Interstellar chemistry. Liv Hornekær

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1 Interstellar chemistry Liv Hornekær

2 Overall structure of the ISM 21 cm 2.6 mm Overview picture

3 21 cm observations Leiden-Dwingeloo/Argentina/Bonn

4 H atom La: nm, 10.2 ev J=L+S F=I+J

5 Relative abundance of the elements: A(X) = 10 6 (N X /N H ) From spectroscopy of the solar photosphere.

6 2.6 mm

7 CO J=0->1

8 CO

9 Overview of interstellar molecules Molecules with Two Atoms AlF AlCl C2 CH CH+ CN CO CO+ CP CS CSi HCl H2 KCl NH NO NS NaCl OH PN SO SO+ SiN SiO SiS HF SH FeO N2 Molecules with Three Atoms C3 C2H C2O C2S CH2 HCN HCO HCO+ HCS+ HOC+ H2O H2S HNC HNO MgCN MgNC N2H+ N20 NaCN OCS SO2 SiC2 CO2 NH2 H3+ AlNC Molecules with Four Atoms C3H C3H C3N C3O C3S C2H2 CH2D+ HCCN HCNH+ HNCO HNCS HOCO+ H2CO H2CN H2CS H3O+ SiC3 NH3 Molecules with Five Atoms C5 C4H C4Si l-c3h2 c-c3h2 CH2CN CH4 HC3N HC2NC HCOOH H2CHN H2C20 H2NCN HNC3 SiH4 H2COH+ Molecules with Six Atoms C5H C5O C2H4 CH3CN CH3NC CH3OH CH3SH HC3NH+ HC2CHO HCONH2 H2C4 C5N Molecules with Seven Atoms C6H CH2CHCN CH3C2H HC5N HCOCH3 NH2CH3 C2H4O CH2CHOH

10 Molecules with Eight Atoms CH3C3N HCOOCH3 CH3COOH C7H H2C6 CH2OHCHO CH2CHCHO Molecules with Nine Atoms CH3C4H CH3CH2CN (CH3)20 CH3CH20H HC7N C8H Molecules with Ten Atoms CH3C5N (CH3)2CO NH2CH2COOH CH3CH2CHO Molecules with Eleven Atoms HC9N Molecules with Thirteen Atoms HC11N

11 Chemical reactions in general Reaction rate: k=ae -E a/kt

12 Stabilizing reaction products

13 Stabilizing reaction products Radiation stabilization Collisional stabilization: AB*+ M AB + M

14 Chemical reactions in general Reaction rate: k=ae -E a/kt Reactions including radicals: E a small Reactions including ions: E a ~ 0

15 Ion-molecule reactions A + + B -> C + + D Typical rate: k=10-9 cm 3 /s Rate equation: dn(a) dt = -k(t)n(a)n(b)

16 Chemical models ~1000 gas phase reactions ~1-2 chemical reactions on dust grain surfaces. A few models include ~40 surface reactions. Reaction rates from: 1) Experimental measurements 2) Extrapolation of experimental measurements at high temperature to interstellar temperatures 3) Theoretical calculations 4) Guess or trial and error (Fitting the model to observations)

17 Experimental measurements CN + C 2 H 6 Reaction rate: k=ae -E a/kt Ian Sims

18 Experimental measurements CN + C 2 H 6 x Reaction rate: k=ae -E a/kt Ian Sims

19 Carbon chemistry I.P. of C: ev < 13.6 ev => majority of carbon as C + C + + H 2 CH hν possible at low T (starting reaction) CH 2 + => fast ion-molecule chemistry giving: CH, C 2, C + + H 2 CH + + H: endothermic by 0.4 ev

20 Average Interstellar Radiation field

21 Average Interstellar Radiation field Sum of CMB (radio/fir), thermal emission from dust (IR), cool stars + OB stars (VIS, UV), hot ionized medium (FUV and X-ray)

22 Carbon chemistry I.P. of C: ev < 13.6 ev => majority of carbon as C + C + + H 2 CH hν possible at low T (starting reaction) CH 2 + => fast ion-molecule chemistry giving: CH, C 2, C + + H 2 CH + + H: endothermic by 0.4 ev

