Probing Cosmic-Ray Acceleration and Propagation with H 3 + Observations
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Probing Cosmic-Ray Acceleration and Propagation
with H3+ Observations
Nick Indriolo, Brian D. Fields, & Benjamin J. McCall
University of Illinois at Urbana-Champaign
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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
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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, A&A, 40, 91
dEEEhigh
low
E
E)()(4
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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
HESS gamma-ray map of W 28Aharonian et al. 2008, A&A, 481, 401
Fermi-LAT gamma-ray map of W 28Abdo et al. 2010, ApJ, 718, 348
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Pionic Gamma-Rays & Supernova Remnants
Supernova remnants accelerate hadronic cosmic raysEkin > 280
MeV
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)H()H( 322 nnkn ee
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Calculating the Ionization Rate)H()H( 322
nnkn ee
)H(
)H(
2
3H2 n
nnxk ee
)H(
)H(
2
3H2 N
Nnxk ee
xe from C+; Cardelli et al. 1996, ApJ, 467, 334
nH from C2;Sonnentrucker et al. 2007, ApJS, 168, 58
Sheffer et al. 2008, ApJ, 687, 1075
N(H2) from N(CH)
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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
Phoenix: Gemini South
CRIRES: VLT UT1
CGS4: UKIRT
NIRSPEC: Keck II
IRCS: Subaru
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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 2–810-16 s-1, with 3 upper limits as low as 710-17 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
HD 254577
HD 254755
HD 43582
HD 43907
HD 43703
ALS 8828
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Indriolo et al. 2010, ApJ, in press
Results
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HD 254577
HD 254755
HD 43582
HD 43907
HD 43703
ALS 8828
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ResultsN(H3
+) ζ2
(1014 cm-2) (10-16 s-1)
ALS 8828 4.4 16±10
HD 254577 2.2 26±16
HD 254755 < 0.6 < 3.5
HD 43582 < 0.8 < 9.0
HD 43703 < 0.6 < 5.7
HD 43907 < 2.1 < 40
H
223
)H()H(
nxk
NN
ee
Either ζ2 is
large, or xenH is small
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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 3×10-4 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, 971Torres 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 0.7–810-16 s-1
• Ionization rates inferred near IC 443 are ~210-15 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