6th IRAM 30m Summer School Star formation near and far

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6th IRAM 30m Summer SchoolStar formation near
and far
Photon Dominated Regions II. Chemistry
A. Fuente Observatorio Astronómico Nacional (OAN,
Spain)
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Some history
Before 1937 Scientists thought that the ISM was
composed by atoms
1937-1941 The first three diatomic molecules
(CH,CN,CH) are detected (optical lines).
- - - - THE BIRTH OF RADIOASTRONOMY - - - -
1963 Weinreb and collaborators detected OH in
the Galactic Center.
1968 Charles Townes and collaborators detected
H2O y NH3.
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Which is the composition of the molecular gas?

• Because of the extinction of the UV radiation
  produced by interstellar grains, complex
  molecules can form and survive within molecular
  clouds
• H2 is the most abundant molecule in the ISM, and
  it is basically 100 of the molecular gas.
• The second most abundant molecule is CO, with a
  fractional abundance wrt H2 of, CO/H2 10-4
• More than 123 molecular species have been
  detected hasta in the ISM. Some are common in the
  Earth such as NH3, H2CO, HCOOH,.... Others can
  survive long enough only in extreme conditions
  (low density and temperatue), and in the Earth
  they are only found in laboratory (CO, C2H,
  HCS,...).
• Moleculas

