Astrochemistry. Lecture 7, Observational astrochemistry. Jorma Harju. Department of Physics

Transcription

1 Astrochemistry Lecture 7, Observational astrochemistry Jorma Harju Department of Physics Friday, March 1, 2013, 12:15-13:45, Lecture room D117 Course web page

2 Historical ISM studies (1) Early 1900s Narrow, static K-line of Ca + in absorption in the spectrum of the spectroscopic binary δ Ori (1904 Hartmann), later Na D-lines, evidence for diffuse ISM gas Dark patches on photographs (1919 Barnard) - dust clouds obscuring star light

3 Historical ISM studies (2) 1919 Diffuse interstellar bands (DIBs) discovered (Mary Lea Heger) 1930 The presence of the interestellar dust confirmed (Robert Trumpler: diameter and photometric distance of stellar associations)

4 Historical ISM studies (3) The first interstellar molecules: CH, CN, CH + (Swings & Rosenfeld, McKellar, Douglas & Herzberg) Absorption lines in the visual (electronic transitions) in the spectra of bright stars Molecular spectroscopy started to develop (Gerhard Herzberg) 1950s Neutral hydrogen (HI) 21-cm line Predicted in 1944 by van de Hulst Detected in 1951 Ewen & Purcell The distribution and kinematics of the neutral gas in the Milky Way

5 Spectroscopy of molecules in space Information of the existence of molecules and their abundances in space can be derived from observations of their emission or absorption lines Molecules interact with radiation through transitions between their electronic, vibrational, and rotational states Molecules thrive predominantly in regions obscured from light and UV radiation Therefore most molecules have been mainly detected in rotational or vibrational lines Large part of the molecular gas is cool where only low-lying energy levels, i.e. rotational levels are excited

6 Rotation spectra Linear rigid rotor (e.g. CO, HCN, HC 3 N,...) E(J) = BJ(J + 1) J = 0, 1, 2,... B =, I is the moment of inertia, 2I Rotational constant B is large for light molecules Selection rules: electric dipole J = ±1, quadrupole J = ±2 Symmetric top (e.g. NH 3, CH 3 CCH,...) E(J, K ) = BJ(J + 1) + (A B)K 2 J = 0, 1, 2,..., K = 0, ±1, ±2,..., ±J B = 2I, A = 2I Selection rules J = ±1, K = 0

7 Vibration spectra Vibrations of diatomics in the Morse potential E(ν) = (ν ) ω (ν )χ e ω ν = 0, 1, 2,... k ω = µ m 1m 2 m 1 +m 2 is the reduced mass, and k is the force constant for the bond, χ e = ω 4D e is the anharmonicity constant, D e is the depth of the potential energy minimum. Selection rule: ν = ±1

8 Molecular excitation (1) Usually the molecules are in their vibrational ground states in dense clouds. Several low-j rotational levels of heavy molecules, can be excited in molecular clouds (true for CO, CS, and especially for long carbon chains like HC 7 N) Light hydrides are usually in their ground rotational states (H 2, CH, OH,...) because of the small moment of inertia

9 Molecular excitation (2) For molecules with non-zero total electronic angular momentum L and spin S (radicals OH, CH, CN,...) the rotational levels are, however, split owing to the coupling between L and S (or actually their projections on the molecular axis, Λ and Σ). Angular momentum J = R + Λ + Σ, or J = R + Λ Σ, R end-over-end rotation Total angular momentum with the nuclear spin I: F = J + I,..., J I The Λ-doubling transitions of these radicals can be observed at low radio frequencies. Rotational transitions lie in the far-infrared.

10 Molecular excitation (3) The inversion transition of NH 3 (λ 1.2 cm) is another case where the molecular structure helps the detection. (The lowest rotational transition is at λ 0.5 mm.)

11 Limitations of observational spectroscopy (1) Atmosphere. Atmospheric transmission affected mainly by the absorption lines of H 2 O, CO 2, O 2, O 3, CH 4 (infrared and submillimetre) Transparency is good at most radio frequencies λ > 1 mm Becomes poorer towards shorter wavelengths The atmosphere becomes almost totally opaque in the far-ir (λ < 0.3)

12 Limitations of observational spectroscopy (2) Telescopes. The progress in astrochemisty has been dependent on the development of radio telescopes (besides theoretical and laboratory work) Needed: large collecting area, high surface accuracy, high-altitude site to alleviate atmospheric absorption Submillimeter interferometry developing quickly

