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1. Accretion–ablation mechanics

   https://doi.org/10.1098/rsta.2022.0373  

   Pradhan, Satya Prakash ; Yavari, Arash ( December 2023 , Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences)

   In this paper, we formulate a geometric nonlinear theory of the mechanics of accreting–ablating bodies. This is a generalization of the theory of accretion mechanics of Sozio & Yavari (Sozio & Yavari 2019J. Nonlinear Sci.29, 1813–1863 (doi:10.1007/s00332-019-09531-w)). More specifically, we are interested in large deformation analysis of bodies that undergo a continuous and simultaneous accretion and ablation on their boundaries while under external loads. In this formulation, the natural configuration of an accreting–ablating body is a time-dependent Riemannian$3$-manifold with a metric that is an unknowna prioriand is determined after solving the accretion–ablation initial-boundary-value problem. In addition to the time of attachment map, we introduce a time of detachment map that along with the time of attachment map, and the accretion and ablation velocities, describes the time-dependent reference configuration of the body. The kinematics, material manifold, material metric, constitutive equations and the balance laws are discussed in detail. As a concrete example and application of the geometric theory, we analyse a thick hollow circular cylinder made of an arbitrary incompressible isotropic material that is under a finite time-dependent extension while undergoing continuous ablation on its inner cylinder boundary and accretion on its outer cylinder boundary. The state of deformation and stress during the accretion–ablation process, and the residual stretch and stress after the completion of the accretion–ablation process, are computed.

   This article is part of the theme issue ‘Foundational issues, analysis and geometry in continuum mechanics’.

    

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2. Bridging Scales in Black Hole Accretion and Feedback: Magnetized Bondi Accretion in 3D GRMHD

   https://doi.org/10.3847/2041-8213/ad1048  

   Cho 조, Hyerin 혜린 ; Prather, Ben S. ; Narayan, Ramesh ; Natarajan, Priyamvada ; Su, Kung-Yi ; Ricarte, Angelo ; Chatterjee, Koushik ( December 2023 , The Astrophysical Journal Letters)

   Fueling and feedback couple supermassive black holes (SMBHs) to their host galaxies across many orders of magnitude in spatial and temporal scales, making this problem notoriously challenging to simulate. We use a multi-zone computational method based on the general relativistic magnetohydrodynamic (GRMHD) code KHARMA that allows us to span 7 orders of magnitude in spatial scale, to simulate accretion onto a non-spinning SMBH from an external medium with a Bondi radius ofR_B≈ 2 × 10⁵GM_•/c², whereM_•is the SMBH mass. For the classic idealized Bondi problem, spherical gas accretion without magnetic fields, our simulation results agree very well with the general relativistic analytic solution. Meanwhile, when the accreting gas is magnetized, the SMBH magnetosphere becomes saturated with a strong magnetic field. The density profile varies as ∼r⁻¹rather thanr^−3/2and the accretion rate$\dot{M}$is consequently suppressed by over 2 orders of magnitude below the Bondi rate${\dot{M}}_{\mathrm{B}}$. We find continuous energy feedback from the accretion flow to the external medium at a level of$\sim {10}^{-2}\dot{M}{c}^2\sim 5\times {10}^{-5}{\dot{M}}_{\mathrm{B}}{c}^2$. Energy transport across these widely disparate scales occurs via turbulent convection triggered by magnetic field reconnection near the SMBH. Thus, strong magnetic fields that accumulate on horizon scales transform the flow dynamics far from the SMBH and naturally explain observed extremely low accretion rates compared to the Bondi rate, as well as at least part of the energy feedback.

    

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3. Core segregation during pebble accretion

   https://doi.org/10.1016/j.epsl.2022.117537  

   Olson, Peter ; Sharp, Zachary ; Garai, Susmita ( June 2022 , Earth and Planetary Science Letters)
4. Centrifugal barriers in magnetospheric accretion

   https://doi.org/10.1093/mnras/stad284  

   Lyutikov, Maxim ( January 2023 , Monthly Notices of the Royal Astronomical Society)

   We reconsider the dynamics of accretion flows onto magnetized central star. For dipolar magnetically aligned case, the centrifugal barrier is at Rcb = (2/3)1/3Rc = 0.87Rc, where Rc = (GM/Ω2)1/3 is the corotation radius. For oblique dipole direct accretion from the corotation radius Rc is possible only for magnetic obliquity satisfying $\tan \theta _\mu \ge 1/(2 \sqrt{3})$ (θμ ≥ 16.1°). The accretion proceeds in a form of funnel flows – along two streams centred on the μ–Ω plane, with azimuthal opening angle $\cos (\Delta \phi) = { \cot ^ 2 {\theta _\mu } }/{12}$. For the magnetosphere distorted by the diamagnetic disc, the centrifugal barrier can be at as small radius as Rcb = 0.719Rc for the fully confined dipole, extending out to Rcb ∼ Rc for the magnetically balanced case. Type-II X-ray bursts in accreting neutron stars may be mediated by the centrifugal barrier; this requires nearly aligned configuration. Centrifugally barriered material trapped in the magnetosphere may lead to periodic obscuration (‘dips’) in the light curve of the host star, e.g. as observed in accreting young stellar objects and X-ray binaries.

    

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5. Bondi accretion in trumpet geometries

   https://doi.org/10.1088/1361-6382/aa51fe  

   Miller, August J. ; Baumgarte, Thomas W. ( January 2017 , Classical and Quantum Gravity)