Magnetorotational Instability Research Articles

Magnetorotational instability in a swirling partially ionized gas

ABSTRACT The magnetorotational instability (MRI) has been proposed as the method of angular momentum transport that enables accretion in astrophysical discs. However, for weakly ionized discs, such as protoplanetary discs, it remains unclear whether the combined non-ideal magnetohydrodynamic (MHD) effects of Ohmic resistivity, ambipolar diffusion, and the Hall effect make these discs MRI stable. While much effort has been made to simulate non-ideal MHD MRI, these simulations make simplifying assumptions and are not always in agreement with each other. Furthermore, it is difficult to directly observe the MRI astrophysically because it occurs on small scales. Here, we propose the concept of a swirling gas experiment of weakly ionized argon gas between two concentric cylinders threaded with an axial magnetic field that can be used to study non-ideal MHD MRI. For our proposed experiment, we derive the hydrodynamic equilibrium flow and a dispersion relation for MRI that includes the three non-ideal effects. We solve this dispersion relation numerically for the parameters of our proposed experiment. We find it should be possible to produce a non-ideal MRI in such an experiment because of the Hall effect, which increases the MRI growth rate when the vertical magnetic field is anti-aligned with the rotation axis. As a proof of concept, we also present experimental results for a hydrodynamic flow in an unmagnetized prototype. We find that our prototype has a small, but non-negligible, α-parameter that could serve as a baseline for comparison to our proposed magnetized experiment, which could be subject to additional turbulence from the MRI.

Linear and non-linear eccentric mode evolution in unstratified MHD discs

ABSTRACT In this paper, we develop a framework for studying unstratified, magnetized eccentric discs and compute uniformly precessing eccentric modes in a cylindrical annulus which provide convenient initial conditions for numerical simulations. The presence of a magnetic field in an eccentric disc can be described by an effective gas with a modified equation of state. At magnetic field strengths relevant to the magneto-rotational instability the magnetic field has negligible influence on the evolution of the eccentric disc, however, the eccentric disc can significantly enhance the magnetic field strength over that in the a circular disc. We verify the suitability of these eccentric disc solutions by carrying out 2D simulations in ramses. Our simulated modes (in 2D) follow a similar evolution to the purely hydrodynamical modes, matching theoretical expectations, provided they are adequately resolved. Such solutions will provide equilibrium states for studies of the eccentric magneto-rotational instability and magnetized parametric instability in unstratified discs and are useful for exploring the response of disc turbulence on top of a fluid flow varying on the orbital time-scale.

Local magneto-shear instability in Newtonian gravity

ABSTRACT The magneto-rotational instability (MRI), which is due to an interplay between a sheared background and the magnetic field, is commonly considered a key ingredient for developing and sustaining turbulence in the outer envelope of binary neutron star merger remnants. To assess whether (or not) the instability is active and resolved, criteria originally derived in the accretion disc literature, thus exploiting the symmetries of such systems, are often used. In this paper, we discuss the magneto-shear instability as a truly local phenomenon, relaxing common symmetry assumptions on the background on top of which the instability grows. This makes the discussion well suited for highly dynamical environments such as binary mergers. We find that, although this is somewhat hidden in the usual derivation of the MRI dispersion relation, the instability crucially depends on the assumed symmetries. Relaxing the symmetry assumptions in the background, we find that the role of the magnetic field is significantly diminished, as it affects the modes’ growth but does not drive it. We conclude by making contact with a suitable filtering operation, as this is key to separating background and fluctuations in highly dynamical systems.

