ALBERT

Description: Motivated by indirect observational evidence for strongly magnetized accretion discs around black holes, and the novel theoretical properties of such solutions, we investigate how a strong magnetization state can develop and persist. To this end, we perform local simulations of accretion discs with an initially purely toroidal magnetic field of equipartition strength. We demonstrate that discs with zero net vertical magnetic flux and realistic boundary conditions cannot sustain a strong toroidal field. However, a magnetic pressure-dominated disc can form from an initial configuration with a sufficient amount of net vertical flux and realistic boundary conditions. Our results suggest that poloidal flux is a necessary prerequisite for the sustainability of strongly magnetized accretion discs.

Description: We characterize magnetically driven accretion at radii between 1 and 100 au in protoplanetary discs, using a series of local non-ideal magnetohydrodynamic (MHD) simulations. The simulations assume a minimum mass solar nebula (MMSN) disc that is threaded by a net vertical magnetic field of specified strength. Confirming previous results, we find that the Hall effect has only a modest impact on accretion at 30 au, and essentially none at 100 au. At 1–10 au the Hall effect introduces a pronounced bimodality in the accretion process, with vertical magnetic fields aligned to the disc rotation supporting a strong laminar Maxwell stress that is absent if the field is anti-aligned. In the anti-aligned case, we instead find evidence for bursts of turbulent stress at 5–10 au, which we tentatively identify with the non-axisymmetric Hall-shear instability. The presence or absence of these bursts depends upon the details of the adopted chemical model, which suggests that appreciable regions of actual protoplanetary discs might lie close to the borderline between laminar and turbulent behaviour. Given the number of important control parameters that have already been identified in MHD models, quantitative predictions for disc structure in terms of only radius and accretion rate appear to be difficult. Instead, we identify robust qualitative tests of magnetically driven accretion. These include the presence of turbulence in the outer disc, independent of the orientation of the vertical magnetic fields, and a Hall-mediated bimodality in turbulent properties extending from the region of thermal ionization to 10 au.

Description: A tidal disruption event, which occurs when a star is destroyed by the gravitational field of a supermassive black hole, produces a stream of debris, the evolution of which ultimately determines the observational properties of the event. Here, we show that a post-periapsis caustic – a location where the locus of gas parcels comprising the stream would collapse into a two-dimensional surface if they evolved solely in the gravitational field of the hole – occurs when the pericentre distance of the star is of the order of the tidal radius of the hole. It is demonstrated that this ‘pancake’ induces significant density perturbations in the debris stream, and, for stiffer equations of state (adiabatic index 5/3), these fluctuations are sufficient to gravitationally destabilize the stream, resulting in its fragmentation into bound clumps. The results of our findings are discussed in the context of the observational properties of tidal disruption events.

Description: Strongly magnetized accretion discs around black holes have attractive features that may explain enigmatic aspects of X-ray binary behaviour. The structure and evolution of these discs are governed by a dynamo-like mechanism, which channels part of the accretion power liberated by the magnetorotational instability (MRI) into an ordered toroidal magnetic field. To study dynamo activity, we performed three-dimensional, stratified, isothermal, ideal magnetohydrodynamic shearing box simulations. The strength of the self-sustained toroidal magnetic field depends on the net vertical magnetic flux, which we vary across almost the entire range over which the MRI is linearly unstable. We quantify disc structure and dynamo properties as a function of the initial ratio of mid-plane gas pressure to vertical magnetic field pressure, $\beta _0^{\rm mid} = p_{\rm gas} / p_B$ . For $10^5 \ge \beta _0^{\rm mid} \ge 10$ the effective α-viscosity parameter scales as a power law. Dynamo activity persists up to and including $\beta _0^{\rm mid} = 10^2$ , at which point the entire vertical column of the disc is magnetic pressure dominated. Still stronger fields result in a highly inhomogeneous disc structure, with large density fluctuations. We show that the turbulent steady state β mid in our simulations is well matched by the analytic model of Begelman et al. describing the creation and buoyant escape of toroidal field, while the vertical structure of the disc can be broadly reproduced using this model. Finally, we discuss the implications of our results for observed properties of X-ray binaries.

