FC10; June 25, 2010 Image credit: Gerhard Bachmayer

Constraining the Flux of Low-Energy Cosmic Rays

Accelerated by the Supernova Remnant IC 443

N. Indriolo1, G. A. Blake2, M. Goto3, T. Usuda4, T. R. Geballe5, T. Oka6, & B. J.

McCall1

1 – University of Illinois at Urbana-Champaign2 – California Institute of Technology3 – Max Planck Institute for Astronomy4 – Subaru Telescope5 – Gemini Observatory6 – University of Chicago

Why look near supernova remnants?

• Observational evidence suggests Galactic cosmic rays are accelerated primarily by supernova remnants (SNRs)

• As cosmic rays propagate, they interact with the ISM– excitation & ionization of atoms & molecules– excitation of nuclear states– spallation of ambient nuclei– production of pions (0, +, -)

IC 443 Basics

• Located at (l,b)=(189°,+3°)• 1.5 kpc away in Gem OB1 association• Estimated to be about 30,000 years old• Known to be interacting with

surrounding molecular material• Lies behind a quiescent molecular

cloud

IC 443 tour: Radio to Gamma-Rays

Troja et al. 2006, ApJ, 649, 258

IC 443 tour: Radio to Gamma-Rays

12CO antenna temperature map:Dickman et al. 1992, ApJ, 400, 203

IC 443 tour: Radio to Gamma-Rays

2MASS JHK bands:Rho et al. 2001, ApJ, 547, 885

IC 443 tour: Radio to Gamma-Rays

XMM 0.3-0.5 keV X-ray map:Troja et al. 2006, ApJ, 649, 258

IC 443 tour: Radio to Gamma-Rays

VERITAS gamma-ray map:Acciari et al. 2009, ApJ, 698, L133

0

H3+ Chemistry

• Formation– CR + H2 H2

+ + e- + CR’

– H2+ + H2 H3

+ + H

• Destruction– H3

+ + e- H2 + H or H + H + H (diffuse cloud)

– H3+ + CO H2 + HCO+ (dense clouds)

• Steady state )H()H( 322 nnkn ee

Calculating the Ionization Rate)H()H( 322

nnkn ee

)H(

)H(

2

32 N

Nnk ee

)H(

)H(

2

3H2 N

Nnxk ee

xe from C+; Cardelli et al. 1996, ApJ, 467, 334

nH from C2 and CN;Hirschauer et al. 2009, ApJ, 696, 1533

Sheffer et al. 2008, ApJ, 687, 1075

N(H2) from N(CH)

Observations

• Transitions– H3

+ ν2 0

– R(1,1)u, R(1,0), R(1,1)l, Q(1,0), Q(1,1), R(3,3)l

• Telescopes– Keck: NIRSPEC– Subaru: IRCS

• 6 target sight lines with CH & CN

Observations

HD 254577

HD 254755

HD 43582

HD 43907

HD 43703

ALS 8828

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±15

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

7.5 pc

1610-16 s-1

2610-16 s-

1

<5.710-16 s-1

<3.510-16 s-1

<9.010-16 s-1

<4010-16 s-1

Conclusions

• We’ve detected large columns of H3+ in 2

sight lines toward IC 443• This is either the result of a high cosmic-

ray ionization rate or low electron density• Unclear whether or not low-energy cosmic

rays accelerated by SNRs can account for the flux necessary in the Galactic ISM to produce the inferred ionization rate

Future Work

• Use COS on Hubble to observe C II, C I, and CO absorption toward IC 443

• Search for H3+ toward other supernova

remnants which are interacting with molecular clouds; e.g. W 44, W 28, W 51