November 6, 2010 MWAM 2010 University of Illinois1 The ortho:para ratio of H 3 + in diffuse molecular clouds Kyle N. Crabtree, Nick Indriolo, Holger Kreckel,

November 6, 2010November 6, 2010MWAM 2010

University of IllinoisMWAM 2010

University of Illinois 11

The ortho:para ratio of H3+

in diffuse molecular cloudsThe ortho:para ratio of H3

+ in diffuse molecular clouds

Kyle N. Crabtree, Nick Indriolo, Holger Kreckel, Brian A. Tom, and Benjamin J. McCall

Kyle N. Crabtree, Nick Indriolo, Holger Kreckel, Brian A. Tom, and Benjamin J. McCall




H3+, Interstellar Chemistry, and

AstrophysicsH3

+, Interstellar Chemistry, and Astrophysics

• Starting point for complex gas-phase chemistry

• Use as an astrophysical probe:

– Gas density at the galactic center

– Imaging of Jupiter’s polar aurorae

– Interstellar cosmic ray ionization rate

– Interstellar temperature

• Starting point for complex gas-phase chemistry

• Use as an astrophysical probe:

– Gas density at the galactic center

– Imaging of Jupiter’s polar aurorae

– Interstellar cosmic ray ionization rate

– Interstellar temperature




H3+ SpectroscopyH3+ Spectroscopy

R(1,0)36685 Å

R(1,1)u

36681 Å

R(1,1)l

37155 Å




H3+ as an Interstellar

ThermometerH3

+ as an Interstellar Thermometer• Observed R(1,0)

and R(1,1)u/R(1,1)l lines n(1,0), n(1,1) T(H3

+)

• Analogous to T01, derived from UV observations of H2 J=0 (para) and J=1 (ortho) levels

• Observed R(1,0) and R(1,1)u/R(1,1)l lines n(1,0), n(1,1) T(H3

+)

• Analogous to T01, derived from UV observations of H2 J=0 (para) and J=1 (ortho) levels




Temperature in Diffuse Molecular CloudsTemperature in Diffuse Molecular Clouds• Diffuse molecular clouds: diffuse clouds

with most hydrogen in molecular form.• Survey of diffuse molecular clouds:

– <T01> ~ 70 K (N = 66) 1

– <T(H3+)> ~ 30 K (N = 18) 2

• Only 2 are in common: ζ-Per and X-Per• Recent observations (N. Indriolo) have

extended this number to 5

• Diffuse molecular clouds: diffuse clouds with most hydrogen in molecular form.

• Survey of diffuse molecular clouds:– <T01> ~ 70 K (N = 66) 1

– <T(H3+)> ~ 30 K (N = 18) 2

• Only 2 are in common: ζ-Per and X-Per• Recent observations (N. Indriolo) have

extended this number to 51 1 B. D. Savage B. D. Savage et al.et al., , ApJApJ, (1977), , (1977), 216216, 291, B. L. Rachford , 291, B. L. Rachford et al.et al., , ApJApJ, (2002), , (2002), 577577, 221, B. L. , 221, B. L. Rachford Rachford et al.et al., , ApJSApJS, (2009), , (2009), 180180, 125., 125.22 N. Indriolo N. Indriolo et al.et al., , ApJApJ, (2007), , (2007), 671671, 1736., 1736.

1 1 B. D. Savage B. D. Savage et al.et al., , ApJApJ, (1977), , (1977), 216216, 291, B. L. Rachford , 291, B. L. Rachford et al.et al., , ApJApJ, (2002), , (2002), 577577, 221, B. L. , 221, B. L. Rachford Rachford et al.et al., , ApJSApJS, (2009), , (2009), 180180, 125., 125.22 N. Indriolo N. Indriolo et al.et al., , ApJApJ, (2007), , (2007), 671671, 1736., 1736.




Temperature DiscrepancyTemperature Discrepancy

ζζ-Per-Per

X-PerX-Per

HD 73882HD 73882

HD 154368HD 154368

HD 110432HD 110432

T(HT(H33++))

TT0101

Which of these Which of these represents the represents the “true” kinetic “true” kinetic temperature?temperature?TT0101

Why is there more Why is there more pp-H-H33

++ than than expected for the expected for the

temperature?temperature?




Chemistry of H3+Chemistry of H3+

• Formation:1. H2 + cosmic ray H2

+ + e- (slow)

2. H2+ + H2 H3

+ + H (fast)

• ThermalizationH3

+ + H2 H2 + H3+

• Destruction:H3

+ + e- H2 + H or 3H

• Nuclear spin dependence?

• Formation:1. H2 + cosmic ray H2

+ + e- (slow)

2. H2+ + H2 H3

+ + H (fast)

• ThermalizationH3

+ + H2 H2 + H3+

• Destruction:H3

+ + e- H2 + H or 3H

• Nuclear spin dependence?