Description: Recently, it has been suggested that the fragmentation boundary in smoothed particle hydrodynamic (SPH) and fargo simulations of self-gravitating accretion discs with β-cooling do not converge as resolution is increased. Furthermore, this recent work suggests that by carefully optimizing the artificial viscosity parameters in these codes, it can be shown that fragmentation may occur for much longer cooling times than earlier work suggests. If correct, this result is intriguing as it suggests that gas giant planets could form, via direct gravitational collapse, reasonably close to their parent stars. This result is, however, slightly surprising and there have been a number of recent studies suggesting that the result is likely an indication of a numerical problem with the simulations. One suggestion, in particular, is that the SPH results are influenced by the manner in which the cooling is implemented. We extend this work here and show that if the cooling is implemented in a manner that removes a known numerical artefact in the shock regions, the fragmentation boundary converges to a value consistent with earlier work and that fragmentation is unlikely for the long cooling times suggested by this recent work. We also investigate the optimization of the artificial viscosity parameters and show that the values that appear optimal are likely introducing numerical problems in both the SPH and fargo simulations. We therefore conclude that earlier predictions for the cooling times required for fragmentation are likely correct and that, as suggested by this earlier work, fragmentation cannot occur in the inner parts ( r  〈 50 au) of typical protostellar discs.

Description: We consider the role of thermal diffusion due to turbulence and radiation on accretion bursts that occur in protoplanetary discs which contain dead-zones. Using 1D viscous disc models, we show that diffusive radial transport of heat is important during the gravomagnetic limit cycle, and can strongly modify the duration and frequency of accretion outbursts. When the Prandtl number is large – such that turbulent diffusion of heat is unimportant – radial radiative diffusion reduces the burst duration compared to models with no diffusive transport of heat. When the Prandtl number is small 25, we find that turbulent diffusion dominates the radial transport of heat, reducing the burst duration to 10 3 yr as well as increasing the burst frequency. Furthermore, inclusion of radial transport of heat extends the range of infall rates under which the disc undergoes accretion bursts from 10 –8 to 10 –6 M  yr –1 with no diffusion, to 10 –8 to 10 –4 M  yr –1 with radiative and strong turbulent diffusion. The relative roles of radiative and turbulent thermal diffusion are likely to vary during an accretion burst, but simple estimates suggest that the expected Prandtl numbers are of the order of 10 in protoplanetary discs, and hence that turbulent diffusion is likely to be an important process during accretion outbursts due to the gravomagnetic limit cycle.

Description: A tidal disruption event (TDE) – when a star is destroyed by the immense gravitational field of a supermassive black hole – transforms a star into a stream of tidally shredded debris. The properties of this debris ultimately determine the observable signatures of tidal disruption events (TDEs). Here we derive a simple, self-similar solution for the velocity profile of the debris streams produced from TDEs, and show that this solution agrees extremely well with numerical results. Using this self-similar solution, we calculate an analytic, approximate expression for the radial density profile of the stream. We show that there is a critical adiabatic index that varies as a function of position along the stream above (below) which the stream is unstable (stable) to gravitational fragmentation. We also calculate the impact of heating and cooling on this stability criterion.