The persistence of magneto-rotational turbulence in gravitationally turbulent accretion disks

Aims. Our main goal is to probe the persistence of turbulence originating from the magneto-rotational instability (MRI) in gravito-turbulent disks. This state is referred to here as GI-MRI coexistence, with GI standing for gravitational instability. We test the influence of GI strength, controlled by the cooling law, and the impact of Ohmic resistivity. Methods. Our starting point was three-dimensional, ideal, magnetohydrodynamic (MHD) simulations of gravitational turbulence in the local shearing-box approximation using the code Athena. We introduced a zero-net-flux magnetic seed field in a GI-turbulent state and investigated the nonlinear evolution. The GI strength was varied by modifying the cooling parameters. We tested the cooling times τcΩ0 = 10, τcΩ0 = 20, and τcΩ0 = 10, with additional background heating. For some resistive cases, ideal-MHD simulations, which had already developed GI-MRI coexistence, were restarted with a finite Ohmic resistivity enabled at the moment of restart. Results. It appears that there are two possible saturated dynamo states in the ideal-MHD regime: a state of GI-MRI coexistence (for low GI activity) and a strong-GI dynamo. The cases with lower GI activity eventually develop a clearly visible butterfly pattern. For the case with the highest GI activity (τcΩ0 = 10, no heating), a clearly visible butterfly pattern is absent, though more chaotic field reversals are observed above (and below) the mid-plane. We were also able to reproduce the results of previous simulations. With Ohmic resistivity, the simulation outcome can be substantially different. There exists a critical magnetic Reynolds number, ⟨Rm⟩ ∼ 500, below which the ideal-MHD outcome is replaced by a new dynamo state. For larger Reynolds numbers, one recovers turbulent states that are more reminiscent of the ideal-MHD states, and especially the strong-GI case. This new state leads to oscillations, which are caused by a significant heat production due to the resistive dissipation of magnetic energy. The additional heat periodically quenches GI, and the quenching events correspond to maxima of the Toomre value, Q.

The Formation of Star-forming Disks in the TNG50 Simulation

We investigate the disk formation process in the TNG50 simulation, examining the profiles of SFR surface density (ΣSFR), gas inflow and outflow, and the evolution of the angular momentum of inflowing gas particles. The TNG50 galaxies tend to have larger star-forming disks, and they also show larger deviations from exponential profiles in ΣSFR when compared to real galaxies in the Mapping Nearby Galaxies at APO survey. The stellar surface density of TNG50 galaxies show good exponential profiles, which is found to be the result of strong radial migration of stars over time. However, this strong radial migration of stars in the simulation produces flatter age profiles in TNG50 disks compared to observed galaxies. The star formation in the simulated galaxies is sustained by a net gas inflow, and this gas inflow is the primary driver for the cosmic evolution of star formation, as expected from simple gas-regulator models of galaxies. There is no evidence for any significant loss of angular momentum for the gas particles after they are accreted on to the galaxy, which may account for the large disk sizes in the TNG50 simulation. Adding viscous processes to the disks, such as the magnetic stresses from magnetorotational instability proposed by Wang & Lilly, will likely reduce the sizes of the simulated disks and the tension with the sizes of real galaxies, and this may produce more realistic exponential profiles.

Angular momentum and lithium transport from main sequence to sub-giant and red giant low-mass stars

Context. Asteroseismology provides a unique opportunity to probe the interiors of evolved stars and constrain their internal rotation. The correct reproduction of the core rotation evolution has not yet been achieved, although it is key to understanding the internal processes involved in low-mass stars. Aims. We explore the efficiency required to reproduce the general behaviour of the transport of angular momentum along the evolution in view of asteroseismic constraints from giant low-mass stars. We analyse the consequences and predictions for lithium and beryllium surface abundances from the main sequence to red giant phase. Methods. We computed a series of models, which included atomic diffusion, rotation-induced mixing, magnetic braking, and additional processes tailored for main sequence low-mass stars. We extended these models to more evolved phases and investigated an updated angular momentum transport by including a time-dependent extra viscosity related to the azimuthal magneto-rotational instability. We compared our predictions to the asteroseismic measurements of the core and surface rotation of a sample of sub-giant and red giant stars. We compared the model predictions for the lithium and beryllium surface evolution with the available observations. Results. We confirm that a time-dependent additional viscosity νadd(t) is required to reproduce the general behaviour of the core rotation rate along successive stellar evolutionary phases given the dependence on the differential rotation and the mass. We show that it results in stronger lithium and beryllium depletions for low-mass stars over evolution. We confirm that predicted lithium abundances at the red giant bump by classical models, commonly used as references, cannot reproduce the lithium depletion along the main sequence and evolved phases of stellar evolution. We show that the observed amount of lithium of stars less massive than 1 M⊙ leads to a discrepancy between model predictions and observations at the red giant bump. Conclusions. We show that a semi-parametric model can reproduce the rotational behaviour along the first phases of evolution well, with the exception of the sharp transition observed during the sub-giant phase. This suggests that two distinct transport processes may be involved. The processes required to transport chemicals during the main sequence phase and angular momentum until the red giant phase impact the lithium depletion all along the evolutionary duration. A good prediction of the lithium abundance at young phases places strong constraints on the predicted one at more evolved phases. It also highlights discrepancies between models and observations for the lowest mass stars and impacts the threshold that defines lithium-rich giant stars, showing that classical models tend to overestimate this threshold.

