsoidal grain proportional to (Mie theory) (2nk/L 2 )/[(1/L-1+n 2 -k 2 ) 2 +(2nk) 2 ] Resonance for sphere (L=1/3) occurs at k 2 -n 2 =2, so at large k (=strong transitions) Important for pure CO, but not for CO diluted in H 2 O and also not for 13 CO. Laboratory and interstellar absorption spectra cannot always be directly compared: – Scattering on large (micron sized) grains leads to 3 um red wing (often observed) – Surface modes in small grains may lead to large absorption profile variations: Ice refractive index m=n+ik Absorption cross section ellipsoidal grain proportional to (Mie theory): (2nk/L 2 )/[(1/L-1+n 2 -k 2 ) 2 +(2nk) 2 ] Resonance for sphere (L=1/3) occurs at k 2 -n 2 =2, so at large k (=strong transitions): important for pure CO, but not for CO diluted in H 2 O and also not for 13 CO.

26. 17/Nov/200926 SUNY Stony Brook Astrochemistry Lecture Ice Column Densities and Abundances Ice column densities: N=  peak *FWHM/A lab A lab integrated band strength measured in laboratory A[H 2 O 3  m]=6.2x10 -16 cm/mol. Order of magnitude in quiescent dense clouds: N(H 2 O-ice)=10 18 cm -2 For reference: this is ice layer of 0.3  m at 1 g/cm 3 in laboratory, but.... Ice abundance: X(H 2 O-ice)=N(H 2 O-ice)/N H ~10 -4 This is comparable to X(CO-gas)

27. 17/Nov/200927 SUNY Stony Brook Astrochemistry Lecture 'Typical' abundances w.r.t. H 2 O ice Ice Inventory COfew-50% CO 2 15-35% CH 4 2-4% CH 3 OH<8, 30% HCOOH3-8% [NH 3 ]<10, 40% (?) H 2 CO<2, 7% [HCOO-]0.3% OCS<0.05, 0.2% [SO 2 ]<=3% [NH 4 + ]3-12% [OCN - ]<0.2, 7% Factors of 2 abundance variations between sight- lines are common! Note uncertain NH 3 abundance. Will Spitzer spectra finally establish presence of NH 3 in interstellar ices? Note far fewer ices detected than gas phase species. This is because ices can only be detected by absorption spectroscopy.

28. 17/Nov/200928 SUNY Stony Brook Astrochemistry Lecture Molecular Evolution Next slides molecular evolution: – Dense Clouds – Young Stars – Hot Cores/Disks – Stars – Stellar Death – Diffuse Clouds – Astrobiology Not independent environments. Cycling of matter is key.

29. 17/Nov/200929 SUNY Stony Brook Astrochemistry Lecture Molecular Evolution: Diffuse vs. Dense Medium Hubble telescope image of M51 shows massive young stars (red) 'normal' stars (white) molecular clouds (black) diffuse clouds in between clouds 'processed' by UV photons massive stars very similar to our own Galaxy Cycling between environments as spiral density wave passes

30. 17/Nov/200930 SUNY Stony Brook Astrochemistry Lecture Molecular Evolution: Diffuse vs. Dense Medium CO J=1-0 image M51 highlighting giant molecular clouds. [Obtained with CARMA array in Owens Valley by Jin Koda]

31. 17/Nov/200931 SUNY Stony Brook Astrochemistry Lecture Molecular Evolution: Dense Core Molecules in core freeze out at sublimation temperature of molecule. H 2 O T=90 K CO T=16 K Background star H2OH2O H2OH2O NH 4 + silicates extinction Wavelength

32. 17/Nov/200932 SUNY Stony Brook Astrochemistry Lecture Molecular Evolution: Dense Core CO sublimation temperature ~16 K In densest part of core, most CO freezes out N 2 and H 2 lower sublimation temperature (<13 K) cosmic rays penetrate deep in core, ionizing H 2, forming N 2 H + H 2 + CR  H 2 + + e- H 2 + + H 2  H 3 + + H H 3 + + N 2  N 2 H + + H 2 N 2 H + observable at sub-mm frequencies (e.g. Herschel) better dense cloud tracer than CO

33. 17/Nov/200933 SUNY Stony Brook Astrochemistry Lecture Molecular Evolution: Young Stars Deep ice bands observed toward young stars. As star ages, ices heated: crystallization and sublimation (most volatile species, e.g. CO) first.

34. 17/Nov/200934 SUNY Stony Brook Astrochemistry Lecture Solid 13 CO 2 : Molecular Evolution: Young Stars Observational evidence for thermal processing of ices near YSOs: Solid 13 CO 2 band profile varies toward different protostars…

35. 17/Nov/200935 SUNY Stony Brook Astrochemistry Lecture Solid 13 CO 2 : Molecular Evolution: Young Stars Observational evidence for thermal processing of ices near YSOs: Solid 13 CO 2 band profile varies toward different protostars… …and laboratory simulated spectra show this is due to CO 2 :H 2 O mixture progressively heated by young star (Boogert et al. 2000; Gerakines et al. 1999)

36. 17/Nov/200936 SUNY Stony Brook Astrochemistry Lecture Molecular Evolution: Young Stars Observational evidence for thermal processing of ices near YSOs: Solid 13 CO 2 band profile varies toward different protostars… …and laboratory simulated spectra show this is due to CO 2 :H 2 O mixture progressively heated by young star (Boogert et al. 2000; Gerakines et al. 1999) H 2 O crystallization (Smith et al. 1989) gas/solid ratio increases (van Dishoeck et al. 1997) Detailed modelling gas phase mm- wave observations (van der Tak et al. 2000) Little evidence for energetic processing of ices, however......

