h 3 + : a case study for the importance of molecular laboratory astrophysics ben mccall dept. of...
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![Page 1: H 3 + : A Case Study for the Importance of Molecular Laboratory Astrophysics Ben McCall Dept. of ChemistryDept. of Astronomy](https://reader035.vdocument.in/reader035/viewer/2022062517/56649edb5503460f94bea651/html5/thumbnails/1.jpg)
HH33++::
A Case Study for the Importance of A Case Study for the Importance of Molecular Laboratory AstrophysicsMolecular Laboratory Astrophysics
Ben McCallBen McCall
Dept. of Chemistry Dept. of Astronomy
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HH33++: Cornerstone of Interstellar Chemistry: Cornerstone of Interstellar Chemistry
H 2
H 2+
C H 3+
C H 5+
C H 4
C H2 3+
C H2 2
C H3+
C H3 3+
C Hm n
C H+
C H 2+
N H2
+
H C O+
O H+
H O2+
H O3
+H O2
O H
C H C N H2 5
+
C H C N H3
+
C H N H3 2
+
H C N2
+
H C N
C H N H3 2
C H N H2
C H C N3
C H C N2 5
C H
C H C O3
+
C H O H3 2
+
C H C O2
C H O H3
H C OC H O H
C H O C H
2
2 5
3 3
C H2 5
+C H2 4
H C O2 3
+C O3
C H2
H C N3 3
+H C N3
H C N5
H C N7
H C N9
H C N11
C H3
C 4
+
C H4+
C H4 2+
C H4 3+
C H4
C H3 2
cosm ic ray
H 2N 2
C OO
H 2
H 2
e
e
e
ee
e
e
N
N H 3
H C N
C H C N3
e
C O
H O2
C H O H , e3
C
H 2
H 2
H 2
e
e
C H 3
+e
e eH C N
C+
e
C+
H 2
e eC
+
H 2H 2
e
C O
C
H
+
e
e
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HH33++ in Dense Clouds in Dense Clouds
1.02
1.00
0.98
0.96
0.94
36700366803666036640 3717037150
AFGL 2136
AFGL 2591
R(1,1)u R(1,0) R(1,1)l
Wavelength (Å)
N(H3+) ~ 31014 cm-2
Consistent with expectations
McCall, Geballe, Hinkle, & OkaApJ 522, 338 (1999)UKIRT Kitt Peak
Nature 384, 334 (1996)
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Role of Laboratory AstrophysicsRole of Laboratory Astrophysics
• Four and a half years – much of it assembling the IR laser system and discharge cell
• Scanned from:– 6/12-8/3 (1978)– 12/18-1/26 (1978-79)– 4/24-12/18 (1980)
• Success on April 25, 1980
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Surprise: HSurprise: H33++ in Diffuse Clouds! in Diffuse Clouds!
observed at UKIRT observed at Kitt Peak
~ 4 × 1014 cm-2 !Similar column density
to dense clouds!!
B. J. McCall, T. R. Geballe, K. H. Hinkle & T. Oka,Science 279, 1910 (1998)
Cygnus OB2 12
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Rate = ke [H3+] [e-]
[H2]
Diffuse Cloud HDiffuse Cloud H33++ Chemistry Chemistry
H2 H2+ + e-
H2 + H2+ H3
+ + H
cosmic ray
H3+ + e- H + H2 or 3H
Rate =
Formation
Destruction
[H3+]
=
ke[e-]
Steady State
[H2]=
(310-17 s-1)
(510-7 cm3 s-1) (2400)
= 10-7 cm-3Density
Independent
If L ~ 3 pc ~ 1019 cm, expect only N(H3
+) ~ 1012 cm-2!
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HH33++ in Lots of Diffuse Clouds! in Lots of Diffuse Clouds!
8
6
4
2
0
H3+
Col
umn
Den
sity
(10
14cm
-2)
6543210
E(B-V) (mag)
OphP Cygni
HD 183143
WR 118
Cyg OB2 12
WR 104
Cyg OB2 5
WR 121
HD 168607
HD 194279
GC IRS 3
2 Ori
HD 20041
1.01
1.00
0.99
0.98
0.97
Rel
ativ
e In
tens
ity
3.7173.7163.7153.6693.6683.667
Wavelength (µm)
R(1,1)u
R(1,1)l
R(1,0)
HD 183143
McCall, et al.ApJ 567, 391 (2002)
Cygnus OB2 12
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Big Problem with the Chemistry!Big Problem with the Chemistry!
Steady State: [H3+]
=
ke[e-][H2]
To increase the value of [H3+], we need:
• Smaller electron fraction [e-]/[H2]
• Smaller recombination rate constant ke
• Higher ionization rate
>1 order of magnitude!!
