Image credit: Gerhard Bachmayer

Probing Cosmic-Ray Acceleration and Propagation

with H3+ Observations

Nick Indriolo, Brian D. Fields, & Benjamin J. McCall

University of Illinois at Urbana-Champaign

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

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

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

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 28Aharonian et al. 2008, A&A, 481, 401

Fermi-LAT gamma-ray map of W 28Abdo et al. 2010, ApJ, 718, 348

Pionic Gamma-Rays & Supernova Remnants

Supernova remnants accelerate hadronic cosmic raysEkin > 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)H()H( 322 nnkn ee

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)

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

Phoenix: Gemini South

CRIRES: VLT UT1

CGS4: UKIRT

NIRSPEC: Keck II

IRCS: Subaru

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 2–810-16 s-1, with 3 upper limits as low as 710-17 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 254577

HD 254755

HD 43582

HD 43907

HD 43703

ALS 8828

Indriolo et al. 2010, ApJ, in press

Results

HD 254577

HD 254755

HD 43582

HD 43907

HD 43703

ALS 8828

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

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

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

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 0.7–810-16 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

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