Astrochimistry Spring 2013 Lecture 4: Interstellar PAHs NGC HST

Transcription

1 Astrochimistry Spring 2013 Lecture 4: Interstellar PAHs NGC HST Julien Montillaud 8th February 2013

2 Outline I. From Unidentified to Aromatic Infrared Bands (7 p.) I.1 Historical background I.2 Observational evidence I.3 The PAH hypothesis II. General properties of chemical PAHs (9 p.) II.1 Overview of the PAH family II.2 Electronic properties II.3 Vibrational properties III. Evolution of interstellar PAHs (15 p.) III.1 Observational evidence III.2 Dissociation of PAHs III.3 Reactivity of PAHs III.4 Formation of PAHs IV. Influence of PAHs on the ISM evolution (3 p.) IV.1 Extinction curve IV.2 Photoelectric effect IV.3 Formation of H2 V. Summary VI. Some bibliographic references 2

3 I. From Unidentified to Aromatic Infrared Bands I.1 Historical background Gillett et al. (1967): IR excess in the?? range compared to free-free radio continuum extrapolation Russell et al. (1977): unidentified IR bands at 3.3, 3.4, 6.2, 7.7, 8.6 & 11.3 µm from reflection nebulea Duley & Williams (1981): notice the match between some PAH spectral features and UIBs Sellgren et al. (1983): near IR emission from reflection nebulae inconsistant with UV-photons thermally excited dust grains => transient heating of small particles? Léger & Puget (1984): the first first attribution of UIBs to PAHs (transient heating excitation model) Allamandola, Tielens & Barker (1985): the second first attribution of UIBs to PAHs (non thermal excitation model) Désert et al. (1990): global model of dust grains including PAHs 1990': first big experiments and detailed theoretical models + Infrared Space Observatory era 2000': multiplication of experiments and detailed models + Spitzer Space Telescope era 3

4 I. From Unidentified to Aromatic Infrared Bands I.2 Observational evidence NGC

5 I. From Unidentified to Aromatic Infrared Bands I.2 Observational evidence HII region Edge of molecular cloud NGC Proto-planetary nebula Planetary nebula

6 I. From Unidentified to Aromatic Infrared Bands I.2 Observational evidence Band positions: typical of C-H and C-C stretches or bends in aromatic material Plateaux at short wavelength: High temperature, small species Simple reasoning: Plateau up to ~3.4 µm T~1000 K (black body approx.) 1 E=10 ev Cv=3kN=E/T => N=39 atoms Possible reading: Sellgren (1984) 6

7 I. From Unidentified to Aromatic Infrared Bands I.3 The PAH hypothesis Astrophysical PAH = carrier of the Aromatic Infrared Bands PAH = polycyclic aromatic hydrocarbon chemical PAH = one model for astrophysical PAH Other models are reasonnable Graphitic grains/ PAH clusters Free flying PAH molecules Hydrogenated amorphos carbon 7

8 I. From Unidentified to Aromatic Infrared Bands I.3 The PAH hypothesis Still no individual spectroscopic identification Kokkin et al Pilleri et al Pilleri et al Not symmetric permanent electric dipole strong rotational emission Undetected in Red Rectangle in the millimetric range (1 3mm) at IRAM-30m Electronic transition in the visible 8

9 I. From Unidentified to Aromatic Infrared Bands I.3 The PAH hypothesis Related detections Fullerene C60: not a PAH, but very close Also: - C70 - C6H6 9

10 II. General properties of chemical PAHs I. From Unidentified to Aromatic Infrared Bands II. General properties of chemical PAHs II.1 Overview of the PAH family II.2 Electronic properties II.3 Vibrational properties III. Grain-catalyzed formation of H2 IV. From processes to interstellar H2 formation rate V. Summary 10

11 II. General properties of chemical PAHs II.1 Overview of the PAH family 5-cycles Catacondensed Irregular Pericondensed 11

12 II. General properties of chemical PAHs II.2 Electronic properties Electron delocalization (=resonance) and aromaticity Molecular orbitals = combination of atomic orbitals original p-atomic orbitals Images from Paula Yurkanis Bruice, Cleveland, OH Resulting molecular (delocalized) orbital Electrostatic potential map of benzene 12