37. 17/Nov/200937 SUNY Stony Brook Astrochemistry Lecture Molecular Evolution: Hot Cores......., but in immediate vicinity of YSO ices evaporate, and warm gas directly observable at submm/radio wavelengths in rotational transitions. (sub)millimeter-wave gas phase measurements orders of magnitude more sensitive to abundances than IR ice observations Regions called hot cores for massive young stars and corinos for low mass stars. Cazaux et al. 2004

38. 17/Nov/200938 SUNY Stony Brook Astrochemistry Lecture A. Wootten, “Science with ALMA” Madrid 2006. SGR B2(N), ALMA Band 6 mixer at SMT Have to be able to separate flowers from the weeds Molecular Evolution: Hot Cores Formic acid Methyl formate Formic acid Dimethyl ether

39. 17/Nov/200939 SUNY Stony Brook Astrochemistry Lecture Herschel/HIFI: 480-1916 GHz (625-157 μm). Resolving Power  up to 10 million, or <0.1 km/s Molecular Evolution: Hot Cores CH 3 OH gas cell measurement using HIFI (Teyssier et al. 2005)

40. 17/Nov/200940 SUNY Stony Brook Astrochemistry Lecture Molecules are (Nearly) Everywhere …even on the Sun T>5000 K, most molecules dissociate Lower T, molecules quite easily formed, as demonstrated by H 2 O detection in sun spots (T~3000 K) ~13 um

41. 17/Nov/200941 SUNY Stony Brook Astrochemistry Lecture Molecular Evolution: Stellar Death Cas A, Spitzer SN 1987A, HST Stars at end burning phase expel massive shells of matter, enriching ISM with new elements and dust Effect on chemistry strongly depends on stellar mass, and episode of explosion. Some form oxygen-rich dust (silicates), others graphitic dust (and PAHs). Supernovae vaporize environment, destroying or modifying dust (graphite →diamond). Molecules (CO and SiO) formed in ejecta Produce cosmic rays

42. 17/Nov/200942 SUNY Stony Brook Astrochemistry Lecture Molecular Evolution: Diffuse Medium, Mystery 1 Diffuse Interstellar Bands discovered in 1922 in optical spectra of diffuse medium. Over 200 bands detected. Probably a large gas phase species Polycyclic Aromatic Hydrocarbons possible spherical C 60, “Buckminster Fullerenes”, “Buckyballs” problem not solved...: 1 DIB, 1 carrier? PAHs Buckyball

43. 17/Nov/200943 SUNY Stony Brook Astrochemistry Lecture Another enigmatic diffuse medium feature.... the 3.4 um absorption band toward the Galactic Center). Triple peaks due to hydrocarbons (-CH, -CH 2, - CH 3 ), but what kind of hydrocarbon? Pendleton et al. 1994, Adamson et al. 1998, Chiar et al. 1998, Chiar et al. 2000 Molecular Evolution: Diffuse Medium, Mystery 2 -CH- -CH 2 - -CH 3 -

44. 17/Nov/200944 SUNY Stony Brook Astrochemistry Lecture Molecular Evolution: Diffuse Medium, Mystery 2 Bacteria? Apples?

45. 17/Nov/200945 SUNY Stony Brook Astrochemistry Lecture Greenberg et al. ApJ 455, L177 (1995): launched processed ice sample in earth orbit exposing directly to solar radiation (EUREKA experiment). Yellow stuff turned brown: highly carbonaceous residue, also including PAH. Molecular Evolution: Diffuse Medium, Mystery 2

46. 17/Nov/200946 SUNY Stony Brook Astrochemistry Lecture Greenberg et al. ApJ 455, L177 (1995): launched processed ice sample in earth orbit exposing directly to solar radiation (EUREKA experiment). Yellow stuff turned brown: highly carbonaceous residue, also including PAH. Molecular Evolution: Diffuse Medium, Mystery 2 Little evidence production by UV/CR bombardment of ices: band not polarized as opposed to silicates/ices: not in processed mantle but separate grains 3.4 um band observed in dense clouds, but not triple peaked. Likely NH 3. H 2 O hydrate. Due to Low infrared sensitivity? Better observe sublimated species (more sensitive) formed in evolved star envelopes, and injected in ISM?

47. 17/Nov/200947 SUNY Stony Brook Astrochemistry Lecture Molecular Evolution: Astrobiology Do molecules formed in interstellar medium have anything to do with formation of life? This is topic of astrobiology. Amino acids building blocks of most complex molecules in living organisms...protein. It has been produced in laboratory by heavy processing interstellar ice analog. Also, chirality of amino acids in protein is left-handed. May have been caused by nearby massive star producing circularly polarized light

48. 17/Nov/200948 SUNY Stony Brook Astrochemistry Lecture Future of Astrochemistry is Bright.... Herschel Space Observatory Atacama Large MM Array James Webb Space Telescope ….plus a lot more……