ALBERT

Description: Giant protoplanets evacuate a gap in their host protoplanetary disc, which gas must cross before it can be accreted. A magnetic field is likely carried into the gap, potentially influencing the flow. Gap crossing has been simulated with varying degrees of attention to field evolution [pure hydrodynamical, ideal, and resistive magnetohydrodynamical (MHD)], but as yet there has been no detailed assessment of the role of the field accounting for all three key non-ideal MHD effects: Ohmic resistivity, ambipolar diffusion, and Hall drift. We present a detailed investigation of gap magnetic field structure as determined by non-ideal effects. We assess susceptibility to turbulence induced by the magnetorotational instability (MRI) and angular momentum loss from large-scale fields. As full non-ideal simulations are computationally expensive, we take an a posteriori approach, estimating MHD quantities from the pure hydrodynamical gap-crossing simulation by Tanigawa, Ohtsuki & Machida. We calculate the ionization fraction and estimate field strength and geometry to determine the strength of non-ideal effects. We find that the protoplanetary disc field would be easily drawn into the gap and circumplanetary disc. Hall drift dominates, so that much of the gap is conditionally MRI unstable depending on the alignment of the field and disc rotation axes. Field alignment also influences the strong toroidal field component permeating the gap. Large-scale magnetic forces are small in the circumplanetary disc, indicating that they cannot drive accretion there. However, turbulence will be key during satellite growth as it affects critical disc features, such as the location of the ice line.

Description: The dynamics of the partially ionized solar photosphere and chromosphere can be described by a set of equations that are structurally similar to the magnetohydrodynamic equations, except now the magnetic field is no longer frozen in the fluid but slips through it due to non-ideal magnetohydrodynamic effects which are manifested as Ohm, ambipolar and Hall diffusion. Macroscopic gas motions are widespread throughout the solar atmosphere and shearing motions couple to the non-ideal effects, destabilizing low-frequency fluctuations in the medium. The origin of this non-ideal magnetohydrodynamic instability lies in the collisional coupling of the neutral particles to the magnetized plasma in the presence of a sheared background flow. Unsurprisingly, the maximum growth rate and most unstable wavenumber depend on the flow gradient and ambient diffusivities. The orientation of the magnetic field, velocity shears and perturbation wavevector play a crucial role in assisting the instability. When the magnetic field and wavevector are both vertical, ambipolar and Ohm diffusion can be combined as Pedersen diffusion and cause only damping; in this case only Hall drift in tandem with shear flow drives the instability. However, for non-vertical fields and oblique wavevectors, both ambipolar diffusion and Hall drift are destabilizing. We investigate the stability of magnetic elements in the network and internetwork regions. The shear scale is not yet observationally determined, but assuming a typical shear flow gradient of ~0.1 s –1 we show that the magnetic diffusion shear instability grows on a time-scale of 1 min. Thus, it is plausible that network–internetwork magnetic elements are subject to this fast growing, diffusive shear instability, which could play an important role in driving low-frequency turbulence in the plasma in the solar photosphere and chromosphere.

Description: Giant protoplanets evacuate a gap in their host protoplanetary disc, which gas must cross before it can be accreted. A magnetic field is likely carried into the gap, potentially influencing the flow. Gap crossing has been simulated with varying degrees of attention to field evolution [pure hydrodynamical, ideal, and resistive magnetohydrodynamical (MHD)], but as yet there has been no detailed assessment of the role of the field accounting for all three key non-ideal MHD effects: Ohmic resistivity, ambipolar diffusion, and Hall drift. We present a detailed investigation of gap magnetic field structure as determined by non-ideal effects. We assess susceptibility to turbulence induced by the magnetorotational instability (MRI) and angular momentum loss from large-scale fields. As full non-ideal simulations are computationally expensive, we take an a posteriori approach, estimating MHD quantities from the pure hydrodynamical gap-crossing simulation by Tanigawa, Ohtsuki & Machida. We calculate the ionization fraction and estimate field strength and geometry to determine the strength of non-ideal effects. We find that the protoplanetary disc field would be easily drawn into the gap and circumplanetary disc. Hall drift dominates, so that much of the gap is conditionally MRI unstable depending on the alignment of the field and disc rotation axes. Field alignment also influences the strong toroidal field component permeating the gap. Large-scale magnetic forces are small in the circumplanetary disc, indicating that they cannot drive accretion there. However, turbulence will be key during satellite growth as it affects critical disc features, such as the location of the ice line.

Description: The character of star formation is intimately related to the supersonic magnetohydrodynamic (MHD) turbulent dynamics of the molecular clouds in which stars form. A significant amount of the turbulent energy dissipates in low-velocity shocks. Fast and slow MHD shocks differ in how they compress and heat the molecular gas, and so their radiative signatures reveal distinct physical conditions. We use a two-fluid model to compare one-dimensional fast and slow MHD shocks propagating at low speeds (a few km s – 1 ). Fast shocks are magnetically driven, forcing ion species to stream through the neutral gas ahead of the shock front. This magnetic precursor heats the gas sufficiently to create a large, warm transition zone where all the fluid variables smoothly change in the shock front. In contrast, slow shocks are driven by gas pressure, and neutral species collide with ion species in a thin hot slab that closely resembles an ordinary gas dynamic shock. We consider shocks at velocities v s  = 2–4 km s – 1 and pre-shock hydrogen nuclei densities n H  = 10 2 –10 4  cm –3 . We include a simple oxygen chemistry and cooling by CO, H 2 and H 2 O. CO rotational lines above J  = 6–5 are more strongly excited in slow shocks. These slow-shock signatures may have already been observed in infrared dark clouds in the Milky Way.

