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Introduction to Astrochemistry

School of Physics and Astronomy FACULTY OF MATHEMATICS & PHYSICAL SCIENCES. Introduction to Astrochemistry. Paola Caselli. Outline. Astrochemical processes: The formation of H 2 H 3 + formation The chemistry initiated by H 3 + Formation and destruction of CO Nitrogen chemistry

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Introduction to Astrochemistry

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  1. School of Physics and Astronomy FACULTY OF MATHEMATICS & PHYSICAL SCIENCES Introduction to Astrochemistry Paola Caselli

  2. Outline • Astrochemical processes: • The formation of H2 • H3+ formation • The chemistry initiated by H3+ • Formation and destruction of CO • Nitrogen chemistry • Deuterium fractionation • Surface chemistry • Examples: pre-stellar cores, protostellar envelopes, outflows, hot cores, protoplanetary disks…

  3. Amino acetonitrile in SgrB2(N) (Belloche et al. 2008) H H O C C N H H O H Glycine - the simplest amino acid Interstellar Molecules Known Interstellar Molecules (Total: 151 as of today)

  4. How do molecules form in the interstellar medium ? The most elementary chemical reaction is the association of A and B to form a molecule AB with internal energy: A + BAB* The molecule AB* must loose the internal energy. In the Earth atmosphere, where the number of particles per cubic centimeter (cc) is very large (~1019), the molecule looses its energy via three-body reactions: AB* + M AB But this is not an efficient process in interstellar clouds (FAB~10-36n3cm-3s-1), where the number of particles per cc ranges between a few hundred and 107.

  5. 1. The formation of H2 The reaction that starts the chemistry in the interstellar medium is the one between two hydrogen atoms to form molecular hydrogen: H + H  H2 This reaction happens on the surface of dust grains.

  6. 1. The formation of H2 The H2 formation rate (cm-3 s-1) is given by (Gould & Salpeter 1963; Hollenbach & Salpeter 1970; Jura 1974; Pirronello et al. 1999; Cazaux & Tielens 2002; Habart et al. 2003; Bergin et al. 2004; Cuppen & Herbst 2005): nH gas number density vH H atoms speed in gas-phase A  grain cross sectional area ng dust grain number density SH sticking probability   surface reaction probability

  7. Dissociation energy (eV) Molecule Once H2 is formed, the fun starts… H2 is the key to the whole of interstellar chemistry. Some important species that might react with H2 are C, C+, O, N… To decide whether a certain reaction is chemically favored, we need to examine internal energy changes. Question: Can the following reactions proceed in the cold interstellar medium? O + H2 OH + H ?? O+ + H2  OH+ + H ?? C + H2 CH + H ?? C+ + H2  CH+ + H ??

  8. C + H2 CH + H ?? 4.48 eV 3.47 eV The bond strength of H2 is larger than that of CH the reaction is not energetically favorable. The reaction is endothermic(by 4.48-3.47 = 1.01 eV) and cannot proceed in cold clouds, where kb T < 0.01 eV ! Dissociation energy or bond strength Once H2 is formed, the fun starts…

  9. Dissociation energy (eV) Molecule  C + H2 CH + H C+ + H2  CH+ + H O+ H2 OH + H  (endothermic by 1.01 eV)  (endothermic by 0.39 eV) (endothermic by 0.09 eV)  (exothermic by 0.62 eV!) O+ + H2  OH+ + H Once H2 is formed, the fun starts…

  10. Some technical details: Ion-Neutral reactions A+ + B  C+ + D Exothermic ion-molecule reactions do not possess activation energy because of the strong long-range attractive force(Herbst & Klemperer 1973; Anicich & Huntress 1986): R V(R) = -  e2/2R4 kLANGEVIN = 2 e(/)1/2  10-9 cm3 s-1 independent on T

  11. Example: O + H2 OH + H (does not proceed in cold clouds) Some techincal details: Neutral-Neutral reactions A + BC  AB + C Energy to break the bond of the reactant. 1 eV for endothermic reactions E 0.1-1 eV for exothermic reactions kb T< 0.01 eV in molecular clouds Energy released by the formation of the new bond. Duley & Williams 1984, Interstellar Chemistry; Bettens et al. 1995, ApJ

