Astronomy 101-lecture 8 Astrochemistry

Astronomy 101 - Lecture 8
Astrochemistry
Adwin Boogert
NASA Herschel Science Center
Caltech
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Contents
–What is Astrochemistry?
–Chemical Reactions in Space
–How to Observe Molecules
–Molecular Evolution:
•Dense Clouds
•Young Stars
•Hot Cores+Disks
•Stellar Death
•Diffuse Clouds
•Astrobiology
–Future: Herschel, ALMA, JWST
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What is Astrochemistry
Astrochemistry studies molecules anywhere in the universe:
–how
are they formed
–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 do molecules in space relate to life as we know it (astrobiology)
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Chemical Reactions in Space
Cosmic
Abundances
H
He
O
C
N
Ne
Si
Mg
S
Fe
0.9
0.1
7e-4
3e-4
1e-4
8e-5
3e-5
3e-5
2e-5
4e-6
H2
inert
CO
CO
N2
inert
dust
dust
–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 to earth standards.
Examples common species:
•HCO+ [formyl ion]
•H3+ [protonated dihydrogen]
dust
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Chemical Reactions in Space
Cosmic
Abundances
H
He
O
C
N
Ne
Si
Mg
S
Fe
0.9
0.1
7e-4
3e-4
1e-4
8e-5
3e-5
3e-5
2e-5
4e-6
H2
inert
CO
CO
N2
inert
dust
dust
–Some
key facts:
•Abundance H factor 1000 larger than any
other (reactive) elements
•Away 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>O
–Oxidizing environment if H<O
–Types
of chemistry:
•Gas phase chemistry
•Grain surface chemistry (freeze out <100 K)
•Energetic processing ices
dust
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Chemical Reactions in Space: Gas Phase
•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 (CC+)
•Cosmic rays also important ionization
sources
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Chemical Reactions in Space: Gas Phase
Some key gas phase reactions:
H3+: (recently discovered, see http://h3plus.uiuc.edu)
H2 + CR H2+ + eH2+ + 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
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H3O + e  H2O + H
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Chemical Reactions in Space: Solid State
Many molecules (H2, H2O) much more
easily formed on grain surfaces. Freeze out
<100 K.
More realistic grain:
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Chemical Reactions in Space: Solid State
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,
Gerakines et al. A&A 357, 793 (2000)
Catania, NASA Ames/Goddard,
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Chemical Reactions in Space: Solid State
Solid 13CO2:
•Solid 13CO2 band profile varies toward
different protostars…
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Chemical Reactions in Space: Solid State
Solid 13CO2:
•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
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Chemical
Reactions in
Space: Inventory
129 molecules currently
detected in space
(123 listed here)
http://www.cv.nrao.edu/~awootten/allmols.html
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How to Observe Molecules
–Molecules
detected (mostly) by vibrational and rotational transitions, at infrared
and radio wavelengths.
–Electronic transitions occur at X-ray/UV wavelengths  extinction-limited
H2O vibration
modes
symmetric stretch v1
bend v2
asymmetric stretch v1
rotation axis A
rotation axis B
rotation axis C
H2O rotation
modes
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How to Observe Molecules
–Molecules
in solid state cannot rotate, just
vibrate
–Spectra solid and gas phase molecules look
very different:
Pure rotational lines occur
mostly in the far-IR/submm
(Herschel!)
922 GHz
807 GHz
691 GHz
576 GHz
461 GHz
231 GHz
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346 GHz
115 GHz
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Molecules are (Nearly) Everywhere
…even on the Sun
–T>5000 K, most molecules dissociate
–Lower T, molecules quite easily formed, as demonstrated by H O detection in sun
2
spots (T~3000 K)
~13 um
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Molecular Evolution
Next slides molecular
evolution:
–Dense Clouds
–Young Stars
–Hot Cores/Disks
–Stellar Death
–Diffuse Clouds
–Astrobiology
Not independent
environments. Cycling
of matter is key.
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Molecular
Evolution: Diffuse
vs. Dense Medium
Hubble 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
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Molecular
Evolution: Diffuse
vs. Dense Medium
CO J=1-0 image M51 highlighting
giant molecular clouds.
[Obtained with CARMA array in
Owens Valley by Jin Koda (Caltech)]
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Molecular Evolution: Dense Core
extinction
Background star
H2O
NH4+
H2O
silicates
Wavelength
•Molecules in core freeze out
at sublimation temperature
of molecule.
•H2O T=90 K
•CO T=16 K
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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+ + eH2+ + H2  H3+ + H
H3+ + N2  N2H+ + H2
•N2H+ observable at sub-mm
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(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.
•Actual chemical processing
observationally not established, but............
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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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Molecular Evolution: Hot Cores
Have to be able to separate flowers from the weeds
Formic acid
Methyl
formate
Formic acid
Dimethyl
ether
SGR B2(N), ALMA Band 6 mixer at SMT
A. Wootten, “Science with ALMA” Madrid 2006.
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Molecular Evolution: Hot Cores
Herschel/HIFI: 480-1916 Ghz (625-157 um)
Resolving Power ν/δ ν up to 10 million,
or <0.1 km/s
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CH3OH gas cell measurement using HIFI
(Teyssier et al. 2005)
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Molecular Evolution: Stellar Death
•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.
Cas A, Spitzer
SN 1987A, HST
•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
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Molecular Evolution: Diffuse Medium, Mystery 1
PAHs
•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?
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Buckyball
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Molecular Evolution: Diffuse Medium, Mystery 2
Another enigmatic diffuse
medium feature.... the 3.4 um
absorption band toward the
Galactic Center).
Triple peaks due to
hydrocarbons (-CH, -CH2,
-CH-CH3-
-CH3), but what kind of
hydrocarbon?
-CH2-
Pendleton et al. 1994, Adamson et al. 1998, Chiar et al. 1998,
Chiar et al. 2000
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Molecular Evolution: Diffuse Medium, Mystery 2
Bacteria? Apples?
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Molecular Evolution: Diffuse Medium, Mystery 2
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.
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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 polarized light
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Future of Astrochemistry is Bright....
Atacama Large MM Array
Herschel Space Observatory
James Webb Space Telescope
….plus a lot more……
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