How Large Is a Disk—What Do Protoplanetary Disk Gas Sizes Really Mean?

It remains unclear what mechanism is driving the evolution of protoplanetary disks. Direct detection of the main candidates, either turbulence driven by magnetorotational instabilities or magnetohydrodynamical disk winds, has proven difficult, leaving the time evolution of the disk size as one of the most promising observables able to differentiate between these two mechanisms. But to do so successfully, we need to understand what the observed gas disk size actually traces. We studied the relation between R CO,90%, the radius that encloses 90% of the 12CO flux, and R c , the radius that encodes the physical disk size, in order to provide simple prescriptions for conversions between these two sizes. For an extensive grid of thermochemical models, we calculate R CO,90% from synthetic observations and relate properties measured at this radius, such as the gas column density, to bulk disk properties, such as R c and the disk mass M disk. We found an empirical correlation between the gas column density at R CO,90% and disk mass: . Using this correlation we derive an analytical prescription of R CO,90% that only depends on R c and M disk. We derive R c for disks in Lupus, Upper Sco, Taurus, and the DSHARP sample, finding that disks in the older Upper Sco region are significantly smaller (〈R c 〉 = 4.8 au) than disks in the younger Lupus and Taurus regions (〈R c 〉 = 19.8 and 20.9 au, respectively). This temporal decrease in R c goes against predictions of both viscous and wind-driven evolution, but could be a sign of significant external photoevaporation truncating disks in Upper Sco.

Nonlinear evolution of magnetorotational instability in a magnetized Taylor-Couette flow: Scaling properties and relation to upcoming DRESDYN-MRI experiment

Nonlinear evolution of magnetorotational instability in a magnetized Taylor-Couette flow: Scaling properties and relation to upcoming DRESDYN-MRI experiment

Joint Action and Competition between Centrifugal, Magnetorotational, and Magnetic Buoyancy Instabilities

Instabilities driven by some combination of rotation, velocity shear, and magnetic field in a stratified fluid under gravity play an important role in many astrophysical settings. Of particular note are the centrifugal instability, the magnetorotational instability, and magnetic buoyancy instability. Here, we consider a Cartesian model of an equatorial region incorporating all the physical ingredients necessary to study their competition. We investigate the linear instability to interchange (“axisymmetric”) modes of an inviscid, perfectly conducting, isothermal gas, including the effects of rotation, velocity shear, and poloidal and toroidal magnetic fields. The stability problem can be reduced to a second-order boundary value problem, with the growth rate as the eigenvalue. We can make analytic progress through consideration of the physically relevant regime in which the transverse horizontal wavenumber k ≫ 1. Via a perturbation analysis, with 1/k as the small parameter, we can derive the growth rate and the spatial dependence of the eigenfunctions: the unstable modes are strongly localized in the vertical direction, being either wall modes (localized near a boundary of the domain) or body modes (localized in the interior). We describe the conditions under which the joint action of the separate instability mechanisms leads to enhancement or suppression of the instability. Our analytical results are supplemented by numerical solutions of the stability problem. The most unstable mode found analytically is typically in excellent agreement with that found numerically through consideration of a wide range of wavenumbers. Finally, we discuss how our results relate to the solar tachocline.