Description: With hydrodynamical simulations, we determine the conditions under which an initially coplanar planet–disc system that orbits a member of a misaligned binary star evolves to form a planet that undergoes Kozai–Lidov (KL) oscillations once the disc disperses. These oscillations may explain the large orbital eccentricities, as well as the large misalignments with respect to the spin of the central star, observed for some exoplanets. The planet is assumed to be massive enough to open a gap in the disc. The planet's tilt relative to the binary orbital plane is subject to two types of oscillations. The first type, present at even small inclination angles relative to the binary orbital plane, is due to the interaction of the planet with the disc and binary companion and is amplified by a secular resonance. The second type of oscillation is the KL oscillation that operates on both the planet and disc at larger binary inclination angles. We find that for a sufficiently massive disc, even a relatively low inclination planet–disc system can force a planet to an inclination above the critical KL angle, as a consequence of the first type of tilt oscillation, allowing it to undergo the second type of oscillation. We conclude that the hydrodynamical evolution of a sufficiently massive and inclined disc in a binary system broadens the range of systems that form eccentric and misaligned giant planets to include a wide range of initial misalignment angles (20° i 160°).

Description: The porosity and permeability of sandstone and carbonate reservoirs (known as reservoir quality) are essential inputs for successful oil and gas resource exploration and exploitation. This chapter introduces basic concepts, analytical and modelling techniques and some of the key controversies to be discussed in 20 research papers that were initially presented at a Geological Society conference in 2014 titled ‘Reservoir Quality of Clastic and Carbonate Rocks: Analysis, Modelling and Prediction’. Reservoir quality in both sandstones and carbonates is studied using a wide range of techniques: log analysis and petrophysical core analysis, core description, routine petrographic tools and, ideally, less routine techniques such as stable isotope analysis, fluid inclusion analysis and other geochemical approaches. Sandstone and carbonate reservoirs both benefit from the study of modern analogues to constrain the primary character of sediment before they become a hydrocarbon reservoir. Prediction of sandstone and carbonate reservoir properties also benefits from running constrained experiments to simulate diagenetic processes during burial, compaction and heating. There are many common controls on sandstone and carbonate reservoir quality, including environment of deposition, rate of deposition and rate and magnitude of sea-level change, and many eogenetic processes. Compactional and mesogenetic processes tend to affect sandstone and carbonate somewhat differently but are both influenced by rate of burial, and the thermal and pressure history of a basin. Key differences in sandstone and carbonate reservoir quality include the specific influence of stratigraphic age on seawater composition (calcite v. aragonite oceans), the greater role of compaction in sandstones and the greater reactivity and geochemical openness of carbonate systems. Some of the key controversies in sandstone and carbonate reservoir quality focus on the role of petroleum emplacement on diagenesis and porosity loss, the role of effective stress in chemical compaction (pressure solution) and the degree of geochemical openness of reservoirs during diagenesis and cementation. This collection of papers contains case study-based examples of sandstone and carbonate reservoir quality prediction as well as modern analogue, outcrop analogue, modelling and advanced analytical approaches.

Description: We investigate the pre-disruption gravitational dynamics and post-disruption hydrodynamics of the tidal disruption of stars by supermassive black hole (SMBH) binaries. We focus on binaries with relatively low mass primaries (10 6 M ), moderate mass ratios, and separations with reasonably long gravitational wave inspiral times (tens of Myr). First, we generate a large ensemble (between 1 and 10 million) of restricted three-body integrations to quantify the statistical properties of tidal disruptions by circular SMBH binaries of initially unbound stars. Compared to the reference case of a disruption by a single SMBH, the binary potential induces a significant variance into the specific energy and angular momentum of the star at the point of disruption. Second, we use Newtonian numerical hydrodynamics to study the detailed evolution of the fallback debris from 120 disruptions randomly selected from the three-body ensemble (excluding only the most deeply penetrating encounters). We find that the overall morphology of the debris is greatly altered by the presence of the second black hole, and the accretion rate histories display a wide range of behaviours, including order of magnitude dips and excesses relative to control simulations that include only one black hole. Complex evolution typically persists for many orbital periods of the binary. We find evidence for power in the accretion curves on time-scales related to the binary orbital period, though there is no exact periodicity. We discuss our results in the context of future wide-field surveys, and comment on the prospects of identifying and characterizing the subset of events occurring in nuclei with binary SMBHs.