The Distribution of Cosmic-Ray Ionization

Rates in Diffuse Molecular Clouds as Probed by H3

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Nick IndrioloJohns Hopkins University

February 10, 2012 Chemistry, Astronomy, & Physics of H3

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In Collaboration with...• Ben McCall (University of Illinois)• Brian Fields (University of Illinois)• Tom Geballe (Gemini Observatory)• Geoff Blake (Caltech)• Miwa Goto (MPIA)• Tomonori Usuda (Subaru Telescope)• David Neufeld (Johns Hopkins)• Takeshi Oka (University of Chicago)

Outline• Introduction to cosmic rays and

interstellar H3+

• Calculating the cosmic-ray ionization rate

• Observations of H3+ and example

spectra• Line-of-sight properties and ζ2

• The distribution of ionization rates• Ionization rate and location• Complementary tracers and future

work

Cosmic Rays• Discovered in 1912 by Victor Hess

during balloon borne experiments that showed increased radiation at higher altitudes

• Later dubbed cosmic rays (Millikan 1926)

• Now known to be highly energetic charged particles (p, e-, e+, α, nuclei)

Energy Distribution• Power law in

energy (φ~E-2.7) spanning 12 decades in E, and 30 decades in flux

• Spectral shape is consistent for all species

Swordy 2001

Ave et al. 2008

Particle Interactions• Ionization

p + H2 H2+ + e- + p'

• Spallation and Fusion[p, ] + [12C, 14N, 16O] [6Li,7Li,9Be,10B,11B]

• Nuclear Excitation[p, ] + 12C 12C* 12C + γ4.44 MeV

• Inelastic Collisionsp + H p' + H + 0 γ +

γ

Ionization by Cosmic Rays• Cosmic rays ionize H, He, and H2

throughout diffuse molecular clouds, forming H+, He+, and H3

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• Initiates the fast ion-molecule reactions that drive chemistry in the ISM

Ion-Molecule Reactions

CRH2+

H2

H2 H3+

CO HCO

+

O

OH+

N2

N2H+

H2H2O+

H2 H3O+

CRH H

+

OO+

H2

OH

H2O

e-e-e-

H+

H

hν

hν

O

H3+

ζ2 over the past 50 years

Hayakawa et al. 1961; Spitzer & Tomasko 1968; O'Donnell & Watson 1974; Hartquist et al. 1978; van Dishoeck & Black 1986; Federman et al. 1996; Webber 1998; McCall et al. 2003; Indriolo et al. 2007; Gerin et al. 2010; Neufeld et al. 2010

H3+ Chemistry

• Formation– CR + H2 H2

+ + e- + CR'–H2

+ + H2 H3+ + H

• Destruction–H3

+ + e- H + H + H (diffuse clouds)–H3

+ + O OH+ + H2 (diffuse & dense clouds)

–H3+ + CO HCO+ + H2 (dense clouds)

–H3+ + N2 HN2

+ + H2 (dense clouds)

Steady State Equation

Necessary Parameters

• ke measured in lab (adopt McCall et al. 2004)• xe approximated by x(C+)≈1.510-4 Cardelli et

al. 1996; Sofia et al. 2004• nH estimated from rotational excitation analysis

of C2 (Sonnentrucker et al. 2007) or thermal pressure analysis of fine structure lines of C I (Jenkins et al. 1983; Jenkins & Tripp 2001)

• N(H2)–UV observations of H2 lines

(Savage et al. 1977; Rachford et al. 2002, 2009)

–N(CH)/N(H2)≈3.510-8 (Sheffer et al. 2008)

–NH≈E(B-V)5.81021 cm-2 mag-1 (Bohlin et al. 1978; Rachford et al. 2002)

• All that remains is N(H3+)

Necessary Parameters (cont.)

