Astrochemistry Lecture 7, Observational Astrochemistry

Astrochemistry Lecture 7, Observational astrochemistry

Jorma Harju

Department of Physics

Friday, March 1, 2013, 12:15-13:45, Lecture room D117 Course web page http://www.courses.physics.helsinki.ﬁ/astro/astrokemia 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”, evi- dence for diffuse ISM gas

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

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) Historical ISM studies (3) 1937-41 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 de- velop (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

Spectroscopy of molecules in space

I Information of the existence of molecules and their abundances in space can be derived from observations of their emission or absorption lines

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

I Molecules thrive predominantly in regions obscured from light and UV radiation

I Therefore most molecules have been mainly detected in rotational or vibrational lines

I Large part of the molecular gas is cool where only low-lying energy 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 = ~ , A = ~ 2I|| 2I⊥ Selection rules ∆J = ±1, ∆K = 0 Vibration spectra

Vibrations of diatomics in the Morse potential

1 1 E(ν) = (ν + ) ω − (ν + )χ ω 2 ~ 2 e~ ν = 0, 1, 2, ... q ω = k , µ = m1m2 is the reduced mass, µ m1+m2 and k is the force constant for the bond, ω χe = ~ is the anharmonicity constant, De 4De is the depth of the potential energy minimum. Selection rule: ∆ν = ±1

Molecular excitation (1)

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

I 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 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 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).

I 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

I The Λ-doubling transitions of these radicals can be observed at low radio frequencies. Rotational transitions lie in the far-infrared. Molecular excitation (3)

The inversion transition of NH3 (λ ∼ 1.2 cm) is another case where the molecular structure helps the detection. (The lowest rotational transition is at λ ∼ 0.5 mm.)

Limitations of observational spectroscopy (1)

Atmosphere. Atmospheric trans- mission affected mainly by the absorption lines of H2O, CO2, O2,O3, CH4 (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) 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 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 Early times of radio spectroscopy (1)

1963 OH Ground-state Λ-doubling line (λ = 18 cm) in absorption towards the supernova remnant Cas A, there- I after emission lines in molecular clouds and in dusty envelopes of dying stars (CSEs). The ﬁrst Galactic OH maser detected in 1965.

I 1968NH 3 J, K = (1, 1) inversion line (λ = 1.3 cm) towards Sagittarius 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 ion Identiﬁed 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 ,H3 , 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 alcolhol CH3CH2OH, etc.) CO - the most common molecule after H2 H2 is usually in its ground state in molecular clouds - not detectable CO is the most commonly used tracer of molecular gas, −4 [CO]/[H2] ∼ 10 )

Milky Way (photograph) Milky Way (photograph + CO) Discovery of H2

1970H 2, HD (Aerobee-150 Electronic, Vibrational, and Rotational Energy Levels in the Hydrogen Molecule rocket, Carruthers) 1975 Copernicus satellite -FUV absorption in diffuse clouds -Lyman band B1Σ+ → X 1Σ+ (1100Å)

u g 0 5 4

. p

1 1 + ) 0 7

-Werner band C Π → X Σ (1010Å) 9 u 1

g , s g n i m m u C / n -H /HI ∼ 1 i

2 m a j n e B (

y r t s i

-Thereafter e.g. Lyman absorp- m e h C

l a c i s y h P

tion with Far Ultraviolet Spec- o

s t n e d u t S

r o f

troscopic Explorer (FUSE, 1999- n o i t c u d o r t n I

n A

2008, H/D etc.) towards diffuse : s e l u c e l o M

clouds s m o t A

, r e t r o P

. N

rotational-vibrational transitions . R

d n a

s u p r a K

emitting in the infrared (warm . M

: m o r f

d e t p

clouds) a d a

Energy spectrum of H2

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

1 + - excited electronic state B Σg

0 5 4

. p

) 0 7 9 1

, s g n i m m

The potential energy, V , of the sys- u C / n i m a j n e B (

tem has a minimum at an internu- y r t s i m e h C

clear distance Re. Depends strongly a c i s y h P

f o

s t

on the electronic state. n e d u t S

r o f

n o i t

Dissociation energy E = 4.5 eV c

d u d o r t n I

n A

: s e l u c e l o M

s m o t A

, r e t r o P

. N

. R

X n a vibrational states X s u p

Xz r a K

AK M

A : m o r f

d e t p a

A d a

1 + rotational states electronic ground state X Σg Vibration and rotation of H2 (1)

orto-H2 XXX XXX para-H2 XX Xz Wolfgang Pauli XX XXXz Vibration and rotation of H2 (2)

