Angular momentum is transported outwards through an accretion disc by magnetohydrodynamical (MHD) turbulence thus allowing material to accrete on to the central object. The magnetorotational ...instability (MRI) requires a minimum ionization fraction to drive turbulence in a disc. The inner parts of the disc around a young stellar object are sufficiently hot to be thermally ionized. Further out, cosmic rays ionize the surface layers and a dead zone forms at the mid-plane where the disc is too cool for the MRI to operate. The surface density in the turbulent active layer is often assumed to be constant with radius because the cosmic rays penetrate a constant layer. However, if a critical magnetic Reynolds number, Re
M, crit, is used to determine the extent of the dead zone, the surface density in the layer generally increases with radius. For small critical magnetic Reynolds number of the order of 1, the constant-layer approximation may be a reasonable fit. However, MHD simulations suggest that the critical magnetic Reynolds number may be much larger, of the order of 104. Analytical fits for the surface density in the magnetic active layer show that
, at temperature T and radius R, are a good fit for higher critical magnetic Reynolds number. For the metallicity variation between our Galaxy, the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC), there should be no significant difference in the extent of the dead zone. Observations suggest an increase in the lifetime of the disc with decreasing metallicity, which cannot be explained by the dead-zone structure (ignoring possible differences in dust abundances).
We consider the response of a protostellar disc to a tidally induced warp and the resultant changes in the spectral energy distribution (SED). We argue that for typical protostellar disc parameters ...the warp is communicated through the disc in a wave-like fashion. We find that the main effects of the warp tend to be at large radii (R≳ 30 au) and, for sufficiently small viscosity, can be quite long-lived. This can result in non-uniform illumination of the disc at these radii and can induce significant changes to the SED at wavelengths λ≳ 100 μm.
Short gamma-ray bursts (sGRBs) are generally thought to result from the merger of two neutron stars or the merger of a neutron star with a black hole. It is becoming standard practice to model these ...mergers with hydrodynamical simulations that employ equations of state that are derived, for example, for determining the behavior of matter in core-collapse supernovae, and which therefore make use of the assumption that the matter is hot and in nuclear statistical equilibrium (NSE). In this Letter we draw attention to the fact that the hydrodynamical timescale (roughly the gravitational timescale of the neutron star) may be several orders of magnitude shorter than the timescale on which such equilibrium can be reestablished in the tidal debris ejected during an sGRB, and that on the initial decompression timescales the unshocked tidal ejecta may remain sufficiently cool that the employed equations of state are not appropriate for modeling the dynamics of this part of the flow. On timescales short compared with the timescale on which NSE can be (re)established, the equation of state can remain relatively stiff and thus the stream of tidal debris can remain narrow and vulnerable to gravitational instability, as has recently been suggested. These findings suggest that estimates of the type and abundances of heavy elements formed in short gamma-ray bursts need to be revisited. We suggest that the most direct method of testing the physical and dynamical properties of tidal ejecta in sGRBs will come from modeling of their lightcurves, which provides the cleanest source of information on the system dynamics.
In this Letter, we analyse the distributions of stellar ages in giant molecular clouds (GMCs) in spiral arms, interarm spurs and at large galactic radii, where the spiral arms are relatively weak. We ...use the results of numerical simulations of galaxies, which follow the evolution of GMCs and include star particles where star formation events occur. We find that GMCs in spiral arms tend to have predominantly young (<10 Myr) stars. By contrast, clouds which are the remainders of spiral arm giant molecular asssociations that have been sheared into interarm GMCs contain fewer young (<10 Myr) stars and more ∼20 Myr stars. We also show that clouds which form in the absence of spiral arms, due to local gravitational and thermal instabilities, contain preferentially young stars. We propose that the age distributions of stars in GMCs will be a useful diagnostic to test different cloud evolution scenarios, the origin of spiral arms and the success of numerical models of galactic star formation. We discuss the implications of our results in the context of Galactic and extragalactic molecular clouds.
A warped accretion disc with a central radiation source is subject to non-axisymmetric radiation pressure forces, which in turn modify the warp. We show here, with a simple analytic approach, that ...even an initially flat disc is unstable to warping through this effect. We give estimates of the radii at which such self-induced warping takes place, and discuss applications to discs in binary X-ray sources and in active galactic nuclei.
Fifteen chemistry‐transport models are used to quantify, for the first time, the export of oxidised nitrogen (NOy) to and from four regions (Europe, North America, South Asia, and East Asia), and to ...estimate the uncertainty in the results. Between 12 and 24% of the NOx emitted is exported from each region annually. The strongest impact of each source region on a foreign region is: Europe on East Asia, North America on Europe, South Asia on East Asia, and East Asia on North America. Europe exports the most NOy, and East Asia the least. East Asia receives the most NOy from the other regions. Between 8 and 15% of NOx emitted in each region is transported over distances larger than 1000 km, with 3–10% ultimately deposited over the foreign regions.
