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School of Physics and Astronomy FACULTY OF MATHEMATICS & PHYSICAL SCIENCES Introduction to QuickTime™ and a decompressor are needed to see this picture. Astrochemistry QuickTime™ and a decompressor are needed to see this picture. Paola Caselli Outline • Astrochemical processes: 1. 2. 3. 4. 5. 6. 7. 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… Interstellar Molecules Known Interstellar Molecules (Total: 151 as of today) Amino acetonitrile in SgrB2(N) (Belloche et al. 2008) H H O C C N H H O H Glycine - the simplest amino acid 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. 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. 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): RH 2 1 n H v H A ng S H 2 1017 cm-3s -1 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 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. Molecule Dissociation energy (eV) H2 4.48 CH 3.47 OH 4.39 CH+ 4.09 OH+ 5.10 Question: Can the following reactions proceed in the cold interstellar medium? C + H2 CH + H ?? C+ + H2 CH+ + H ?? O + H2 OH + H ?? O+ + H2 OH+ + H ?? Once H2 is formed, the fun starts… 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… Molecule Dissociation energy (eV) H2 4.48 CH OH CH+ OH+ C + H2 CH + H C+ + H2 CH+ + H 3.47 4.39 4.09 5.10 (endothermic by 1.01 eV) (endothermic by 0.39 eV) OH + H (endothermic by 0.09 eV) O+ H2 OH+ + H O+ + H2 (exothermic by 0.62 eV!) 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 Some techincal details: Neutral-Neutral reactions Energy to break the bond of the reactant. A + BC AB + C 1 eV for endothermic reactions E 0.1-1 eV for exothermic reactions Energy released by the formation of the new bond. kb T < 0.01 eV in molecular clouds Example: O + H2 OH + H (does not proceed in cold clouds) Duley & Williams 1984, Interstellar Chemistry; Bettens et al. 1995, ApJ 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: H H H2+ + H2 H3+ + H H The cosmic-ray ionization rate, 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) (Tielens 2005, The Physics and Chemistry of the Interstellar Medium) 3. The chemistry initiated by H3+ H3 + + Once H3 is formed, a cascade of reactions greatly enhance the chemical complexity of the ISM. O OH+ H2 In fact, H3+ can easily donate a proton and allow larger molecules to build. H2 O+ H2 H3 O+ e Example OXYGEN CHEMISTRY (the formation of water in the ISM) H2 O e e OH O O2 O 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: C H3 + CH+ H2 CH2+ H2 e CH2 CH3+ e 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. 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 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 = 610-17 s-1 and nC/nH = 10-4, the above expression gives a value of 105 yr. 5. Nitrogen Chemistry Nitrogen chemistry differs from that of oxygen and carbon: N + H3+ NH+ + H2 The N-chemistry starts with a neutralneutral reaction (e.g.): CH + N CN + H + tN2 ~ 106 yr N2 vs. CO Chemistry in Photodissociation Regions (PDRs) Sternberg & Dalgarno 1995 CO N2 Chemical Evolution? Suzuki et al. 1992 Chemical Evolution? Dust has to be taken into account! Freeze-out vs. free-fall: tdep 1 9 1 10 m /T (n ) yr X H 2 nd ad v t 3 1/ 2 7 1/ 2 t ff yr 4 10 (n H ) 32G Walmsley 1991 van Dishoeck et al. 1993 Evidences of freeze-out: solid features QuickTime™ and a decompressor are needed to see this picture. Spitzer from van Dishoeck et al. 2003 QuickTime™ and a decompressor are needed to see this picture. Pontoppidan et al. 2007 Evidences of freeze-out: the missing CO C17O(1-0) emission (Caselli et al. 1999) CO hole dust peak 0.05 ly Dust emission in a pre-stellar core (Ward-Thompson et al. 1999) Molecules freeze out onto dust grains in the center of pre-stellar cores Dust grain Evidences of freeze-out: deuterium fractionation N2D+(2-1) N2H+(1-0) Dust emission in the pre-stellar core L1544 (Ward-Thompson et al. 1999) D-fractionation increases towards the core center (~0.2; Caselli