I H2 is homonuclear - no electric dipole moment I Pure rotational transitions ∆J = ±2 caused by interaction between the electric quadrupole moment with radiation

I Because H2 is a light molecule, the rotational levels have large separations in energy Vibration-rotation transitions observable from shock-heated gas 1 I H nuclei are fermions (nuclear spin I = 2 ) - 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 also space telescopes operating in the far-infrared (ISO, Spitzer, Herschel) have contributed signiﬁcantly

I See http://www.astrochymist.org/astrochymist_ism.html -a chronological list of detections with references, method and target 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 an astronomical target 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”

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: tentative detection towards three sources (Kuan et al. 2005) disputed

I Glycine formation possible on grain surfaces, see Lecture 5 by Julien, to be detectable it should be released into the gas phase

I Detection amino acids probably requires careful preparatory observations of chemically related species (e.g., HCOOH, CH3COOH, NH2CH2CN,...) and high spatial & spectral resolution observations Search for prebiotic molecules (1)

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

I Several lines at different frequency ranges need to be identiﬁed

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

I Detection should be supported by modelling (a source model used to calculate the expected relative intensities of various line components)

I Precursors (chemically related species) should be present 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

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 with massive envelope of gas and dust Most species in CSEs (∼ 50) are detected towards this object

Circumstellar envelopes (1)

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

First detections of molecules: CO 1971, SiO 1975 towards I IRC+10216, rich in carbon molecules

Circumstellar envelopes (2)

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

Planetary nebulae are more developed objects. Molecular I gas found in the outer parts (AV > 1), e.g. NGC7027 Molecules in circumstellar envelopes (1)

I 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.

I Tens of molecules and radicals have been detected in a large shell-like structure: CN, HNC, C4H, SiC2, etc. Molecules in circumstellar envelopes (2)

I Non-polar species like C2,C3,C4,C5 detected through their IR and FIR bands in circumstellar envelopes − − − − − I Anions C8H ,C4H , CN ,C3N ,C5N detected in the mm spectrum 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 in molecular clouds or in circumstellar envelopes: SiC2, c-C3H, C3H2, c-C2H4O, c-C3H2O

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

I Fullerenes C60 and C70 detected towards a planetary nebula (Cami et al. 2010) - carriers of some DIBs in the mid IR Diffuse Interstellar Bands (DIBs)

I So far mainly small organic molecules identiﬁed. ISM contains probably also large molecules I About 300 absorption bands (Diffuse Interstellar Bands, UV→IR), strength proportional to the interstellar extinction, probably caused by complex carbon combounds I 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 Cool stars (1)

I Some two- and three-atomic molecules found in the atmospheres 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 and broad molecular absorption features can be seen.

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

I The relative abundances correspond to the thermodynamic equilibrium (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 of H2O 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 they have to be taken into account in atmospheric models.

I Brown dwarf (M < 0.08M , T < 1500 K) atmospheres have lines of H2O and CH4.

Possibly below this temperature CH4 replaces CO as the principal reservoir of carbon Comets

I Mosty water ice. Formed on the outskirts of the solar system I Evaporating gas can be studied spectroscopically when a comet approaches the Sun

I 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, NH2CHO + + + + molecular cations: H2O ,H3O , HCO , CO (photoionization + gas phase 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 solid phase, 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 about 1% of the gas mass in the ISM

Size distribution: from the extinction curve Extinction in the visual and infrared can be I 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 ∼ 0.016 µm). 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 important characteristic 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 to reﬂect different size distributions of dust.

I The bump at 2175 Å in the extinction curve is probably caused by aromatic carbon compounds or graphitic dust. Spectroscopy of dust

I 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)

I Absorption bands from the ice mantles of dust particles in dark clouds:

3.1 ja 6.0 µm (H2O), 4.67 µm (CO), 4.27 ja 15.2 µm (CO2), 3.54 ja 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 development of a chemical theory called ion-molecule chemistry (Herbst & Klemperer) which dominates in the gas phase.

I 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.)

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