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24 Oxygen chemistry I.P. of O: > ev => majority of oxygen as O Ionization by cosmic rays H 2 or H + C.R. H 2 + or H + + C.R. + e H H 2 H H (fast) H + or H 3 + reacts with oxygen: H + + O H + O +, O + + H 2 OH + + H H O OH + + H 2 When OH + is formed fast ion-molecule reactions give OH, H 2 O and CO OH abundace is proportional to the ionization rate by cosmic radiation: ζcr => observed OH abundance used to determine ζcr

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26 Overview of interstellar molecules Molecules with Two Atoms AlF AlCl C2 CH CH+ CN CO CO+ CP CS CSi HCl H2 KCl NH NO NS NaCl OH PN SO SO+ SiN SiO SiS HF SH FeO N2 Molecules with Three Atoms C3 C2H C2O C2S CH2 HCN HCO HCO+ HCS+ HOC+ H2O H2S HNC HNO MgCN MgNC N2H+ N20 NaCN OCS SO2 SiC2 CO2 NH2 H3+ AlNC Molecules with Four Atoms C3H C3H C3N C3O C3S C2H2 CH2D+ HCCN HCNH+ HNCO HNCS HOCO+ H2CO H2CN H2CS H3O+ SiC3 NH3 Molecules with Five Atoms C5 C4H C4Si l-c3h2 c-c3h2 CH2CN CH4 HC3N HC2NC HCOOH H2CHN H2C20 H2NCN HNC3 SiH4 H2COH+ Molecules with Six Atoms C5H C5O C2H4 CH3CN CH3NC CH3OH CH3SH HC3NH+ HC2CHO HCONH2 H2C4 C5N Molecules with Seven Atoms C6H CH2CHCN CH3C2H HC5N HCOCH3 NH2CH3 C2H4O CH2CHOH

27 H 2 formation H No dipole allowed transitions No radiation stabilization 3-body collisions ok at high density Diffuse/Dense cloud ISM densities => ~No 3-body collisions

28 Coalsack nebula in the Southern Cross

29 H 2 absorption bands in diffuse clouds HD behind the Coalsack Nebula

30 ISM and star formation

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32 H2

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34 H2

35 H 2 is an important cooling agent and key to developing chemical complexity in ISM - But how is H 2 formed under ISM conditions?

36 Destruction mechanisms for H 2 Lyman transition + Cosmic radiation Werner 202 nm transition 202 nm

37 H 2 formation rate from observations Typical diffuse cloud rate: k~ n(h) cm 3 s -1

38 H 2 formation H No dipole allowed transitions No radiation stabilization 3-body collisions ok at high density Diffuse/Dense cloud ISM densities => ~No 3-body collisions

39 H 2 formation in the gas phase Radiative association (slow): H + e - H - + hn Associative detachment: H + H - H 2 + e - Typical diffuse cloud rate: k~10-21 n(h) cm 3 s -1 H 2 formation rate: k~ n(h) cm 3 s -1

40 Surface reactions H

41 Surface reactions

42 Surface reactions

43 Objects: Dark nebulae

44 Molecules and dust grains

45 Depletion D(X) = log(n X /N H )-log(n X /N H ) ISM Relative depletion: d(x) = 1-10 D(X) =1-(N X /N H ) / (N X /N H ) ISM Measured in UV => diffuse and intercloud medium.

46 Absorption spectrum CH H 2 0 CO CH H 2 O Silicates (Mg 2 SiO 4, Fe 2 SiO 4)

47 Silicates MgSiO 3 (enstatite): Si-O stretch 9.7 mm O-Si-O bend 19.0 mm Mg 2 SiO 4 (fosterite): Si-O stretch 10.0 mm O-Si-O bend 19.5 mm FeSiO 3 (ferrosilite): Si-O stretch 9.5 mm O-Si-O bend 20.0 mm SiC (Silicon carbide): Si-C stretch 11.2 mm

48 Carbon Hydrogenated amorphous carbon: C-H stretch: 3.4 mm Observed in the diffuse ISM Measured towards Sgr A.

49 C 60 +

50 C60 and C70 detected in protoplanetary nebula Tc 1 C60 C70 A few % of C Cami et al., Science 329, 1180, Sept. 2010

51 Diamond ISO spectra of two pre-main-sequence stars Lower curves are laboratory absorption spectra for diamond nano-crystals

52 Aromatic hydrocarbon related features microns

53 Aromatic hydrocarbon related features 3.3 mm: C-H stretch 6.2 mm: C-C stretch 7.7 mm: C-C stretch 8.7 mm: C-H in plane bend 11.2 mm: C-H out of plane bend microns

