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7/29/2019 Astrochem Sb Nov2009 Www
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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)
Ice Band ProfilesPolar vs Apolar Ices
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Ice Band ProfilesPolar vs Apolar Ices
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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Molecular Evolution: Young Stars
Observational evidence for thermalprocessing of ices near YSOs:
Solid 13CO2 band profile variestoward different protostars
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Molecular Evolution: Young Stars
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-
Molecular Evolution: Diffuse Medium
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Molecular Evolution: Diffuse Medium,Mystery 2
Bacteria? Apples?
Molecular Evolution: Diffuse Medium
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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.
Molecular Evolution: Diffuse Medium,Mystery 2
Molecular Evolution: Diffuse Medium
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Molecular Evolution: Diffuse Medium,Mystery 2
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
Recommended