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17/Nov/2009 1SUNY Stony Brook Astrochemistry Lecture
Astrochemistry
Adwin BoogertNASA Herschel Science
Center,Caltech, Pasadena, CA
17/Nov/2009 2SUNY Stony Brook Astrochemistry Lecture
Contents
What is Astrochemistry? Chemical Reactions in Space
Gas Phase neutral and ion reactionsGrain surface chemistry
TunnelingMantle growthIce formation threshold
Ice processingLaboratory simulationsThermal processingEnergetic processing
Observing Interstellar MoleculesGas Phase
IR versus radio observationsDetected Species
17/Nov/2009 3SUNY Stony Brook Astrochemistry Lecture
Contents
Observing Interstellar MoleculesSolid State
Band profilesPolar versus apolar ices; SublimationAmorphous versus Crystalline ices; Time scalesGrain size/shape effectsColumn densities
Molecular Evolution:Dense CloudsLow and High Mass Young StarsHot Cores+DisksStarsStellar DeathDiffuse Clouds
Astrobiology Future: Herschel, ALMA, JWST
17/Nov/2009 4SUNY Stony Brook Astrochemistry Lecture
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 the Interstellar 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.
17/Nov/2009 5SUNY Stony Brook Astrochemistry Lecture
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)?
17/Nov/2009 6SUNY Stony Brook Astrochemistry Lecture
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 determine chemical environment:
Reducing environment if H>OOxidizing environment if H<O
CosmicAbundances
H 0.9 H2
He 0.1 inertO 7e-4 COC 3e-4 CON 1e-4 N2
Ne 8e-5 inertSi 3e-5 dustMg 3e-5 dustS 2e-5 Fe 4e-6 dust
17/Nov/2009 7SUNY Stony Brook Astrochemistry Lecture
Chemical Reactions in Space
More key factors in interstellar chemistry:
Densities atoms and molecules in interstellar medium extremely low: 1-105 particles/cm3. Compare:
earth atmosphere 1019
ultra-high vacuum 108
Therefore chemistry quite unusual compared to earth standards. Rare earth species (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 <100 K)Energetic processing ices
17/Nov/2009 8SUNY Stony Brook Astrochemistry Lecture
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 <13.6 eV likely photo-ionized (CC+)
Cosmic rays also important ionization sources
17/Nov/2009 9SUNY Stony Brook Astrochemistry Lecture
Some Key Gas Phase Reactions H3
+: (recently discovered, see http://h3plus.uiuc.edu)
H2 + CR H2+ + e-
H2+ + H2 H3
+ + H
HCO+:
H3+ + CO HCO+ + H2
H2O:
O + H+ O+ + H
O+ + H2 OH+ + H
OH+ + H2 H2O+ + H
H2O+ + H2 H3O
+ + H
H3O+ + e- H2O + H
Collides and excites H2, source of UV in dense clouds
17/Nov/2009 10SUNY Stony Brook Astrochemistry Lecture
More realistic grain:
Many molecules (H2, H2O) much more
easily formed on grain surfaces. Freeze out
<100 K.
Interstellar ‘ice’ or ‘dirty ice’: any frozen volatile, e.g. H2O, H2O mixtures, pure CO.
Grain Surface Chemistry
17/Nov/2009 11SUNY Stony Brook Astrochemistry Lecture
Grain Surface Chemistry “Grain surfaces are the watering holes of
astrochemistry where species come to meet and mate.” (Tielens 2005)
Species accreted from gas are chemisorbed or physisorbed on grains, allowing for much longer time to find reaction partner than in gas phase
Species move fast over surface, meeting partners many times, allowing for tunneling through activation barriers. e.g. H atom has 50% probability of tunneling through 3400 K barrier.
At molecular cloud densities (104-105 cm-3) it takes a few days for an atom to stick to a grain and 5*105 yrs for all gas to deplete on grains, much less than cloud lifetime.
