Heavy Nuclei Enrichment of the Galactic Cosmic Rays at High Energy: Astrophysical Interpretation

Details Heavy Nuclei Enrichment of the Galactic Cosmic Rays at High Energy: Astrophysical Interpretation Heavy Nuclei Enrichment of the Galactic Cosmic Rays at High Energy: Astrophysical Interpretation

2002-02-18

arxiv.org/abs/astro-ph/0202334v3

Astropart.Phys. 18 (2003) 471-486

Astronomy and Astrophysics

Astrophysics

26 pages, 7 PostScript figures. Astropart. Phys, accepted; misprint in fig6 corrected

Scientific paper

10.1016/S0927-6505(02)00163-9

A substantial increase of the mean logarithmic mass $<\ln A>$ of galactic cosmic rays {\em vs} energy has been observed . We study three effects that could explain this trend i) different source spectra for protons and heavy nuclei ii) a selective nuclear destruction in flight of heavies iii) a gradient of the source number and chemical composition in the galactic disk. We take advantage of the diffusive cosmic ray propagation model developed at LAPTH to study specifically the geometrical aspects of the propagation and extend it to high energy. Using a simple modeling of the spectral knee around $10^{15}$ eV, a bump in $<\ln A>$ appears. This feature is smoother when the spectral index of protons is steeper than Fe's. We analyze the effects of the rigidity dependence of the diffusion coefficient and the scale height of the confinement halo and we show that $<\ln A>$ is most sensitive to the first parameter. Pure geometrical effects are less determining than the diffusion coefficient spectral index. Subsequently, we conclude that the physics of cosmic ray confinement is the essential cause of the heavy nuclei enrichment until $\sim 10^{15}$ eV.

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