  12. H H H 2. Cosmic-ray ionization of H2 After the formation of molecular hydrogen,cosmic rays ionize H2 initiating fast routes towards the formation of complex molecules in dark clouds: H2 + c.r. H2+ + e- + c.r. Once H2+ is formed (in small percentages), it very quickly reacts with the abundant H2 molecules to form H3+, the most important molecular ion in interstellar chemistry: H2+ + H2 H3+ + H

  13.   10-18 s-1 from the known spectrum of high energy cosmic rays.  < 10-14 s-1 from thermal equilibrium in diffuse clouds. •  6x10-17 s-1 from thermal equilibrium in dark clouds. • (Dalgarno 2006, PNAS) The cosmic-ray ionization rate,  (Tielens 2005, The Physics and Chemistry of the Interstellar Medium)

  14. H3+ O OH+ H2 H2O+ H2 H3O+ e e e H2O O OH O O2 3. The chemistry initiated by H3+ Once H3+ is formed, a cascade of reactions greatly enhance the chemical complexity of the ISM. In fact, H3+ can easily donate a proton and allow larger molecules to build. Example  OXYGEN CHEMISTRY (the formation of water in the ISM)

  15. CH2 e H3+ H2 H2 C CH+ CH2+ CH3+ e 3. The chemistry initiated by H3+ CARBON CHEMISTRY (the formation of hydrocarbons) The formation of more complicated species from neutral atomic carbon begins with a sequence very similar to that which starts the oxygen chemistry: CH A. Proton transfer from H3+ to a neutral atom; B. Hydrogen abstraction reactions terminating in a molecular ion that does not react with H2; C. Dissociative recombination with electrons.

  16. 4. Formation and destructio1n of CO [a] C + H3O+ HCO+ + H2 [b] O + CH3+ HCO+ + H2 [c]HCO+ + e  CO + H is the most important source of CO. CO is very stable and difficult to remove. It reacts with H3+: [d] H3+ + CO  HCO+ + H2 but reaction [c] immediately reform CO. The main mechanisms for removing CO are: [e] He+ + CO  He + C+ + O [f] h + CO  C + O Some of C+ react with OH and H2O (but not with H2): [g] C+ + OH  CO+ + H [h] CO+ + H2 HCO+ + H [i] C+ + H2O  HCO+ + H

  17. The timescale to form CO Assume: dark region where all H is in H2 and all atoms more massive than He are in neutral atomic form. The timescale on which almost all carbon becomes contained in CO (nO > nC) is at least equal to the timescale for one hydrogen molecule to be ionized for every C: nC/[ n(H2)] = 2 nC/[ nH] For  = 610-17 s-1 and nC/nH = 10-4, the above expression gives a value of 105 yr.

  18. 5. Nitrogen Chemistry Nitrogen chemistry differs from that of oxygen and carbon: N + H3+ NH+ + H2 The N-chemistry starts with a neutral- neutral reaction (e.g.): CH + N  CN + H  + tN2 ~ 106 yr

  19. CO N2 N2 vs. CO Chemistry in Photodissociation Regions (PDRs) Sternberg & Dalgarno 1995 The Orion Bar

  20. Chemical Evolution? Suzuki et al. 1992

  21. Chemical Evolution? Dust has to be taken into account! Freeze-out vs. free-fall: Walmsley 1991 van Dishoeck et al. 1993

  22. Evidences of freeze-out: solid features Spitzer from van Dishoeck et al. 2003 Pontoppidan et al. 2007

  23. Evidences of freeze-out: the missing CO C17O(1-0) emission (Caselli et al. 1999) CO hole Dust grain Molecules freeze out onto dust grains in the center of pre-stellar cores  dust peak 0.05 ly Dust emission in a pre-stellar core (Ward-Thompson et al. 1999)