•Accretion discs are central to much of modern astronomy•The disc structure is sensitive to the inner accretion or decretion boundary•Here we provide the theory for a more general inner disc boundary ...condition•For many astrophysical systems such a boundary condition is more appropriate•We provide analytical and numerical solutions to the disc evolution
We present analytical and numerical solutions for accretion discs subject to a non-zero central torque. We express this in terms of a single parameter, f, which is the ratio of outward viscous flux of angular momentum from the inner boundary to the inward advected flux of angular momentum there. The standard “accretion” disc, where the central boundary condition is zero-torque, is represented by f=0. A “decretion” disc, where the radial velocity at the inner boundary is zero, is represented by f → ∞. For f > 0 a torque is applied to the disc at the inner boundary, which feeds both angular momentum and energy into the disc. This can arise, for example, in the case of a circumbinary disc where resonances transfer energy and angular momentum from the binary to the disc orbits, or where the disc is around a rotating magnetic star which can allow the disc orbits to be accelerated outwards at the magnetospheric radius. We present steady-state solutions to the disc structure as a function of f, and for arbitrary kinematic viscosity ν. For time-dependent discs, we solve the equations using a Green’s function approach for the specific case of ν ∝ R and provide an example numerical solution to the equations for the case of ν ∝ R3/2. We find that for values of f ≲ 0.1 the disc solutions closely resemble “accretion” discs. For values of f ≳ 10 the solutions initially resemble “decretion” discs, but at sufficiently late times exhibit the properties of “accretion” discs. We discuss the application of this theory to different astrophysical systems, and in particular the values of the f parameter that are expected in different cases.
ABSTRACT
We have made high-precision polarimetric observations of the polluted white dwarf G29-38 with the HIgh Precision Polarimetric Instrument 2. The observations were made at two different ...observatories – using the 8.1-m Gemini North Telescope and the 3.9-m Anglo-Australian Telescope – and are consistent with each other. After allowing for a small amount of interstellar polarization, the intrinsic linear polarization of the system is found to be 275.3 ± 31.9 parts per million at a position angle of 90.8 ± 3.8° in the SDSS g′ band. We compare the observed polarization with the predictions of circumstellar disc models. The measured polarization is small in the context of the models we develop, which only allows us to place limits on disc inclination and Bond albedo for optically thin disc geometries. In this case, either the inclination is near-face-on or the albedo is small – likely in the range 0.05–0.15 – which is in line with other debris disc measurements. A preliminary search for the effects of G29-38’s pulsations in the polarization signal produced inconsistent results. This may be caused by beating effects, indicate a clumpy dust distribution, or be a consequence of measurement systematics.
We consider the evolution of a warped disc around a Kerr black hole, under conditions such that the warp propagates in a wave-like manner. This occurs when the dimensionless effective viscosity, α, ...that damps the warp is less than the characteristic angular semi-thickness, H/R, of the disc. We adopt linearized equations that are valid for warps of sufficiently small amplitude in a Newtonian disc, but also account for the apsidal and nodal precession that occur in the Kerr metric. Through analytical and time-dependent studies, we confirm the results of Demianski & Ivanov and of Ivanov & Illarionov, that such a disc takes on a characteristic warped shape. The inner part of the disc is not necessarily aligned with the equator of the hole, even in the presence of dissipation. We draw attention to the fact that this might have important implications for the directionality of jets emanating from discs around rotating black holes.
•Observations suggest that the dimensionless viscosity parameter α∼0.2−0.3.•In fully ionised discs the strength of the turbulence is always limited to subsonic.•This limit arises due to the ...fundamental nature of the turbulence.•Partially ionised discs have lower viscosity implying magnetism plays a dominant role.•This lends observational support for magneto-rotational instability driven turbulence.
We use well–established observational evidence to draw conclusions about the fundamental nature of the viscosity in accretion discs. To do this, we first summarise the observational evidence for the value of the dimensionless accretion disc viscosity parameter α, defined by Shakura and Sunyaev (1973, 1976). We find that, for fully ionized discs, the value of α is readily amenable to reliable estimation and that the observations are consistent with the hypothesis that α∼0.2−0.3. In contrast in discs that are not fully ionized, estimates of the value of α are generally less direct and the values obtained are generally < 0.01 and often ≪ 0.01. We conclude that this gives us crucial information about the nature of viscosity in accretion discs. First, in fully ionized discs the strength of the turbulence is always limited by being at most trans-sonic. This implies that it is necessary that credible models of the turbulence reflect this fact. Second, the smaller values of α found for less ionized, and therefore less strongly conducting, discs imply that magnetism plays a dominant role. This provides important observational support for the concept of magneto-rotational instability (MRI) driven hydromagnetic turbulence.
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