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arXiv:2307.16829 (Published 2023-07-31)
Effects of Grain Magnetic Properties and Grain Growth on Synthetic Dust Polarization of MHD Simulations in Protostellar Environments
Comments: 30 pages, 23 figures, 10 pages in AppendixCategories: astro-ph.GA, astro-ph.SRThermal dust polarization is a powerful tool to probe magnetic fields ($\textbf{B}$), grain magnetic properties, and grain sizes. However, a systematic study of the dependence of synthetic dust polarization on grain properties in protostellar environments is not yet available. In this paper, we post-process a non-ideal MHD simulation of a collapsing protostellar core with our updated POLARIS to study in detail the effects of grain magnetic properties and grain growth on dust polarization. We found that superparamagnetic (SPM) grains can produce high polarization degree $p \sim 10-40\%$ beyond $\sim 500$ au because of their efficient magnetic alignment by magnetically enhanced Radiative Torque (MRAT) mechanism. The magnetic field tangling due to turbulence in the envelope causes the decrease of $p$ with emission intensity $I$ as $p\propto I^{\alpha}$ with the slope $\alpha \sim -0.3$. But within 500 au, SPM grains tend to have weak internal alignment and be aligned with $\textbf{B}$ by RAdiative Torque mechanism only, producing lower $p \sim 1\%$ and larger $\alpha \sim -0.6$. For paramagnetic (PM) grains, their inefficient magnetic alignment produces $p << 1\%$, and the depolarization happens with a steep slope of $\alpha \sim -0.9$ owing to the alignment loss of large grains toward the protostar. Grain growth can help to increase $p$ and weaken the depolarization effect caused by turbulence beyond $500$ au for SPM grains. But for SPM grains within $\sim 500$ au and for PM grains, increasing $a_{\rm max}$ enhances the depolarization effect due to the increasing amount of large grains with inefficient alignment. Finally, we found that the polarization angle dispersion function $S$ increases with increasing iron inclusions and $a_{\rm max}$. Our findings reveal the dependence of magnetic field strength measured using the Davis-Chandrashekhar-Fermi technique on grain alignment degree.
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arXiv:2306.15761 (Published 2023-06-27)
MHD Simulation of The Inner Galaxy with Radiative Cooling and Heating
Comments: Submitted to ApJ; 21 pages, 18 figures 3 tables. Comment are welcomeCategories: astro-ph.GA, astro-ph.HEWe investigate the role of magnetic field on the gas dynamics in the Galactic bulge region by three dimensional simulations with radiative cooling and heating. While high-temperature corona with $T>10^6\ {\rm K}$ is formed in the halo regions, the temperature near the Galactic plane is $\lesssim 10^4\ {\rm K}$ following the thermal equilibrium curve determined by the radiative cooling and heating. Although the thermal energy of the interstellar gas is lost by radiative cooling, the saturation level of the magnetic field strength does not significantly depend on the radiative cooling and heating. The magnetic field strength is amplified to $10\ {\rm \mu G}$ on average, and reaches several hundred ${\rm \mu G}$ locally. We find the formation of magnetically dominated regions at mid-latitudes in the case with the radiative cooling and heating, which is not seen in the case without radiative effect. The vertical thickness of the mid-latitude regions is $50-150\ {\rm pc}$ at the radial location of $0.4-0.8 \ {\rm kpc}$ from the Galactic center, which is comparable to the observed vertical distribution of neutral atomic gas. When we take the average of different components of energy density integrated over the Galactic bulge region, the magnetic energy is comparable to the thermal energy. We conclude that the magnetic field plays a substantial role in controlling the dynamical and thermal properties of the Galactic bulge region.
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arXiv:1601.04361 (Published 2016-01-17)
MHD simulation of the formation of clumps and filaments in quiescent diffuse clouds by thermal instability
Comments: 18 pages, 10 figures, submitted to MNRASCategories: astro-ph.GAWe have used the AMR hydrodynamic code, MG, to perform 3D MHD simulations of the formation of clumpy and filamentary structure in a thermally unstable medium. A stationary thermally unstable spherical diffuse cloud with uniform density in pressure equilibrium with low density surroundings was seeded with random density variations and allowed to evolve. A range of magnetic field strengths threading the cloud have been explored, from beta=0.1 to beta=1.0 to the zero magnetic field case (beta=infinity), where beta is the ratio of thermal pressure to magnetic pressure. Once the density inhomogeneities had developed to the point where gravity started to become important, self-gravity was introduced to the simulation. With no magnetic field, clumps form within the cloud with aspect ratios of around unity, whereas in the presence of a relatively strong field (beta=0.1) these become filaments, then evolve into interconnected corrugated sheets that are predominantly perpendicular to the magnetic field. With magnetic and thermal pressure equality (beta=1.0), both filaments and clumps are formed. At any particular instant, the projection of the 3D structure onto a plane parallel to the magnetic field, i.e. a line of sight perpendicular to the magnetic field, resembles the appearance of filamentary molecular clouds. The filament densities, widths, velocity dispersions and temperatures resemble those observed in molecular clouds. In contrast, in the strong field case beta=0.1, projection of the 3D structure along a line of sight parallel to the magnetic field reveals a remarkably uniform structure.