Description: During the final growth phase of giant planets, accretion is thought to be controlled by a surrounding circumplanetary disc. Current astrophysical accretion disc models rely on hydromagnetic turbulence or gravitoturbulence as the source of effective viscosity within the disc. However, the magnetically coupled accreting region in these models is so limited that the disc may not support inflow at all radii, or at the required rate. Here, we examine the conditions needed for self-consistent accretion, in which the disc is susceptible to accretion driven by magnetic fields or gravitational instability. We model the disc as a Shakura–Sunyaev α disc and calculate the level of ionization, the strength of coupling between the field and disc using Ohmic, Hall and Ambipolar diffusevities for both a magnetorotational instability (MRI) field and vertical field, and the strength of gravitational instability. We find that the standard constant-α disc is only coupled to the field by thermal ionization within 30 R J with strong magnetic diffusivity prohibiting accretion through the bulk of the mid-plane. In light of the failure of the constant-α disc to produce accretion consistent with its viscosity, we drop the assumption of constant-α and present an alternate model in which α varies radially according to the level magnetic turbulence or gravitoturbulence. We find that a vertical field may drive accretion across the entire disc, whereas MRI can drive accretion out to ~ 200 R J , beyond which Toomre's Q  = 1 and gravitoturbulence dominates. The discs are relatively hot ( T 800 K), and consequently massive ( M disc ~ 0.5 M J ).

Description: The formation of stars occurs in the dense molecular cloud phase of the interstellar medium. Observations and numerical simulations of molecular clouds have shown that supersonic magnetized turbulence plays a key role for the formation of stars. Simulations have also shown that a large fraction of the turbulent energy dissipates in shock waves. The three families of MHD shocks – fast, intermediate and slow – distinctly compress and heat up the molecular gas, and so provide an important probe of the physical conditions within a turbulent cloud. Here, we introduce the publicly available algorithm, shockfind , to extract and characterize the mixture of shock families in MHD turbulence. The algorithm is applied to a three-dimensional simulation of a magnetized turbulent molecular cloud, and we find that both fast and slow MHD shocks are present in the simulation. We give the first prediction of the mixture of turbulence-driven MHD shock families in this molecular cloud, and present their distinct distributions of sonic and Alfvénic Mach numbers. Using subgrid one-dimensional models of MHD shocks we estimate that ~0.03 per cent of the volume of a typical molecular cloud in the Milky Way will be shock heated above 50 K, at any time during the lifetime of the cloud. We discuss the impact of this shock heating on the dynamical evolution of molecular clouds.

Description: The common envelope (CE) interaction describes the swallowing of a nearby companion by a growing, evolving star. CEs that take place during the asymptotic giant branch phase of the primary may lead to the formation of a planetary nebula (PN) with a post-CE close binary in the middle. We have used published observations of masses and kinematics of jets in four post-CE PN to infer physical characteristics of the CE interaction. In three of the four systems studied, Abell 63, ETHOS 1 and the Necklace PN, the kinematics indicate that the jets were launched a few thousand years before the CE and we favour a scenario where this happened before Roche lobe overflow, although better models of wind accretion and wind Roche lobe overflow are needed. The magnetic fields inferred to launch pre-CE jets are of the order of a few gauss. In the fourth case, NGC 6778, the kinematics indicate that the jets were launched about 3000 yr after the CE interaction. Magnetic fields of the order of a few hundreds to a few thousands gauss are inferred in this case, approximately in line with predictions of post-CE magnetic fields. However, we remark that in the case of this system, we have not been able to find a reasonable scenario for the formation of the two jet pairs observed: the small orbital separation may preclude the formation of even one accretion disc able to supply the necessary accretion rate to cause the observed jets.

Description: The Circumnuclear Disc (CND) is a torus of dust and molecular gas rotating about the Galactic Centre and extending from approximately 1.6 pc to 7 pc from the central massive black hole, SgrA*. Large Velocity Gradient modelling of the intensities of the HCN 1–0, 3–2 and 4–3 transitions is used to infer hydrogen density and HCN optical depth. From HCN observations we find the molecular hydrogen density ranges from 0.1 to 2 10 6 cm –3 , about an order of magnitude less than inferred previously. The 1–0 line is weakly inverted with line-centre optical depth approx –0.1, in stark contrast to earlier estimates of 4. The estimated mass of the ring is approximately 3–4 10 5  M , consistent with estimates based on thermal dust emission. The tidal shear in the disc implies that star formation is not expected to occur without some significant triggering event.