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1. Modelling coexisting GSF and shear instabilities in rotating stars

   https://doi.org/10.1093/mnras/stab1927  

   Chang, Eonho; Garaud, Pascale (August 2021, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT Zahn’s widely used model for turbulent mixing induced by rotational shear has recently been validated (with some caveats) in non-rotating shear flows. It is not clear, however, whether his model remains valid in the presence of rotation, even though this was its original purpose. Furthermore, new instabilities arise in rotating fluids, such as the Goldreich–Schubert–Fricke (GSF) instability. Which instability dominates when more than one can be excited, and how they influence each other, were open questions that this paper answers. To do so, we use direct numerical simulations of diffusive stratified shear flows in a rotating triply periodic Cartesian domain located at the equator of a star. We find that either the GSF instability or the shear instability tends to take over the other in controlling the system, suggesting that stellar evolution models only need to have a mixing prescription for each individual instability, together with a criterion to determine which one dominates. However, we also find that it is not always easy to predict which instability ‘wins’ for given input parameters, because the diffusive shear instability is subcritical, and only takes place if there is a finite-amplitude turbulence ‘primer’ to seed it. Interestingly, we find that the GSF instability can in some cases play the role of this primer, thereby providing a pathway to excite the subcritical shear instability. This can also drive relaxation oscillations, which may be observable. We conclude by proposing a new model for mixing in the equatorial regions of stellar radiative zones due to differential rotation. 

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2. Simulations of radiatively cooled magnetic reconnection driven by pulsed power

   https://doi.org/10.1017/S0022377824000448  

   Datta, Rishabh; Crilly, Aidan; Chittenden, Jeremy P.; Chowdhry, Simran; Chandler, Katherine; Chaturvedi, Nikita; Myers, Clayton E.; Fox, William R.; Hansen, Stephanie B.; Jennings, Chris A.; et al (April 2024, Journal of Plasma Physics)

   Magnetic reconnection is an important process in astrophysical environments, as it reconfigures magnetic field topology and converts magnetic energy into thermal and kinetic energy. In extreme astrophysical systems, such as black hole coronae and pulsar magnetospheres, radiative cooling modifies the energy partition by radiating away internal energy, which can lead to the radiative collapse of the reconnection layer. In this paper, we perform two- and three-dimensional simulations to model the MARZ (Magnetic Reconnection on Z) experiments, which are designed to access cooling rates in the laboratory necessary to investigate reconnection in a previously unexplored radiatively cooled regime. These simulations are performed in GORGON, an Eulerian two-temperature resistive magnetohydrodynamic code, which models the experimental geometry comprising two exploding wire arrays driven by 20 MA of current on the Z machine (Sandia National Laboratories). Radiative losses are implemented using non-local thermodynamic equilibrium tables computed using the atomic code Spk, and we probe the effects of radiation transport by implementing both a local radiation loss model and$$P_{1/3}$$multi-group radiation transport. The load produces highly collisional, super-Alfvénic (Alfvén Mach number$$M_A \approx 1.5$$), supersonic (Sonic Mach number$$M_S \approx 4-5$$) strongly driven plasma flows which generate an elongated reconnection layer (Aspect Ratio$$L/\delta \approx 100$$, Lundquist number$$S_L \approx 400$$). The reconnection layer undergoes radiative collapse when the radiative losses exceed the rates of ohmic and compressional heating (cooling rate/hydrodynamic transit rate =$$\tau _{\text {cool}}^{-1}/\tau _{H}^{-1}\approx 100$$); this generates a cold strongly compressed current sheet, leading to an accelerated reconnection rate, consistent with theoretical predictions. Finally, the current sheet is also unstable to the plasmoid instability, but the magnetic islands are extinguished by strong radiative cooling before ejection from the layer. 

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3. Cooling flows as a reference solution for the hot circumgalactic medium

