Transcript Probing Cosmic-Ray Acceleration and Propagation with H3

Probing Cosmic-Ray Acceleration and
Propagation with H3+ Observations
Nick Indriolo, Brian D. Fields, &
Benjamin J. McCall
University of Illinois at Urbana-Champaign
Image credit: Gerhard Bachmayer
Collaborators
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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
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
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
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
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
+
Cross Sections
  4 
Ehigh
Elow
 ( E ) ( E )dE
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
Pionic Gamma-Rays &
Supernova Remnants
Pionic Gamma-Rays &
Supernova Remnants
VERITAS gamma-ray map of IC 443:
Acciari et al. 2009, ApJ, 698, L133
Pionic Gamma-Rays &
Supernova Remnants
HESS gamma-ray map of W 28
Aharonian et al. 2008, A&A, 481, 401
Fermi-LAT gamma-ray map of W 28
Abdo et al. 2010, ApJ, 718, 348
Pionic Gamma-Rays &
Supernova Remnants
Supernova remnants accelerate hadronic cosmic rays
Ekin > 280 MeV
Abdo et al. 2010, ApJ, 718, 348
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
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
 2 n(H 2 )  ke ne n(H )

3
Calculating the Ionization Rate
 2 n(H 2 )  ke ne n(H )

3
N(H2) from N(CH)

3
n( H )
 2  ke xe nH
n( H 2 )

3
N (H )
 2  ke xe nH
N (H 2 )
xe from C+;
Cardelli et al. 1996, ApJ, 467, 334
Sheffer et al. 2008, ApJ, 687, 1075
nH from C2;
Sonnentrucker et al. 2007, ApJS, 168, 58
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
Instruments/Telescopes
IRCS: Subaru
CGS4: UKIRT
NIRSPEC: Keck II
Phoenix: Gemini South
CRIRES: VLT UT1
Select H3+ Spectra
Crabtree et al. 2010, ApJ, submitted
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 2–810-16 s-1,
with 3 upper limits as low as 710-17 s-1
Dame et al. 2001, ApJ, 547, 792
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
Target Sight Lines
HD 43703
ALS 8828
HD 254755
HD 43582
HD 254577
HD 43907
Results
Indriolo et al. 2010, ApJ, in press
HD 43703
ALS 8828
HD 254755
HD 43582
HD 254577
HD 43907
Results
N(H3+)
(1014 cm-2)
ALS 8828
HD 254577
HD 254755
4.4
2.2
< 0.6
HD 43582
HD 43703
HD 43907
< 0.8
< 0.6
< 2.1

3
N (H ) 
 2 N (H 2 )
ke xe nH
ζ2
(10-16 s-1)
16±10
26±16
< 3.5
< 9.0
< 5.7
< 40
Either ζ2 is large,
or xenH is small
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
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
Spitzeret&al.
Torres
Tomasko
2008, MNRAS,
1968, ApJ,
387,
152,
L59971
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)
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
Summary
• H3+ has been detected in 20 of ~50 diffuse
cloud sight lines studied, and ionization
rates range from 0.7–8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 lowenergy cosmic rays
• Propagation effects and proximity to the
acceleration site may cause nonuniformity in the cosmic-ray spectrum
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