17/Nov/2009 12SUNY Stony Brook Astrochemistry Lecture
Ice Mantle Growth
H2O formed by grain surface reactions, CO formed in gas and inertly condenses on grains.Grain mantle thickness: Mass growth rate: dm/dt=S**a2*n*<v>*<m> Radius growth rate: da/dt=(dm/dt)/(4**a2*) da/dt=S*n*<v>*<m>/(4*)
Mantle thickness independent of grain radius Dense clouds can have mantles as thick as 0.1 um, and in deeply embedded protostars even more. Mantle thicker than most grain cores according to MRN grain size distribution
n(a)~a-3.5, amin=0.005 μm, amax=0.25 μm
17/Nov/2009 13SUNY Stony Brook Astrochemistry Lecture
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
Col
umn
Den
sity
COH2O
Extinction (AV)
17/Nov/2009 14SUNY Stony Brook Astrochemistry Lecture
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, NASA
Ames/Goddard, Paris
Gerakines et al. A&A 357, 793 (2000)
Simulating Interstellar Ices
17/Nov/2009 15SUNY Stony Brook Astrochemistry Lecture
Thermal Processing of Ices
New molecules easily produced by heating acid/base mixtures.
Example shown H
2O/NH
3/HNCO=120/10/1 at 15,
52, 122 K NH3+HNCO -->NH4
+ + OCN-
NH4+ and OCN- have spectral
characteristics that fit interstellar 4.62 and 6.85 μm bands.
Relative intensities not in agreement with observations, however, when requiring charge balance; further study needed.
Van Broekhuizen et al., A&A 415, 425 (2004)
17/Nov/2009 16SUNY Stony Brook Astrochemistry Lecture
Energetic Processing of Ices
Chemical processing of ices by UV photons and cosmic rays can be simulated
Top figure shows H2O/CO/NH
3 ice
mixture after photo-processing with hard UV photons
Bottom figure shows similar spectra compared to a YSO. Heating after irradiation can explain the 6.85 μm band.
Long exposure to photons or particles can form very complex molecules, incl. Amino acids and PAHs.
Relevance to interstellar medium is hard to prove.
See slides on diffuse medium
415, 425-436 (2004)
17/Nov/2009 17SUNY Stony Brook Astrochemistry Lecture
Observing Gas Phase Molecules
symmetric stretch v1 bend v2 asymmetric stretch v1
rotation axis A rotation axis Crotation axis B
H2O vibration modes
H2O rotation modes
Molecules detected (mostly) by vibrational and rotational transitions, at infrared and radio wavelengths.
Electronic transitions occur at X-ray/UV wavelengths →extinction-limited
17/Nov/2009 18SUNY Stony Brook Astrochemistry Lecture
Observing Gas Phase MoleculesRo-vibrational transition rules lead to characteristic P and R branch spectrum, if there is
permanent (e.g. CO) or induced (e.g. CH4)
dipole moment. N2 and O
2 cannot be observed
this way. Example CO fundamental (J=1, v=1):
Pure rotational lines occur mostly in the far-IR/submm for species with permament dipole moments (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 GHz 346 GHz
17/Nov/2009 19SUNY Stony Brook Astrochemistry Lecture
Observing Gas Phase Molecules: Inventory
129 gas phase molecules currently detected in space(123 listed here)
http://www.cv.nrao.edu/~awootten/allmols.html
17/Nov/2009 20SUNY Stony Brook Astrochemistry Lecture
Observing Solid State Molecules
H2O ice has many broad absorption bands:
● Symmetric stretch● Asymmetric stretch● Bending mode● Libration mode● Combination modes● Lattice mode● etc...
Width, position and shape determined by solid state (dipole) interactions → band profile powerful diagnostic of ice environment and structure
17/Nov/2009 21SUNY Stony Brook Astrochemistry Lecture
Ice Band ProfilesPolar vs Apolar Ices
Molecular dipole moment determines physical and spectral characteristics. Compare solid H
2O
and CO: Sublimation temperature much
higher for H2O (90 K vs. 18 K in space)
Bands much broader for H2O H2O/CO mixtures: distinct polar
and apolar ices with different H2O/CO ratios that can spectroscopically be distinguished and sublimate at different T.
