Organic Dust in Space Sun Kwok University of British Columbia, - - PowerPoint PPT Presentation

Organic Dust in Space

Sun Kwok

University of British Columbia, Vancouver, Canada & Laboratory for Space Research, University of Hong Kong, Hong Kong

Cosmic Dust and Magnetism, Daejeon, Korea October 31, 2018

Organic Matter

• Before the 19th century, organic matter was

assumed to be associated only with living

• rganisms (amino acid asparagine from asparagus,

1806; leucine from cheese, 1819; glycine from gelatin, 1820) and was assumed to possess a “vital

force”

• Living yeast was needed for fermentation of

sugar into alcohol

Laboratory synthesis of organics

• Urea from ammonium cyanate (1823)
• Amino acid alanine from acetaldehyde,

ammonia, and hydrogen cyanide (1850)

• Sugars from formaldehyde (1861)
• Nucleobase adenine from HCN and NH3

(1960)

Organics: a group of molecules and compounds based on the element carbon, together with H, O, N, S, and P. Vital force not necessary

Carbon Reservoirs on Earth

Pools Quantity (Giga tons) Atmosphere 720 Oceans 38,400 Total inorganic 37,400 Surface layer 670 Deep layer 36,730 Total organic 1,000 Lithosphere Sedimentary carbonates >60,000,000 Kerogen 15,000,000 Terrestrial biosphere (total) 2,000 Living biomass 600-1,000 Dead biomass 1,200 Aquatic biosphere 1-2 Fossil fuels 4,130 Coal 3,510 Oil 230 Gas 140 Other (peat) 250

Table adapted from Falkowski et al. (2000)

Organic matter

• n Earth is the

result of life Is there organic matter in the Universe?

Organic matter on Earth

• Kerogen, coal, oil, natural gas
• random arrays of aromatic carbon sites, aliphatic

chains and linear chains with functional groups made up of H, O, N, and S attached

• a solid sedimentary, insoluble, organic material

found in the upper crust of the Earth

• Under pressure and thermal processing, transform

into more stable forms of carbon such as graphite and diamond

• Biological in origin

Organics beyond the Earth

• Earth was thought to be the sole domain of
• rganics
• Existence of complex organics in space was

proposed but not believed (Hoyle & Wickramasinghe

1977)

• Organic molecules and solids are now found

throughout the Universe, from our solar system to distant galaxies (Kwok 2016, A&AR, 24, 8)

Carbonaceous chondrites

• Paraffins in Orgueil meteorite (Nagy et al.

1961)

• Aromatic and aliphatic compounds in

Murchison meteorite (Cronin et al. 1987)

• From amino acids to 30-C-long nonpolar

hydrocarbons

The soluble component of carbonaceous chondrites

• Carboxylic acids, sulfonic and phosphonic acids, amino

acids, aromatic hydrocarbons, heterocyclic compounds, aliphatic hydrocarbons, amines and amides, alcohols, aldehydes, ketones, and sugar related compounds

• 14,000 compounds with millions of diverse structures
• Almost all biologically relevant organic compounds are

present in carbonaceous meteorites Decreasing abundance with increasing C number within the same class of compounds suggests abiotic origin.

Schmidt-Kopplin et al. 2010, PNAS, 107, 2763

Non-terrestrial origin

• Amino acids: equal mixture of D and L

chirality amino acids, non-protein amino acids not found in the biosphere, non- terrestrial values of deuterium

• Nucleobases are achiral
• Unusual nucleobases (Callahan et al. 2011, PNAS,

108, 13995).

Insoluble Organic Matter (IOM) in carbonaceous chondrites

• Insoluble macromolecular solids
• 70% of organic matter in IOM
• Destructive: thermal and chemical degradations

followed by GC/MS

• Nondestrutive: NMR, FTIR, XANES, EPR,

HRTEM

• Small (1-4) aromatic rings, short aliphatic chains,

heteroelements (O, S, N) (Derenne & Robert 2010)

• Average abundance C100H46N10O15S4.5 (Pizzarello &

Shock 2010)

Comets: the volatile component

• Mm and IR spectroscopy: CH4, C2H2, C2H6,

CH3OH, H2CO, HOCH2CH2OH, HCOOH, HCOOCH3, CH3CHO, H2CHO,NH3,HCN, HNCO, HNC, CH3CN, HC3N

• ROSATA: methyl isocyanate (CH3NCO),

acetone (CH3COCH3), propanal (C2H5CHO), and acetamide (CH3CONH2)