13 II. General properties of chemical PAHs II.2 Electronic properties Electron delocalization (=resonance) and aromaticity Bonding MO: constructive (in-phase) overlap Antibonding MO: destructive (out-of-phase) overlap 13

14 II. General properties of chemical PAHs II.2 Electronic properties Electron delocalization (=resonance) and aromaticity n(e) = 4n+2 => extra stabilisation (aromatic) n(e) = 4n => extra de-stabilisation (anti-aromatic) n(e) = 4n+1 or 4n+3 => nothing special NB: - changing the charge changes the aromaticity - changing the number of H-atoms changes the aromaticity 14 Le Page et al. 2001

15 II. General properties of chemical PAHs II.2 Electronic properties Ionization potential Empirical classical model: PAH = thin conducting disk Reasonable agreement with detailed quantum calculations (Density Functional Theory) Z=1 Z=2 Malloci et al

16 II. General properties of chemical PAHs II.2 Electronic properties Extinction curve Visible UV * Lyman cut (13.6 ev) Electronic spectrum * * * 16

17 II. General properties of chemical PAHs II.2 Electronic properties Electronic spectrum En ~ n / L n=level number En = energy of level n L = well width 17

18 II. General properties of chemical PAHs II.3 Vibrational properties Experimental spectra Theoretical spectra Joblin et al

19 II. General properties of chemical PAHs II.3 Vibrational properties PAH spectroscopy: only little variations Anion Neutral

20 III. Evolution of interstellar PAHs I. From Unidentified to Aromatic Infrared bands II. General properties of chemical PAHs III. Evolution of interstellar PAHs III.1 Observational evidence III.2 Photodissociation of PAHs III.3 Reactivity of PAHs III.4 Formation of PAHs IV. From processes to interstellar H2 formation rate V. Summary 20

21 III. Evolution of interstellar PAHs III.1 Observational evidence In HII regions and reflection nebulae: from region to region Peeters et al &

22 III. Evolution of interstellar PAHs III.1 Observational evidence In the reflection nebulae NGC 7023: within one region Berné et al Peeters et al &

23 III. Evolution of interstellar PAHs III.2 Photodissociation of PAHs Direct photodissociation? apparently not efficient (Buch 1989, Jochims et al. 1994) Other possibility: statistical photodissociation 23

24 III. Evolution of interstellar PAHs III.2 Photodissociation of PAHs Relaxation processes of an isolated PAH IVR: internal vibrational redistribution IC: internal conversion 24

25 III. Evolution of interstellar PAHs III.2 Photodissociation of PAHs (diagram for a PAH cluster, but similar to H-loss or C-loss for an isolated PAH molecule) 25

26 III. Evolution of interstellar PAHs III.2 Photodissociation of PAHs Arrhenius law kdiss(t) = Adiss exp(-e0/kt) Laplace transform Approximated microcanonical expression kdiss = dissociation rate coefficient T = vibrational temperature also kinetic temperature if enough collisions for thermalization Adiss = prefactor E0 = dissociation energy kdiss = dissociation rate coefficient E = internal (vibrational) energy Adiss = prefactor vibrational frequency entropy cost (structural changes) difficult to estimate theoretically, but not impossible E0 = dissociation energy = vibrational density of state(vdos) of the parent molecule => one has to know 26 the vibrational modes

27 III. Evolution of interstellar PAHs III.2 Photodissociation of PAHs Anharmonicity = different from a parabolic potential well always the case for dissociation mode frequency vary with internal energy much more difficult to find the frequencies, and to compute VDOS from frequencies VDOS directly accessible from molecular dynamics simulations In practice, astronomers still use the harmonic VDOS 27

28 III. Evolution of interstellar PAHs III.2 Photodissociation of PAHs General trends Ekern et al Experimental results: More compact => more photostable Bigger => more photostable Loss E0 [ev] Adiss [s-1] H(even) e17 H(odd) e17 C e15 C e17 C e18 (Joblin et al. 2013, Léger et al. 1989) 28

29 III. Evolution of interstellar PAHs III.3 Reactivity of PAHs Reactivity with hydrogen Montillaud et al No data on neutral Only qualitative data on super-hydrogenated (more H-atoms than in normal PAHs) Measurement/calculation for dehydrogenated only for small PAHs Fast reactions with H No or slow reactions with H2 29