Highly relevant for icy bodies (e.g. comets) as well, as dipole moment determines outgassing behaviour. 'Pockets' of apolar CO may result in sudden sublimation.
17/Nov/2009 22SUNY Stony Brook Astrochemistry Lecture
• CO band consists of 3 components, explained by laboratory simulations as originating from CO in 3 distinct mixtures:
– 'polar' H2O:CO
– 'apolar' CO2:CO
– 'apolar' pure CO
(Boogert, Hogerheijde & Blake, ApJ 568,761, 2002)
Ice Band ProfilesPolar vs Apolar Ices
17/Nov/2009 23SUNY Stony Brook Astrochemistry Lecture
Ice Band ProfilesPolar vs Apolar Ices
Indeed, CO ice profiles vary dramatically in different lines of sight, as apolar component highly volatile. 'Older' YSOs have less apolar CO
17/Nov/2009 24SUNY Stony Brook Astrochemistry Lecture
Ice Band ProfilesAmorphous vs. Crystalline
Interstellar H2O ices formed in amorphous phase, as evidencedby prominent 'blue' wing. Crystallization by protostellar heat.[long wavelength wingoriginates from scattering on large
grains and NH3:H2O complexes]
Crystallization temperature ~120 Kin laboratory, but ~70 K in spacedue 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)].
17/Nov/2009 25SUNY Stony Brook Astrochemistry Lecture
Ice Band ProfilesGrain Shape and Size Effects
Laboratory and interstellar absorption spectra cannot always be directly compared: Scattering on large (micron sized) grains leads to 3 μm 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 for 13CO.
17/Nov/2009 26SUNY Stony Brook Astrochemistry Lecture
Ice Column Densities and Abundances
Ice column densities:N=peak*FWHM/Alab
Alab integrated band strength measured in laboratory
A[H2O 3 m]=6.2x10-16cm/mol. Order of magnitude in quiescent dense clouds:
N(H2O-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)
17/Nov/2009 27SUNY Stony Brook Astrochemistry Lecture
'Typical' abundances w.r.t. H2O ice
Ice Inventory
CO few-50%
CO215-35%
CH42-4%
CH3OH <8, 30%
HCOOH 3-8%
[NH3]<10, 40% (?)
H2CO <2, 7%
[HCOO-] 0.3%OCS <0.05, 0.2%
[SO2]<=3%
[NH4+] 3-12%
[OCN-] <0.2, 7%
Note far fewer ices detected than gas phase species. This is because ices can only be detected by absorption spectroscopy.
17/Nov/2009 28SUNY Stony Brook Astrochemistry Lecture
Molecular Evolution
Next slides molecular evolution:
–Dense Clouds–Young Stars–Hot Cores/Disks–Stars–Stellar Death–Diffuse Clouds–Astrobiology
Not independent environments. Cycling of matter is key.
17/Nov/2009 29SUNY Stony Brook Astrochemistry Lecture
Molecular Evolution: 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
17/Nov/2009 30SUNY Stony Brook Astrochemistry Lecture
Molecular Evolution: Diffuse vs.
Dense MediumCO J=1-0 image M51 highlighting
giant molecular clouds.
[Obtained with CARMA array in
Owens Valley by Jin Koda]
17/Nov/2009 31SUNY Stony Brook Astrochemistry Lecture
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
extin
ctio
n
Wavelength
17/Nov/2009 32SUNY Stony Brook Astrochemistry Lecture
Molecular Evolution: Dense Core•CO sublimation temperature ~16 K•In densest part of core, most CO
freezes out•N2 and H2 lower sublimation
temperature (<13 K)•cosmic rays penetrate deep in core, ionizing H2, forming N2H
+
•H2 + CR H2+ + e-
H2+ + H2 H3
+ + H
H3+ + N2 N2H
+ + H2
•N2H+ observable at sub-mm
frequencies (e.g. Herschel)•better dense cloud tracer than CO
17/Nov/2009 33SUNY Stony Brook Astrochemistry Lecture
Molecular Evolution: Young Stars
•Deep ice bands observed toward young
stars. •As star ages, ices heated: crystallization
and sublimation (most volatile species, e.g.