• STARDUST: glycine of extraterrestrial origin

(Elsila et al. 2009)

Comets: not dirty snow balls

• ROSETTA:

27000 particles collected from Comet 67P

• Large

macromolecular compound similar to IOM

Fray et al. 2016, Nature, 538, 72

3.3 and 3.4 features in Comet Wild2 (Keller et al. 2006)

Complex organics in comets

• Comparison between

cometary dust, IDP, meteorites, and kerogen

• D and 15N enrichment

suggests presolar

• rigin

Raman spectra (Sandford et al. 2006)

Asteroids

• Extreme red color and low (0.01-0.15)

albedos of some asteroids are inconsistent with minerals or ice.

• Optical properties of 5145 Pholus can be

fitted with tholins (Cruikshank et al. 1998)

• Terrestrial coal, tar sands, asphaltite,

anthraxolite, kerite, etc., show low albedo and red colors similar to those of asteroids (Roush &

Cruikshank 2004).

Interplanetary dust particles

• Because of their small sizes (5-50 μm,

mass~nanogram), cannot be analyzed by traditional techniques

• Scanning transmission X-ray microscope

& X-ray absorption near-edge structure spectroscopy

• Carbonaceous materials (Messenger 2000,

Keller et al. 2002)

• 3.4 µm aliphatic feature and sometimes

C=O group (Flynn et al. 2003)

Interplanetary Dust Particles

• Few microns to tens of microns in

size (Brownlee 1978)

• Silicates (olivine & pyroxene)
• 10-12% carbon content
• 3.4 µm aliphatic feature and

sometimes C=O group (Flynn et al.

2003)

O-XANES spectrum of IDP

Titan

• Organic haze in the

atmosphere of Titan (Waite

2007)

• These nanoparticles are blown

into dunes by wind

• Lakes of liquid methane and

ethane

• Total amount of hydrocarbons
• n Titan is larger than the oil

and gas reserves on Earth

Tholins

• Tholins: refractory organic materials formed by UV

photolysis of reduced gas mixtures (N2, NH3, CH4) (Sagan & Khare 1979)

• Tholins=amorphous hydrogenated carbon nitrides
• Colors from yellow to dark brown
• Optical properties depend on sp2/sp3 ratio

Planets and satellites

Thiophenic, aromatic, and aliphatic compounds—in drill samples from Mars’ Gale crater (Eigenbrode et al. 2018,

Science, 360, 1096) (sample analysis at Mars on

Curiosity rover)

Kerogen as precursor, abiological in

• rigin

Complex macromolecular

• rganics in ocean on

Saturn’s moon Enceladus

(Postberg et al. 2018, Nature, 558, 564) (cosmic dust analyser and ion and

neutral mass spectrometer on Cassini)

Elemental synthesis in the late stages of stellar evolution

• Triple-α reaction

(He→C)

• Slow neutron capture (s-

process) (Y, Zr, Ba, La, Ce, Pr,

Nd, Sm, Eu, etc)

• Thermal pulse and

dredge up

• Synthesis of C2, C3, CN

in the stellar atmosphere

3 M⊙ track

Mass loss

Bloecker

Molecular synthesis in the stellar winds of AGB stars

• Rotational transitions of over 70 molecules

have been detected in the circumstellar envelopes of AGB stars

• Inorganics: CO, SiO, SiS, NH3, AlCl, ..
• Organics: C2H2, CH4, H2CO, CH3CN, ..
• Radicals: CN, C2H, C3, HCO+
• Rings (C3H2), chains (HC9N)

AGB stars are prolific molecular factories

Unidentified infrared emission (UIE) bands

(Russell et al. 1977)

2 4 6 8 10 12 14 16 18 20

Wavelength (µm)

500 1000 1500

λFλ(10-10erg cm-2 s-1 )

NGC 7027

6.2

3.3 7.7 11.3

[NeV] [SiIV] [NeIII] [MgV] 3.3: sp2 C-H stretch 6.2: sp2 C=C stretch 7.7: sp2 C-C stretch 8.6: sp2 =C-H in-plane bend

8.6

11.3: sp2 =C-H out-of-plane bend

12.0 12.7 13.5

Stretching and bending modes of aromatic compounds Aromatic nature first proposed by: Knacke 1977,

