AstrochemistryLecture 7, Observational astrochemistry

Jorma Harju

Department of Physics

Friday, March 1, 2013, 12:15-13:45, Lecture room D117Course web page

http://www.courses.physics.helsinki.fi/astro/astrokemia

Historical ISM studies (1)

Early 1900s Narrow, static “K-line” ofCa+ in absorption in the spectrum ofthe spectroscopic binary δ Ori (1904Hartmann), later Na “D-lines”, evi-dence for diffuse ISM gas

Dark patches on photographs (1919Barnard) - dust clouds obscuring starlight

Historical ISM studies (2)

1919 Diffuse interstellar bands(DIBs) discovered (Mary Lea Heger)

1930 The presence of the interestellar dust confirmed (RobertTrumpler: diameter and photometric distance of stellar associations)

Historical ISM studies (3)1937-41 The first interstellarmolecules: CH, CN, CH+ (Swings &Rosenfeld, McKellar, Douglas & Herzberg)

Absorption lines in the visual (elec-tronic transitions) in the spectra ofbright stars

Molecular spectroscopy started to de-velop (Gerhard Herzberg)

1950s Neutral hydrogen (HI) 21-cmline

Predicted in 1944 by van de Hulst

Detected in 1951 Ewen & Purcell

The distribution and kinematics of theneutral gas in the Milky Way

Spectroscopy of molecules in space

I Information of the existence of molecules and their abundancesin space can be derived from observations of their emission orabsorption lines

I Molecules interact with radiation through transitions betweentheir electronic, vibrational, and rotational states

I Molecules thrive predominantly in regions obscured from lightand UV radiation

I Therefore most molecules have been mainly detected inrotational or vibrational lines

I Large part of the molecular gas is cool where only low-lyingenergy levels, i.e. rotational levels are excited

Rotation spectra

I Linear rigid rotor (e.g. CO, HCN, HC3N,...)

E(J) = BJ(J + 1)

J = 0, 1, 2, ...

B = ~2I , I is the moment of inertia,

Rotational constant B is large for light molecules

Selection rules: electric dipole ∆J = ±1, quadrupole ∆J = ±2

I Symmetric top (e.g. NH3, CH3CCH,...)

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

Vibration spectra

Vibrations of diatomics in the Morse potential

E(ν) = (ν +12

)~ω − (ν +12

)χe~ω

ν = 0, 1, 2, ...ω =

√kµ

, µ = m1m2m1+m2

is the reduced mass,and k is the force constant for the bond,χe = ~ω

4Deis the anharmonicity constant, De

is the depth of the potential energy mini-mum.Selection rule: ∆ν = ±1

Molecular excitation (1)

I Usually the molecules are in their vibrational ground statesin dense clouds.

I Several low-J rotational levels of heavy molecules, can beexcited in molecular clouds (true for CO, CS, andespecially for long carbon chains like HC7N)

I Light hydrides are usually in their ground rotational states(H2, CH, OH, ...) because of the small moment of inertia

Molecular excitation (2)

I For molecules with non-zero total electronic angularmomentum L and spin S (radicals OH, CH, CN, ...) therotational levels are, however, split owing to the couplingbetween L and S (or actually their projections on themolecular axis, Λ and Σ).

I Angular momentum J = R + Λ + Σ, or J = R + |Λ− Σ|, Rend-over-end rotationTotal angular momentum with the nuclear spin I:F = J + I, ..., J − I

I The Λ-doubling transitions of these radicals can beobserved at low radio frequencies. Rotational transitionslie in the far-infrared.

Molecular excitation (3)

The inversion transition of NH3 (λ ∼ 1.2 cm) is another casewhere the molecular structure helps the detection. (The lowestrotational transition is at λ ∼ 0.5 mm.)

