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?