Targeted Transitions• Transitions of the 2 0

band of H3+ are available

in the infrared• Given average diffuse

cloud temperatures (70 K) only the (J,K)=(1,0) & (1,1) levels are significantly populated

• Observable transitions are:– R(1,1)u: 3.668083 μm– R(1,0): 3.668516 μm– R(1,1)l: 3.715479 μm– Q(1,1): 3.928625 μm– Q(1,0): 3.953000 μm

Energy level diagram for theground vibrational state of H3

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Instruments & Telescopes

Phoenix: Gemini South

CRIRES: VLT UT1

CGS4: UKIRT

NIRSPEC: Keck II

IRCS: Subaru

Survey Status

• Observations targeting H3+ in diffuse

clouds have been made in 50 sight lines

• H3+ is detected in 21 of those

Dame et al. 2001

Example Spectra

CRIRES at VLT

S/N Wλ (10-6 μm) N(H3+) (1014 cm-2)

HD 110432

1200 0.7 0.5

HD 313599

300 4.5 3.2

ζ2 vs. Position

ζ2 vs. Total Column Density

Particle Range

Padovani et al. 2009

Range for a 1 MeV proton is ~31020 cm-2

Range for a 10 MeV proton is ~21022 cm-2

Diffuse cloud column densities are about 1021 ≤ NH ≤ 1022 cm-2

Distribution of Ionization Rates

Where do we stand?

Regional Distributions

Ophiuchus-Scorpius region

Image Credit: Rogelio Bernal Andreo

o Sco HD 147889

ρ Oph D

χ Oph

3 pc

N

E

Per OB2 region

4.6 pc

10.5 pc

Image Credit: Rogelio Bernal Andreo

PSR J0357+3205

ζ Per

o Per

ξ Per

X Per

40 PerBD +31 643

N

E

N(H2)=4.1×1020 cm-2

N(H2)=4.8×1020 cm-2

Why the Differences?• Particle range determined by energy• Cosmic rays must get from

acceleration site to observed clouds• Ionization rate controlled by

proximity of cloud to nearest acceleration site

• If true, ζ2 should be large near known source, e.g. IC 443, a supernova remnant

IC 443 Survey

ALS 8828

HD 254577

HD 254755

HD 43582

Image credit: Gerhard Bachmayer

ALS 8828

HD 254577

HD 43582

HD 254755

IC 443 Results

• SNRs accelerate cosmic rays• Proximity to IC 443 boosts ζ2

• Ionization rates vary over a few pc

Conclusions• ζ2 is NOT a constant!!!!!• !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

!!!!• The cosmic-ray ionization rate is

controlled by the distance between a cloud and acceleration site

• Different regions of the sky show different distributions of ζ2

• ζ2 can vary on size scales of a few pc

H3+ work in progress/development

• Observe H3+ near W 28 & Vela SNRs

• High S/N survey of compact regions (Sco-Oph & Per OB2)

• Southern hemisphere survey (CRIRES)

• ESO Archive survey

Ion-Molecule Reactions

CRH2+

H2

H2 H3+

CO HCO

+

O

OH+

N2

N2H+

H2H2O+

H2 H3O+

CRH H

+

OO+

H2

OH

H2O

e-e-e-

H+

H

hν

hν

O

H3+

ζ from OH+ and H2O+

• Oxygen chemistry closely linked to ionization rate of H (ζH)

• ε is fraction of H+ that forms OH+

• Needs to be determined

W 51

Preliminary results• from H3

+: ζ2=(12.5±9.3)×10-16 s-1

• from OH+: ζHε=(0.52±0.28)×10-16 s-1

• taking 2.3ζH=1.5ζ2: ζ2ε=(0.79±0.43)×10-16 s-1

• ε=0.06 ± 0.03• >90% of the time, cosmic-ray

ionization of H does not result in OH+

• Grain neutralization on PAH-

γ-ray Signatures

IC 443, VERITAS;Acciari et al. 2009

W28, Fermi-LAT;Abdo et al. 2010

W51C, Fermi-LAT;Carmona et al. 2011

Diffuse γ-ray Signatures

Thanks!