13 Limitations of observational spectroscopy (3) Receivers. Much progress in the receiver techniques since 1960s - cooled, low-noise heterodyne receivers (amplitude & phase conserved) operating at THz regime Very wide band, high spectral resolution correlating and Fourier transfrom spectrometers available Radiative transfer. Converting line intensities to column densities is usually tricky Advanced programs for the solution of the radiative transfer problem available

14 Early times of radio spectroscopy (1) 1963 OH Ground-state Λ-doubling line (λ = 18 cm) in absorption towards the supernova remnant Cas A, thereafter emission lines in molecular clouds and in dusty envelopes of dying stars (CSEs). The first Galactic OH maser detected in NH 3 J, K = (1, 1) inversion line (λ = 1.3 cm) towards Sagittarius B2 (Sgr B2) (microwave spectroscopy in the laboratory: C.H. Townes) 1969 Rotational transitions of H 2 O (strong maser line at 1.3 cm ) and H 2 CO (6 cm, the first organic molecule)

15 Early times of radio spectroscopy (2) 1970 CO J = 1 0 (λ = 2.6 mm) Penzias, Wilson & Jefferts, 12-m NRAO Kitt Peak telescope -CO is the most stable molecule with non-zero dipole moment 1970 U89.2 GHz: HCO + - the first molecular ion Identified by Klemperer & Herbst (ion-molecule chemistry) Other molecules detected at that time: CH 3 OH, HC 3 N, HCN, HCOOH, HNC (U90.7 GHz), U93.2 GHZ: N 2 H + - the second molecular ion (cations detected to date: HCO +, N 2 H +, H + 3, HCNH+, HCS +, HOCO +, H 2 O +, H 3 O +, H 2 COH +, H 2 Cl +, SH + - protonated atoms and closed shell molecules) late 1970s more radicals (CCH, C 3 N, C 4 H,), long carbon chains (HC 5N, HC 7N, HC 9 N), and more complex organic molecules (dimethyl ether CH 3 OCH 3, vinyl cyanide CH 2 CHCN, ethyl alcolhol CH 3 CH 2 OH, etc.)

16 CO - the most common molecule after H 2 H 2 is usually in its ground state in molecular clouds - not detectable CO is the most commonly used tracer of molecular gas, [CO]/[H 2 ] 10 4 ) Milky Way (photograph) Milky Way (photograph + CO)

17 Discovery of H H 2, HD (Aerobee-150 rocket, Carruthers) 1975 Copernicus satellite -FUV absorption in diffuse clouds -Lyman band B 1 Σ + u X 1 Σ + g (1100Å) -Werner band C 1 Π u X 1 Σ + g (1010Å) -H 2 /HI 1 -Thereafter e.g. Lyman absorption with Far Ultraviolet Spectroscopic Explorer (FUSE, , H/D etc.) towards diffuse clouds rotational-vibrational transitions emitting in the infrared (warm clouds) Electronic, Vibrational, and Rotational Energy Levels in the Hydrogen Molecule adapted from: M. Karpus and R. N. Porter, Atoms & Molecules: An Introduction for Students of Physical Chemistry (Benjamin/Cummings, 1970) p. 450

18 Energy spectrum of H 2 Electronic, Vibrational, and Rotational Energy Levels in the Hydrogen Molecule excited electronic state B 1 Σ + g The potential energy, V, of the system has a minimum at an internuclear distance R e. Depends strongly on the electronic state. Dissociation energy E d = 4.5 ev vibrational states 3 electronic ground state X 1 Σ + g rotational states adapted from: M. Karpus and R. N. Porter, Atoms & Molecules: An Introduction for Students of Physical Chemistry (Benjamin/Cummings, 1970) p. 450

19 Vibration and rotation of H 2 (1) Wolfgang Pauli orto-h 2 para-h 2 3 3

20 Vibration and rotation of H 2 (2) H 2 is homonuclear - no electric dipole moment Pure rotational transitions J = ±2 caused by interaction between the electric quadrupole moment with radiation Because H 2 is a light molecule, the rotational levels have large separations in energy Vibration-rotation transitions observable from shock-heated gas H nuclei are fermions (nuclear spin I = 1 2 ) - total wave function Ψ is antisymmetric (the sign changes in a permutation of nuclei) ortho-h 2 (s) can only be in odd (a) rotational levels J = 1, 3, 5... para-h 2 (a)can only be in even (s) rotational levels J = 0, 2, 4,... HD has a small permanent dipole moment