Gas molecular

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List of detected molecules
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Orion KL
Spectral survery at 3mm, 2mm and 1mm carried out
with the 30m telescope by B. Tercero and
collaborators (Tercero et al., 2010 AA 517, 96
,2011, AA 528, 26)
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Orion KL
Spectral survery at 3mm, 2mm and 1mm carried out
with the 30m telescope by B. Tercero and
collaborators (Tercero et al., 2010 AA 517, 96
,2011, AA 528, 26)
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Charateristics of the ISM composition
There are two important characteristics that
defines the chemical composition of the ISM and
are different from that in Earth
1. There are many molecular ions and radicals in
the list of ISM molcules. These species are very
reactive and consequently unstable in the
physical conditions prevailing in the Earth (e.g.
C3H2, C3H, o C3N )
2. Organic compounds are usually un-saturated
(few H atoms per molecule). Saturated
compounds (CH2CHCN, CH3CH2CN) are
only found in warm (Tkgt100 K) and dense (gt106
cm-3) regions (hot cores).
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How are molecules formed in the ISM?
Most molecules are formed in gas-phase by
chemical reactions between more simple species
Earth Three-body reactions AB ?AB ABC? AB
C Reactions neutral-neutral with activation
energies, kTa 0.3 eV.
ISM No three-body reactions The first
important consequence is that H2 cannot be formed
in gas-phase. Reactions without activation
energy, mainly exothermic (kTMI 0.01 eV). The
most important reactions are ion-molecule.
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Chemical reactions in Space
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Photochemistry
The FUV photons permeating the diffuse ISM are a
dominant destruction agent for small molecules.
Typical bonding energies of molecules are in the
range 5 10 eV. However, direct absorption into
dissociating continuum is usually negligible.
More frequently, the dissociation occurs through
a transition to a electronic state followed by
dissociation. The photodissociation rates are
usually expressed as
Different at each layer of the PDR
The rates are expressed in units of the Habind
field. Therefore, the characteristic time is
given by t1/(kpd G0)
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Photochemistry
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Grain-Surface reactions
The most abundant molecule in space, H2, cannot
be formed in gas phase. Must be formed on grain
surfaces. Grain-surface reactions are also key
to explain the abundances of other molecules such
as CH3OH, H2CO, NH3 or H2O.
Evaporation Photo-desorption Chemi-desorption Sput
tering
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Grain-Surface reactions
The accretion rate is
The depletion time for a cold core is (4 x109)/n,
i.e., less than 105 yr in a dense core. We can
also express this lifetime as the time required
for a species to arrive at a grain. In a gas with
kinetic temperature of 10 K, for CO
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Grain-Surface reactions
The evaporation time is
Ebbinding energy
The evaporation time is very sensitive to the
dust temperature Td.
In PDRs, the main mechanism to release molecules
from the grain mantles is photo-desorption.
Assuming the mean interstellar UV field, the
grains will remain clean of ices for
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Grain-Surface reactions
Other important concept is migration time.
Migration time is the time to move from one
position to another on the grain surface. When
the migration time is lower than the evaporation
time, surface reactions occur.
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PAHs
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PDR Chemical Models
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Stationary Chemical Models (Sternberg Dalgarno
1995)
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Stationary Chemical Models (Sternberg Dalgarno
1995,ApJ SS 99, 565)
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Reactive ions (CO,HOC,SO)
Reactive ions are destroyed in every collision
with e- and H2. For this reason, their abundance
are expected negligible in molecular clouds. Only
in the HI/HII warm interface of PDRs, their
rapid formation can overcome destruction and they
reach measureable abundances. The first reactive
ion, CO, was detected toward the planetary
nebular NG7027.
Formation of CO
O H2 OH H Endothermicity3000K
Formation of HOC
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Reactive ions (HOC)
Branching ratio0.8a Branching
ratio0.6 Branching ratio0.48
Destruction HOC H2 HCO H2
(1) HCO e- products (2)
HCO/HOCk1nH2/a k2ne
Usero et al. AA 419, 897
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Reactive ions (CO,HOC,SO) (Fuente et al. 2003,
AA 406, 899)
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Reactive ions (CO,HOC,SO) (Fuente et al. 2003,
AA 406, 899)
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Reactive ions
Sternberg Dalgarno, 1995, ApJSS 99, 565
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Reactive ions
Sternberg Dalgarno, 1995, ApJSS 99, 565
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Herschel (CH,OH,SH...)
Herschel has provided for the first time the
opportunity of observing light reactive ions
(CH, OH, SH...) in the interstellar
medium. Light molecules such as CH, OH, NH,
NH2,.. has been extensively observed Some light
molecules like H2Cl has been discovered for the
first time. Others, like H2O and CH turned to
be more abundant than expected.
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HF, CH,SH,OH,H2O
HF is a good tracer of H2 The high abundances of
CH and SH are only undersand in the context of
turbulent models. OH and H2O proves that the
absorbing layer is highly ionized.
Falgarone et al., SF2A 2010
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Nitrogen hydrides (NH, NH2,NH3)
Although the NH/NH3 and NH2/NH3 ratios in
diffusse clouds are consistent with gas-phase PDR
models, but their large abundances derived
require of grain surface chemistry.
Persson et al., 2010, AA 521, L45
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H2Cl
Lis et al. 2010, AA 521, L9
Formation route
HCl/H2Cl1 -10 in agreement with PDR
chemical models
H2Cl column densities 1013 cm-2, in excess of
model predictions
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Failures
Sternberg Dalgarno, 1995, ApJSS 99, 565
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Other chemical diagnostics
1.- Millimeter observations provides chemical
diagnostics of PDRs 1.1 CN/HCN 1.2 HCO/H13CO
1.2 Small hydrocarbons (PAHs)
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NGC 7023
CFHT
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CN/HCN ratio (Fuente et al. 1993, AA 276, 473)
The CN/HCN ratio was first proposed as a tracer
of PDRs by Fuente et al. (1993) based on their
observations in NGC 7023.
CN/HCN gt 1 is considered a good evidence for PDR.
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CN/HCN ratio in Orion (Fuente et al. 1996, AA
312, 599 Rodríguez-Franco et al. 1998, AA 329,
1097)
The CN/HCN ratio is 3 across the Orion Bar
C.R. O'Dell (Rice University) y NASA/ESA
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S140 Two PDRs with different physcial conditions
Ionized by IRS 1
IRS1 RA 221918.21, Dec631846.9
IF RA 221911.53, Dec 631746.9
Ionized by the B0V star HD 211880 Draine field
150
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HCO (Schilke et al. 2001, AA 372, 291)
In a pioneering work Schenewerk et al. (1988)
proposed that HCO is easily detected in PDRs.
Schilke et al. (2001) concluded that HCO is
associated to PDRs and HCO/H13CO varies
between 30 in the Orion Bar and 3 in NGC 7023
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Horsehead
Hily-Blant et al., 2005 AA 440, 909
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Horsehead (Gerin et al. 2009, AA 494, 977)
N (HCO)/N (H13CO) 50 in the FIR peak of the
Horsehead!!!!
They proposed a new gas-phase route to explain
the high HCO abundance
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Small hydrocarbons (C2H, c-C3H2,
C4H...) (Teyssier et al. 2004, AA 417, 135)
Hily-Blant et al., 2005 AA 440, 909
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Horsehead (Pety et al. 2005, AA 435,885)
The abundance of small hydrocarbons in the PDR
position is similar to that of these species in
dark clouds. This cannot be understood in terms
of gas -phase chemistry. The correlation between
the abundances of these species and the PAHs
suggests to propose an alternative path C2H2,
CH3,... are released from PAHs by photolysis and
subsequent chemistry in gas phase produce
enchanced abundance of small hydrocarbons.
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Summary
I. Observations
1.- Millimeter observations provides chemical
diagnostics of PDRs 1.1 CN/HCN (well explained
gas-phase stationary models) 1.2 HCO/H13CO
(grain surface chemistry?) 1.2 Small
hydrocarbons (PAHs) 1.3 Reactive ions 2.- Far-IR
spectroscopy (Herschel) 2.1 Hydrides (turbulence
is required to explain some species) 2.2 New
molecules (H2Cl)
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References
1.- The Physics and Chemistry of the
Interstellar Medium, A.G.G.M. Tielens, de.
Cambridge University Press
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Summary
II. Models
1.- Include grain-surface chemistry (UCL and
under developement in Meudon PDR code) 2.-
Include PAH chemistry (under development in
Meudon) 3.- 2-D chemistry (Leiden code) and 3-D
(under developement in Meudon PDR code) 4.-
Coupling dynamics and chemistry in a time
dependent code (UCL) 5.- Include clumpyness
(KOSMA)