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17/Nov/2009 1SUNY Stony Brook Astrochemistry Lecture

Astrochemistry

Adwin Boogert

NASA Herschel ScienceCenter,

Caltech, Pasadena, CA


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Contents

What is Astrochemistry? Chemical Reactions in Space

Gas Phase neutral and ion reactionsGrain surface chemistry

TunnelingMantle growth

Ice formation thresholdIce processing

Laboratory simulationsThermal processingEnergetic processing

Observing Interstellar Molecules

Gas PhaseIR versus radio observationsDetected Species


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Contents

Observing Interstellar MoleculesSolid State

Band profilesPolar versus apolar ices; SublimationAmorphous versus Crystalline ices; Time scalesGrain size/shape effects

Column densities Molecular Evolution:

Dense CloudsLow and High Mass Young StarsHot Cores+DisksStarsStellar DeathDiffuse Clouds Astrobiology Future: Herschel, ALMA, JWST


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Reading

Material covered in this lecture is described at a similar level in

Complex Organic Interstellar Molecules, E. Herbst and E. F. van Dishoeck,ARA&A 2009, 47, 427-480. No need to read sections 2, 3.3, 5.2, 5.3, 6.4-6.6.

For the interested:

More advanced astrochemistry chapters in The Physics and Chemistry of theInterstellar Medium, A. G. G. M. Tielens, ISBN 0521826349. Cambridge,UK: Cambridge University Press, 2005.

Astrobiology: An Introduction to Astrobiology, eds. I. Gilmour and M. A.Sephton, ISBN 0521546214. Cambridge, UK: Cambridge University Press,

2003, 2004.


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What is Astrochemistry?

Astrochemistry studies molecules anywhere in the universe:

how are they formed?how are they destroyed?

how complex can they get ?how does molecular composition vary from place to place?use them as tracer of physical conditions (temperature, density)?how are molecules in space related to life as we know it (astrobiology)?


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Chemical Reactions in Space

Key factors in interstellar chemistry:

Abundance H factor 1000 larger than any other

(reactive) elementsAway from very strong UV fields: H,N,C,O atoms'locked up' in H2, N2, CO. Left over atoms determinechemical environment:

Reducing environment ifH>OOxidizing environment ifH


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Chemical Reactions in Space

More key factors in interstellar chemistry:

Densities atoms and molecules in interstellar medium extremely low: 1-105particles/cm3. Compare:

earth atmosphere 1019

ultra-high vacuum 108

Therefore chemistry quite unusual compared to earth standards. Rare earthspecies (discussed in a few slides) are abundant in the ISM:

HCO+ [formyl ion]H3

+ [protonated dihydrogen]

Types of chemistry:

Gas phase chemistryGrain surface chemistry (freeze out


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Gas Phase Chemical Networks

Despite extreme vacuum conditions,

long time scales allow for complex

gas phase chemistry.

Ion-neutral reactions orders of

magnitude faster than neutral-neutral.

Species with ionization potential


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Some Key Gas Phase ReactionsH3

+: (recently discovered, see http://h3plus.uiuc.edu)

H2 + CRH2+ + e-H2

+ + H2 H3+ + H

HCO+:

H3+ + CO HCO+ + H2

H2O:

O + H+ O+ + HO+ + H2 OH

+ + HOH+ + H2 H2O

+ + H

H2O+

+ H2

H3O+

+ HH3O+ + e- H2O + H

Collides and excites H2

, source of UV in dense clouds


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More realistic grain:

Many molecules (H2, H2O) much more

easily formed on grain surfaces. Freeze out


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Grain Surface Chemistry

Grain surfaces are the watering holes of

astrochemistry where species come to meetand mate. (Tielens 2005)

Species accreted from gas are chemisorbedorphysisorbed on grains, allowing for muchlonger time to find reaction partner than in

gas phase

Species move fast over surface, meetingpartners many times, allowing fortunnelingthrough activation barriers. e.g. H atom has50% probability of tunneling through 3400K barrier.

At molecular cloud densities (104-105 cm-3)it takes a few days for an atom to stick to agrain and 5*105 yrs for all gas to deplete ongrains, much less than cloud lifetime.


