Constraining Galactic cosmic-ray parameters with Z  ≤  2 nuclei

¹ Laboratoire de Physique Subatomique et de Cosmologie, Université Joseph Fourier Grenoble 1, CNRS/IN2P3, Institut Polytechnique de Grenoble, 53 avenue des Martyrs, 38026 Grenoble, France
e-mail: coste@lpsc.in2p3.fr
² The Oskar Klein Centre for Cosmoparticle Physics, Department of Physics, Stockholm University, AlbaNova, 10691 Stockholm, Sweden

Received: 22 August 2011
Accepted: 18 December 2011

Abstract

Context. The secondary-to-primary B/C ratio is widely used for studying Galactic cosmic-ray propagation processes. The ²H/⁴He and ³He/⁴He ratios probe a different Z/A regime, which provides a test for the “universality” of propagation.

Aims. We revisit the constraints on diffusion-model parameters set by the quartet (¹H, ²H, ³He, ⁴He), using the most recent data as well as updated formulae for the inelastic and production cross-sections.

Methods. Our analysis relies on the USINE propagation package and a Markov Chain Monte Carlo technique to estimate the probability density functions of the parameters. Simulated data were also used to validate analysis strategies.

Results. The fragmentation of CNO cosmic rays (resp. NeMgSiFe) on the interstellar medium during their propagation contributes to 20% (resp. 20%) of the ²H and 15% (resp. 10%) of the ³He flux at high energy. The C to Fe elements are also responsible for up to 10% of the ⁴He flux measured at 1 GeV/n. The analysis of ³He/⁴He (and to a lesser extent ²H/⁴He) data shows that the transport parameters are consistent with those from the B/C analysis: the diffusion model with δ ~ 0.7 (diffusion slope), V_c ~ 20 km s^-1 (galactic wind), V_a ~ 40 km s^-1 (reacceleration) is favoured, but the combination δ ~ 0.2, V_c ~ 0, and V_a ~ 80 km s^-1 is a close second. The confidence intervals on the parameters show that the constraints set by the quartet data can compete with those derived from the B/C data. These constraints are tighter when adding the ³He (or ²H) flux measurements, and the tightest when the He flux is added as well. For the latter, the analysis of simulated and real data shows an increased sensitivity to biases. Using the secondary-to-primary ratio along with a loose prior on the source parameters is recommended to obtain the most robust constraints on the transport parameters.

Conclusions. Light nuclei should be systematically considered in the analysis of transport parameters. They provide independent constraints that can compete with those obtained from the B/C analysis.