H 3 + : A Case Study for the Importance of Molecular Laboratory Astrophysics Ben McCall Dept. of...

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

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

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)

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

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

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!

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

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!!

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)

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

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

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

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)

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

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

(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)

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!)

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)

Cosmic Ray ObservationsCosmic Ray Observations

W.R.Webber, ApJ 506, 329 (1998)

Inferred interstellar

Theoretical SpectraTheoretical Spectra

Consider This SpectrumConsider This Spectrum

Inferred Ionization RateInferred Ionization Rate

dense~6×10-17 s-1

Inferred Ionization RateInferred Ionization Rate

dense~6×10-17 s-1

diffuse~3.1×10-16 s-1

See poster 05.04(Nick Indriolo)

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

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

Astronomer's Periodic TableAstronomer's Periodic Table

H He

C N O Ne

Mg

Fe

Si S Ar

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

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

[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)

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

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

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)

Theoretical CalculationsTheoretical Calculations

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”!

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+

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!

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

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)