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T. Oka,PRL 45,531 (1980)
What is HWhat is H33+ + ??
2y
2x
Equilateral triangle structure Simplest stable polyatomic molecule No stable excited electronic states No allowed rotational spectrum Laboratory spectrum obtained in 1980
Infrared inactivestretching mode
Infrared activedegenerate bending mode
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Formation of HFormation of H33++
e2ray cosmic
2 HH
H H HH 322
Step 1: Cosmic-ray ionization of H2:
)n(H Rate 2
Step 2: Ion-Molecule reaction with H2:
[occurs on every collision]
The cosmic-ray ionization rate is estimated from various methods to be ~ 10-17 s-1. For a dense
cloud with n(H2) ~ 105 cm-3, the rate of formationof H3
+ is ~ 10-12 cm-3 s-1.
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Ion-Neutral ChemistryIon-Neutral Chemistry
23 H HCOCO H
23 H HXX H
H3+ initiates a network of
ion-molecule chemicalreactions, leading to the production of H2O and other molecules. Were
Earth’s oceans made by H3+?
The destruction of H3+ in
molecular cloud isdominated by reactionwith the most abundantreaction partner, CO.
This rate can be expressedas kCO n(H3
+) n(CO).
kCO ~ 210-9 cm3 s-1
(measured in lab)
HXY YHX
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HH33++ Number Density Number Density
Assuming steady-state, the H3+ number density
can be derived by equating the rates of formationand destruction.
Formation Rate (cosmic rays): n(H2)Destruction Rate (rxn with CO): kCO n(CO) n(H3
+)
constant!CO)(n
)n(H
k)n(H 2
CO3
Rearrange the equation n(H2) = kCO n(CO) n(H3+) to find:
Adopted values: ~ 10-17 s-1 (derived from observations)kCO = 210-9 cm3 s-1 (measured in lab)n(H2)/n(CO) ~ 6.7103 (from model calculations)
n(H3+) is constant ~ 310-5 cm-3 which is
independent of the density of the cloud!
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HH33++ Transitions Transitions
2
3I
2
1I
ortho-H3+
para-H3+
= G
ortho para para
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Telescopes & InstrumentsTelescopes & Instruments
United Kingdom Infrared Telescope (UKIRT)Mauna Kea, Hawaii
Cooled Grating Spectrometer 4 (CGS4)R ~ 20,000
Nicholas U. Mayall TelescopeKitt Peak, AZ
Phoenix SpectrometerR ~30,000
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GL 2136GL 2136
Molecular Cloud GL2136. This source provided the first detection of interstellar H3
+. Using the CGS4 spectrometer at UKIRT, it observed at two times separated by nearly three months. The Earth’s orbital motion around the Sun caused the spectral lines of H3
+ to be Doppler shifted – compelling evidence that the lines are genuine.
Npara = 4.0(9) 1014 cm-2
Northo = 3.0(6) 1014 cm-2
T. R. Geballe & T. OkaNature 384, 334 (1996)
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HH33++ – Interstellar Probe – Interstellar Probe
Measurements of H3+ provide:
path length of cloud number density of H2
kinetic temperature
Path Length:
Number Density:
3pccm10cm103
cm103
)n(H
)N(HL 19
35
214
3
3
3519
2242
2 cm10cm10
cm10
L
)N(H)n(H
Temperature:
T
87.32
kT
E
para
ortho
3para
3ortho 2g
g
)(HN
)(HN
ee
T ~ 27 K
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0.98
0.99
1
1.01
1.02
1.03
1.04
36620 36640 36660 36680 36700 36720
0.98
0.99
1
1.01
1.02
36620 36640 36660 36680 36700 36720
0.94
0.95
0.96
0.97
0.98
0.99
1
1.01
1.02
1.03
36620 36640 36660 36680 36700 36720
Other DetectionsOther Detections
Npara = 3.32(54) x1014 cm-2
Northo = 2.20(33) x 1014 cm-2
Npara = 1.16(21) x1014 cm-2
Northo = 0.58(13) x 1014 cm-2
Npara = 1.38(21) x1014 cm-2
Northo = 0.71(11) x 1014 cm-2
GL961E
MonR2 IRS3
W33A