Observations of H3

1
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
2
Astronomers Periodic Table
H
He
O
Ne
C
N
Si
S
Ar
Mg
Fe
3
Molecules in Space
H2 H3 NH3 CH4 C2H4 C6H CH3C3N CO H2O C2H2 SiH4 CH
3CN 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 HC2C
HO HC7N AlF C2S HCCN HC2NC HCONH2 C8H AlCl CH2 H
CNH HCOOH l-H2C4 (CH3)2CO CP HCS HNCO H2CHN C5N
HC9N CS H2S HNCS H2C2O HC11N CSi HNC HOCO H2N
CN HCl HNO H2CN HNC3 KCl MgCN H2CS H2COH NH MgN
C SiC3 NS N2H NaCl N2O PN NaCN SO OCS SO SO
2 SiN c-SiC2 SiO C2H SiS NH2 HF
116 molecules...and counting!
http//www.cv.nrao.edu/awooten/allmols.html
updated 05/06/99
4
Ion-Neutral Reactions
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
5
Tree of Interstellar Chemistry
6
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
n2y
Infrared inactive stretching mode
Infrared active degenerate bending mode
7
Searching for H3
Cooled Grating Spectrometer 4 (CGS4) R 20,000
United Kingdom Infrared Telescope (UKIRT) Mauna
Kea, Hawaii
Nicholas U. Mayall Telescope Kitt Peak, AZ
Phoenix Spectrometer R 60,000
8
H3 in Molecular Clouds
Geballe Oka Nature, 384, 334 (1996)
N(H3) 1014 cm-2
McCall, Geballe, Hinkle, Oka Astrophysical
Journal, 522, 338 (1999)
9
H3 Chemistry
Formation
Rate
? n(H2)
Destruction
kCO n(H3) n(CO)
Steady State
( 10-4 cm-3)
10
H3 as a Probe
Path Length
1 parsec
Mean Density
104105 cm-3
Kinetic Temperature
para-H3
ortho-H3
2550 K
Agreement with canonical dense cloud values
confirms ion-neutral reaction scheme.
McCall, Geballe, Hinkle, Oka Astrophysical
Journal, 522, 338 (1999)
11
Galactic 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, Oka Astrophysical
Journal, 510, 251 (1999)
12
Galactic Rotation
13
About Cyg OB2 12
? 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 with n few hundred
cm-3 ? Less than 5 of C atoms in CO ? long
path of diffuse material
Morgan, Johnson, Roman PASP 66, 85 (1954)
14
Diffuse Cloud H3
observed at Kitt Peak
observed at UKIRT
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)
15
CO Observations
N(CO) few 1016 cm-2
? CO scarce ? diffuse ? velocity consistent
with H3, C2
16
Diffuse H3 Chemistry
Formation
Rate
? n(H2)
Destruction
ke n(H3) n(e-)
Steady State
( 10-6 cm-3)
17
H3 vs. Density
18
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),
20 cm-3
Seems unreasonably low... also inconsistent with
C2 and CO observations!
19
Dalgarnos Model
Cecchi-Pestellini Dalgarno, MNRAS, submitted
clump L 6.7 pc n(H2) 100 cm-3 H3, OH
core L 1 AU n 106 CO, HCO
cloudlet L 0.01 pc n 104 C2
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 abundance.
Scappini, Cecchi-Pestellini, Codella, Dalgarno,
AA, submitted
20
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.
21
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?