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Ion Acceleration in Protostellar Jets: Simulations

Supervisors: Dr. Brandt Gaches & Prof. Stefanie Walch-Gassner

Cosmic rays (CRs) are found to be ubiquitous in the interstellar medium (ISM), roughly in energy equipartition with the thermal and magnetic energy. In regions shielded from ultraviolet radiation, CRs control the chemistry and help drive the increase in molecular complexity in dense gas. The ionization rates within the diffuse and dense gas of molecular clouds has been found to vary by nearly two orders of magnitude, with the average cosmic- ray ionization rate (CRIR) being ζ ≈ 10−16 s−1. The variation of the CRIR may be due to the heterogeneity of external mechanisms which accelerate CRs, notably supernova remnants and stellar winds. However, many star-forming regions are clustered near each other. As such, the star formation occurring in one region could influence a nearby region.

Protostellar jets are a general result of the star formation process: the jets provide a mechanism to release angular momentum from the disk. These jets can be highly collimated and may extend to parsec distances away from the central protostar. Furthermore, jets have been observed to be radio bright with synchrotron-like spectra. Thus, it is thought that protostellar jets may be the sites of the acceleration of energetic particles (EPs). The EPs from protostellar jets would escape when the jet breaks out and flood the nearby molecular gas. Previous theoretical work has calculated the spectrum of a model protostellar jet using an analytic framework. The jet was assumed to accelerate particles via Fermi Acceleration (Diffusive Shock Acceleration). Jet acceleration is substantially more complicated through interactions with the numerous shock interfaces such as the reverse shock and the bow show. However, to date, there have been no simulations which directly model the acceleration of EPs in the protostellar jets and their transport along the jet. Therefore, the exact (re)acceleration mechanisms which act within the jet are unknown.s This project will directly investigate the acceleration of EPs in protostellar jets using numerical simulations. The simulations will use the Ramsey & Clarke 2.5D setups to allow for a high dynamic resolution: it is essential to resolve both the launching radius and the large scales to capture the interactions with the surrounding envelope. We will utilize the Pluto magneto-hydrodynamic code which now has Hybrid MHD-Particle-in-Cell (PIC) capabilities in 2D with the Chombo Adaptive Mesh Refinement package. The results of this project will be:

  • Simulations of model protostellar jets resolving the from launching radius (a few solar radii) to O(0.1 pc) scales with the MHD-PIC module.

  • EP spectrum throughout the domain. The particle spectrum will be post-processed to calculate the emerging synchrotron emission along the jet.

  • The escaping-EP spectrum will be used to model how protostellar jets may influence the chemistry of nearby star-forming regions.