Limitations of observational spectroscopy (1)

Atmosphere.Atmospheric trans-mission affectedmainly by the absorp-tion lines of H2O, CO2,O2, O3, CH4 (infraredand submillimetre)

Transparency is good at most radio frequencies λ > 1 mmBecomes poorer towards shorter wavelengthsThe atmosphere becomes almost totally opaque in the far-IR (λ < 0.3)

Limitations of observational spectroscopy (2)

Telescopes. The progress in astrochemisty has beendependent on the development of radio telescopes (besidestheoretical and laboratory work)

Needed: large collecting area, high surface accuracy, high-altitude site toalleviate atmospheric absorptionSubmillimeter interferometry developing quickly

Limitations of observational spectroscopy (3)

Receivers. Much progress inthe receiver techniques since 1960s- cooled, low-noise heterodyne re-ceivers (amplitude & phase conserved)operating at THz regimeVery wide band, high spectral resolu-tion correlating and Fourier transfromspectrometers available

Radiative transfer. Converting line intensities to column densities isusually trickyAdvanced programs for the solution of the radiative transfer problemavailable

Early times of radio spectroscopy (1)

I

1963 OH Ground-state Λ-doubling line(λ = 18 cm) in absorption towardsthe supernova remnant Cas A, there-after emission lines in molecular cloudsand in dusty envelopes of dying stars(CSEs). The first Galactic OH maserdetected in 1965.

I 1968 NH3 J,K = (1,1) inversion line (λ = 1.3 cm) towardsSagittarius B2 (Sgr B2)(microwave spectroscopy in the laboratory: C.H. Townes)

I 1969 Rotational transitions of H2O (strong maser line at 1.3 cm )and H2CO (6 cm, the first organic molecule)

Early times of radio spectroscopy (2)

I 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

I 1970 U89.2 GHz: HCO+ - the first molecular ionIdentified by Klemperer & Herbst (ion-molecule chemistry)

Other molecules detected at that time: CH3OH, HC3N, HCN,HCOOH, HNC (U90.7 GHz), ...

I 1974 U93.2 GHZ: N2H+ - the second molecular ion (cations detected to

date: HCO+, N2H+, H+3 , HCNH+, HCS+, HOCO+, H2O+, H3O+, H2COH+, H2Cl+, SH+ - protonated

atoms and closed shell molecules)

I late 1970s more radicals (CCH, C3N, C4H,), long carbon chains(HC5N, HC7N, HC9N), and more complex organic molecules(dimethyl ether CH3OCH3, vinyl cyanide CH2CHCN, ethyl alcolholCH3CH2OH, etc.)

CO - the most common molecule after H2H2 is usually in its ground state in molecular clouds - notdetectableCO is the most commonly used tracer of molecular gas,[CO]/[H2] ∼ 10−4)

Milky Way (photograph) Milky Way (photograph + CO)

Discovery of H2

1970 H2, HD (Aerobee-150rocket, Carruthers)1975 Copernicus satellite-FUV absorption in diffuseclouds-Lyman band B1Σ+

u → X1Σ+g (1100Å)

-Werner band C1Πu → X1Σ+g (1010Å)

-H2/HI ∼ 1-Thereafter e.g. Lyman absorp-tion with Far Ultraviolet Spec-troscopic Explorer (FUSE, 1999-2008, H/D etc.) towards diffusecloudsrotational-vibrational transitionsemitting in the infrared (warmclouds)

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Electronic, Vibrational, and Rotational Energy Levels in the Hydrogen Molecule

Energy spectrum of H2

The potential energy, V , of the sys-tem has a minimum at an internu-clear distance Re. Depends stronglyon the electronic state.Dissociation energy Ed = 4.5 eV

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Electronic, Vibrational, and Rotational Energy Levels in the Hydrogen Molecule

excited electronic state B1Σ+g

-

electronic ground state X 1Σ+g

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vibrational states XXXz

rotational statesAAAK

Vibration and rotation of H2 (1)

orto-H2

para-H2

XXXXXXXzXXXXXXXzWolfgang Pauli

Vibration and rotation of H2 (2)

I H2 is homonuclear - no electric dipole momentI Pure rotational transitions ∆J = ±2 caused by interaction

between the electric quadrupole moment with radiationI Because H2 is a light molecule, the rotational levels have large

separations in energyVibration-rotation transitions observable from shock-heated gas

I H nuclei are fermions (nuclear spin I = 12 ) - total wave function Ψ

is antisymmetric (the sign changes in a permutation of nuclei)⇒ortho-H2 (s) can only be in odd (a) rotational levels J = 1, 3, 5...para-H2 (a)can only be in even (s) rotational levels J = 0, 2, 4, ...