21 Search for molecules in space Since 1970s about 180 intestellar molecules have been found -mainly thanks to the development of radio telescopes but also space telescopes operating in the far-infrared (ISO, Spitzer, Herschel) have contributed significantly See -a chronological list of detections with references, method and target indicated New molecules are constantly searched for. The usual procedure is to synthesize a molecule in a laboratory, measure the spectrum, and try to observe spectral lines in an astronomical target

22 Famous objects 1 - Sagittarius B2 Sagittarius B2 - a giant molecular cloud 120 pc from the centre of the Galaxy Most detections of complex organic molecules made towards Sgr B2 N - the Large Molecule Heimat

23 Search for prebiotic molecules (1) A large number of prebiotic molecules detected in Sgr B2 (and in nearby hot molecular cores): formaldehyde, methanol, ethanol, vinyl alcohol, acetone,... aldehydes, e.g., glycolaldehyde (CH 2 OHCHO) formamide (NH 2 CHO), acetamide (CH 3 CONH 2 ) Glycine NH 2 CH 2 COOH, the simplest amino acid: tentative detection towards three sources (Kuan et al. 2005) disputed Glycine formation possible on grain surfaces, see Lecture 5 by Julien, to be detectable it should be released into the gas phase Detection amino acids probably requires careful preparatory observations of chemically related species (e.g., HCOOH, CH 3 COOH, NH 2 CH 2 CN,...) and high spatial & spectral resolution observations

24 Search for prebiotic molecules (1) Problems. Complex molecules have numerous weak lines which are difficult to disentangle amongst the weed caused by other organic molecules Several lines at different frequency ranges need to be identified A single missing line (a relatively strong line which should be there) can ruin the detection Detection should be supported by modelling (a source model used to calculate the expected relative intensities of various line components) Precursors (chemically related species) should be present

25 Famous objects 2 - Orion KL Orion Kleinman-Low Nebula - a region of massive star formation about 500 pc away The hot molecular core has been target for numerous spectral scans

26 Spectral scans (Orion KL) left OVRO ( GHz), right Herschel (2010, 500&1000 GHz)

27 Famous objects 3 - IRC IRC or CW Leonis - a carbon star some pc away with massive envelope of gas and dust Most species in CSEs ( 50) are detected towards this object

28 Circumstellar envelopes (1) A star can loose up to 90% of its mass through stellar winds and outflows First detections of molecules: CO 1971, SiO 1975 towards IRC+10216, rich in carbon molecules

29 Circumstellar envelopes (2) Mira variables are oxygenrich asymptotic giant branch (AGB) stars -all carbon locked up in CO, oxygencontaining species: SiO, H 2 O, OH Planetary nebulae are more developed objects. Molecular gas found in the outer parts (A V > 1), e.g. NGC7027

30 Molecules in circumstellar envelopes (1) Several metal-bearing, closed-shell molecules detected early in circumstellar envelopes: NaCl, AlCl, KCl, AlF, NaCN,... Cernicharo & Guélin et co. since mid-1980 s These molecules form in the atmosphere of the star (T 2300 K), and are condensed on dust grains further out in the cool envelope. The observed distribution concentrates on the central star. Tens of molecules and radicals have been detected in a large shell-like structure: CN, HNC, C 4 H, SiC 2, etc.

31 Molecules in circumstellar envelopes (2) Non-polar species like C 2, C 3, C 4, C 5 detected through their IR and FIR bands in circumstellar envelopes Anions C 8 H, C 4 H, CN, C 3 N, C 5 N detected in the mm spectrum of IRC (Cernicharo et al. 2008) -radiative association between a carbon chain and an electron -theoretically predicted in early 1980s (Herbst) -laboratory spectroscopy needed to characterize the spectrum (McCarthy et al. 2006)

32 Ring molecules Five ring molecules have been detected with certainty in molecular clouds or in circumstellar envelopes: SiC 2, c-c 3 H, C 3 H 2, c-c 2 H 4 O, c-c 3 H 2 O IR band of Benzene, C 6 H 6, detected in the protoplanetary nebula CRL 618 (Cernicharo 2003) Fullerenes C 60 and C 70 detected towards a planetary nebula (Cami et al. 2010) - carriers of some DIBs in the mid IR