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PerseiPersei
HH33++ toward toward Persei Persei
Cardelli et al. ApJ 467, 334 (1996)Savage et al. ApJ 216, 291 (1977)
N(H2) from Copernicus
N(C+) from HST
[e-]/[H2]not to blame
McCall, et al. Nature 422, 500 (2003)
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Big Problem with the Chemistry!Big Problem with the Chemistry!
Steady State: [H3+]
=
ke[e-][H2]
To increase the value of [H3+], we need:
• Smaller electron fraction [e-]/[H2]
• Smaller recombination rate constant ke
• Higher ionization rate
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Enigma of HEnigma of H33++ Recombination Recombination
• Laboratory values of ke have varied by 4 orders of magnitude!
• Problem: not measuring H3
+ in ground states
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Ion Storage Ring MeasurementsIon Storage Ring Measurements
20 ns 45 ns
electron beam
H3+
H, H2
+ Very simple experiment
+ Complete vibrational relaxation
+ Control H3+ – e- impact energy
+ Rotationally cold ions from supersonic expansion source
CRYRING
30 kV30 kV
900 keV900 keV
12.1 MeV12.1 MeV
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CRYRING ResultsCRYRING Results
• Considerable amount of structure (resonances) in the cross-section
• ke = 2.6 10-7 cm3 s-1
• Factor of two smallerMcCall, et al.Phys. Rev. A 70, 052716 (2004)
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Agreement with Other WorkAgreement with Other Work
• Reasonable agreement between:– CRYRING
• Supersonic
expansion
– TSR• 22-pole trap
– Theory
S.F. dos Santos, V. Kokoouline and C. H. Greene, J. Chem. Phys 127 (2007) 124309
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Big Problem with the Chemistry!Big Problem with the Chemistry!
Steady State: [H3+]
=
ke[e-][H2]
To increase the value of [H3+], we need:
• Smaller electron fraction [e-]/[H2]
• Smaller recombination rate constant ke
• Higher ionization rate
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(71013 cm-2)
Implications for Implications for Persei Persei
[H3+]
L =
ke N(e-)
N(H2)==L
N(H3+)
N(H2)
N(e-)
L = 5300 cm s-1
ke N(H3+) (1.610-7 cm3 s-1) (4.710-4)
Adopt =310-17 s-1
Adopt n = 215 cm-3 → L=2.4 pc
L = 60 pcn = 9 cm-3
=7.410-16 s-1
(25x higher!)
(firm)(densecloudvalue)
Similar results in many other sightlines N. Indriolo, T. R. Geballe, T. Oka, & B. J. McCall, ApJ 671, 1736 (2007)
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Surprise Surprise → Conventional Wisdom→ Conventional Wisdom
• Higher in diffuse (vs. dense) clouds initially greeted with “skepticism”
• Incorporated into models without incident
• Now generally accepted (but not understood!)
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Low Energy CRs?Low Energy CRs?• Could there be
a large flux of low energy cosmic rays?
M.D. Stage, G. E. Allen, J. C. Houck, J. E. Davis, Nat. Phys. 2, 614 (2006)
T. E. Cravens & A. Dalgarno, ApJ 219, 750 (1978)
.13
.44830160600
AV
1 MeV
2 MeV
10 MeV
20 MeV
50 MeV
(diffuse) (dense)
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Cosmic Ray ObservationsCosmic Ray Observations
W.R.Webber, ApJ 506, 329 (1998)
Inferred interstellar
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Theoretical SpectraTheoretical Spectra
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Consider This SpectrumConsider This Spectrum
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Inferred Ionization RateInferred Ionization Rate
dense~6×10-17 s-1
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Inferred Ionization RateInferred Ionization Rate
dense~6×10-17 s-1
diffuse~3.1×10-16 s-1
See poster 05.04(Nick Indriolo)
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SummarySummary
• H3+ surprisingly abundant in diffuse clouds
– Enabled by laboratory spectroscopy
• H3+ is now a direct probe of ionization rate
– Enabled by storage ring measurements & theory
• Ionization rate ~10× higher than thought– only in diffuse clouds– would still be unknown if not for laboratory
astrophysics work
• Two proposed explanations– MHD self-confinement (Padoan & Scalo)– high flux of low energy cosmic rays
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AcknowledgmentsAcknowledgments
http://astrochemistry.uiuc.edu
Nick Indriolo(U. Illinois)
Brian Fields (U. Illinois)
Tom Geballe(Gemini)