30 III. Evolution of interstellar PAHs III.3 Reactivity of PAHs Reactivity with other atoms Le Page et al. (1999) Fast reactions for normal and dehydrogenated cations with O and N => it is likely that a more general population than strict PAHs populate the ISM Tentative family identification, using NASA Ames database, but not convincing (so far...) 30

31 III. Evolution of interstellar PAHs III.3 Reactivity of PAHs Physisorption of bigger atoms (Si, Fe,...) Joalland et al. (2009) Thermodynamically favorable Weak spectral signature => not impossible but difficult E0 = 1.5 ev => easily photodissociated 31

32 III. Evolution of interstellar PAHs III.3 Reactivity of PAHs Aggregation of PAHs (C24H12)11 (C96H24)4 32

33 III. Evolution of interstellar PAHs III.3 Reactivity of PAHs Aggregation of PAHs Formation of PAH clusters: dilution of the excess energy within the numerous vibrational degrees of freedom (see movie, credit: Mathias Rapacioli, IRSAMC, Toulouse, France) Destruction of PAH clusters: analogous to PAH photodissociation difficulty to deal with all the degrees of freedom => rigid molecules approximation? Spectroscopy of PAH clusters: are they responsible for continuum emission in the mid-ir? 33

34 III. Evolution of interstellar PAHs III.4 Formation of PAHs Asymptotic Giant Branch Stars (AGB) Cherchneff et al IRC Shocks local dense and cold points condensation of some dust grains acceleration of grains by radiation pressure wind acceleration by dragging coupling stellar wind dynamics dust formation 34 Model of AGB circumstellar material

35 III. Evolution of interstellar PAHs III.4 Formation of PAHs Most classical model: the HACA mechanism in circumstellar regions of AGB stars (H-abstraction, C-addition) Frenklach et al Consistent with observations? CRL 2688 IRC IC 418 AGB stars No UV Planetary nebula Intense UV RF Protoplanetary nebula: some UV 35

36 IV. From processes to interstellar H2 formation rate I. From Unidentified to Aromatic Infrared Bands II. General properties of chemical PAHs III. Evolution of interstellar PAHs IV. Influence of PAHs on ISM evolution IV.1 Photoelectric heating IV.2 Extinction curve IV.3 Formation of H2 V. Summary 36

37 IV. Influence of PAHs on the ISM evolution IV.1 Photoelectric heating PDR model for NGC

38 IV. Influence of PAHs on the ISM evolution IV.2 Extinction curve 38

39 IV. Influence of PAHs on the ISM evolution IV.3 Formation of H2 Rauls & Hornekaer

40 V. Summary Astronomical PAHs are defined as the carriers of the AIBs Chemical PAHs are one good model for astronomical PAHs no individual spectroscopic identification but identification of 3 large aromatic molecules: C6H6, C60, C70 Many theoretical and experimental studies provide molecular data, but still not enough Theoretical spectral database of PAHs (Cagliari, Italy) NASA Ames PAH IR Spectroscopic Database Understanding is limited by: - confusion in mid-ir spectroscopy - scarcity in UV-visible spectroscopy - the huge number of parameters (e.g. molecule size, structure, charge,...) - the absence of emission in UV-poor environments - the large size of interstellar PAHs => difficult (if not impossible) for both theoretical and experimental studies Deep coupling between PAH and ISM evolutions through photoelectric heating, extinction curve and H2 formation In the future, progresses should be achieved from on-going laboratory and theoretical studies for large species + spectroscopic observations (DIBs, AIBs, far-ir rotation lines) + coupling40 with chemistry of other species (H2 formation/excitation, small hydrocarbons, evaporating VSGs)

41 VI. Some bibliographic references PAH review: - Tielens, A. G. G. M., The Physics and Chemistry of the Interstellar Medium, Cambridge University Press (2005) - Tielens, A. G. G. M., ARA&A, (2008) Chemical evolution: - Le Page et al., ApJSS, 132: , (2001) - Le Page et al., ApJ, 584: (2003) - Theoretical chemistry aspects (mainly for astronomers) - Malloci et al., A&A, 462: , (2007) - Léger et al. A&A, 213: (1989) - Experimental chemistry aspects - Modelling aspects 41