  24. Evidences of freeze-out: deuterium fractionation N2D+(2-1) N2H+(1-0) D-fractionation increases towards the core center (~0.2; Caselli et al. 2002; Crapsi et al. 2004, 2005) Dust emission in the pre-stellar core L1544 (Ward-Thompson et al. 1999)

  25. CO/H2 (ii) when the abundance of gas phase neutral species decreases. (Dalgarno & Lepp 1984) Roberts, Millar & Herbst 2003 N2  N2D+ + H2 H2D++ CO  DCO+ + H2 D/H  0.3 ! Evidences of freeze-out: deuterium fractionation 6. Deuterium fractionation H2D+ / H3+(and D/H) increases: (i) in cold gas H3+ + HD  H2D+ + H2 + 230 K

  26. Evidences of freeze-out: deuterium fractionation H2D+ in L1544 Vastel et al. 2006 o-H2D+ CSO Caselli et al. 2003, 2008 N2H+(1-0) IRAM N2D+(2-1) IRAM

  27. Evidences of freeze-out: deuterium fractionation D-fractionation and ion fraction g- e- PAH-, PAH Wootten et al. 1979 Guelin et al. 1982 Bergin et al. 1998 Caselli et al. 1998 Dalgarno 2006 H3+ neutrals /n(H2) HCO+, N2H+… HD H2 e- neutrals Uncertainties: * PAHs, PAH-s * neutrals (O) * ortho:para H2 DCO+, N2D+… H2D+ 1/3 g- PAH- PAH HD H2 e- neutrals DCO+, N2D+… D2H+ 2/3 g- PAH- PAH HD H2 e- neutrals DCO+, N2D+… D3+ g- PAHs PAH- PAH

  28. What happens after a protostar is born?

  29. What happens after a protostar is born? Large abundances of multiply deuterated species in (Class 0) protostellar envelopes (Ceccarelli et al. 1998; Parise et al. 2002, 2004, 2006; van der Tak et al. 2002; Vastel et al. 2003) D2CO/H2CO = 0.1 CHD2OH/CH3OH = 0.02 D2S/H2S = 0.02 ND3/NH3 = 0.001 CD3OH/CH3OH = 0.02

  30. SO HCN HCO+ CH3OH CH3CH2CN HCOOCH3 What happens after a protostar is born? Complex organic molecules in hot cores and hot corinos (e.g. Wright et al. 1996; Cazaux et al. 2003; Bottinelli et al. 2004,2008; Kuan et al. 2004)

  31. What happens after a protostar is born? Strong H2O, SiO, CH3OH, NH3, emission (e.g. Bachiller 1996) and complex molecules (C2H5OH, HCOOCH3: Arce et al. 2008) along outflows. Jørgensen et al. 2004

  32. What happens after a protostar is born? • dust heating, X-rays nearby protostars (mantle processing and evaporation) • dust (mantles and cores) sputtering + vaporization along protostellar outflows

  33. 7. Surface Chemistry thermal hopping quantum tunneling 106 sites Tielens & Hagen (1982); Tielens & Allamandola (1987); Hasegawa et al. (1992); Tielens 1993; Cazaux & Tielens (2002); Cuppen & Herbst (2005); Cazaux et al. (2008); Garrod (2008)

  34. 7. Surface Chemistry REACTANTS: MAINLY MOBILEATOMS AND RADICALS A + B AB association H + H H2 H + X XH (X = O, C, N, CO, etc.) Accretion WHICH CONVERTS O  OH  H2O C CH  CH2  CH3  CH4 N  NH  NH2  NH3 CO  HCO  H2CO  H3CO  CH3OH 10/[Tk1/2 n(H2)] days Diffusion+Reaction tqt(H)10-5-10-3 s Watson & Salpeter 1972; Allen & Robinson 1977; Pickes & Williams 1977; d’Hendecourt et al. 1985; Hasegawa et al. 1992; Caselli et al. 1993

  35. What happens in protoplanetary disks? Aikawa & Herbst 1999; Markwick & Charnley 2003; Aikawa & Nomura 2006; Bergin et al. 2007; Dutrey et al. 2007; Meijerink, Poelman et al. 2008; Semenov et al. 2008 Henning & Semenov 2008

  36. What happens in protoplanetary disks? Chemical Structure of PPDs surface UV, X-rays intermediate              midplane                   UV, c.r. Surface layer : n~104-5cm-3, T>50K Photochemistry (high CN/HCN) Intermediate: n~106-7cm-3, 20<T<40K Dense cloud chemistry (freeze-out, D-fractionation) Midplane: n>107cm-3, T<20K Freeze-out (are parent-cloud species preserved?)

