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Observations of H3+
The Initiator of Interstellar Chemistry
Benjamin McCallOka Ion Factory
University of Chicago
Thomas GeballeGemini Observatory (HI)
Kenneth HinkleNational Optical Astronomy Observatories
Kitt Peak National Observatory (AZ)
Takeshi OkaOka Ion Factory
University of Chicago
Astronomer’s Periodic TableAstronomer’s Periodic Table
C N O Ne
Mg
Fe
Si S Ar
H He
Molecules in SpaceMolecules in Space
H2 H3+ NH3 CH4 C2H4 C6H CH3C3N
CO H2O C2H2 SiH4 CH3CN CH2CHCN HCOOCH3
CH CO2 H2CO c-C3H2 C5H CH3C2H C7H
CH+ HCO H3O+ l-C3H2 C5O HC5N H2C6
OH HCO+ c-C3H C5 CH3NC HCOCH3 CH3C4H
C2 HOC+ l-C3H C4H CH3OH NH2CH3 CH3CH2CN
CN HCN C3N C4Si CH3SH c-C2H4O (CH3)2O
CO+ C3 C3O CH2CN HC3NH+ CH3CH2OH
NO C2O C3S HC3N HC2CHO HC7N
AlF C2S HCCN HC2NC HCONH2 C8H
AlCl CH2 HCNH+ HCOOH l-H2C4 (CH3)2CO
CP HCS+ HNCO H2CHN C5N HC9N
CS H2S HNCS H2C2O HC11N
CSi HNC HOCO+ H2NCN
HCl HNO H2CN HNC3
KCl MgCN H2CS H2COH+
NH MgNC SiC3
NS N2H+
NaCl N2O
PN NaCN
SO OCS
SO+ SO2
SiN c-SiC2
SiO C2H
SiS NH2
HF
116 molecules...and counting!
http://www.cv.nrao.edu/~awooten/allmols.html [updated 05/06/99]
Ion-Neutral ReactionsIon-Neutral Reactions
eHH 2ray cosmic
2
H H HH 322
H3+ is abundantly produced in the interstellar medium
through the cosmic-ray ionization of H2
H3+ initiates a network of ion-neutral reactions,
which is responsible for most observed molecules
23 H HXX H
HXY YHX
23 HOH O H
HO H HOH 22
HO H HOH 322
HO He OH 2-
3
Tree of Interstellar Chemistry
T. Oka,PRL 45,531 (1980)
What is HWhat is H33+ + ??
2y
2x
Equilateral triangle structure No allowed rotational spectrum No electronic spectrum Infrared spectrum obtained in 1980
Infrared inactivestretching mode
Infrared activedegenerate bending mode
Searching for HSearching for H33++
United Kingdom Infrared Telescope (UKIRT)Mauna Kea, Hawaii
Cooled Grating Spectrometer 4 (CGS4)R ~ 20,000
Nicholas U. Mayall TelescopeKitt Peak, AZ
Phoenix SpectrometerR ~60,000
HH33++ in Molecular Clouds in Molecular Clouds
1.02
1.00
0.98
0.96
0.94
0.92
0.9036720367003668036660
MonR2 IRS 3
AFGL 961E
AFGL 490
1.02
1.00
0.98
0.96
0.94
36700366803666036640 3717037150
AFGL 2136
AFGL 2591
McCall, Geballe, Hinkle, & OkaAstrophysical Journal, 522, 338 (1999)
Geballe & OkaNature, 384, 334 (1996)
N(H3+) ~ 1014 cm-2
HH33++ Chemistry Chemistry
Formation:
Destruction:
23 HHCO COH
kCO n(H3+) n(CO)
eHH 2ray cosmic
2
H H HH 322
n(H2)
Steady State:
constantn(CO)
)n(H
k
ζ)n(H 2
CO3
(~ 10-4 cm-3)
Rate:
para-H3+
HH33++ as a Probe as a Probe
)n(H
)N(HL
3
3
L
)N(H)n(H 2
2
kT
E
para
ortho
3para
3ortho
g
g
)(HN
)(HN
e
~ 1 parsec
~ 104–105 cm-3
~ 25–50 K
Agreement with canonical dense cloudvalues confirms ion-neutral reaction scheme.