I HD has a small permanent dipole moment

Search for molecules in space

I Since 1970s about 180 intestellar molecules have been found

-mainly thanks to the development of radio telescopes but alsospace telescopes operating in the far-infrared (ISO, Spitzer,Herschel) have contributed significantly

I Seehttp://www.astrochymist.org/astrochymist_ism.html

-a chronological list of detections with references, method andtarget indicated

I 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 anastronomical target

Famous objects 1 - Sagittarius B2

Sagittarius B2 - a giant molecular cloud 120 pc from the centre of theGalaxyMost detections of complex organic molecules made towards Sgr B2N - the “Large Molecule Heimat”

Search for prebiotic molecules (1)

I 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 (CH2OHCHO)formamide (NH2CHO), acetamide (CH3CONH2)

I Glycine NH2CH2COOH, the simplest amino acid: tentativedetection towards three sources (Kuan et al. 2005)disputed

I Glycine formation possible on grain surfaces, see Lecture5 by Julien, to be detectable it should be released into thegas phase

I Detection amino acids probably requires carefulpreparatory observations of chemically related species(e.g., HCOOH, CH3COOH, NH2CH2CN,...) and highspatial & spectral resolution observations

Search for prebiotic molecules (1)

I Problems. Complex molecules have numerous weak lineswhich are difficult to disentangle amongst the “weed”caused by other organic molecules

I Several lines at different frequency ranges need to beidentified

I A single “missing” line (a relatively strong line which shouldbe there) can ruin the detection

I Detection should be supported by modelling (a sourcemodel used to calculate the expected relative intensities ofvarious line components)

I Precursors (chemically related species) should be present

Famous objects 2 - Orion KL

Orion Kleinman-Low Nebula - a region of massive star formationabout 500 pc awayThe hot molecular core has been target for numerous spectral scans

Spectral scans (Orion KL)

left OVRO (1987 ∼ 300 GHz), right Herschel (2010, ∼ 500&1000 GHz)

Famous objects 3 - IRC+10216

IRC+10216 or CW Leonis - a carbon star some 120-150 pc away withmassive envelope of gas and dustMost species in CSEs (∼ 50) are detected towards this object

Circumstellar envelopes (1)

I A star can loose up to 90% of its mass through stellarwinds and outflows

I

First detections of molecules:CO 1971, SiO 1975 towardsIRC+10216, rich in carbonmolecules

Circumstellar envelopes (2)

I

Mira variables are oxygen-rich asymptotic giant branch(AGB) stars -all carbonlocked up in CO, oxygen-containing species: SiO,H2O, OH

I

Planetary nebulae are moredeveloped objects. Moleculargas found in the outer parts(AV > 1), e.g. NGC7027

Molecules in circumstellar envelopes (1)

I Several metal-bearing, closed-shell molecules detected early incircumstellar envelopes: NaCl, AlCl, KCl, AlF, NaCN, ...Cernicharo & Guélin et co. since mid-1980’sThese molecules form in the atmosphere of the star (T ∼ 2300K), and are condensed on dust grains further out in the coolenvelope. The observed distribution concentrates on the centralstar.

I Tens of molecules and radicals have been detected in a largeshell-like structure: CN, HNC, C4H, SiC2, etc.

Molecules in circumstellar envelopes (2)

I Non-polar species like C2, C3, C4, C5 detected through their IRand FIR bands in circumstellar envelopes

I Anions C8H−, C4H−, CN−, C3N−, C5N− detected in the mmspectrum of IRC+10216 (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)

Ring molecules

I Five ring molecules have been detected with certainty inmolecular clouds or in circumstellar envelopes: SiC2,c-C3H, C3H2, c-C2H4O, c-C3H2O

I

IR band of Benzene, C6H6, de-tected in the protoplanetary nebulaCRL 618 (Cernicharo 2003)

I Fullerenes C60 and C70 detected towards a planetarynebula (Cami et al. 2010) - carriers of some DIBs in themid IR

Diffuse Interstellar Bands (DIBs)I So far mainly small organic molecules identified. ISM contains probably

also large moleculesI About 300 absorption bands (Diffuse Interstellar Bands, UV→IR),

strength proportional to the interstellar extinction, probably caused bycomplex carbon combounds

I Candidates: PAHs (polycyclic aromatic hydrocarbons, merged benzenerings), long carbon chains (12-18 C), cyclic molecules and fullerenes(pure carbon spheres or tubes)See Lecture 4 by Julien

Cool stars (1)

I Some two- and three-atomic molecules found in theatmospheres of stars, e.g. the solar photosphere (T ∼ 5800 K).

