Thermal Instability and Multiphase Gas in the Simulated Interstellar Medium with Conduction, Viscosity and Magnetic Fields (2012.05252v2)

Abstract: Thermal instability (TI) plays a crucial role in the formation of multiphase structures and their dynamics in the Interstellar Medium (ISM) and is a leading theory for cold cloud creation in various astrophysical environments. In this paper we use two-dimensional (2D) simulations to investigate thermal instability under the influence of various initial conditions and physical processes. We experiment with Gaussian random field (GRF) density perturbations of different initial power spectra. We also enroll thermal conduction and physical viscosity in isotropic hydrodynamic and anisotropic magnetohydrodynamic (MHD) simulations. We find that the initial GRF spectral index $\alpha$ has a dramatic impact on the pure hydrodynamic development of thermal instability, influencing the size, number and motions of clouds. Cloud fragmentation happens due to two mechanisms: tearing and contraction rebound. In the runs with isotropic conduction and viscosity, the structures and dynamics of the clouds are dominated by evaporation and condensation flows in the non-linear regime, and the flow speed is regulated by viscosity. Cloud disruptions happen as a result of the Darrieus--Landau instability (DLI). Although at very late times, all individual clouds merge into one cold structure in all hydrodynamic runs. In the MHD case, the cloud structure is determined by both the initial perturbations and the initial magnetic field strength. In high $\beta$ runs, anisotropic conduction causes dense filaments to align with the local magnetic fields and the field direction can become reoriented. Strong magnetic fields suppress cross-field contraction and cold filaments can form along or perpendicular to the initial fields.

• The paper uses 2D hydrodynamic and MHD simulations to explore thermal instability and multiphase gas in the ISM, examining the roles of conduction, viscosity, and magnetic fields under various initial conditions.
• Thermal conduction and viscosity significantly influence instability evolution and cloud dynamics by regulating flow speeds, with the Prandtl number governing the degree of turbulence and cloud merging.
• Magnetic fields, especially their initial strength quantified by plasma beta, critically determine cloud structure formation, alignment, and the limitations on cross-field dynamics in the simulated interstellar medium.

Analyzing Thermal Instability and Multiphase Gas in the Simulated Interstellar Medium

This paper explores crucial aspects of thermal instability (TI) and the formation of multiphase gas within the interstellar medium (ISM), emphasizing the roles played by conduction, viscosity, and magnetic fields. The authors employed two-dimensional simulations to explore the intricacies of these processes through both hydrodynamic and magnetohydrodynamic (MHD) models. The paper is comprehensive, involving a variety of initial conditions and considering the interplay between gaussian random field (GRF) density perturbations and different physical processes.

Key Findings and Analysis

1. Influence of Initial Perturbation: The research demonstrates that the initial GRF spectral index significantly impacts the development and characteristics of TI. Specifically, variations in the spectral index affect the number, size, and movement of clouds within the ISM. The paper reveals that larger spectral index magnitudes push power into longer wavelengths, fostering conditions for larger cloud structures through isochoric collapse, whereas smaller indices lead to numerous, smaller clouds formed through isobaric collapse and slower dynamics.
2. Role of Conduction and Viscosity: The addition of thermal conduction and physical viscosity markedly influences the TI evolution. Without these elements, TI progresses unfettered, dominated by small-scale perturbations. However, when included, they regulate flow speeds and structural dynamics through evaporation and condensation, aligning with the observations of evaporation-driven dynamics regulated by viscosity. The evolution is significantly governed by the Prandtl number; lower values enhance turbulence and cloud merging due to increased evaporation flows, while higher values reduce turbulence and these merging processes owing to more substantial viscous forces counteracting the evaporative pressure gradients.
3. Fragmentation Mechanisms: The paper identifies specific modes of cloud fragmentation — notably, one where high vorticity induced by Richtmyer-Meshkov-like instabilities breaks down clouds and another wherein merging pressures between clouds disrupt and fragment smaller clouds. With conduction present, the Darrieus–Landau instability (DLI) facilitates fragmentation by enhancing curvature-driven phase changes on cloud surfaces, indicating how different physical processes can initiate distinct fragmentation behaviors.
4. Magnetic Field Impacts: Introduction of magnetic fields through MHD simulations reveals the complexities of anisotropic conduction's role. The strength of the initial magnetic field, quantified through the plasma beta parameter, critically determines cloud structure formation and alignment. High beta runs, where magnetic tension is weaker, enable significant field realignment and a wider variety of cloud structures, whereas low beta conditions confine motions along magnetic field lines, severely restricting cross-field dynamics and maintaining field orientation.
5. Density and Magnetic Field Alignment: The anisotropy measured between density structures and magnetic field lines aligns with observed astronomical phenomena, suggesting that appropriately configured TI can produce filament structures reminiscent of those seen in observations. The simulations indicate varying alignments based on initial conditions, demonstrating how dense filaments could either align with or orient perpendicular to magnetic fields, especially for configurations like the Taurus B211/3 cloud or Musca filamentary regions.

Implications for ISM Studies and Beyond

The findings in this paper have significant implications for astrophysical models pertaining to TI and cold cloud creation. The detailed investigation of TI mechanisms, conduction, and MHD effects provides a refined framework to understand ISM dynamics and potentially enhance predictions about star formation processes. The paper further underscores the importance of considering varying initial conditions and physical effects when modeling ISM environments, particularly in regard to MHD phenomena, which are increasingly relevant for understanding the cosmic structural formation.

In future research, extending these simulations to three-dimensional models may offer deeper insights, potentially validating or altering findings related to cloud dynamics, fragmentation processes, and magnetic field impacts observed here. This research demonstrates that a systematic analysis of various forces and processes within the ISM can vastly enhance our understanding of these astrophysical systems, suggesting a promising route for more complex exploration in this domain.