Probing Cosmic-Ray Acceleration and Propagation with H3 Observations

1
Probing Cosmic-Ray Acceleration and Propagation
with H3 Observations

• Nick Indriolo, Brian D. Fields,
• Benjamin J. McCall
• University of Illinois at Urbana-Champaign

2
Collaborators

• Takeshi Oka University of Chicago
• Tom Geballe Gemini Observatory
• Tomonori Usuda Subaru Telescope
• Miwa Goto Max Planck Institute for Astronomy
• Geoff Blake California Institute of Technology
• Ken Hinkle NOAO

3
Cosmic Ray Basics

• Energetic charged particles and nuclei
• Thought to be primarily accelerated in supernova
  remnants
• Diffuse throughout the interstellar medium along
  magnetic field lines
• Generally assumed that the cosmic-ray spectrum is
  uniform in the Galaxy

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Example Cosmic-Ray Spectra
1 - Nath, B. B., Biermann, P. L. 1994, MNRAS,
267, 447 2 - Hayakawa,
S., Nishimura, S., Takayanagi, T. 1961, PASJ,
13, 184 3 - Valle, G., Ferrini, F., Galli,
D., Shore, S. N. 2002, ApJ, 566, 252
4 - Kneller, J. P., Phillips, J. R., Walker, T.
P. 2003, ApJ, 589, 217 5 - Spitzer,
L., Jr., Tomasko, M. G. 1968, ApJ, 152, 971
6 Indriolo, N.,
Fields, B. D., McCall, B. J. 2009, ApJ, 694, 257
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Interactions with the ISM

• Ionization and excitation of atoms and molecules
• CR H ? CR p e-
• CR H2 ? CR H2 e-
• Spallation of ambient nuclei and of heavier
  cosmic rays
• CR C,N,O ? CR Li,Be,B fragments

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Interactions with the ISM

• Excitation of nuclear states, resulting in
  gamma-ray emission
• CR 12C ? CR 12C ? 12C ?4.44
• CR 16O ? CR 16O ? 16O ?6.13
• Production of mesons (?, ?-, ?0) during
  inelastic collisions
• CR H ? CR H ?0

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Cross Sections
Bethe, H. 1933, Hdb. d Phys. (Berlin J.
Springer), 24, Pt. 1, 491 Read, S. M., Viola,
V. E. 1984, Atomic Data Nucl. Data, 31, 359
Meneguzzi, M. Reeves, H. 1975, AA, 40, 91
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Pionic Gamma-Rays Supernova Remnants
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Pionic Gamma-Rays Supernova Remnants
VERITAS gamma-ray map of IC 443 Acciari et al.
2009, ApJ, 698, L133
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Pionic Gamma-Rays Supernova Remnants
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Pionic Gamma-Rays Supernova Remnants
Abdo et al. 2010, ApJ, 718, 348
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Tracing Lower-Energy Cosmic Rays

• Formation of molecular ion H3 begins with
  ionization of H2
• CR H2 ? H2 e- CR
• H2 H2 ? H3 H
• Cross section for ionization increases as
  cosmic-ray energy decreases, so H3 should trace
  MeV particles

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H3 Chemistry

• Formation
• CR H2 ? H2 e- CR
• H2 H2 ? H3 H
• Destruction
• H3 CO ? HCO H2 (dense clouds)
• H3 e- ? H2 H or H H H (diffuse clouds)
• Steady state in diffuse clouds

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Calculating the Ionization Rate
xe from C Cardelli et al. 1996, ApJ, 467, 334
nH from C2 Sonnentrucker et al. 2007, ApJS, 168,
58
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Observations

• Transitions of the ?2 ? 0 band of H3 are
  available in the infrared
• R(1,1)u 3.66808 ?m R(1,0) 3.66852 ?m
• R(1,1)l 3.71548 ?m Q(1,1) 3.92863 ?m
• Q(1,0) 3.95300 ?m R(3,3)l 3.53367 ?m
• Weak absorption lines (typically 1-2) require
  combination of a large telescope and high
  resolution spectrograph

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Instruments/Telescopes
IRCS Subaru
CGS4 UKIRT
NIRSPEC Keck II
Phoenix Gemini South
CRIRES VLT UT1
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Select H3 Spectra
Crabtree et al. 2010, ApJ, submitted
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Current Survey Status

• Searched for H3 in about 50 diffuse cloud sight
  lines
• Detected absorption in 20 of those
• Column densities range from a few times 1013 cm-2
  to a few times 1014 cm-2
• Inferred ionization rates of 28?10-16 s-1, with
  3? upper limits as low as 7?10-17 s-1

Dame et al. 2001, ApJ, 547, 792
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Implications

• Variations in the ionization rate suggest that
  the cosmic-ray spectrum may not be uniform at
  lower energies
• If true, the cosmic-ray flux should be much
  higher in close proximity to the site of particle
  acceleration
• Search for H3 near the supernova remnant IC 443

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Target Sight Lines
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Results
Indriolo et al. 2010, ApJ, in press
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(No Transcript)
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Results
N(H3) ?2
(1014 cm-2) (10-16 s-1)
ALS 8828 4.4 1610
HD 254577 2.2 2616
HD 254755 lt 0.6 lt 3.5
HD 43582 lt 0.8 lt 9.0
HD 43703 lt 0.6 lt 5.7
HD 43907 lt 2.1 lt 40
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Case 1 Low electron density

• By taking an average value from C, have we
  overestimated the electron density?
• xe decreases from 10-4 in diffuse clouds to
  10-8 in dense clouds
• C2 rotation-excitation and CN restricted chemical
  analyses indicate densities of 200-400 cm-3
  (Hirschauer et al. 2009)
• Estimated values of x(CO) are 10-6, much lower
  than 310-4 solar system abundance of carbon

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Case 2 High Ionization Rate

• How can we explain the large difference between
  detections and upper limits?
• Cosmic-ray spectrum changes as particles
  propagate
• Perhaps ALS 8828 HD 254577 sight lines probe
  clouds closer to SNR

Spitzer Tomasko 1968, ApJ, 152, 971
Torres et al. 2008, MNRAS, 387, L59
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Propagation Acceleration

• MHD effects
• May exclude lower-energy particles from entering
  denser regions
• Damping of Alfvén waves may limit time spent in
  denser regions
• Acceleration effects
• In models of diffusive shock acceleration, the
  highest energy particles escape upstream while
  the others are advected downstream (into the
  remnant)

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Applications

• With sufficient spatial coverage (i.e. sight
  lines), it may be possible to track particle flux
  in supernova remnants
• This may be useful in constraining particle
  acceleration/escape efficiency in models
• Allow for better constraints on the interstellar
  cosmic-ray spectrum

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Summary

• H3 has been detected in 20 of 50 diffuse cloud
  sight lines studied, and ionization rates range
  from 0.78?10-16 s-1
• Ionization rates inferred near IC 443 are
  2?10-15 s-1, suggesting that the supernova
  remnant accelerates a large flux of low-energy
  cosmic rays
• Propagation effects and proximity to the
  acceleration site may cause non-uniformity in the
  cosmic-ray spectrum

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Future Work

• Continue survey of H3 in diffuse cloud sight
  lines
• Search for H3 near more supernova remnants
  interacting with the ISM
• Where possible, perform necessary ancillary
  observations (H2, CH, CO, C, C) to constrain
  sight line properties

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p H2 Ionization Cross Section
Padovani et al. 2009, AA, 501, 619