Duley & Williams 1979, 1981; Puetter et al. 1979

UIE also seen in galaxies

The same aromatics are also widely seen in galaxies

AIB=aromatic infrared bands Smith et al. 2007

From a few to 20% of total luminosity

UIE observed to z~2

2 4 6 8 10 12 14 16 18 20

Wavelength (µm)

• 20

20 40 60 80 100 120 140 160 180 200

λFλ(10-10erg cm-2 s-1)

IRAS 21282+5050

3.3 6.2 7.7 8.6 11.3 6.2: sp2 C=C stretch 7.7: sp2 C-C stretch 11.3: sp2 C-H out-of-plane bend 8.6: sp2 C-H in-plane bend 12.4: sp2 C-H out-of-plane bend 12.4

UIE are detected in many planetary nebulae. Since the carrier is synthesized in situ, PN are the best objects to study their origins

A young PN

3.4 μm aliphatic C-H stretch

• 3.38 μm: asymmetric CH3
• 3.42 μm: asymmetric CH2
• 3.46 μm: lone C-H group
• 3.49 μm: symmetric CH3
• 3.51 μm: asymmetric CH2

Infrared spectroscopy reveals aliphatic features

Aliphatic in-plane and out-of- plane bending modes

• 8µm plateau: -CH3 (7.25 µm), -C(CH3)3 (8.16 µm, “e”), =(CH3)2 (8.6 µm, “f”)
• 12 µm plateau: C-H out-of-plane bending modes of alkene (“a”, “b”), cyclic

alkanes (9.5-11.5 µm, “c”), long chains of -CH2- groups (13.9 µm, “d”).

Kwok et al. 2001

2 4 6 8 10 12 14 16 18

Wavelength (µm)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

normalized spectrum

IRAS 22272+5435

11.4 12.1 6.2 6.9 13.4 14.2 7.3 7.7

The UIE phenomenon

• Aromatic features: 3.3, 6.2, 7.7, 8.6, and

11.3 µm

• Aliphatic features: 3.4 and 6.9 µm
• Features at 15.8, 16.4, 17.4, 17.8, and 18.9

µm (in PPN, Kwok et al. 1999, in reflection nebulae, Sellgren et al.

2007, in galaxies, Sturm et al. 2000)

• Broad plateau features at 8, 12, and 17 µm.

What is the chemical structure of the carrier?

The PAH hypothesis

(Allamandola et al. 1989, Puget & Léger 1989)

• the UIE features are the result of infrared

fluorescence from small (~50 C atoms) gas-phase PAH molecules being pumped by far-ultraviolet photons (Tielens 2008, Ann. Rev. Astr. Ap., 46, 289)

• The central argument for the PAH hypothesis is

that single-photon excitation of PAH molecules can account for the 12 µm excess emission

• bserved in cirrus clouds in the diffuse interstellar

medium by IRAS (Sellgren 1984, 2001).

Problems with the PAH model

• PAH molecules have well-defined sharp features but

the UIE features are broad

• PAHs primarily excited by UV, with little absorption in

the visible, but UIE features are seen in PPN and reflection nebulae with no UV radiation

• The strong and narrow predicted gas phase features in

the UV are not seen in interstellar extinction curves → upper limits of 10-10-10-8 (Clayton et al. 2003, Salama et al. 2011,

Gredel et al. 2011)

• No specific PAH molecules have been detected in spite
• f the fact that the vibrational and rotational frequencies

are well known

Expected from IR: 3x10-7 (Tielens 2008)

Problems with the PAH model

• “No PAH emission spectrum has been able to

reproduce the UIE spectrum w.r.t. either band positions

• r relative intensities” (Schlemmer et al. 1994, Cook et al. 1996,

Cook & Saykally 1998, Wagner et al. 2000)

• The shapes and peak wavelengths of UIE features are

independent of temperature of exciting star

• In order to fit the astronomical observations, the PAH

model has to appeal to a mixture of PAH of different sizes, structures (compact, linear, branched) and ionization states, as well as artificial broad intrinsic line profiles (Cami 2011).

Note the pioneering work of Bert Donn and Alan Tokunaga

Vibrational modes of PAHs

“In order to reproduce the narrow 6.2 and 11.2 μm UIR bands, the carriers must consistently exhibit bands at these positions with a consistency similar to that which is

• bserved with the 3.3 μm emission.

