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We showcase a study on the physical properties of the circumnuclear disk (CND) surrounding the supermassive black hole (SMBH) Sgr A* of the Galactic Center, emphasizing the role of magnetic field (⁠|$\boldsymbol {B}$| field) with |$0.50\,$| pc spatial resolution. Based on the sensitive |$\lambda = 850\, \mu$| m polarization data taken with the JCMT SCUBA2/POL2 (James Clerk Maxwell Telescope Submillimetre Common-User Bolometer Array 2), we analyzed ancillary datasets: CS |$J = 2$| –1 emission taken with ALMA (Atacama Large Millimeter/submillimeter Array), continuum emissions taken at |$\lambda = 6\,$| cm and at |$\lambda = 37\, \mu$| m taken with the VLA (Very Large Array) and SOFIA (the Stratospheric Observatory for Infrared Astronomy telescope). The |$\boldsymbol {B}$| field within the CND exhibits a coherent spiral pattern. Applying the model described by Wardle and Königl (1990, ApJ, 362, 12; the WK model) to the observed |$\boldsymbol {B}$| field pattern, it favors gas-pressure-dominant models without dismissing a gas-and- |$\boldsymbol {B}$| field comparable model, leading us to estimate the |$\boldsymbol {B}$| -field strength in the ionized cavity around Sgr A* as |$0.24^{+0.06}_{-0.04}\,$| mG. Analysis based on the WK model further allows us to derive representative |$\boldsymbol {B}$| -field strengths for the radial, azimuthal, and vertical components as |$(B_r, B_\phi , B_z) = (0.4 \pm 0.1, -0.7 \pm 0.2, 0.2 \pm 0.05)\,$| mG, respectively. A key finding is that the |$|B_\phi |$| component is dominant over |$B_r$| and |$B_z$| components, consistent with the spiral morphology, indicating that the CND's |$\boldsymbol {B}$| -field is predominantly toroidal, possibly shaped by accretion dynamics. Considering the turbulent pressure, estimated plasma |$\beta$| values indicate that the effective gas pressure should surpass the magnetic pressure. Assessing the CND of our MWG in the toroidal-and-vertical stability parameter space, we propose that such an "effective" magneto-rotational instability (MRI) may likely be active. The estimated maximum unstable wavelength, |$\lambda _{\rm max} = 0.1 \pm 0.1\,$| pc, is smaller than the CND's scale height (⁠|$0.2 \pm 0.1\,$| pc), which indicates the potential for the effective MRI intermittent cycles of |$\sim 10^6\,$| yr, which should profoundly affect the CND's evolution, considering the estimated mass accretion rate of |$10^{-2} M_{\odot }\,$| yr |$^{-1}$| to the SMBH. [ABSTRACT FROM AUTHOR]

We investigate the roles of magnetically driven disc wind (MDW) and thermally driven photoevaporative wind (PEW) in the long-time evolution of protoplanetary discs. We start simulations from the early phase in which the disc mass is |$0.118\, \rm M_{\odot }$| around a |$1\, \rm M_{\odot }$| star and track the evolution until the disc is completely dispersed. We incorporate the mass-loss by PEW and the mass-loss and magnetic braking (wind torque) by MDW, in addition to the viscous accretion, viscous heating, and stellar irradiation. We find that MDW and PEW, respectively, have different roles: magnetically driven wind ejects materials from an inner disc in the early phase, whereas photoevaporation has a dominant role in the late phase in the outer (≳1 au) disc. The disc lifetime, which depends on the combination of MDW, PEW, and viscous accretion, shows a large variation of ∼1–20 Myr; the gas is dispersed mainly by the MDW and the PEW in the cases with a low viscosity and the lifetime is sensitive to the mass-loss rate and torque of the MDW, whereas the lifetime is insensitive to these parameters when the viscosity is high. Even in discs with very weak turbulence, the cooperation of MDW and PEW enables the disc dispersal within a few Myr. [ABSTRACT FROM AUTHOR]