54 PAH er Polycykliske Aromatiske Kulbrinter Grænsen mellem molekyle og nano-partikel Benzene Pyrene

55 Emission fra PAH er teori 850 K PAH er Gennemsnitlig interstellar emission, => kun ~5% af C i PAH er

56 PAH er

57 Carbon

58 Interstellar carbon

59 Ice H 2 O: O-H stretch: 3.05 mm H-O-H bend: 6.0 mm CO: C-O stretch: 4.67 mm CH 3 OH: O-H stretch: 3.05 mm C-H stretch: 3.53 mm Dust enshrouded protostar

60 Water ice - morphology

61 CO ice Spectral lines vibrational bands modified by local environment Rotationel splitting Hindered rotation

62 Mixed ice High shielding, low temperature: Even very volatile molecules (e.g. CO) condenses out on dust grains. Observations of 4.67 mm absorption in C-O stretch, shows that CO and water are not mixed Different molecules condenses at different temperatures

63 Average interstellar extinction curve Large grains: ~100 nm 2. Surface structure or smaller grains? 3. Small carbon grains < 20 nm 4. PAHs or other very small grains (VSG) < 10nm Greenberg 1996

64 Dust grain sizes Large grains: 20 nm - 1 mm Silicate or carbon Separate populations? / composite grains? / Grains with carbon mantles? Very small grains: 1-20 nm, Carbon PAHs

65 Onion-like graphite particles Pre-solar dust grains in meteorites Silicates: Olivines (Mg 2 SiO 4, Fe 2 SiO 4 )

66 Atoms and molecules on surfaces Sticking: S = Probability of 2a+2b Sticking => adsorbed 1 2a 2b Scattering Sticking Sticking + hot atom

67 Possible mechanisms for H 2 formation on surfaces Langmuir-Hinshelwood: Adsorption Eley-Rideal: Diffusion Recombination Desorption H(ads) + H(gas) H 2 (gas) Hot Atom: Strong/Weak Adsorption Diffusion Recombination Desorption

68 The effect on the ISM of H 2 formation => E released ~ 4.5 ev ( o C) v Kinetic energy? J = 0 n = 0 Molecular excitation? Dust grain heating?

69 The effect on the ISM of H 2 formation Dashed line: No energy branching into kinetic energy Curve 1: 0.5 ev in kinetic energy Curve 2: 1.5 ev in kinetic energy Curve 3: 2.25 ev in kinetic energy Flores

70 Bringing the Interstellar medium to a laboratory near you

71 Re-creating interstellare conditions? Interstellar pressure: P= atm Interstellar temperatures: T = K Relevant surfaces: Ice, graphite, PAHs, amorphous carbon, silicates

72 Surfaces of interstellar relevance Graphite Water ice Atomic deposition Cu shield T

73 H 2 formation at high temperatures bare grains Orion nebula Orion bar

74 H on graphite Neumann et al. Appl. Phys. A 55, 489 (1992) Jeloica & Sidis, Chem. Phys. Lett. 300, 157 (1999) Eva Rauls Sha et al, Surface Science 496, 318 (2002)

75 103 x 114 Å 2 H-Dimers on graphite Dimer A Dimer B V t = 884 mv, I t = 0.16 na

76 Dimers: Theori vs. Experiment Ortho dimer - Dimer A Para dimer - Dimer B V t =0.9 V, LDOS=1x10-6 (ev) -1 Å -3 e. f. V t = 884 mv, I t = 0.16 na

77 Pair formation Hornekær et al. Phys. Rev. Lett. 97, (2006)

78 H 2 formation on graphite - TDS dq dt = -k 0 e - E k / T Q n B B n=1 => First order desorption 490 K => 1.4 ev 580 K => 1.6 ev Zecho et al, J. Chem. Phys. 117, 8486 (2002)

79 H 2 formation Ortho Meta Para Hornekær et al. Phys. Rev. Lett. 96, (20

80 Eley Rideal - Abstraction Jeloaica & Sidis (2001) Meijer et al. (2001) Sha et al. (2002) Zecho et al. (2002) Matinazzo & Tantardini (2006) Morisset et al. (2004) Bachellerie et al. (2007) Thomas et al. (2008)

