17/Nov/20091 SUNY Stony Brook Astrochemistry Lecture Astrochemistry Adwin Boogert NASA Herschel...

17/Nov/2009 1SUNY Stony Brook Astrochemistry Lecture

Astrochemistry

Adwin BoogertNASA Herschel Science

Center,Caltech, Pasadena, CA


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


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


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.


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)?


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


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


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


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


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


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.


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


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)


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


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)


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)


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


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


Observing Gas Phase Molecules: Inventory

129 gas phase molecules currently detected in space(123 listed here)

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


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


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.


• 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)




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


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)].


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.


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)


'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.


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.


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


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]


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


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


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.



Observational evidence for thermal processing of ices near YSOs:

Solid 13CO2 band profile varies toward different protostars…



Observational evidence for thermal processing of ices near YSOs:


…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)


Molecular Evolution: Young Stars Observational evidence for thermal

processing of ices near YSOs:


…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......


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


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


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)


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


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


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


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


-CH-

-CH2--CH3-



Bacteria? Apples?


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.




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?


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


Future of Astrochemistry is Bright....

Herschel Space Observatory

Atacama Large MM Array

James Webb Space Telescope

….plus a lot more……

17/Nov/20091 SUNY Stony Brook Astrochemistry Lecture Astrochemistry Adwin Boogert NASA Herschel...

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