Abstract: Cosmic rays (CRs) play a pivotal role in various astrophysical systems, delivering feedback that spans a broad range of scales. However, modeling their transport remains challenging due to the in-
herent multi-scale nature and microphysical complexities. Recent advancements in two-moment CR hydrodynamics have alleviated some of these challenges and deepened our understanding of CR feedback. Yet, current two-moment methods may not fully incorporate all possible CR transport processes, while the outcome of CR feedback sensitively depends on these underlying microphysics. Furthermore, numerical challenges persist, including instabilities from stiff streaming terms and ambiguities in solver design for coupled CR-MHD systems.
We develop a two-moment description for CR hydrodynamics from first principles, and implement our formulation as a new CR fluid module to the ATHENA++ code. To test the code, we investigate the characteristic CR-MHD waves in such systems as benchmarks, and provide a detailed analysis of their properties. In this talk, I will introduce our formulations, and show the improved performance of our module. I will also share our preliminary findings of the CR-MHD waves.
Bio: Xihui Zhao is a PhD student in astrophysics at Tsinghua University, before that he completed undergraduate study at the University of Science and Technology of China. He is interested in the dynamics of multiphase systems in astrophysical environments (such as the circumgalactic medium), with a specific focus on understanding the influence of magnetic fields and cosmic rays (CRs). Recently He developed a CR fluid module in the ATHENA++ code, aiming to enhance its capabilities for studying macroscopic CR feedback. We hope this work can provide opportunities for diverse future applications.