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Ice Mantle Growth

H2O formed by grain surface reactions, CO formed in gas and inertly condenseson grains.Grain mantle thickness:

Mass growth rate: dm/dt=S**a2*n**Radius growth rate: da/dt=(dm/dt)/(4**a2*)

da/dt=S*n**/(4*)

Mantle thickness independent of grain radius

Dense clouds can have mantles as thick as 0.1 um, and in deeply embeddedprotostars even more.

Mantle thicker than most grain cores according to MRN grain size distribution

n(a)~a-3.5, amin=0.005 m, amax=0.25 m


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Ice Mantle GrowthDue to grain temperature and interstellar radiation field ices form only if visual extinction

(AV) large enough: the ice formation thresholdTaurus cloud: H2O ices absent below visual extinction AV~3 and CO ices below AV~7.

Difference due to lower Tsub of CO.

Variation between clouds due to different temperature/radiation field

COH2O

Extinction (AV)


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Chemical processes occurring in

space can be simulated in laboratory

at low T (10 K) and low pressure.

Thin films of ice condensed on a

surface and absorption or reflection

spectrum taken.

Temperature and irradiation by

UV light or energetic particles of ice

sample can be controlled.

Astrophysical laboratories:

Leiden, Catania, NASAAmes/Goddard, Paris

Gerakines et al. A&A 357, 793 (2000)

Simulating Interstellar Ices


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Thermal Processing of Ices

New molecules easily produced byheating acid/base mixtures.

Example shownH

2

O/NH3

/HNCO=120/10/1 at 15,

52, 122 KNH3+HNCO -->NH4

+ + OCN-

NH4+ and OCN- have spectral

characteristics that fit interstellar4.62 and 6.85 m bands.

Relative intensities not in

agreement with observations,however, when requiring chargebalance; further study needed.

Van Broekhuizen et al., A&A 415, 425 (2004)


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Energetic Processing of Ices

Chemical processing of ices by UVphotons and cosmic rays can besimulated

Top figure shows H2O/CO/NH

3ice

mixture after photo-processingwith hard UV photons

Bottom figure shows similarspectra compared to a YSO.Heating after irradiation canexplain the 6.85 m band.

Long exposure to photons orparticles can form very complex

molecules, incl. Amino acids andPAHs. Relevance to interstellar medium is

hard to prove. See slides on diffuse medium

415, 425-436 (2004)


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Observing Gas Phase Molecules

symmetric stretch v1 bend v2 asymmetric stretch v1

rotation axis A rotation axis Crotation axis B

H2O vibration

modes

H2O rotationmodes

Molecules detected (mostly) by vibrational and rotational transitions, atinfrared and radio wavelengths.

Electronic transitions occur at X-ray/UV wavelengths extinction-limited


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Observing Gas Phase MoleculesRo-vibrational transition rules lead to

characteristic P and R branch spectrum, if there ispermanent (e.g. CO) or induced (e.g. CH

4) dipole

moment.N2

and O2

cannot be observed this way.Example CO fundamental (J=1, v=1):

Pure rotational lines occur

mostly in the far-IR/submm forspecies with permament dipolemoments (e.g. CO, but not CH

4)

Note that in solid state, no rotations allowed, leadingto one broad vibrational spectrum

115 GHz

807 GHz

576 GHz

922 GHz

691 GHz

461 GHz

231 GHz346 GHz


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ObservingGas Phase

Molecules:Inventory

129 gas phase molecules

currentlydetected in space

(123 listed here)

http://www.cv.nrao.edu/~awootten/allmols.html


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Observing Solid State Molecules

H2O ice has many broadabsorption bands:

Symmetric stretch Asymmetric stretch Bending mode Libration mode

Combination modes Lattice mode etc...

Width, position and shapedetermined by solid state(dipole) interactions band

profile powerful diagnostic ofice environment and structure


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Ice Band ProfilesPolar vs Apolar Ices

Molecular dipole moment determinesphysical and spectralcharacteristics. Compare solid H

2O

and CO: Sublimation temperature much

higher for H2O (90 K vs. 18 K inspace)

Bands much broader for H2O H2O/CO mixtures: distinctpolar

and apolarices with differentH2O/CO ratios that canspectroscopically be distinguishedand sublimate at different T.