33 Diffuse Interstellar Bands (DIBs) So far mainly small organic molecules identified. ISM contains probably also large molecules About 300 absorption bands (Diffuse Interstellar Bands, UV IR), strength proportional to the interstellar extinction, probably caused by complex carbon combounds Candidates: PAHs (polycyclic aromatic hydrocarbons, merged benzene rings), long carbon chains (12-18 C), cyclic molecules and fullerenes (pure carbon spheres or tubes) See Lecture 4 by Julien

34 Cool stars (1) Some two- and three-atomic molecules found in the atmospheres of stars, e.g. the solar photosphere (T 5800 K). Sunspots (T 3200 K): penumbra OH, umbra H 2 O In stars with the surface temperature below 4000 K strong and broad molecular absorption features can be seen. The lines are either vibrational lines (CO, H 2 O, HCN) or caused by electronic transitions (TiO, VO, ZrO, FeH). The relative abundances correspond to the thermodynamic equilibrium (unlike in molecular clouds)

35 Cool stars (2) C and O form CO (stable when T 3000 K) M-type stars: oxygen left over from the formation of CO. Lines of H 2 O and TiO visible in the spectra. C-type stars: carbon left over. Carbon compounds, e.g., HCN, C 3,H 2 C 2, microdiamonds? S-type stars, C/O 1 Molecular lines deform the stellar blackbody spectrum, and they have to be taken into account in atmospheric models. Brown dwarf (M < 0.08M, T < 1500 K) atmospheres have lines of H 2 O and CH 4. Possibly below this temperature CH 4 replaces CO as the principal reservoir of carbon

36 Comets Mosty water ice. Formed on the outskirts of the solar system Evaporating gas can be studied spectroscopically when a comet approaches the Sun The gas halo of Hale-Bopp: simple neutral molecules: H 2 O, HDO, CO, CO 2, H 2 S, SO, SO 2, OCS, CS, NaCl, NH 3 radicals: OH, CN, NH 2, NH, C 3, C 2 hydrocarbons, nitriles, amides, etc: HCN, DCN, CH 3 CN, HNC, HC 3 N, HNCO, C 2 H 2, C 2 H 6 CH 4, NH 2 CHO molecular cations: H 2 O +, H 3 O +, HCO +, CO + (photoionization + gas phase chemical reactions)

37 Interstellar dust (1) Composition: The observed depletion of some species in the gaseous ISM (with respect to the cosmic abundances) imply that the solid phase, so called dust, contains e.g. the following species: C, O, Mg, Si, Fe From this one can also deduce that the mass of the dust is about 1% of the gas mass in the ISM Size distribution: from the extinction curve Extinction in the visual and infrared can be explained by large dust particles (radius a λ/2π 0.1 µm), whereas the strong increase in the UV requires very small particles a λ/2π µm).

38 Interstellar dust (2) Usually the size distribution is assumed to follow a power law: dn/da a 3.5, 50 Å a 0.25µm (MRN: Mathis, Rumpl & Nordsieck 1977) From the chemistry point of view the most important characteristic is the total surface area of dust per H atom: n g σ g /n H The changes of the parameter R V A V /E(B V ) is believed to reflect different size distributions of dust. The bump at 2175 Å in the extinction curve is probably caused by aromatic carbon compounds or graphitic dust.

39 Spectroscopy of dust Infrared absorption bands originating in diffuse cloud when observed againts bright background stars: 3.4 µm (C-H stretching, aliphatic hydrocarbons), 9.7 µm ja 18 µm (Si-O streching, O-Si-O bending, amorphous silicates) Absorption bands from the ice mantles of dust particles in dark clouds: 3.1 ja 6.0 µm (H 2 O), 4.67 µm (CO), 4.27 ja 15.2 µm (CO 2 ), 3.54 ja 9.75 µm (CH 3 OH), 2.97 µm (NH 3 ), 7.68 µm (CH 4 ), 5.81 µm (H 2 CO), 4.62 µm (XCN-) PAH emission bands from warm dust heated by stellar radiation: 3.3, 6.2, 7.7, 8.6, 11.3 µm

40 Summary Astrochemical research is driven by observations. (In the beginning astronomers did not care about chemistry.) The discovery of simple molecules in space led to development of a chemical theory called ion-molecule chemistry (Herbst & Klemperer) which dominates in the gas phase. The importance of gas-grain interaction has become increasingly evident through observations of very cold cores (depletion) and star forming regions (shocks). Astrochemical models need to account for both gas-phase and grain-surface reactions (see Lecture5 by Julien) (For theorists this has been clear for a long time.) At present astrochemical research is a joint effort of theorists, observers, and laboratory workers.