81 H 2 formation at lower temperatures - ice covered surfaces

82 Water ice - morphology

83 Thermal Desorption Spectroscopy D 2 from graphite HD from amorphous water ice

84 Thermal Desorption Spectroscopy D 2 from graphite HD from amorphous water ice

85 Kinetic energy of formed H2

86 Laser desorption Time-of-flight H D ASW Laser QMS

87 D(E) Kinetic energy of D 2 formed on graphite ~1.3 ev i translation Translational Energy (ev) S. Baouche et al, J. Chem. Phys. 2006

88 Kinetic energy distribution Baouche et al., J. Chem. Phys. 125, (2006 )

89 Kinetic energy of HD formed on porous water ice HD D 2 Solid line: 45 K Maxwell-Boltzmann velocity dist. L. Hornekær et al., Science, 302, 1943 (2003)

90 H 2 formed on porous water ice Surface structure determines energy partitioning Porous surface: Slow H 2 Grain heating

91 Result Surface structure determines energy partitioning

92 Energy distribution in H 2 formation on graphite L1630 Orion bar Latimer et al., CPL 455, 174 (2008)

93 Dust grain morphology Bare grains porous and non-porous Ice covered grains maybe compacted by formation/processing

94 Orion nebula Orion bar

95 Energy distribution in H 2 formation and PDR observations Photo Dissociation Regions: gas temperatures: K Observation show overpopulation in v=4 Does not fit shock and UV fluorescence models - WHY? L1630 Orion bar Blue: PAH emission Green: H 2 vibrational line emission (FAST) Red: CO emission (30 m Telescope, IRAM) Star at NW

96 Energy distribution in H 2 formation and PDR observations Photo Dissociation Regions: gas temperatures: K Observation show overpopulation in v=4 Does not fit Shock and UV fluorescense models - formation pumping? L1630 Orion bar Blue: PAH emission Green: H 2 vibrational line emission (FAST) Red: CO emission (30 m Telescope, IRAM) Star at NW

97 H 2 formation in different ISM environments? H gas +H gas H 2 Fine under Dense cloud and PDR conditions Hornekær et al., Science (2003) Hornekær et al., PRL (2006) H + H + H 2 Problematic under diffuse cloud conditions Katz et al., ApJ (1999) Cuppen et al., MNRAS (2005) H + H + H 2 Alternative candidate Hornekær et al., PRL (2006) Rauls and Hornekær (2007)

98 PAHs as an H 2 catalyst Correlations between high H 2 formation rates and PAH emission observed in PDRs with low UV flux Habart et al.* PAH cations considered for H 2 formation Snow et al.^ LePage et al. # Here: the role of neutral PAHs? *Habart et al., A&A, 397, 623 (2003) Habart et al. A&A, 414, 531 (2004) ^Snow et al. Nature 391, 259 (1998) # LePage et al. Ap.J., 704, 274, (2009)

99 H-PAH interaction Density Functional Theory (DFT) calculations reveal low barrier routes to H-PAH formation and PAH catalyzed H 2 formation 60 mev -1.4 ev C 24 H 13 C 24 H 12 + H Rauls and Hornekær, Astrophys. J. 679, 531 (2008)

100 H-PAH interaction Density Functional Theory (DFT) calculations reveal low barrier routes to H-PAH formation and PAH catalyzed H 2 formation Rauls and Hornekær, Astrophys. J. 679, 531 (2008)

101 H-PAH formation

102 Superhydrogenated PAHs Evidence in IR emission C-H stretching mode 3.3 μm aromatic 3.4 μm aliphatic High UV flux (Orion bar) Limited excess hydrogen Low UV flux (IRAS 05341) Significant excess hydrogen -CH 3 or -H M. P. Bernstein, et al., ApJ, 472, L127 (1996).

103 Reactions out of mass 300 σ = 0.6 ± 0.3 Å 2

104 H-PAH formation

105 CDH groups CD 2 groups

106 Subsequent H irradiation

107 IR spectroscopy Menella et al., Astrophys. J. Lett. 2012

108 Experiments reveal many different pathways to H 2 formation, which together make H 2 formation efficient under the many different physical and chemical conditions in the ISM

109 Interstellar surface reactions O H H 2 N C

110 ISM in the Milkyway Overview picture

111 ISM and star formation

112 Eagle-Nebula -Pillars

113 Eagle-Nebula

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