In addition, the carriers of the UIRs must, in general, exhibit an absence

• f strong bands in the gap between

the 6.2 and 7.7 μm UIR features. The PAHs used in these model spectra simply do not meet these criteria; hence they do not reproduce the details of the UIR

• spectra. “

Cook and Saykally 1998

Experimental spectra of 12 PAH molecules

Vibrational modes of PAH

• The wavelengths of C-H stretch of PAH

molecules are shortward of 3.3 µm (Sakata et al. 1990,

Kwok & Zhang 2013)

• To fit the 11.3 µm by OOP of PAH molecules

needs an exotic mix (Sadjadi et al. 2015)

• C-C stretch of PAH are very weak and are at

wavelengths longer than 6.2 µm (Hudgins et al. 2005)

• Origin of 7.7 µm unclear (blending of several C-C

stretching modes)

Fitting of UIE by PAHs

• NASA Ames PAH database and fitting routines

(Boersma et al. 2014).

• 700 computational and 75 experimental spectra
• f PAH molecules and ions.
• Size range from 6 to 384 C atoms
• Charged states: neutral, anion (−), and cations (

+ , ++, and + + +).

silicates

Coal

Hydrogenated Amorphous Carbon

C55H56

Zhang and Kwok 2015, ApJ, 798, 37

5 artificial features

10 Artificial features

The PAH database model can fit anything!

Amorphous carbonaceous solids

• By introducing H into

graphite (sp2) and diamond (sp3), a variety

• f amorphous C-H alloys

can be created

• Geometric structures of

different long- and short- range can be created by varying the aromatic to aliphatic ratio

• Different sp2/sp3

hybridization ratios, mixed hybridization states

Robertson 2002

PAH graphite

Laboratory synthesis of carbonaceous solids

• Microwave irradiation of plasma of 4-torr methane (Sakata et al.

1987, Godard et al. 2011)

• Hydrocarbon flame or arc-discharge in a neutral of hydrogenated

atmosphere (Colangeli et al. 1995, Mennella et al. 2003)

• laser ablation of graphite in a hydrogen atmosphere (Scott and Duley

1996, Mennella et al. 1999, Jäger et al. 2008)

• Infrared laser pyrolysis of gas phase molecules (C2H4, C4H6)⇒C-

based nanoparticles (Herlin et al. 1998)

• Photolysis of methane at low temperatures (Dartois et al. 2004)
• Flame combustion forming soot (Pino et al. 2008, Carpentier et al. 2012)

(C2H2, C2H4, C3H6 mixed with O2)

Laboratory infrared spectra of hydrogenated amorphous carbon (top, Dischler 1983) compared to the astronomical spectrum of the planetary nebula IRAS 21282+5050. Band profiles naturally broad

Comparison of the laboratory spectrum of nanoparticles produced by laser pyrolysis of hydrocarbons (Herlin et al. 1998) (top panel) with the astronomical spectrum of the planetary nebula IRAS 21282+5050 .

Mixed aromatic/aliphatic organic nanoparticles (MAON) as a component

• f interstellar dust

Kwok & Zhang 2011, Nature, 479. 80

• Small units of aromatic rings

linked by aliphatic chains

• Impurities of O, N, S
• A typical nanoparticle may

contain multiple of this structures

Complex organic solids with disorganized structures

C (black), H (grey), S (yellow), O (red) and N (blue). There are 101 C, 120 H, 14 O, 4 N and 4 S atoms in this example.

Properties of MAON

• Amorphous (no fixed structure)
• Contains rings of different sizes and chains
• f different lengths and random orientations
• Contains impurities
• 3-D (not 2-D)
• Exact aromatic to aliphatic ratio depends on

radiation environment (photochemistry),

• riginal gas-phase components, and H

content

Flux (2.4-27.6 µm): continuum 65%, AIB: 13%, aliphatic 17%

A schematic of the possible structure of stellar organics.

This structure is characterized by a highly disorganized arrangement of small units of aromatic rings linked by aliphatic

• chains. Other impurities such as

O (in red), N (in blue), and S (in yellow) are commonly present. This structure contains 169 C atoms and a typical particle may consist of multiple structures similar to this one. C169H225N7O4S3

O in red, N in blue, S in yellow

Quantum chemistry calculations

• Calculations based on density functional theory where the non-

relativistic Schrödinger equation is solved under the Born– Oppenheimer approximation

• The two hybrid functionals Becke-Half-and-Half-LYP

(BHandHLYP) and Becke-Lee-Yang-Parr (B3LYP) are applied to obtain the equilibrium geometries and the fundamental vibrational frequencies

• Molecular structure calculated from minimum potential energy

surfaces

• Displacement vector analysis used to identify the atoms

participating in each normal mode (total: 3N-6 modes)

• The contribution from each vibrational mode to a spectral band is

quantitatively determined

• A Drude model of 500 K is used to simulate astronomical spectra

Understanding of vibrational spectra of complex organics

• Start with PAH molecules
• PAH molecules with aliphatic side groups (Sadjadi,

Zhang, & Kwok, 2015, ApJ, 801, 34)

• Nature of the 3.3/3.4, 6.2, and 11.3 µm UIE

bands (Sadjadi, Zhang & Kwok 2015, ApJ, 807, 95; Hsia et al.

