The Chemistry of Stellar and Planetary

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The Chemistry of Stellar and Planetary Formation
Eric Herbst Departments of Physics, Astronomy,
and Chemistry The Ohio State University
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The Center of the Milky Way
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100,000 lt yr
Andromeda a nearby spiral galaxy
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The Eagle Nebula active star forming region in
our galaxy
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The Horsehead Nebula (also in our galaxy Orion)
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Dense Interstellar Cold Core
10 K
10(4) cm-3
H2 dominant
sites of star formation
0.5 lt yr
500 lt yr away
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Extinction consistent with size distribution
from 5-250 nm. Dust constitutes 1 of mass in a
cloud. IR spectral studies yield
information about cores of dust
particles (silicates, carbon) as well as icy
mantles
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Dominant Mantle Species water, CO2, CO, CH3OH
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Work-horse method for gas-phase molecules
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Must compare with results of laboratory spectra
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The soon-to-be Herschel Space Observatory
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Cold Core
Low-mass Star Formation
adiabatic collapse
Protostar
T 10 K
n 104 cm-3
Molecule factory
Star Disk
hot core
100 K
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Via high-resolution gas-phase analytical
spectroscopy

• 133 neutral molecules (February 2008)
• 18 molecular ions (main isotopes)
• 14 positive
• 4 negative
• H
• C, N, O
• S, Si, P, K, Na, Mg, Al, F

Most in our own galaxy only.
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Gaseous interstellar molecules (151)

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The Chemistry of Cold Cores

• Do chemical reactions take place under low
  density and low temperature conditions?
• Collision interval 1 day to 1 millenium k(T)
  A(T)exp(-Ea/RT)
• How can we convert atoms into molecules?
• H H ? H H

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Cosmic rays produce ions
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T 10-20 K
Gould Salpeter
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FORMATION OF GASEOUS WATER IN COLD CORES
H2 COSMIC RAYS ? H2 e

H2 H2 ? H3 H H3 O ? OH
H2 OHn H2 ? OHn1 H H3O e ?
H2O H OH 2H, etc
longer pathways to unsaturated organic species
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Formation of Ices In Cold Cores
H
O
OH
H
H2O
Other ices formed methane, ammonia, CO, CO2,
formaldehyde, methanol (all confirmed by
experiments at low temperature.)
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What is a model?

• Simulates chemistry in the gas and on surfaces
• 6000 gas-phase reactions 200 surface reactions
• Physical conditions can be homogeneous and
  time-independent, or can be heterogeneous and/or
  time dependent
• Molecular concentrations can be calculated.
  Comparison with observation yields physical
  conditions and history of object.
• Cold cores (gas ices) fit best at age of 105 yr
  (80 of molecules fit to within observational
  error).
• Can even simulate what ices look like!

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Development of ice mantle in cold interstellar
core
Cuppen Herbst, 2007
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Hot Core Chemistry
100-300 K
evaporation
Saturated organic molecules such as ethers,
alcohols
10 K
Surface chemistry involving heavy radicals
(photochem-istry)
Cold phase accretion surface chemistry (H-rich)
Garrod Herbst (2006)
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ORGANIC MOLECULES PREDICTED IN HOT CORES

• Dimethyl ether, methyl formate, formic acid,
  glycolaldehyde, acetic acid, ethanol,
  acetaldehyde, ketene, acetone, ethylene glycol
• Methyl amine, urea, formamide, acetamide,
  methoxyamine, hydroxymethylamine
• Garrod, Widicus Weaver, Herbst (2008)

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Protoplanetary Disk
Cosmic rays
UV
X-ray
midplane
UV
500 AU
0.01-0.1 M0
T Tauri star 106 yr old
Keplerian rotation
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ALMA the future.
http//www.physics.ohio-state.edu/eric/
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Vertical Distribution

109 cm-3 108 107 106 105
R 105 AU
densiy cm-3
photodissociation
0 20 40 60 80 100
Z(AU)
accretion
Too detailed for observers
Icy Layer
Molecular Layer
PDR
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HOT CORE IN ORION
Molecular inventory contains gaseous saturated
(H-rich) normal molecules, not detected in
colder regions. Ice mantles no longer exist.
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Negative Ions in Clouds

• Herbst (1982) considered the possible abundance
  of anions in cold regions of the ISM based on
  radiative attachment mechanism
• A e ? A- hn
• and estimated their maximum abundance to be 1 of
  the neutral counterparts.

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