[Paper Review] Propagation of Light Elements in the Galaxy

I. V. Moskalenko, A. W. Strong+2 more|arXiv (Cornell University)|Jun 17, 2003

Astrophysics and Star Formation Studies3 citations

TL;DR

This study uses the GALPROP numerical code to model the Galactic propagation of light isotopes (H, He, Li, Be, B) via spallation of cosmic rays in the interstellar medium, comparing a conventional reacceleration model with a new model including a local component from the Local Bubble. The key finding is that the local component model better fits low-energy B/C ratio and antiproton data, requiring a ~2× higher local He-4 abundance to match observations, suggesting dilution of material before acceleration in weak shock waves.

ABSTRACT

The origin and evolution of isotopes of the lightest elements H2, He3, Li, Be, B in the universe is a key problem in such fields as astrophysics of CR, Galactic evolution, non-thermal nucleosynthesis, and cosmological studies. One of the major sources of these species is spallation by CR nuclei in the interstellar medium. On the other hand, it is the B/C ratio in CR and Be10 abundance which are used to fix the propagation parameters and thus the spallation rate. We study the production and Galactic propagation of isotopes of elements Z<6 using the numerical propagation code GALPROP and updated production cross sections.

Motivation & Objective

• To investigate the origin and propagation of light isotopes (H, He, Li, Be, B) in the Galaxy using updated spallation cross sections.
• To test whether a conventional reacceleration model or a model with a local component (from the Local Bubble) better explains cosmic ray data.
• To constrain propagation parameters using the B/C ratio and antiproton flux, particularly at low and high energies.
• To assess the isotopic composition of light elements and its implications for Galactic cosmic ray origin and chemical evolution.
• To evaluate the role of helium-4 in producing H-2 and He-3, and how source abundance variations affect model fits.

Proposed method

• Uses the GALPROP 2D numerical propagation code to simulate cosmic ray transport in a Galactic disk with a 4 kpc halo.
• Applies a modified power-law injection spectrum for nucleons and a power-law spectrum with exponential cutoff for the Local Bubble component.
• Employs self-fitted cross sections for p+He, C, N, O reactions producing H-2, He-3,4, Li, Be, B based on experimental data.
• Treats heliospheric modulation via the force-field approximation with Φ = 500 MV.
• Calibrates model A (conventional) using B/C ratio and high-energy data (ACE, HEAO-3), while model B includes a local component tuned to low-energy B/C and antiproton flux.
• Introduces a spectral break in He-4 injection at 14 GV (model A) and 10 GV (model B) to better match the He spectrum.

Experimental results

Research questions

• RQ1 Can a conventional reacceleration model simultaneously fit the B/C ratio and antiproton flux at low and high energies?
• RQ2 Does introducing a local component from the Local Bubble improve the fit to low-energy B/C and antiproton data?
• RQ3 What is the required local He-4 abundance in the Local Bubble to reproduce the He-3/He-4 ratio and He spectrum?
• RQ4 How do variations in source abundances (especially He/Si) affect the production of secondary light isotopes?
• RQ5 What do the isotopic ratios of Li, Be, and B imply about the origin and acceleration history of Galactic cosmic rays?

Key findings

• The H-2/He-4 ratio is measured to be ~1.8 times higher than predicted, though systematic uncertainties remain under investigation.
• Both models reproduce the Li, Be, and B isotopic ratios within large error bars, but the He-3/He-4 ratio is more sensitive and requires a ~2× higher local He-4 abundance in model B.
• Model B requires a local He-4 abundance of 220 (relative to Si) compared to the solar value of 2400, indicating significant dilution before acceleration.
• The higher He-4 abundance in model B supports the hypothesis that low-energy cosmic rays originate from weak shock waves accelerating material from the interstellar medium.
• The model suggests that low-energy proton and helium spectra may be flatter than previously thought, with implications for diffuse Galactic γ-ray emission.
• The data favor a scenario where high-energy cosmic rays originate from fresh supernova ejecta (with lower He abundance), while low-energy particles are derived from pre-diluted interstellar material.