Takeshi Oka(U. Chicago)
NASA LaboratoryAstrophysics
NSF Divisions of Chemistry & Astronomy
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Astronomer's Periodic TableAstronomer's Periodic Table
H He
C N O Ne
Mg
Fe
Si S Ar
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Observing Interstellar HObserving Interstellar H33++
• Equilateral triangle• No rotational spectrum• No electronic spectrum• Vibrational spectrum is
only probe
• Absorption spectroscopy against background or embedded star
1
2
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Interstellar Cloud Classification*Interstellar Cloud Classification*Diffuse clouds:
• H ↔ H2
• C C+
• n(H2) ~ 101–103 cm-3
– [~10-18 atm]
• T ~ 50 K
PerseiPersei
Photo: Jose Fernandez Garcia
• Diffuse atomic clouds– H2 << 10%
• Diffuse molecular clouds– H2 > 10% (self-shielded)
* Snow & McCall, ARAA, 44, 367 (2006)
Barnard 68 (courtesy João Alves, ESO)
Dense molecular clouds:
• H H2
• C CO• n(H2) ~ 104–106 cm-3
• T ~ 20 K
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[H2]
Dense Cloud HDense Cloud H33++ Chemistry Chemistry
H2 H2+ + e-
H2 + H2+ H3
+ + H
cosmic ray
H3+ + CO HCO+ + H2
Rate =
Formation
Destruction
Rate = k [H3+] [CO]
[H3+]
=
k [CO]
Steady State
=(310-17 s-1)
(210-9 cm3 s-1)
[H2]
(6700)
= 10-4 cm-3Density
Independent!
(fast)
McCall, Geballe, Hinkle, & OkaApJ 522, 338 (1999)
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HH33++ as a Probe of Dense Clouds as a Probe of Dense Clouds
• Given n(H3+) from model, and N(H3
+) from infrared observations:– path length L = N/n ~ 31018 cm ~ 1 pc
– density n(H2) = N(H2)/L ~ 6104 cm-3
– temperature T ~ 30 K
• Unique probe of clouds• Consistent with expectations
– confirms dense cloud chemistry (forbidden)
32.9 K
K
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R(1
,0)
R(1
,1)u
R(2
,2)l
33 K151 K(0,0)(0,0)
(2,0)(2,0)
(J,G)(J,G)
probe of temperature
not detected
HH33++ Energy Level Structure Energy Level Structure
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Spectroscopy of HSpectroscopy of H33++ Source Source
• Confirmed that H3
+ produced is rotationally cold, as in interstellar medium
Infrared Cavity Ringdown Laser Absorption Spectroscopy
McCall, et al.Nature 422, 500 (2003)
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Theoretical CalculationsTheoretical Calculations
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TSR ResultsTSR Results
H. Kreckel, et al.Phys. Rev. Lett. 95, 263201 (2005)
CRYRING TSR
Ion sourceSupersonic expansion
RF 22-pole ion trap @ 13 K
Electron targetkT~ 2 meV
ne~6.3×106 cm-3
kT~ 500 µeV
ne~4.5×105 cm-3
Electron cooler (same as target)kT~ 10.5 meV
ne~1.6×107 cm-3
Beam energy 12.1 MeV 5.25 MeV
CRYRINGTSR
theory• Good agreement
• Different ion production
• Different conditions
• Experiments likely “right”!
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Supersonic Expansion Ion SourceSupersonic Expansion Ion Source
• Similar to sources used for laboratory spectroscopy
• Pulsed nozzle design• Supersonic expansion
leads to rapid cooling• Discharge from ring
electrode downstream• Spectroscopy used to
characterize ions
H2Gas inlet
2 atm
Solenoid valve
-900 Vring
electrode
Pinhole flange/ground
electrode
H3+
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Observational Consequences?Observational Consequences?
• Energy required for acceleration– about 0.2 × 1051 ergs/century
• Heating of diffuse clouds– about 1/10 of photoelectric heating
• Production of LiBeB (spallation)– roughly consistent with observed abundances
• γ-ray line production (nuclear excitation)– below detectable limits
our spectrum is not excluded by observations!
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The Future (The Dream?)The Future (The Dream?)
• Improved precision in determinations– improved density estimates– more sophisticated cloud models
• Measure H3+ in wider range of sightlines
– diffuse, translucent, dense clouds
• Infer (AV) → cosmic ray spectrum
– information on acceleration mechanism(s)– information on galactic propagation
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Recent Astronomical ResultsRecent Astronomical Results
• Range of ζ from 1.1-7.3 10-16 s-1
• Biggest uncertainty is in adopted n N. Indriolo, T. R. Geballe, T. Oka, & B. J. McCall, ApJ 671, 1736 (2007)