Modeling the variability of the cosmic ray ionization rate within 500 parsecs from Earth

Cosmic rays (CRs) affect nearly all aspects of the star and planet formation process. They are crucial in driving the chemistry that takes place in molecular clouds, prestellar cores, and protoplanetary disks. The abundance of low-energy cosmic rays is a major uncertainty underlying modeling of the star and planet formation process. The project will improve our understanding of CR transport in molecular clouds, as well as the energy spectrum of CRs in the surrounding diffuse medium. This will enable the CR ionization rate to be calculated as a function of depth within the molecular cloud. The project also provides funding and tuition support for a PhD student, as well as funding for one undergraduate research student for all three years of the proposal. Researchers who need the CR ionization rate as an input to their models of star formation, or for interpreting molecular abundances, are currently faced with order-of-magnitude uncertainties in what rates to use. Using existing magneto-hydrodynamic simulations of star-forming regions, the project will assess the importance of magnetic pockets and field-line twisting. The project will also evaluate the role of magnetic effects by employing a field-line tracing algorithm to reconstruct the magnetic field line passing through the point of interest. Dedicated magnetohydrodynamic simulations with driven turbulence in a periodic box threaded with an initially uni-axial magnetic field will be run, and the team will use these to analyze the role of magnetic effects in modulating the CR ionization rate. In addition, this proposal will develop a state-of-the-art CR transport model within the local interstellar medium, which will be compared with observational tracers of CR ionization in nearby molecular clouds. Better knowledge of the local CR energy spectra will enable a more reliable estimate of the variation of the CR ionization rate as a function of gas density within a molecular cloud. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.