Investigating the cosmic-ray ionization rate in the galactic interstellar medium through observations of H3+

Indriolo, Nicholas

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https://hdl.handle.net/2142/26029 Copy

Description

Title

Investigating the cosmic-ray ionization rate in the galactic interstellar medium through observations of H3+

Author(s)

Indriolo, Nicholas

Issue Date

2011-08-25T22:09:51Z

Director of Research (if dissertation) or Advisor (if thesis)

McCall, Benjamin J.

Doctoral Committee Chair(s)

McCall, Benjamin J.

Committee Member(s)

• Fields, Brian D.
• Looney, Leslie W.
• Crutcher, Richard M.
• Chu, You-Hua

Department of Study

Astronomy

Discipline

Astronomy

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• cosmic rays
• Interstellar Medium (ISM)
• trihydrogen cation (H3+)
• astrochemistry

Abstract

Observations of H3+ in the Galactic diffuse interstellar medium (ISM) have led to various surprising results, including the conclusion that the cosmic-ray ionization rate (zeta_2) is about 1 order of magnitude larger than previously thought. The present survey expands the sample of diffuse cloud sight lines with H3+ observations to 50, with detections in 21 of those. Ionization rates inferred from these detections are in the range (1.7+-1.0)x10^-16 s^-1 < zeta_2 < (10.6+-6.8)x10^-16 s^-1 with a mean value of zeta_2=(3.3+-0.4)x10^-16 s^-1. Upper limits (3sigma) derived from non-detections of H3+ are as low as zeta_2 < 0.4x10^-16 s^-1. These low upper-limits, in combination with the wide range of inferred cosmic-ray ionization rates, indicate variations in zeta_2 between different diffuse cloud sight lines. Calculations of the cosmic-ray ionization rate from theoretical cosmic-ray spectra require a large flux of low-energy (MeV) particles to reproduce values inferred from observations. Given the relatively short range of low-energy cosmic rays --- those most efficient at ionization --- the proximity of a cloud to a site of particle acceleration may set its ionization rate. Variations in zeta_2 are thus likely due to variations in the cosmic-ray spectrum at low energies resulting from the effects of particle propagation. To test this theory, H3+ was observed in sight lines passing through diffuse molecular clouds known to be interacting with the supernova remnant IC 443, a probable site of particle acceleration. Where H3+ is detected, ionization rates of zeta_2=(20+-10)x10^-16 s^-1 are inferred, higher than for any other diffuse cloud. These results support both the concept that supernova remnants act as particle accelerators, and the hypothesis that propagation effects are responsible for causing spatial variations in the cosmic-ray spectrum and ionization rate. Future observations of H3+ near other supernova remnants and in sight lines where complementary ionization tracers (OH+, H2O+, H3O+) have been observed will further our understanding of the subject.

Graduation Semester

2011-08

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http://hdl.handle.net/2142/26029

Copyright and License Information

Copyright 2011 Nicholas Indriolo

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