FU Orionis disk outburst: Evidence for a gravitational instability scenario triggered in a magnetically dead zone

Context. FUors outbursts are a crucial stage of accretion in young stars. However, a complete mechanism at the origin of the outburst still remains missing. Aims. We aim to constrain the instability mechanism in the star FU Orionis itself by directly probing the size and evolution in time of the outburst region with near-infrared (NIR) interferometry, and to confront it with physical models of this region. Methods. As the prototype of the FUors class of objects, FU Orionis has been a regular target of NIR interferometry. In this paper, we analyze more than 20 years of NIR interferometric observations to perform a temporal monitoring of the region of the outburst, and compare it to the spatial structure deduced from 1D magneto-hydrodynamic (MHD) simulations. Results. We measure from the interferometric observations that the size variation of the outburst region is compatible with a constant or slightly decreasing size over time: -0.56-0.36+0.14 AU/100 yr and -0.30-0.19+0.19 AU/100 yr in the H and K bands, respectively. The temporal variation and the mean size probed by NIR interferometry are consistently reproduced by our 1D MHD simulations. We find that the most compatible scenario is a model of an outburst occurring in a magnetically layered disk, where a magneto-rotational instability (MRI) is triggered by a gravitational instability (GI) at the outer edge of a dead zone. The scenario of a pure thermal instability (TI) fails to reproduce our interferometric sizes because it can only be sustained in a very compact zone of the disk <0.1 AU. The comparison between the data and the MRI-GI models favors MHD parameters of αMRI = 10−2, TMRI = 800 K, and Σcrit = 10 g cm−2, with more work needed in terms of observations and modeling in order to improve the precision of these values. Locally, in the very inner part of the disk, TI can be triggered in addition to MRI-GI, which qualitatively better matches our observation but is not strongly constrained by the currently available data. The scenario of MRI-GI could be compatible with an external perturbation that enhances the GI, such as tidal interaction with a stellar companion, or a planet at the outer edge of the dead zone. Conclusions. We favor a layered-disk model driven by MRI turbulence in order to explain the spatial structure and temporal evolution of the outburst region on FU Orionis. Understanding this phase will provide a crucial link between the early phase of disk evolution and the process of planet formation in the first inner astronomical units.

A new 2D stochastic methodology for simulating variable accretion discs: propagating fluctuations and epicyclic motion

ABSTRACT Accretion occurs across a large range of scales and physical regimes. Despite this diversity in the physics, the observed properties show remarkable similarity. The theory of propagating fluctuations, in which broad-band variability within an accretion disc travel inwards and combine, has long been used to explain these phenomena. Recent numerical work has expanded on the extensive analytical literature but has been restricted to using the 1D diffusion equation for modelling the disc behaviour. In this work we present a novel numerical approach for 2D (vertically integrated), stochastically driven α-disc simulations, generalizing existing 1D models. We find that the theory of propagating fluctuations translates well to 2D. However, the presence of epicyclic motion in 2D (which cannot be captured within the diffusion equation) is shown to have an important impact on local disc dynamics. Additionally, there are suggestions that for sufficiently thin discs the log-normality of the light curves changes. As in previous work, we find that the break frequency in the luminosity power spectrum is strongly dependent on the driving time-scale of the stochastic perturbations within the disc, providing a possible observational signature for probing the magnetorotational instability dynamo. We also find that thinner discs are significantly less variable than thicker ones, providing a compelling explanation for the greater variability seen in the hard state versus the soft state of X-ray binaries. Finally, we consider the wide-ranging applications of our numerical model for use in other simulations.