McCall, Geballe, Hinkle, & OkaAstrophysical Journal, 522, 338 (1999)
2
3I
2
1I
ortho-H3+
Path Length:
Mean Density:
Kinetic Temperature:
Galactic CenterGalactic Center
Npara = 5.1(1.7) × 1014 cm-2 Northo = 2.4(1.1) × 1014 cm-2
Nbroad = 17.5(3.9) × 1014 cm-2
Geballe, McCall, Hinkle, & OkaAstrophysical Journal, 510, 251 (1999)
Galactic RotationGalactic Rotation
About Cyg OB2 #12About Cyg OB2 #12
Morgan, Johnson, & RomanPASP 66, 85 (1954)
Bill Morgan, 1954
distance ~ 1.7 kpc
L ~ 106 L
visual extinction ~ 10 mag
N(H) ~ 2 × 1022 cm-2
no 3.08 µm ice feature no dense clouds
strong 3.4 µm C-H band diffuse clouds
C2 consistent withn ~ few hundred cm-3
Less than 5% of C atomsin CO
long path of diffuse material
Diffuse Cloud HDiffuse Cloud H33++
observed at UKIRT observed at Kitt Peak
Npara = 2.4(3) × 1014 cm-2
Northo = 1.4(2) × 1014 cm-2
Similar column density to dense clouds!!McCall, Geballe, Hinkle, & OkaScience 279, 1910 (1998)
CO ObservationsCO Observations
N(CO) ~ few ×1016 cm-2
N(CO)5%
N( C)
CO scarce diffuse
velocity consistent with H3
+, C2
constant)n(e
)n(H
k
ζ)n(H
-2
e3
Diffuse HDiffuse H33++ Chemistry Chemistry
(25%) HH
(75%)H HHeH
2
3
ke n(H3+) n(e-)
Formation:
Destruction:
eHH 2ray cosmic
2
H H HH 322
n(H2)
Steady State:
Rate:
(~ 10-6 cm-3)
10-9
10-8
10-7
10-6
10-5
10-4
10-3
10-2
10-1
100
101
102
103
104
105
106
n(X
)
10-1
100
101
102
103
104
105
106
n(H)
Dense Clouds Diffuse Clouds
H2
HCO
C+H3
+
HH33++ vs. Density vs. Density
Long Path, Low DensityLong Path, Low Density
Observation: N(H3+) = 4 ×1014 cm-2
L ~ 1021 cm ~ 300 pc!Seems unreasonably long...
For N(H2) ~ 2 × 1022 cm-2 (inferred from visual extinction),
21
222
2 10
102~
L
)N(H~)n(H
~ 20 cm-3
Model: n(H3+) = 4 ×10-7 cm-3
Seems unreasonably low...also inconsistent with C2 and CO observations!
Dalgarno’s ModelDalgarno’s Model
Several “clumps” with hierarchical structure are invoked to produce H3
+.
In this model, the vast majority of the H3+ is
not associated with CO or C2!
“clump”L = 6.7 pc
n(H2) = 100 cm-3
H3+, OH
“cloudlet”L ~ 0.01 pc
n = 104
C2
“core”L ~ 1 AUn = 106
CO, HCO+
Cecchi-Pestellini & Dalgarno, MNRAS, submitted
HCO+ recently observed with consistent abundance. Scappini, Cecchi-Pestellini, Codella, &
Dalgarno, A&A, submitted
Problems RemainProblems Remain
N = 2.3(3) × 1014 cm-2
N = 2.0(5) × 1014 cm-2
Cyg OB2 #12
Cyg OB2 #5
Atmosphere
CH4 CH4
Pressure balance between components difficult
Cyg OB2 #5 (2.5 pc away) shows similarH3
+ and C2
All parameters at extremes to maximize H3+
A more general solution would be desirable.
Impact of HImpact of H33++
Dense Clouds:
Path lengthH2 number densityKinetic temperature
Confirmation of ion-neutral scheme
Diffuse Clouds:
Apparently long path lengths?
Rich chemistry even in rarefiedenvironment?
Insight into Diffuse Interstellar Bands?
Probing a new ISM component?
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