2016, ApJ, 832, 213; Sadjadi, Zhang & Kwok 2017, ApJ, 845, 123)

• MAONs (Sadajadi, Kwok & Zhang 2016, IoP, 728, 062003)

Many spectral bands are due to coupled vibrational modes and their origin is not trivial

MAON model and potential energy surface calculations

calculated geometry of C155 H240

B3LYP/PC1 (extra care on polarization effects) 30 CPUs , 20 days

Long wavelength modes

15.3 µm

Sadjadi, Zhang, & Kwok 2015, ApJ, 801, 34

17.0 µm

Aliphatic bridges introduce flexibility in the structure. These vibrational modes have never been investigated by chemists.

3.39 3.29 NGC7027

asymmetric methylene C-H stretching

C155 H240 (vibrational motion at 3.4 µm)

C155 H240 (vibrational motion at 6.35 µm)

6.22 6.98 7.32 7.65 7.9 8.6 NGC7027

C155 H240 (vibrational motion at 8.13 µm)

6.22 6.98 7.32 7.65 7.9 8.6 NGC7027

methylene wagging

C155 H240 (vibrational motion at 11.16 µm)

11.24 12.8 13.1 13.5 NGC7027

aromatic C-H, OOP coupled with methyl & methylene vibrations

C155 H240 (vibrational motion at 13.20 µm)

11.24 12.8 13.1 13.5 NGC7027

aromatic C-H, OOP

C155 H240 (vibrational motion at 19.06 µm : 32% aromatic, 68% aliphatic)

11.24 12.8 13.1 13.5 NGC7027

complex vibrations

Simulated infrared spectra of 56 MAONs

The number of aromatic rings ranges from 2 to 11 and they are plotted in separate

• colors. The theoretical

spectra are broadened by Drude profiles of temperature 500 K.

PAH vs MAON

• Is the carrier of UIE free-flying molecules or

solids?

• Is the chemical structure regular with repeatable

patterns or amorphous with variable sizes and random orientations?

• Is it aromatic or mixed sp2/sp3?
• Is it 2D or 3D?
• Small (<50 C) or large (~103 C)?
• Pure CH or with impurities?
• Pure rings or with rich functional groups?

Naturally broad profiles

Excitation problem

• The 3.3 and 7.7 µm radiate at too short a

wavelength for the grains to be in thermal equilibrium

• Stochastic heating by single photon: good

for particles of 1-2 nm size.

• Alternate explanation: sudden release of

chemical energy as a source of transient heating (Duley and Williams 2011).

How do they form?

• Surface temperature of red giants: 3000 degrees
• Solid grains condensed from gas in the stellar

wind under near vacuum conditions

• Theoretically impossible, especially during the

PPN phase

• Observationally we see aliphatics and aromatics

form in PPN on time scales as short as hundreds of years

• In novae, they form on a time scale of days

Circumstellar synthesis

• Chemical timescales is constrained by

dynamical and evolutionary time scales

• Sequence of chemical synthesis can be

followed by observing objects in consequent evolutionary stages Complex organics can be formed under very low density conditions over very short (103 yr) time scales

Summary

• The UIE bands have consistent behavior from

circumstellar, interstellar and galactic sources, over a wide range of UV background

• The UIE bands (including the plateau features) are

naturally broad are unlikely to be results of random mixtures of PAH molecules

• The vibrational modes of complex organics (MAONs) have

never been calculated and have the potential to yield a consistent set of spectral features

Summary

• Complex organic matter of abiological origin is

commonly found in Solar System objects

• All biologically relevant molecules can be identified
• Organic solids of mixed aromatic-aliphatic structures

are found in meteorites, comets, and Titan

• Were these organic matter synthesized in the Solar

System or brought in from the outside?

• Complex organics are synthesized by old stars on very

short time scales and ejected into the interstellar medium

• To what extent the primordial solar system and the

early Earth was enriched by stellar organics?