ALBERT

Description: We use 2D (axisymmetric) and 3D hydrodynamic simulations to study Bondi–Hoyle–Lyttleton accretion with and without transverse upstream gradients. We mainly focus on the regime of high (upstream) Mach number, weak upstream gradients, and small accretor size, which is relevant to neutron star accretion in wind-fed supergiant X-ray binaries (SgXBs). We present a systematic exploration of the flow in this regime. When there are no upstream gradients, the flow is always stable regardless of accretor size or Mach number. For finite upstream gradients, there are three main types of behaviour: stable flow (small upstream gradient), turbulent unstable flow without a disc (intermediate upstream gradient), and turbulent flow with a disc-like structure (relatively large upstream gradient). When the accretion flow is turbulent, the accretion rate decreases non-convergently as the accretor size decreases. The flow is more prone to instability and the disc is less likely to form than previously expected; the parameters of most observed SgXBs place them in the regime of a turbulent, disc-less accretion flow. Among the SgXBs with relatively well-determined parameters, we find OAO 1657−415 to be the only one that is likely to host a persistent disc (or disc-like structure); this finding is consistent with observations.

Description: We investigate the formation and early evolution of a protostellar disc from a magnetized pre-stellar core using non-ideal magnetohydrodynamic (MHD) simulations including ambipolar diffusion and Ohmic dissipation. The dynamical contraction of the pre-stellar core ultimately leads to the formation of a first hydrostatic core, after ambipolar diffusion decouples the magnetic field from the predominantly neutral gas. The hydrostatic core accumulates angular momentum from the infalling material, evolving into a rotationally supported torus; this ‘first hydrostatic torus’ then forms an accreting protostar and a rotationally supported disc. The disc spreads out by gravitational instability, reaching ∼30 au in diameter at ∼3 kyr after protostar formation. The total mass and angular momentum of the protostar-disc system are determined mainly by accretion of gas from an infalling pseudo-disc, which has low specific angular momentum because of magnetic braking; their removal from the protostar-disc system by outflow and disc magnetic braking are negligible, in part because the magnetic field is poorly coupled there. The redistribution of angular momentum within the protostar-disc system is facilitated mainly by gravitational instability; this allows formation of relatively large discs even when the specific angular momentum of infalling material is low. We argue that such discs should remain marginally unstable as they grow (with Toomre Q ∼ 1–2), an idea that is broadly consistent with recent observational estimates for Class 0/I discs. We discuss the numerical convergence of our results, and show that properly treating the inner boundary condition is crucial for achieving convergence at an acceptable computational cost.

Description: We use a 3D radiative non-ideal magnetohydrodynamic simulation to investigate the formation and evolution of a young protostellar disc from a magnetized pre-stellar core. The simulation covers the first ${sim }10, { m kyr}$ after protostar formation and shows a massive, weakly magnetized disc with radius that initially grows and then saturates at ${sim }30, { m au}$. The disc is gravitationally unstable with prominent large-amplitude spiral arms. We use our simulation results and a series of physical arguments to construct a predictive and quantitative physical picture of Class 0/I protostellar disc evolution from several aspects, including (i) the angular-momentum redistribution in the disc, self-regulated by gravitational instability to make most of the disc marginally unstable; (ii) the thermal profile of the disc, well-approximated by a balance between radiative cooling and accretion heating; and (iii) the magnetic-field strength and magnetic-braking rate inside the disc, regulated by non-ideal magnetic diffusion. Using these physical insights, we build a simple 1D semi-analytic model of disc evolution. We show that this 1D model, when coupled to a computationally inexpensive simulation for the evolution of the surrounding pseudo-disc, can be used reliably to predict disc evolution in the Class 0/I phase. The predicted long-term evolution of disc size, which saturates at ${sim }30, { m au}$ and eventually shrinks, is consistent with a recent observational survey of Class 0/I discs. Such hierarchical modelling of disc evolution circumvents the computational difficulty of tracing disc evolution through Class 0/I phase with direct, numerically converged simulations.