Magnetically dominated discs in tidal disruption events and quasi-periodic eruptions

ABSTRACT The classical radiation pressure instability has been a persistent theoretical feature of thin, radiatively efficient accretion discs with accretion rates $\sim $ 1 per cent–100 per cent of the Eddington rate. But there is only limited evidence of its occurrence in nature: rapid heartbeat oscillations of a few X-ray binaries and now, perhaps, the new class of hourly X-ray transients called quasi-periodic eruptions (QPEs). The accretion discs formed in tidal disruption events (TDEs) have been observed to peacefully trespass through the range of unstable accretion rates without exhibiting any clear sign of the instability. We try to explain the occurrence or otherwise of this instability in these systems, by constructing steady state 1D models of thin magnetic accretion discs. The local magnetic pressure in the disc is assumed to be dominated by toroidal fields arising from a dynamo sourced by magneto-rotational instability (MRI). We choose a physically motivated criterion of MRI saturation, validated by recent magnetohydrodynamic simulations, to determine the disc magnetic pressure. The resulting magnetic pressure support efficiently shrinks: (1) the parameter space of unstable mass accretion rates, explaining the absence of instability in TDEs and (2) the range of unstable radii in the inner accretion disc, which can shorten the quasi-periods of instability limit-cycles by more than three orders of magnitude, explaining the short periods of QPEs. In addition to examining stability of strongly magnetized discs, we predict other observational signatures such as spectral hardening and jet luminosities to test the compatibility of our disc models with observations of TDE discs.

Magnetically activated accretion outbursts of pre-main-sequence discs

ABSTRACT We investigate whether triggering of the magnetorotational instability (MRI) in protoplanetary discs can account for the wide diversity of observed accretion outbursts. We show that short-lived, relatively low accretion rate events probably result from triggering in the inner disc and can occur at low surface densities, comparable to or smaller than the minimum mass solar nebula, and thus are very unlikely to result from MRI triggering by gravitational instability. We develop time-dependent accretion disc models using an α-viscosity approach and calculate light curves to compare with observations. Our modelling indicates that the lag time between infrared and optical bursts seen in Gaia 17bpi can be explained with an outside-in propagation with an α ∼ 0.1 in the MRI-active region, consistent with other estimates. While outbursts in inner discs can show time delays of a few yr between infrared and optical light curves, our models indicate that large, FU Ori-like bursts can exhibit infrared precursors decades before optical bursts. Detecting such precursors could enable analysis of the central star before it is overwhelmed by the rapid accreting material, as well as constraining outburst physics. Our results emphasize the importance of near-infrared monitoring of young stellar objects in addition to optical surveys. In addition, our findings emphasize the need for more sophisticated, three-dimensional, non-ideal magnetohydrodynamic simulations to fully exploit observational results.

General-relativistic neutrino-radiation magnetohydrodynamics simulation of seconds-long black hole-neutron star mergers: Dependence on the initial magnetic field strength, configuration, and neutron-star equation of state

As a follow-up study of our previous work [Phys. Rev. D 106, 023008 (2022)], numerical-relativity simulations for seconds-long black hole-neutron star mergers are performed for a variety of setups. Irrespective of the initial and symmetry conditions, we find qualitatively universal evolution processes: The dynamical mass ejection takes place together with a massive accretion disk formation after the neutron star is tidally disrupted; subsequently, the magnetic field in the accretion disk is amplified by the magnetic winding, Kelvin-Helmholtz instability, and magnetorotational instability, which establish a turbulent state inducing the dynamo and angular momentum transport; the postmerger mass ejection by the effective viscous processes stemming from the magnetohydrodynamics turbulence sets in at $\ensuremath{\sim}300--500\text{ }\text{ }\mathrm{ms}$ after the merger and continues for several hundred ms; a magnetosphere near the black-hole spin axis is developed and the collimated strong Poynting flux is generated with its lifetime of $\ensuremath{\sim}0.5--2\text{ }\text{ }\mathrm{s}$. We have newly found that the model of no equatorial-plane symmetry shows the reverse of the magnetic-field polarity in the magnetosphere, which is caused by the dynamo associated with the magnetorotational instability in the accretion disk. The model with initially toroidal fields shows the tilt of the disk and magnetosphere in the late postmerger stage because of the anisotropic postmerger mass ejection. These effects could terminate the strong Poynting-luminosity stage within the timescale of $\ensuremath{\sim}0.5--2\text{ }\text{ }\mathrm{s}$.

