Comparing the Ortho-to-Para Ratios of H2 and H3 in Diffuse Interstellar Clouds

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Comparing the Ortho-to-Para Ratios of H2 and H3
in Diffuse Interstellar Clouds

• N. Indriolo1, T. Oka2, T. R. Geballe3,
  K. H. Hinkle4, G. A. Blake5, B. J. McCall1

1 University of Illinois at Urbana-Champaign 2
University of Chicago 3 Gemini Observatory 4
NOAO 5 California Institute of Technology
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Motivations

• Astronomers use excitation temperatures to
  estimate cloud kinetic temperatures
• For H2, it is assumed that T01?Tkin
• Originally assumed that H3 excitation
  temperature for (J,K)(1,1) (1,0) states was
  also a good approximation
• However, observations argue differently
• ltT01gt 70 K
• ltT(H3)gt 30 K
• Given that H3 forms from H2, why do they have
  different excitation temperatures?

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Background
para ortho
H2
J0 J1
H2 thermalized by collisions with protons
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Background
para ortho
H2
J0 J1
H3
(J,K)(1,1) (J,K)(1,0)
H2 thermalized by collisions with protons
H3 thermalized by collisions with H2
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Chemistry

• Resulting nuclear spin configurations depend on
  selection rules
• An H3 molecule suffers on the order of 100
  collisions with H2 during its lifetime
• WJ08 Kyle Crabtree 2015 McPherson 329pm

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Observations

• Targeted sight lines where J0 and J1 column
  densities of H2 are known
• Searched for the R(1,1)u, R(1,0), and R(1,1)l
  transitions of the ?2?0 band of H3
• Instruments/Telescopes
• CGS4/UKIRT (previously acquired data)
• Phoenix/Gemini South
• CRIRES/VLT
• NIRSPEC/Keck

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Observations
UKIRT Gemini S. Keck VLT
? Per HD 154368 62 Tau HD 73882
X Per ? Oph ? Cep HD 110432
? Per ? Nor HD 21483 HD 53367
? Per HD 149404 HD 21856
? Per HD 152236 ? Cam
40 Per
? Oph D
HD 206267
? Oph
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Spectra
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Results
T01 (K) Tex(H3) (K) p-H2 p-H3
? Per 58 28 0.68 0.62
X Per 57 30 0.69 0.60
HD 73882 51 21 0.76 0.71
HD 110432 69 27 0.57 0.62
HD 154368 51 20 0.76 0.73
Indriolo et al. 2007, ApJ, 671, 1736
Rachford et al. 2009, ApJS, 180, 125 Rachford et
al. 2002, ApJ, 577, 221 Savage et al.
1977, ApJ, 216, 291
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Para Fractions
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Para Fractions
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Para Fractions
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Estimating T01 from H3

• Laboratory work and theory give us various
  relationships between the para-fractions of H2
  and H3
• Astronomical data are consistent with laboratory
  results, but do not provide a well-constrained
  relationship

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Estimating T01 from H3
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Estimating T01 from H3

• Laboratory work and theory give us various
  relationships between the para-fractions of H2
  and H3
• Astronomical data are consistent with laboratory
  results, but do not provide a well-constrained
  relationship
• More H3 observations with less uncertainty will
  better constrain how p-H3 varies with p-H2
• Given such a relationship, we may be able to
  convert T(H3) to T01 in the future

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Estimating T01 from H3
2 K in T(H3) means 15 K in T01
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Conclusions

• Weve expanded the sample of sight lines with H2
  and H3 detections from 2 to 5
• T01 and T(H3) do not agree in any sight line
  where both are measured
• Agreement with laboratory data suggests that the
  H2 H3 ? H3 H2 reaction may determine T(H3)
  in the ISM

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Future Work

• Colleagues will continue to study the H3
  para-fraction in the laboratory
• Search for H3 absorption in sight lines with
  measured H2
• Using COS on Hubble, observe H2 in the UV in
  sight lines with measured H3

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Thanks
Dr. Holger Kreckel Kyle Crabtree Carrie Kauffman
Lt. Col. Brian Tom