Highly relevant for icy bodies (e.g.comets) as well, as dipole momentdetermines outgassing behaviour.'Pockets' of apolar CO may resultin sudden sublimation.


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CO band consists of 3 components,explained by laboratory simulationsas originating from CO in 3 distinctmixtures:

'polar' H2O:CO

'apolar' CO2:CO

'apolar' pure CO

(Boogert, Hogerheijde & Blake, ApJ 568,761, 2002)



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Indeed, CO ice profiles vary dramatically in different lines of sight, as apolarcomponent highly volatile. 'Older' YSOs have less apolar CO


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Ice Band ProfilesAmorphous vs. Crystalline

Interstellar H2O ices formed

in amorphous phase, as evidencedbyprominent 'blue' wing.

Crystallization by protostellar heat.

[long wavelength wing

originates from scattering on large

grains and NH3:H2O complexes]

Crystallization temperature ~120 K

in laboratory, but ~70 K in space

due to longer time scales.

[Time scale ~exp(Ebarrier

/T)

(~1 hour in lab, 105 yr in space).

For same reason sublimation

temperature in lab (~180 K)

higher than in space (~90 K)].


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Ice Band ProfilesGrain Shape and Size Effects

Laboratory and interstellarabsorption spectra cannot always be directly compared:

Scattering on large (micron sized) grains leads to 3 m red wing (often observed)

Surface modes in small grains may lead to large absorption profile variations:

For ice refractive index m=n+ik, absorption cross section ellipsoidal grain

proportional to (Mie theory) (2nk/L2)/[(1/L-1+n2-k2)2+(2nk)2]Resonance for sphere (L=1/3) occurs at k2-n2=2, so at large k (=strong transitions)

Important for pure CO, but not for CO diluted in H2O and also not for13

CO.


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Ice Column Densities and Abundances

Ice column densities:N=peak*FWHM/AlabAlab integrated band strength measured in laboratoryA[H2O 3 m]=6.2x10

-16cm/mol.

Order of magnitude in quiescent dense clouds:N(H

2O-ice)=1018 cm-2

For reference: this is ice layer of 0.3 m at 1 g/cm3 in laboratory, but....

Ice abundance:X(H2O-ice)=N(H2O-ice)/NH~10

-4

This is comparable to X(CO-gas)


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'Typical' abundances w.r.t. H2O ice

Ice Inventory

CO few-50%

CO2 15-35%

CH4 2-4%

CH3OH


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Molecular Evolution

Next slides molecular

evolution:

Dense CloudsYoung StarsHot Cores/DisksStars

Stellar DeathDiffuse CloudsAstrobiology

Not independent

environments. Cyclingof matter is key.

Molecular


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MolecularEvolution:Diffuse vs.

Dense MediumHubble telescope image of M51

shows

massive young stars (red)

'normal' stars (white)

molecular clouds (black)

diffuse clouds in between

clouds 'processed' by UV photons

massive stars

very similar to our own Galaxy

Cycling between environments as

spiral density wave passes

o ecu ar


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o ecu arEvolution:Diffuse vs.

Dense MediumCO J=1-0 image M51 highlightinggiant molecular clouds.

[Obtained with CARMA array in

Owens Valley by Jin Koda]


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Molecular Evolution: Dense Core

Molecules in core freeze out at

sublimation temperature

of molecule.

H2O T=90 K

CO T=16 K

Background star

H2O

H2ONH4

+

silicates

extinctio

n

Wavelength


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Molecular Evolution: Dense CoreCO sublimation temperature ~16 K

In densest part of core, most CO

freezes out

N2 and H2 lower sublimation

temperature (


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Molecular Evolution: Young Stars

Deep ice bands observed toward young

stars.

As star ages, ices heated: crystallizationand sublimation (most volatile species, e.g.

CO) first.


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Observational evidence for thermalprocessing of ices near YSOs:

Solid 13CO2 band profile variestoward different protostars


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Observational evidence for thermalprocessing of ices near YSOs:

Solid 13CO2 band profile varies

toward different protostars and laboratory simulated

spectra show this is due toCO2:H2O mixture progressivelyheated by young star(Boogert etal. 2000; Gerakines et al. 1999)


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Molecular Evolution: Young StarsObservational evidence for thermal

processing of ices near YSOs:

Solid 13CO2 band profile variestoward different protostars

and laboratory simulatedspectra show this is due toCO2:H2O mixture progressively

heated by young star(Boogert etal. 2000; Gerakines et al. 1999)

H2O crystallization (Smith et al.