  37. What happens in protoplanetary disks? DCO+ (van Dishoeck et al. 2003; Guilloteau et al. 2006) and H2D+ (Ceccarelli et al. 2004) detected. DCO+/HCO+ and DCN/HCN ~0.05 in TW Hydrae DCO+/HCO+~0.05 in L1544 QI, WILNER, AIKAWA, BLAKE, HOGERHEIJDE 2008 HCO+(3-2) DCO+(3-2) first image! HCN(3-2) ALMAis needed ! DCN(3-2) first image!

  38. Links to the Solar System ? HDO IN THE DISK OF DM Tau: HDO/H2O~0.01 Ground transition at 464 GHz with JCMT (Ceccarelli et al. 2005) Herschel is needed !

  39. Links to the Solar System ? The assemblage of planets…

  40. Links to the Solar System ? Chondrites: interstellar ovens? cement condrule

  41. Links to the Solar System ? ‘Cement’ between chondrules: • Consists of tiny particles (~ interstellar dust) • Often contains water and carbon • Often contains hydrous minerals resulting from ancient interaction of liquid water and primary minerals. Must have been liquid water in planetesimals!

  42. Links to the Solar System ? Carbonaceous chondrites contain a substantial amount of C, up to 3% by weight. ~70 amino acids have been identified in carbonaceous chondrites; 8 of these are found in terrestrial proteins (Botta & Bada 2002, Survey Geophys.) L-Alanine L-Aspartic Acid L-Glutamine Glycine

  43. Exoplanets Brown Dwarf 2M1207 and its planetary companion (~14 MJ; Chauvin et al. 2005).

  44. Exoplanets Initial studies of hot Jupiters’ atmospheres: Richardson et al. 2007 (Spitzer) de Mooij et al. 2009 (WHT+UKIRT) Sing & Lopez-Morales 2009 (Magellan+VLT)

  45. Exoplanets Darwin & TPF will detect Biomarkers: O3 H2O vapor. Spectroscopic Chemical Analysis of Atmophere. Methane: Disequilibrium chemicals. CH4 + O2 --> CO2 +H2O Courtesy: Prof. G.W. Marcy, University of California, Berkeley 17

  46. Exoplanets NASA's Kepler spacecraft, scheduled to launch in March on a journey to search for other Earths, has arrived in Cape Canaveral, FL For four years, Kepler will monitor 100,000 stars in our Galaxy,looking for (Earthlike) planetary transits. http://planetquest.jpl.nasa.gov/news/keplerArrival.cfm

  47. Summary Prestellar cores: CN, N2H+, NH3, N2D+, DCO+, o-H2D+… Ion-molecule reactions, freeze-out, deuterium fractionation, surface chemistry Outflows: H2O, CH3OH, NH3, SiO, S-bearing species Grain sputtering, grain-grain collisions, neutral-neutral reactions Hot Cores: CH3CN, HCOOCH3, complex saturated molecules Grain mantle evaporation, neutral-neutral reactions, surface chemistry PP Disks: CO, CN, HCN,N2H+,HCO+, DCO+, o-H2D+ Ion-molecule reactions, freeze-out, D-fractionation, surface chemistry, photochemistry, X-rays, dust coagulation

  48. Main Uncertainties • Cosmic-ray ionization rate • Elemental abundance in dark clouds (e.g. metals) • Oxygen chemistry ( Herschel) • PAHs abundance • Surface chemistry and gas phase high-T chemistry • H2 ortho-to-para ratio Constant need of interaction with real chemists (theory + lab, gas-phase+solid state), who provide rate coefficients, collisional rates, transition frequencies…

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