Recently, we explored new, meshless finite-volume Lagrangian methods for hydrodynamics: the "meshless finite mass" (MFM) and "meshless finite volume" (MFV) methods; these capture advantages of both smoothed-particle hydrodynamics (SPH) and adaptive mesh-refinement (AMR) schemes. We extend these to include ideal magnetohydrodynamics (MHD). The MHD equations are second-order consistent and conservative. We augment these with a divergence-cleaning scheme, which maintains ∇ · B ≈ 0. We implement these in the code GIZMO, together with stateof-the-art SPH MHD. We consider a large test suite, and show that on all problems the new methods are competitive with AMR using constrained transport (CT) to ensure ∇ · B = 0. They correctly capture the growth/structure of the magnetorotational instability (MRI), MHD turbulence, and launching of magnetic jets, in some cases converging more rapidly than state-of-the-art AMR. Compared to SPH, the MFM/MFV methods exhibit convergence at fixed neighbor number, sharp shock-capturing, and dramatically reduced noise, divergence errors, & diffusion. Still, "modern" SPH can handle most test problems, at the cost of larger kernels and "by hand" adjustment of artificial diffusion. Compared to non-moving meshes, the new methods exhibit enhanced "grid noise" but reduced advection errors and diffusion, easily include self-gravity, and feature velocity-independent errors and superior angular momentum conservation. They converge more slowly on some problems (smooth, slow-moving flows), but more rapidly on others (involving advection/rotation). In all cases, we show divergence-control beyond the Powell 8-wave approach is necessary, or all methods can converge to unphysical answers even at high resolution.

In the seconds following their formation in core-collapse supernovae, “proto”-magnetars drive neutrino-heated magneto-centrifugal winds. Using a suite of two-dimensional axisymmetric MHD simulations, we show that relatively slowly rotating magnetars with initial spin periods of P⋆0 = 50 − 500 ms spin down rapidly during the neutrino Kelvin-Helmholtz cooling epoch. These initial spin periods are representative of those inferred for normal Galactic pulsars, and much slower than those invoked for gamma-ray bursts and super-luminous supernovae. Since the flow is non-relativistic at early times, and because the Alfvén radius is much larger than the proto-magnetar radius, spindown is millions of times more efficient than the typically-used dipole formula. Quasi-periodic plasmoid ejections from the closed zone enhance spindown. For polar magnetic field strengths B0 ≳ 5 × 1014 G, the spindown timescale can be shorter than than the Kelvin-Helmholtz timescale. For B0 ≳ 1015 G, it is of order seconds in early phases. We compute the spin evolution for cooling proto-magnetars as a function of B0, P⋆0, and mass (M). Proto-magnetars born with B0 greater than $\simeq 1.3\times 10^{15}\, {\rm \, G}\, (P_{\star 0}/{400\, \rm \, ms})^{-1.4}(M/1.4\, {\rm M}_\odot )^{2.2}$ spin down to periods >1 s in just the first few seconds of evolution, well before the end of the cooling epoch and the onset of classic dipole spindown. Spindown is more efficient for lower M and for larger P⋆0. We discuss the implications for observed magnetars, including the discrepancy between their characteristic ages and supernova remnant ages. Finally, we speculate on the origin of 1E 161348-5055 in the remnant RCW 103, and the potential for other ultra-slowly rotating magnetars.

Shock revival in core-collapse supernovae (CCSNe) may be due to the neutrino mechanism. While it is known that in a neutrino-powered CCSN, explosion begins when the neutrino luminosity of the proto-neutron star exceeds a critical value, the physics of this condition in time-dependent, multidimensional simulations are not fully understood. Pejcha & Thompson (2012) found that an 'antesonic condition' exists for time-steady spherically symmetric models, potentially giving a physical explanation for the critical curve observed in simulations. In this paper, we extend that analysis to time-dependent, spherically symmetric polytropic models. We verify the critical antesonic condition in our simulations, showing that models exceeding it drive transonic winds whereas models below it exhibit steady accretion. In addition, we find that (1) high spatial resolution is needed for accurate determination of the antesonic ratio and shock radius at the critical curve, and that low resolution simulations systematically underpredict these quantities, making explosion more difficult at lower resolution; (2) there is an important physical connection between the critical mass accretion rate at explosion and the mass loss rate of the post-explosion wind: the two are directly proportional at criticality, implying that, at criticality, the wind kinetic power is tied directly to the accretion power; (3) the value of the post-shock adiabatic index Γ has a large effect on the length and time scales of the post-bounce evolution of the explosion larger values of Γ result in a longer transition from the accretion to wind phases.