   https://doi.org/10.1093/mnras/staf786  

   Sultan, Imran; Faucher-Giguère, Claude-André; Stern, Jonathan; Rotshtein, Shaked; Byrne, Lindsey; Wijers, Nastasha (May 2025, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT The circumgalactic medium (CGM) in $$\gtrsim 10^{12}\ \mathrm{M}_{\odot }$$ haloes is dominated by a hot phase ($$T \gtrsim 10^{6}$$ K). While many models exist for the hot gas structure, there is as yet no consensus. We compare cooling flow models, in which the hot CGM flows inwards due to radiative cooling, to the CGM of $$\sim 10^{12}{\,\rm to\,}10^{13}\ \mathrm{M}_{\odot }$$ haloes in galaxy formation simulations from the Feedback in Realistic Environments (FIRE) project at $$z\sim 0$$. The simulations include realistic cosmological evolution and feedback from stars but neglect AGN feedback. At both mass scales, CGM inflows are typically dominated by the hot phase rather than by the ‘precipitation’ of cold gas. Despite being highly idealized, we find that cooling flows describe $$\sim 10^{13}\ \mathrm{M}_{\odot }$$ haloes very well, with median agreement in the density and temperature profiles of $$\sim 20{{\ \rm per\ cent}}$$ and $$\sim 10{{\ \rm per\ cent}}$$, respectively. This indicates that stellar feedback has little impact on CGM scales in those haloes. For $$\sim 10^{12}\ \mathrm{M}_{\odot }$$ haloes, the thermodynamic profiles are also accurately reproduced in the outer CGM. For some of these lower-mass haloes, cooling flows significantly overpredict the hot gas density in the inner CGM. This could be due to multidimensional angular momentum effects not well captured by our one-dimensional cooling flow models and/or to the larger cold gas fractions in these regions. Turbulence, which contributes $$\sim 10{\!-\!}40{{\ \rm per\ cent}}$$ of the total pressure, must be included to accurately reproduce the temperature profiles. Overall, cooling flows predict entropy profiles in better agreement with the FIRE simulations than other idealized models in the literature. 

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4. The structure of 3-D collisional magnetized bow shocks in pulsed-power-driven plasma flows

   https://doi.org/10.1017/S0022377822001118  

   Datta, R.; Russell, D.R.; Tang, I.; Clayson, T.; Suttle, L.G.; Chittenden, J.P.; Lebedev, S.V.; Hare, J.D. (December 2022, Journal of Plasma Physics)

   We investigate three-dimensional (3-D) bow shocks in a highly collisional magnetized aluminium plasma, generated during the ablation phase of an exploding wire array on the MAGPIE facility (1.4 MA, 240 ns). Ablation of plasma from the wire array generates radially diverging, supersonic ( $$M_S \sim 7$$ ), super-Alfvénic ( $$M_A > 1$$ ) magnetized flows with frozen-in magnetic flux ( $$R_M \gg 1$$ ). These flows collide with an inductive probe placed in the flow, which serves both as the obstacle that generates the magnetized bow shock, and as a diagnostic of the advected magnetic field. Laser interferometry along two orthogonal lines of sight is used to measure the line-integrated electron density. A detached bow shock forms ahead of the probe, with a larger opening angle in the plane parallel to the magnetic field than in the plane normal to it. Since the resistive diffusion length of the plasma is comparable to the probe size, the magnetic field decouples from the ion fluid at the shock front and generates a hydrodynamic shock, whose structure is determined by the sonic Mach number, rather than the magnetosonic Mach number of the flow. The 3-D simulations performed using the resistive magnetohydrodynamic (MHD) code Gorgon confirm this picture, but under-predict the anisotropy observed in the shape of the experimental bow shock, suggesting that non-MHD mechanisms may be important for modifying the shock structure. 

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5. Radiative feedback for supermassive star formation in a massive cloud with H2 molecules in an atomic-cooling halo

   https://doi.org/10.1093/mnras/staa3227  

   Sakurai, Yuya; Haiman, Zoltán; Inayoshi, Kohei (November 2020, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT Recent three-dimensional cosmological simulations of protogalaxy formation have suggested that supermassive stars (SMSs) can form in gas clouds in which H2 cooling is suppressed by dynamical heating prior to the activation of atomic cooling, but they stopped short of the following growth of a central protostar. Here, we examine whether accretion on the protostellar core in this cloud is sufficiently rapid, in the face of the radiation feedback, to produce an SMS. We perform one-dimensional radiation-hydrodynamical simulations of the hot collapsing cloud with non-equilibrium chemical reactions directly adopting the cloud properties from Wise et al. as an initial condition. We find that the stellar Lyman–Werner (LW) radiation from the SMS dissociates H2 in the inner regions of the gas flow, increasing gas temperature and thermal pressure, and temporarily stopping the accretion. However, this negative feedback ceases when the self-gravity and inward ram pressure force on larger scales push the gas inwards. The central protostar is unable to expand an H ii region due to the high density, and grows to a mass of $${\gtrsim}10^5\, {\rm M}_{\odot }$$. Our results suggests the successful formation of SMSs, and resulting massive ($${\sim}10^5\, {\rm M}_{\odot }$$) remnant black holes in the clouds, but need to be confirmed in two- or three-dimensional simulations. 

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