et al. 2002; Crapsi et al. 2004, 2005) Evidences of freeze-out: 6. Deuterium fractionation deuterium fractionation H2D+ / H3+ (and D/H) increases: (i) in cold gas CO/H2 H3+ + HD H2D+ + H2 + 230 K (ii) when the abundance of gas phase neutral species decreases. (Dalgarno & Lepp 1984) Roberts, Millar & Herbst 2003 H2D+ + N2 N2D+ + H2 CO DCO+ + H2 D/H 0.3 ! Evidences of freeze-out: H2D+ in L1544 deuterium fractionation Vastel et al. 2006 Caselli et al. 2003, 2008 oH 2D + CSO N2H+(1-0) IRAM N2D+(2-1) IRAM Evidences of freeze-out: D-fractionation and ion fraction deuterium fractionation ge- PAH-, PAH /n(H2) neutrals H3+ ege- g- HD H2 H2D+ HD H2 D2H+ HCO+, N2H+… Wootten et al. 1979 Guelin et al. 1982 Bergin et al. 1998 Caselli et al. 1998 Dalgarno 2006 neutrals DCO+, N2D+… PAHPAH neutrals DCO+, N2D+… Uncertainties: 1/3 * PAHs, PAH-s * neutrals (O) * ortho:para H2 2/3 PAHPAH eg- HD H2 D3+ neutrals PAHPAH DCO+, N2D+… QuickTime™ and a decompressor are needed to see this picture. PAHs What happens after a protostar is born? e are ne and a Time™ Quick ompress or icture. dec ee this p s ded to QuickTime™ and a decompressor are needed to see this picture. 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) are n ee Quic kT de ime™ ded t compres and a so o see this p r icture . 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) SO HCN Quic kTim are n e eede decompr ™ and a d to s e ee th ss or is pic ture. HCO+ HCOOCH3 CH3OH CH3CH2CN 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 are n ee Quic kT de ime™ ded t compres and a so o see this p r icture . What happens after a protostar is born? QuickTime™ and a decompressor are needed to see this picture. • dust heating, X-rays nearby protostars (mantle processing and evaporation) • dust (mantles and cores) sputtering + vaporization along protostellar outflows 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) 7. Surface Chemistry REACTANTS: MAINLY MOBILE ATOMS AND RADICALS A + B AB association H + H H2 H + X XH (X = O, C, N, CO, etc.) WHICH CONVERTS Accretion 10/[Tk1/2 n(H2)] days O OH H2O C CH CH2 CH3 CH4 N NH NH2 NH3 CO HCO H2CO H3CO CH3OH 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 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 QuickTime™ and a decompressor are needed to see this picture. Henning & Semenov 2008 What happens in protoplanetary disks? Chemical Structure of PPDs UV, X-rays surface 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?) What happens in protoplanetary disks? DCO+ (van Dishoeck et al. 2003; Guilloteau et al. 2006) and H2D+ (Ceccarelli et al. 2004) detected. QI, WILNER, AIKAWA, BLAKE, HOGERHEIJDE 2008 HCO+(3-2) DCO+(3-2) first image! DCO+/HCO+ and DCN/HCN ~0.05 in TW Hydrae HCN(3-2) DCO+/HCO+~ 0.05 in L1544 DCN(3-2) first image! ALMA is needed ! Links to the Solar System ? HDO IN THE DISK OF DM Tau: HDO/H2O~0.01 SOURCE Class 0 protostars (HC) Protoplanetary disks Comets Carbonaceous chondrites Ground transition at 464 GHz Oceans with JCMT (Ceccarelli et al. 2005) Herschel is needed ! HDO/H2O ~0.03 ~0.01? ~3.0x10-4 ~1.510-4 1.610-4 Links to the Solar System ? The assemblage of planets… Links to the Solar System ? Chondrites: interstellar ovens? cement condrule 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! 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 Exoplanets Brown Dwarf 2M1207 and its planetary companion (~14 MJ; Chauvin et al. 2005). 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) Exoplanets Darwin & TPF will detect Biomarkers: O3 Spectroscopic Chemical Analysis of Atmophere. Courtesy: Prof. G.W. Marcy, University of California, Berkeley H2O vapor. Methane: Disequilibrium chemicals. CH4 + O2 --> CO2 +H2O 17 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. QuickTime™ and a decompressor are needed to see this picture. http://planetquest.jpl.nasa.gov/news/keplerArrival.cfm 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 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 …