Previous axisymmetric investigations in two dimensions (2D) have shown that rings and gaps develop naturally in non-ideal magnetohydrodynamic disc–wind systems, especially in the presence of ambipolar diffusion. Here, we extend these 2D simulations to three dimensions (3D) and find that rings and gaps are still formed in the presence of a moderately strong ambipolar diffusion. The rings and gaps form from the same basic mechanism that was identified in the 2D simulations, namely, the redistribution of the poloidal magnetic flux relative to the disc material as a result of the reconnection of a sharply pinched poloidal magnetic field lines. Thus, the less dense gaps are more strongly magnetized with a large poloidal magnetic field compared to the less magnetized (dense) rings. The rings and gaps start out rather smoothly in 3D simulations that have axisymmetric initial conditions. Non-axisymmetric variations arise spontaneously at later times, but they do not grow to such an extent as to disrupt the rings and gaps. These disc substructures persist to the end of the simulations, lasting up to 3000 orbital periods at the inner edge of the simulated disc. The longevity of the perturbed yet still coherent rings make them attractive sites for trapping large grains that would otherwise be lost to rapid radial migration due to gas drag. As the ambipolar diffusivity decreases, both the disc and the wind become increasingly turbulent, driven by the magnetorotational instability, with tightly wound spiral arms becoming more prominent in the disc. [ABSTRACT FROM AUTHOR]

By performing a global magnetohydrodynamical (MHD) simulation for the Milky Way with an axisymmetric gravitational potential, we propose that spatially dependent amplification of magnetic fields possibly explains the observed noncircular motion of the gas in the Galactic centre (GC) region. The radial distribution of the rotation frequency in the bulge region is not monotonic in general. The amplification of the magnetic field is enhanced in regions with stronger differential rotation, because magnetorotational instability and field-line stretching are more effective. The strength of the amplified magnetic field reaches ≳ 0.5 mG, and radial flows of the gas are excited by the inhomogeneous transport of angular momentum through turbulent magnetic field that is amplified in a spatially dependent manner. As a result, the simulated position-velocity diagram exhibits a time-dependent asymmetric parallelogram-shape owing to the intermittency of the magnetic turbulence; the present model provides a viable alternative to the bar-potential-driven model for the parallelogram shape of the central molecular zone. In addition, Parker instability (magnetic buoyancy) creates vertical magnetic structure, which would correspond to observed molecular loops, and frequently excited vertical flows. Furthermore, the time-averaged net gas flow is directed outward, whereas the flows are highly time dependent, which would contribute to the outflow from the bulge. [ABSTRACT FROM PUBLISHER]

We develop a framework for magnetohydrodynamical (MHD) simulations in a local cylindrical shearing box by extending the formulation of the Cartesian shearing box. We construct shearing-periodic conditions at the radial boundaries of a simulation box from the conservation relations of the basic MHD equations, taking into account the explicit radial dependence of physical quantities. We demonstrate quasi-steady mass accretion, which cannot be handled by the standard Cartesian shearing box model, with an ideal MHD simulation in a vertically unstratified cylindrical shearing box for up to 200 rotations. In this demonstrative run we set up (i) net vertical magnetic flux, (ii) a locally isothermal equation of state, and (iii) a sub-Keplerian equilibrium rotation, whereas the sound velocity and the initial Alfvén velocity have the same radial dependence as that of the Keplerian velocity. Inward mass accretion is induced to balance the outward angular momentum flux of the MHD turbulence triggered by the magnetorotational instability in a self-consistent manner. We discuss detailed physical properties of the saturated magnetic field, in comparison to the results of a Cartesian shearing box simulation. [ABSTRACT FROM AUTHOR]

Various observations show peculiar features in the Galactic Center region, such as loops and filamentary structure. It is still unclear how such characteristic features are formed. Magnetic field is believed to play very important roles in the dynamics of gas in the Galaxy Center. Suzuki et al. (2015) performed a global magneto-hydrodynamical simulation focusing on the Galactic Center with an axisymmetric gravitational potential and claimed that non-radial motion is excited by magnetic activity. We further analyzed their simulation data and found that vertical motion is also excited by magnetic activity. In particular, fast down flows with speed of ~100 km/s are triggered near the footpoint of magnetic loops that are buoyantly risen by Parker instability. These downward flows are accelerated by the vertical component of the gravity, falling along inclined field lines. As a result, the azimuthal and radial components of the velocity are also excited, which are observed as high velocity features in a simulated position-velocity diagram. Depending on the viewing angle, these fast flows will show a huge variety of characteristic features in the position-velocity diagram. [ABSTRACT FROM PUBLISHER]