Ionization fractions and magnetic field in evolving protoplanetary disks

Halder, Richa (2025) Ionization fractions and magnetic field in evolving protoplanetary disks. Masters thesis, Indian Institute of Science Education and Research Kolkata.

Text (MS Dissertation of Richa Halder (20MS112)) 20MS112_Thesis_file.pdf - Submitted Version Restricted to Repository staff only Download (4MB)

Abstract

The discovery of thousands of exoplanets has revealed the efficiency and widespread nature of planet formation, along with the surprising diversity of planetary systems. This highlights the need to understand protoplanetary disks (PPDs), where planets form. PPDs are rotating structures of gas and dust, influenced by complex physical processes such as gas dynamics, dust growth, ionization chemistry, and magnetic field interactions. Large-scale magnetic fields play a key role, particularly in angular momentum transport through magnetized accretion processes, like the magnetorotational instability (MRI). This work develops a numerical model to study the long-term evolution of PPDs, incorporating magnetic field evolution and ionization physics. A mean-field approach models large-scale poloidal magnetic fields, using advanced numerical techniques for improved accuracy. The model simulates gas evolution driven by MRI turbulence, dust dynamics, and non-ideal magnetohydrodynamics effects like Ohmic and ambipolar diffusion. Magnetic flux transport is modeled self-consistently, considering MRI criteria. Simulations show that magnetic field strength and distribution affect pressure bumps and dust traps, which are key for planetesimal formation. Stronger fields promote MRI activity near the star, creating inner disk dust traps, while weaker fields delay MRI, favoring outer disk accumulation. Ionization, crucial for MRI, is modeled using sources like cosmic rays and stellar X-rays, with realistic attenuation profiles to determine ionization rates, which influence MRI activity. The analysis of various disk models reveals significant variation in ionization, magnetic Reynolds number, and Elsasser numbers, influencing MRI-driven turbulence and -viscosity. Some regions show ”dead zones” with suppressed turbulence, while others sustain MRI-driven accretion. An analytical framework developed in this work can be used to reduce the computational cost of chemical calculations used previously, enabling longer simulations and a broader parameter space. Our new numerical simulations will be the first of their kind where gas evolution by magnetized accretion processes (here the MRI), complete ionization prescription, dust dynamics and growth factors, and the large-scale poloidal magnetic fields evolution are self consistently modeled together. This highlights the importance of magnetic conditions and local ionization in disk structure and planetesimal formation, providing insights into the diversity of exoplanetary systems. 1

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