CO) first.
17/Nov/2009 34SUNY Stony Brook Astrochemistry Lecture
Molecular Evolution: Young Stars
Observational evidence for thermal processing of ices near YSOs:
Solid 13CO2 band profile varies toward different protostars…
17/Nov/2009 35SUNY Stony Brook Astrochemistry Lecture
Molecular Evolution: Young Stars
Observational evidence for thermal processing of ices near YSOs:
Solid 13CO2 band profile varies toward different protostars…
…and laboratory simulated spectra show this is due to CO2:H2O mixture progressively heated by young star (Boogert et al. 2000; Gerakines et al. 1999)
17/Nov/2009 36SUNY Stony Brook Astrochemistry Lecture
Molecular Evolution: Young Stars Observational evidence for thermal
processing of ices near YSOs:
Solid 13CO2 band profile varies toward different protostars…
…and laboratory simulated spectra show this is due to CO2:H2O mixture progressively heated by young star (Boogert et al. 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 et al. 2000)
Little evidence for energetic processing of ices, however......
17/Nov/2009 37SUNY Stony Brook Astrochemistry Lecture
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
17/Nov/2009 38SUNY Stony Brook Astrochemistry Lecture
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
17/Nov/2009 39SUNY Stony Brook Astrochemistry Lecture
Herschel/HIFI: 480-1916 GHz (625-157 μm). Resolving Power up to 10 million, or <0.1 km/s
Molecular Evolution: Hot Cores
CH3OH gas cell measurement using HIFI
(Teyssier et al. 2005)
17/Nov/2009 40SUNY Stony Brook Astrochemistry Lecture
Molecules are (Nearly) Everywhere…even on the Sun•T>5000 K, most molecules dissociate•Lower T, molecules quite easily formed, as demonstrated by H2O detection in sun spots (T~3000 K)
~13 um
17/Nov/2009 41SUNY Stony Brook Astrochemistry Lecture
Molecular Evolution: Stellar Death
Cas A, SpitzerSN 1987A, HST
Stars at end burning phase expel massive shells of matter, 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), othersgraphitic dust (and PAHs).
Supernovae vaporize environment, destroying or modifying dust (graphite →diamond).
Molecules (CO and SiO) formed in ejecta
Produce cosmic rays
17/Nov/2009 42SUNY Stony Brook Astrochemistry Lecture
Molecular Evolution: Diffuse Medium, 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 possible•spherical C60, “Buckminster Fullerenes”,
“Buckyballs”•problem not solved...: 1 DIB, 1 carrier?
PAHs
Buckyball
17/Nov/2009 43SUNY Stony Brook Astrochemistry Lecture
Another enigmatic diffuse medium feature.... the 3.4 um absorption 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-
17/Nov/2009 44SUNY Stony Brook Astrochemistry Lecture
Molecular Evolution: Diffuse Medium, Mystery 2
Bacteria? Apples?
17/Nov/2009 45SUNY Stony Brook Astrochemistry Lecture
Greenberg et al. ApJ 455, L177 (1995): launched processed ice sample in earth orbit exposing directly to solar radiation (EUREKA experiment). Yellow stuff turned brown: highly carbonaceous residue, also including PAH.
Molecular Evolution: Diffuse Medium, Mystery 2
17/Nov/2009 46SUNY Stony Brook Astrochemistry Lecture
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?
17/Nov/2009 47SUNY Stony Brook Astrochemistry Lecture
Molecular Evolution: AstrobiologyDo 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
17/Nov/2009 48SUNY Stony Brook Astrochemistry Lecture
Future of Astrochemistry is Bright....
Herschel Space Observatory
Atacama Large MM Array
James Webb Space Telescope
….plus a lot more……