The impact of dust evolution on the dead zone outer edge in magnetized protoplanetary disks

Context. The dead zone outer edge corresponds to the transition from the magnetically dead to the magnetorotational instability (MRI) active regions in the outer protoplanetary disk midplane. It has been previously hypothesized to be a prime location for dust particle trapping. A more consistent approach to access such an idea has yet to be developed, since the interplay between dust evolution and MRI-driven accretion over millions of years has been poorly understood. Aims. We provide an important step toward a better understanding of the MRI–dust coevolution in protoplanetary disks. In this pilot study, we present a proof of concept that dust evolution ultimately plays a crucial role in the MRI activity. Methods. First, we study how a fixed power-law dust size distribution with varying parameters impacts the MRI activity, especially the steady-state MRI-driven accretion, by employing and improving our previous 1+1D MRI-driven turbulence model. Second, we relax the steady-state accretion assumption in this disk accretion model, and partially couple it to a dust evolution model in order to investigate how the evolution of dust (dynamics and grain growth processes combined) and MRI-driven accretion are intertwined on million-year timescales, from a more sophisticated modeling of the gas ionization degree. Results. Dust coagulation and settling lead to a higher gas ionization degree in the protoplanetary disk, resulting in stronger MRI-driven turbulence as well as a more compact dead zone. On the other hand, fragmentation has an opposite effect because it replenishes the disk in small dust particles which are very efficient at sweeping up free electrons and ions from the gas phase. Since the dust content of the disk decreases over millions of years of evolution due to radial drift, the MRI-driven turbulence overall becomes stronger and the dead zone more compact until the disk dust-gas mixture eventually behaves as a grain-free plasma. Furthermore, our results show that dust evolution alone does not lead to a complete reactivation of the dead zone. For typical T-Tauri stars, we find that the dead zone outer edge is expected to be located roughly between 10 au and 50 au during the disk lifetime for our choice of the magnetic field strength and configuration. Finally, the MRI activity evolution is expected to be crucially sensitive to the choice made for the minimum grain size of the dust distribution. Conclusions. The MRI activity evolution (hence the temporal evolution of the MRI-induced α parameter) is controlled by dust evolution and occurs on a timescale of local dust growth, as long as there are enough dust particles in the disk to dominate the recombination process for the ionization chemistry. Once that is no longer the case, the MRI activity evolution is expected to be controlled by gas evolution and occurs on a viscous evolution timescale.

Short Gamma rays bursts from binary black holes merger

In this paper, we introduced a model related to the astronomical events having the co-detection of GW associated with Short Gamma Ray Bursts (SGRBs). Study shows that the existence of magnetized accretion disks is responsible for creating the events of GWs associated with electromagnetic (EM) counterparts from binary BHs mergers. Our model leads from space-time instability to the emission of EM radiations as its counterparts. Starting from Maxwell's stress tensor, we found the condition for the growth of instability that is proportional to the angular velocity and is independent of the magnetic field strength. Considering the event GW150914 with the final product as a Kerr BHs in an equatorial plan, we discussed the motion of particles under an effective potential on circular orbits around it and determined its frequency, redshift factor, and epicyclic frequency. Next, considering the evolution of mass and angular momentum, we calculated the remnant mass, rotational energy, and the magnetic field strength acting along the axis of rotation. The accretion rate and its luminosity are determined by the restoring and shear forces of the magnetic field. The energy extraction efficiency of the flow is determined to be very low. Results show the presence of a weak transient caused by magneto-rotational instability with a strong poloidal magnetic field that causes turbulence in the accretion disk onto black holes.