I Sunspots (T ∼ 3200 K): penumbra OH, umbra H2O

I In stars with the surface temperature below 4000 K strong andbroad molecular absorption features can be seen.

I The lines are either vibrational lines (CO, H2O, HCN) or causedby electronic transitions (TiO, VO, ZrO, FeH).

I The relative abundances correspond to the thermodynamicequilibrium (unlike in molecular clouds)

Cool stars (2)

I C and O form CO (stable when T ≤ 3000 K)

I M-type stars: oxygen left over from the formation of CO. Lines ofH2O and TiO visible in the spectra.

I C-type stars: carbon left over. Carbon compounds, e.g., HCN,C3,H2C2, microdiamonds?

I S-type stars, C/O ∼ 1

I Molecular lines deform the stellar blackbody spectrum, and theyhave to be taken into account in atmospheric models.

I Brown dwarf (M < 0.08M�, T < 1500 K) atmospheres havelines of H2O and CH4.Possibly below this temperature CH4 replaces CO as theprincipal reservoir of carbon

CometsI Mosty water ice. Formed on the outskirts of the solar systemI Evaporating gas can be studied spectroscopically when a comet

approaches the SunI The gas halo of Hale-Bopp:

simple neutral molecules: H2O, HDO, CO, CO2, H2S, SO, SO2, OCS,CS, NaCl, NH3

radicals: OH, CN, NH2, NH, C3, C2

hydrocarbons, nitriles, amides, etc: HCN, DCN, CH3CN, HNC, HC3N,HNCO, C2H2, C2H6 CH4, NH2CHOmolecular cations: H2O+, H3O+, HCO+, CO+ (photoionization + gasphase chemical reactions)

Interstellar dust (1)

I Composition:The observed depletion of some species in the gaseous ISM(with respect to the cosmic abundances) imply that the solidphase, so called dust, contains e.g. the following species: C, O,Mg, Si, Fe

I From this one can also deduce that the mass of the dust is about1% of the gas mass in the ISM

I

Size distribution: from the extinctioncurveExtinction in the visual and infrared can beexplained by “large” dust particles (radiusa ≥ λ/2π ∼ 0.1µm), whereas the strongincrease in the UV requires very small par-ticles a ≤ λ/2π ∼ 0.016µm).

Interstellar dust (2)

I 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)

I From the chemistry point of view the most importantcharacteristic is the total surface area of dust per H atom:ngσg/nH

I The changes of the parameter RV ≡ AV/E(B − V ) is believed toreflect different size distributions of dust.

I The bump at 2175 Å in the extinction curve is probably causedby aromatic carbon compounds or graphitic dust.

Spectroscopy of dust

I Infrared absorption bands originating in diffuse cloud whenobserved againts bright background stars:3.4 µm (C-H stretching, aliphatic hydrocarbons), 9.7 µm ja 18µm (Si-O streching, O-Si-O bending, amorphous silicates)

I Absorption bands from the ice mantles of dust particles in darkclouds:3.1 ja 6.0 µm (H2O), 4.67 µm (CO), 4.27 ja 15.2 µm (CO2), 3.54ja 9.75 µm (CH3OH), 2.97 µm (NH3), 7.68 µm (CH4), 5.81 µm(H2CO), 4.62 µm (XCN-)

I PAH emission bands from warm dust heated by stellar radiation:3.3, 6.2, 7.7, 8.6, 11.3 µm

Summary

I Astrochemical research is driven by observations.(In the beginning astronomers did not care about chemistry.)

I The discovery of simple molecules in space led to developmentof a chemical theory called ion-molecule chemistry (Herbst &Klemperer) which dominates in the gas phase.

I The importance of gas-grain interaction has becomeincreasingly evident through observations of very cold cores(depletion) and star forming regions (shocks). Astrochemicalmodels need to account for both gas-phase and grain-surfacereactions (see Lecture5 by Julien)(For theorists this has been clear for a long time.)

I At present astrochemical research is a joint effort of theorists,observers, and laboratory workers.