文章基本信息

标题：Cosmic-ray propagation in the bi-stable interstellar medium
其他标题：I. Conditions for cosmic-ray trapping
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作者：Benoît Commerçon ; Alexandre Marcowith ; Yohan Dubois 等
期刊名称：Astronomy & Astrophysics
印刷版ISSN：0004-6361
电子版ISSN：1432-0746
出版年度：2019
卷号：622
DOI：10.1051/0004-6361/201833809
语种：English
出版社：EDP Sciences
摘要：Context.Cosmic rays propagate through the galactic scales down to the smaller scales at which stars form. Cosmic rays are close to energy equipartition with the other components of the interstellar medium and can provide a support against gravity if pressure gradients develop.Aims.We study the propagation of cosmic rays within the turbulent and magnetised bi-stable interstellar gas. The conditions necessary for cosmic-ray trapping and cosmic-ray pressure gradient development are investigated.Methods.We derived an analytical value of the critical diffusion coefficient for cosmic-ray trapping within a turbulent medium, which follows the observed scaling relations. We then presented a numerical study using 3D simulations of the evolution of a mixture of interstellar gas and cosmic rays, in which turbulence is driven at varying scales by stochastic forcing within a box of 40 pc. We explored a large parameter space in which the cosmic-ray diffusion coefficient, the magnetisation, the driving scale, and the amplitude of the turbulence forcing, as well as the initial cosmic-ray energy density, vary.Results.We identify a clear transition in the interstellar dynamics for cosmic-ray diffusion coefficients below a critical value deduced from observed scaling relations. This critical diffusion depends on the characteristic length scaleLofDcrit≃ 3.1 × 1023cm2s−1(L/1 pc)q+1, where the exponentqrelates the turbulent velocity dispersionσto the length scale asσ~Lq. Hence, in our simulations this transition occurs aroundDcrit≃ 1024–1025cm2s−1. The transition is recovered in all cases of our parameter study and is in very good agreement with our simple analytical estimate. In the trapped cosmic-ray regime, the induced cosmic-ray pressure gradients can modify the gas flow and provide a support against the thermal instability development. We discuss possible mechanisms that can significantly reduce the cosmic-ray diffusion coefficients within the interstellar medium.Conclusions.Cosmic-ray pressure gradients can develop and modify the evolution of thermally bi-stable gas for diffusion coefficientsD0≤ 1025cm2s−1or in regions where the cosmic-ray pressure exceeds the thermal one by more than a factor of ten. This study provides the basis for further works including more realistic cosmic-ray diffusion coefficients, as well as local cosmic-ray sources.
关键词：Key wordsenmagnetohydrodynamics (MHD)methods: numericalISM: structurediffusioncosmic raysISM: individual objects: molecular clouds