H3+(Geballe)

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Transcript H3+(Geballe)

Adventures with H3+
Tom Geballe (Gemini Observatory)
with thanks to:
Takeshi Oka (U. Chicago)
Ben McCall (U. Illinois), Miwa Goto (ESO-Garching), Nick
Indriolo (U. Michigan),
Tomonori Usuda (Subaru Telescope)
and the staffs of the
United Kingdom Infrared Telescope (UKIRT), Gemini,
Subaru, and the Very Large Telescope (VLT)
The molecule at the starting point of
gas phase interstellar chemistry
and
a unique probe of conditions in interstellar clouds
H2
H
1p, 1e
2p, 2e
3p, 2e
H3+
Discovery of H3+ in 1911
J.J. Thomson (Cambridge) –
discoverer of the electron;
inventor of the mass spectrometer
“On several plates taken when the
discharge tube contains hydrogen, the
existence of a primary line for which
m/e=3 has been detected. There can
be little doubt that this line is due to
H3 … the existence of this substance
is interesting from a chemical point of
view, as it is not possible to reconcile
its existence with the ordinary
conceptions about valency, if
hydrogen is regarded as always
monovalent.”
- Thomson, Phil Mag., 24, 209 (1912)
H3 +
ions follow paths of
constant charge/mass
Thomson’s discovery photograph
H3+ can be abundant in pure hydrogen
discharges
 H3+ is stable
(dissociation energy ~4 eV, same as H2)
Dempster 1916)
Produced by the ion-molecule reaction
H2+ + H2  H3+ + H + ΔE
-
Hogness & Lumm (1925)
The same reaction produces H3+ in interstellar gas.
But what ionizes H2 in interstellar gas?
supernova
Answer
Accreting massive
black hole
H+
H2 +
H3 +
Why is H3+ important?
Follow the sequence of
reactions (bottom to top)
Tree of interstellar
gas phase chemistry
in dense (n>103 cm-3)
C H
clouds
CmHn
C4H3+
H
e
4
C4H2+
H2
H2
C4
+
HC11N
+
H2 + CR  H2 +
ζ = 3 x 10-17 sec-1 (~109 yr)
(but 1040 H2 molecules per sec
are ionized in a 1M cloud !)
+
e-
Watson (1973);
Herbst & Klemperer (1973)
C3H2
…
C3O
+
e
+Y
XY+ +
H (Y often H2)
…
H3+ is the starting point of
gas phase chemistry.
HC5N
e
CO
+
C
H
2
3
+
+
CH3
e
e
OH
H3O
e
HC3N
e
+
C2H
C
+
CH5
CH3OH, e
e
e
+
+
e
CH 3OH2
CH3CO
+
+
HCN
+
CH3
CO
e
C2H5CNH
CH3CN
H2
H2O
CH 3CNH
NH3
CH3NH2
N
H2
+
CH2
CH
H2O
HCN H C N
3 3
CH4
H2CO
C2H 5OH
CH 3OCH 3
CH 2CO
HX+
HC7N
C2H2
e
H2C3O
CH3OH
..
+
+
H 2+ + H 2  H 3+ + H
H3 + X 
+ H2
(X ≠ He, N, O2)
C3H3
HC9N
C3H
C
C2H 5
HX+
C
e
C
C3H+
…
+
C4H
e
H2
C2H 4
…
+
H2CN
H2
H2
+
+
H2O
H2
CH
C
+
OH
O
+
CO
N2
H2
+
H2
cosmic ray
H2
+
HCO
N2H
+
+
+ e
+ e
+ e
e
e
C2H5CN
CH3CN
CH2NH
CH3NH2
HCN
Takeshi Oka (measured IR spectrum of H3+ in the lab in 1980)
Oka
JAPAN, 1956
UKIRT, July 15, 1996: two lines of H3+ detected in dense interstellar clouds
H3+
bright
star
bright
star
Dense cloud
(optical)
Measured amounts of H3 consistent
with ion-molecule chemistry models
Huge amount of H3+ toward the Galactic center
H3+ doublet
UKIRT July 11 1997
GC IRS 3
AV = 30 mag
GALACTIC CENTER SPECTRA
GCS 3-2
(30 pc from IRS3)
AV = 30 mag
MOLECULAR CLOUD SPECTRUM
GL 2136
AV = 60 mag
• ~30X more H3+ toward GC than in typical Galactic dense clouds
• Only half as much gas toward GC as toward GL2136
Needless to say, we were puzzled.
Later that same night …
Surprisingly large amounts of H3+ in diffuse clouds
Dense cloud
Cygnus OB2 No. 12
diffuse cloud
July 11, 1997
Dense cloud
6X less gas and much higher ne than dense cloud, but same N(H3+) !
JULY11, 1997: TWO PUZZLES
1. Why is there so much H3+ in diffuse clouds ?
1. Even if H3+ is much more abundant in diffuse clouds
than we had thought, how could there be such a vast
amount of H3+ toward and/or in the Galactic center ?
Spitzer+VLA (Far-IR + Radio) view of the Galactic center
1. Why is there so much H3+ in diffuse clouds ?
There is a large population
of low energy cosmic rays that penetrates
diffuse clouds and ionizes H2,
but does not penetrate dense clouds.
So
ionization rate of H2 is 10X higher
in diffuse clouds.
Warm, diffuse, in GC
local
4.5 kpc
3 kpc
Local arm
Galactic Center
4.5 kpc arm
3 Kpc arm
Some of H3+ is in
gas in foreground spiral arms.
(produces narrow absorption features)
Lots of H3+ is in
warm and diffuse gas within the GC
(previously thought to take up an
insignificant fraction of volume there).
2. Why is there such
a vast amount of H3+
in the Galactic center ?
(a) Previously unsuspected large
volume of warm diffuse gas
(b) higher density of cosmic rays
in the GC produces more H3+
than in diffuse clouds elsewhere
H3+
H3+ in JUPITER
(disc. 1988)
H3+ in SATURN (disc 1993)
H3+
H3+
H3+
H3+ IN THE
SOLAR SYSTEM
H3+ lines in URANUS (disc. 1993)
FUTURE SCIENCE
WITH H3+
Jupiter
(1) Exo-planet “Jupiters”
much closer to their suns
Some could have extremely
bright H3+ line emission.
HISTORY OF THE UNIVERSE
(2) The first Stars
The early universe
contained only H and He.
Radiation by H3+
probably was an
important coolant
allowing the
gravitational collapse
of clouds into the first
generation of stars.
Research involving H3+ will continue to contribute to our understanding of the universe.