1989) gas/solid ratio increases (van

Dishoeck et al. 1997)

Detailed modelling gas phase mm-wave observations (van der Tak etal. 2000)

Little evidence for energeticprocessing of ices, however......


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Molecular Evolution: Hot Cores......., but in immediate vicinity of YSO ices evaporate, and warm gas directly

observable at submm/radio wavelengths in rotational transitions.(sub)millimeter-wave gas phase measurements orders of magnitude more sensitive

to abundances than IR ice observations

Regions called hot cores for massive young stars and corinos for low mass stars.

Cazaux et al. 2004


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A. Wootten, Science with ALMA Madrid 2006.

SGR B2(N), ALMA Band 6 mixer at SMT

Have to be able to separate flowers from the weeds

Molecular Evolution: Hot Cores

Formic acid

Methyl formate

Formic acid

Dimethyl ether


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Herschel/HIFI: 480-1916 GHz (625-157m). Resolving Power up to 10

million, or


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Molecules are (Nearly) Everywhereeven on the SunT>5000 K, most molecules dissociateLower T, molecules quite easily formed, as demonstrated by H2O detection in sunspots (T~3000 K)

~13 um

M l l E l ti


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Molecular Evolution:Stellar Death

Cas A, SpitzerSN 1987A, HST

Stars at end burning phase expel massive shells ofmatter, enriching ISM with new elements and dust

Effect on chemistry strongly depends on stellar

mass, and episode of explosion.

Some form oxygen-rich dust (silicates), others

graphitic dust (and PAHs).

Supernovae vaporize environment,

destroying or modifying dust (graphite diamond).

Molecules (CO and SiO) formed in ejecta

Produce cosmic rays

Mo ecu ar Evo ut on: D use Me um


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Mo ecu ar Evo ut on: D use Me um,Mystery 1

Diffuse Interstellar Bands discovered in 1922 in

optical spectra of diffuse medium.

Over 200 bands detected.

Probably a large gas phase species

Polycyclic Aromatic Hydrocarbons possiblespherical C60, Buckminster Fullerenes,

Buckyballs

problem not solved...: 1 DIB, 1 carrier?

PAHs

Buckyball

Molecular Evolution: Diffuse Medium


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Another enigmatic diffuse

medium feature.... the 3.4 umabsorption band toward the

Galactic Center).

Triple peaks due to

hydrocarbons(-CH, -CH2

, -

CH3), but what kind of

hydrocarbon?

Pendleton et al. 1994, Adamson et al. 1998, Chiar et al. 1998,Chiar et al. 2000

Molecular Evolution: Diffuse Medium,Mystery 2

-CH-

-CH2--CH3-



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Bacteria? Apples?



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Greenberg et al. ApJ 455,L177 (1995): launched

processed ice sample in earthorbit exposing directly to solarradiation (EUREKAexperiment).Yellow stuffturnedbrown:highly carbonaceous residue,

also including PAH.




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Little evidence production by UV/CR bombardment of ices: band not polarized as opposed to silicates/ices: not in processed mantle but

separate grains 3.4 um band observed in dense clouds, but not triple peaked. Likely NH3.H2O

hydrate. Due to Low infrared sensitivity? Better observe sublimated species(more sensitive)

formed in evolved star envelopes, and injected in ISM?


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Molecular Evolution: Astrobiology

Do molecules formed in interstellar medium have anything to do with

formation of life?This is topic of astrobiology.

Amino acids building blocks of most complex molecules in living

organisms...protein.

It has been produced in laboratory by heavy processing interstellar ice

analog.Also, chirality of amino acids in protein is left-handed. May have been

caused by nearby massive star producing circularly polarized light


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Future of Astrochemistry is Bright....

Herschel Space Observatory

Atacama Large MM Array

James Webb Space Telescope

.plus a lot more

astrochem sb nov2009 www

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