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Comparing the Ortho -to- Para Ratios of H 2 and H 3 + in Diffuse Interstellar Clouds

This research compares the excitation temperatures of H2 and H3+ in diffuse interstellar clouds to understand why they have different temperatures. The study includes observations, chemistry, and estimations based on laboratory work.

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Comparing the Ortho -to- Para Ratios of H 2 and H 3 + in Diffuse Interstellar Clouds

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  1. 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

  2. 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 • <T01> = 70 K • <T(H3+)> = 30 K • Given that H3+ forms from H2, why do they have different excitation temperatures?

  3. Background H2 thermalized by collisions with protons

  4. + + Background H2 thermalized by collisions with protons H3+ thermalized by collisions with H2

  5. 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; 3:29pm

  6. Observations • Targeted sight lines where J=0 and J=1 column densities of H2 are known • Searched for the R(1,1)u, R(1,0), and R(1,1)l transitions of the ν20 band of H3+ • Instruments/Telescopes • CGS4/UKIRT (previously acquired data) • Phoenix/Gemini South • CRIRES/VLT • NIRSPEC/Keck

  7. Observations

  8. Spectra

  9. Results 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

  10. Para Fractions

  11. Para Fractions

  12. Para Fractions

  13. 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

  14. Estimating T01 from H3+

  15. 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

  16. Estimating T01 from H3+ ±2 K in T(H3+) means ±15 K in T01

  17. Conclusions • We’ve 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

  18. 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+

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

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