Download What is H 3 +

Observations of
+
H3
The Initiator of Interstellar Chemistry
Benjamin McCall
Oka Ion Factory
University of Chicago
Thomas Geballe
Gemini Observatory (HI)
Kenneth Hinkle
National Optical Astronomy Observatories
Kitt Peak National Observatory (AZ)
Takeshi Oka
Oka Ion Factory
University of Chicago
Astronomer’s Periodic Table
He
C
Si
Mg
Fe
N
O
Ne
S
Ar
Molecules in Space
H2
H3+
NH3
CH4
C2H4
C6 H
CH3C3N
CO
H2O
C 2 H2
SiH4
CH3CN
CH2CHCN
HCOOCH3
CH
CO2
H2CO
c-C3H2
C5 H
CH3C2H
C7 H
CH+
HCO
H3O+
l-C3H2
C5 O
HC5N
H2C6
OH
HCO+
c-C3H
C5
CH3NC
HCOCH3
CH3C4H
C2
HOC+
l-C3H
C4 H
CH3OH
NH2CH3
CH3CH2CN
CN
HCN
C3 N
C4Si
CH3SH
c-C2H4O
(CH3)2O
CO+
C3
C3 O
CH2CN
HC3NH+
CH3CH2OH
NO
C2 O
C3 S
HC3N
HC2CHO
HC7N
AlF
C2 S
HCCN
HC2NC
HCONH2
C8 H
AlCl
CH2
HCNH+
HCOOH
l-H2C4
(CH3)2CO
CP
HCS+
HNCO
H2CHN
C5 N
HC9N
CS
H2S
HNCS
H2C2O
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
C2 H
SiS
NH2
HC11N
116 molecules...and counting!
HF
http://www.cv.nrao.edu/~awooten/allmols.html [updated 05/06/99]
Ion-Neutral Reactions
H3+ is abundantly produced in the interstellar medium
through the cosmic-ray ionization of H2

2
H 2  H  e
cosmic ray

H2  H  H  H

2

3
H3+ initiates a network of ion-neutral reactions,
which is responsible for most observed molecules
H 3  X  HX   H 2
HX   Y  XY   H
H 3  O  OH   H 2
OH  H 2  H 2O  H
H 2O   H 2  H 3O   H
H 3O   e-  H 2O  H
Tree of Interstellar Chemistry
What is
+
H3 ?
 Equilateral triangle structure
 No allowed rotational spectrum
 No electronic spectrum
 Infrared spectrum obtained in 1980
T. Oka,
PRL 45,531 (1980)
n2x
n1
Infrared inactive
stretching mode
n2y
Infrared active
degenerate bending mode
Searching for H3
United Kingdom Infrared Telescope (UKIRT)
Mauna Kea, Hawaii
Nicholas U. Mayall Telescope
Kitt Peak, AZ
+
Cooled Grating Spectrometer 4 (CGS4)
R ~ 20,000
Phoenix Spectrometer
R ~60,000
+
H3 in Molecular Clouds
Geballe & Oka
Nature, 384, 334 (1996)
N(H3+) ~ 1014 cm-2
1.02
MonR2 IRS 3
1.02
1.00
AFGL 2136
1.00
0.98
AFGL 961E
0.98
0.96
AFGL 2591
0.96
0.94
AFGL 490
0.94
0.92
36640
36660
36680
36700
37150
37170
0.90
36660
36680
36700
36720
McCall, Geballe, Hinkle, & Oka
Astrophysical Journal, 522, 338 (1999)
+
H3 Chemistry
Formation:
Rate:
H 2  H 2  e 
cosmic ray
H 2  H 2  H 3  H
 n(H2)
Destruction:

3

H  CO  HCO  H 2
kCO n(H3+) n(CO)
Steady State:
ζ n(H 2 )
n(H ) 
 constant
k CO n(CO) (~ 10 cm )

3
-4
-3
+
H3 as a Probe
Path Length:
N(H 3 )
L
n(H 3 )
~ 1 parsec
Mean Density:
N(H 2 )
n(H 2 ) 
L
~ 104–105 cm-3
Kinetic Temperature:
I
3
2
ortho-H3+

3

3
N ortho(H ) g ortho

e
N para (H ) g para

I
1
2
para-H3+
E
kT
~ 25–50 K
Agreement with canonical dense cloud
values confirms ion-neutral reaction scheme.
McCall, Geballe, Hinkle, & Oka
Astrophysical Journal, 522, 338 (1999)
Galactic Center
Npara
Northo
Nbroad
Geballe, McCall, Hinkle, & Oka
Astrophysical Journal, 510, 251 (1999)
= 5.1(1.7) × 1014 cm-2
= 2.4(1.1) × 1014 cm-2
= 17.5(3.9) × 1014 cm-2
Galactic Rotation
About Cyg OB2 #12
 Bill Morgan, 1954
 distance ~ 1.7 kpc
 L ~ 106 L
 visual extinction ~ 10 mag
Morgan, Johnson, & Roman
PASP 66, 85 (1954)
 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 with
n ~ few hundred cm-3
 Less than 5% of C atoms
in CO
 long path of diffuse material
Diffuse Cloud H3
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, & Oka
Science 279, 1910 (1998)
CO Observations
N(CO) ~ few ×1016 cm-2
N(CO)
 5%
N( C)
 CO scarce  diffuse
 velocity consistent
with H3+, C2
+
Diffuse H3 Chemistry
Formation:
Rate:
H 2  H 2  e 
cosmic ray
H 2  H 2  H 3  H
 n(H2)
Destruction:
H 3  e   H  H  H (75%)
 H  H2
(25%)
ke n(H3+) n(e-)
Steady State:
ζ n(H 2 )
n(H ) 
 constant
(~ 10 cm )
k e n(e )

3
-6
-3
+
H3 vs. Density
6
10
H2
5
10
4
10
3
10
2
10
CO
1
10
H
0
10
n(X)
-1
10
-2
10
-3
10
+
H3
-4
10
+
C
-5
10
-6
10
-7
10
-8
10
Diffuse Clouds
Dense Clouds
-9
10
6
10
5
10
4
10
3
2
10
10
n(H)
1
10
0
10
-1
10
Long Path, Low Density
Model:
n(H3+) = 4 ×10-7 cm-3
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),
N(H 2 ) 2 1022
-3
n(H 2 ) ~
~
~
20
cm
L
10 21
Seems unreasonably low...
also inconsistent with C2 and CO observations!
Dalgarno’s Model
“clump”
L = 6.7 pc
n(H2) = 100 cm-3
H3+, OH
“cloudlet”
L ~ 0.01 pc
n = 104
C2
Cecchi-Pestellini & Dalgarno,
MNRAS, submitted
“core”
L ~ 1 AU
n = 106
CO, HCO+
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!
HCO+ recently observed with consistent
Scappini, Cecchi-Pestellini, Codella, &
abundance.
Dalgarno, A&A, submitted
Problems Remain
 Pressure balance between components difficult
 Cyg OB2 #5 (2.5 pc away) shows similar
H3+ and C2
 All parameters at extremes to maximize H3+
 A more general solution would be desirable.
Atmosphere
CH4
Cyg OB2 #12
Cyg OB2 #5
CH4
N = 2.3(3) × 1014 cm-2
N = 2.0(5) × 1014 cm-2
Impact of H3
+
Dense Clouds:
Path length
H2 number density
Kinetic temperature
Confirmation of ion-neutral scheme
Diffuse Clouds:
Apparently long path lengths?
Rich chemistry even in rarefied
environment?
Insight into Diffuse Interstellar Bands?
Probing a new ISM component?