H3+ FormationH3+ Formation

1. H2 + cosmic ray H2+ + e- (slow)

2. H2+ + H2 H3

+ + H (fast)

1. H2 + cosmic ray H2+ + e- (slow)

2. H2+ + H2 H3

+ + H (fast)

ReactionCollision Fraction

Branching Fraction p-H3

+ p-H3+ Fraction

p-H2+ + p-H2

(p2)2 1 (p2)2

p-H2+ + o-H2

(p2) (1-p2) 2/3(2/3) (p2) (1-

p2)

o-H2+ + p-H2

(1-p2) (p2) 2/3(2/3) (1-p2)

(p2)

o-H2+ + o-H2

(1-p2)2 1/3 (1/3)(1-p2)2

Total(1/3) + (2/3)p2




“Nascent” p-H3+ Fraction (p3)“Nascent” p-H3+ Fraction (p3)




Thermalization: H3+ + H2Thermalization: H3+ + H2

“identity”“identity”

“hop”“hop”

“exchange”“exchange”

H5+H5+

11

33

66

• Branching fractions: Sid, Shop, and Sexch

• α ≡ Shop/Sexch (0.5?)• Selection Rules

• Branching fractions: Sid, Shop, and Sexch

• α ≡ Shop/Sexch (0.5?)• Selection Rules

Does the steady state of this reaction give a thermal ortho:para H3

+ ratio at low temperature?

Does the steady state of this reaction give a thermal ortho:para H3

+ ratio at low temperature?




Bimolecular Reactive Equilibrium (BRE)Bimolecular Reactive Equilibrium (BRE)• n(H2)/n(e-) ~ 104; k(H2,H3

+)/kDR ~ 10-9/10-7 ~ 10-2

• H3+ sees ~100 collisions with H2 during its

lifetime

• Assume steady state [p-H3+] is determined

by nuclear-spin-changing collisions with H2:

• Express in terms of p-H3+ fraction (p3):

• n(H2)/n(e-) ~ 104; k(H2,H3+)/kDR ~ 10-9/10-7 ~

10-2

• H3+ sees ~100 collisions with H2 during its

lifetime

• Assume steady state [p-H3+] is determined

by nuclear-spin-changing collisions with H2:

• Express in terms of p-H3+ fraction (p3):

o-H3+ + p-H2 p-H3

+ + o-H2o-H3+ + p-H2 p-H3

+ + o-H2




Nuclear Spin Rate CoefficientsNuclear Spin Rate Coefficients

Parameter

Value(s)

Trot 10 K

Tcoll 10-160 K

Sid 0.1-0.9

Shop 0-1

Sexch 0-1

koooo kooop koopo koopp

kopoo kopop koppo koppp

kpooo kpoop kpopo kpopp

kppoo kppop kpppo kpppp

f (T,Sid,)




p3 vs. p2 Determinationp3 vs. p2 Determination

Trot = 10 KChoose Sid,

α




p3 vs. p2 Determinationp3 vs. p2 Determination

Tcoll p2




BRE ResultsBRE Results

Sid = 0.9

Sid = 0.1




BRE ResultsBRE Results




Steady State ModelSteady State Model

• Include nuclear spin dependent formation and destruction reactions:

• Include nuclear spin dependent formation and destruction reactions:

ke,o = o-H3+ DRke,o = o-H3+ DR

ke,p = p-H3+ DRke,p = p-H3+ DR

xe = Electron fraction (1.5 x 10-4)

f = Molecular fraction (0.9)

xe = Electron fraction (1.5 x 10-4)

f = Molecular fraction (0.9)




Steady State: DR Rates EqualSteady State: DR Rates Equal




Steady State: DR Rates EqualSteady State: DR Rates Equal




Steady State: Theoretical1 DR RatesSteady State: Theoretical1 DR Rates

p-H3+ DR 10x faster

than o-H3+

1dos Santos et al., J. Chem. Phys. (2007), 127, 124309.1dos Santos et al., J. Chem. Phys. (2007), 127, 124309.




Steady State: DR OnlySteady State: DR Only




ConclusionsConclusions• H3

+ ortho:para ratio in diffuse molecular clouds likely governed by competition between thermalization (H3

+-H2 collisions) and destruction by DR with electrons

• H3+ + H2 reaction based on

microcanonical statistical model– quantum scattering calculations and experimental measurements needed

• State-selective DR measurements of H3+

also needed to verify/invalidate model

• H3+ ortho:para ratio in diffuse molecular

clouds likely governed by competition between thermalization (H3

+-H2 collisions) and destruction by DR with electrons

• H3+ + H2 reaction based on

microcanonical statistical model– quantum scattering calculations and experimental measurements needed

• State-selective DR measurements of H3+

also needed to verify/invalidate model




AcknowledgementsAcknowledgements

• McCall group• Takeshi Oka• Steve Federman• Brian Rachford

• McCall group• Takeshi Oka• Steve Federman• Brian Rachford

Documents

November 6, 2010 MWAM 2010 University of Illinois1 The ortho:para ratio of H 3 + in diffuse molecular clouds Kyle N. Crabtree, Nick Indriolo, Holger Kreckel,