Meridional circulation driven by planetary spiral wakes in radiative and magnetized protoplanetary discs

ABSTRACT We study a Jupiter-mass planet formation for the first time in radiative magnetohydrodynamics (MHD) simulations and compare it with pure hydrodynamical simulations, and also with different isothermal configurations. We found that the meridional circulation is the same in every set-up. The planetary spiral wakes drive a vertical stirring inside the protoplanetary disc and the encounter with these shock fronts also helps in delivering gas vertically on to the Hill sphere. The accretion dynamics are unchanged: the planet accretes vertically, and there is outflow in the mid-plane regions inside the Hill sphere. We determined the effective α-viscosity generated in the disc by the various angular momentum loss mechanisms, which showed that magnetic fields produce high turbulence in the ideal MHD limit, and grows from α ∼ 10−2.5 up to ∼10−1.5 after the planet spirals develop. In the HD simulations, the planetary spirals contribute to α ∼ 10−3, making this a very important angular momentum transport mechanism. Due to the various α values in the different set-ups, the gap opening is different in each case. In the radiative MHD set-ups, the high turbulent viscosity prevents gap opening, leading to a higher Hill mass, and no clear dust trapping regions. While the Hill accretion rate is $10^{-6}~ \rm {M_{Jup}\,yr^{-1}}$ in all set-ups, the accretion variability is orders of magnitude higher in radiative runs than in isothermal ones. Finally, with higher resolution runs, the magnetorotational instability started to be resolved, changing the effective viscosity and increasing the heating in the disc.

Signatures of wakefield acceleration in astrophysical jets via gamma-rays and UHECRs

ABSTRACT We present six case studies from a comprehensive mass range (1–109 M⊙) of astrophysical objects, each of which possess jets, emit high-energy gamma radiation and in some instances spatially identifiable ultra-high-energy cosmic rays (UHECRs). All sources are strong candidates for UHECR emission, if not already known to emit them. We surmise that wakefield acceleration in conjunction with the magnetorotational instability of the accretion disc explains both structural properties of the jets and details in their emission signals, such as correlations in neutrino and gamma-ray bursts, and in the case of blazars, anticorrelations in flux and spectral index. Furthermore, our model predicts an upper bound for the energy of UHECRs emitted from a source given the mass of its central compact object and total jet luminosity. To provide context for our model predictions, we quantitatively compare them with observational data, however, we have not accounted for the GZK limit and assumed universal values for several model parameters (e.g. jet-spreading index, p) that likely differ between sources. Since the accretion and acceleration mechanisms are independent of mass, aside from determining maximum values, blazars (∼109 M⊙), radio galaxies ($\sim 10^8\, {\rm M}_{\odot }$ ), Seyfert galaxies ($\sim 10^6 \, {\rm M}_{\odot }$ ), starburst galaxies ($\sim 10^{3}\, {\rm M}_{\odot }$ ), even microquasars (1–10 M⊙) interestingly exhibit the same physics. Other radiation bands, such as X-ray, ultraviolet, or radio, may harbour additional information, but we chose not to focus on them for brevity. However, such an endeavour may open the door to a new multimessenger approach for understanding these objects.

3D Radiation Hydrodynamic Simulations of Gravitational Instability in AGN Accretion Disks: Effects of Radiation Pressure

We perform 3D radiation hydrodynamic local shearing-box simulations to study the outcome of gravitational instability (GI) in optically thick active galactic nuclei (AGNs) accretion disks. GI develops when the Toomre parameter Q T ≲ 1, and may lead to turbulent heating that balances radiative cooling. However, when radiative cooling is too efficient, the disk may undergo runaway gravitational fragmentation. In the fully gas-pressure-dominated case, we confirm the classical result that such a thermal balance holds when the Shakura–Sunyaev viscosity parameter (α) due to the gravitationally driven turbulence is ≲0.2, corresponding to dimensionless cooling times Ωt cool ≳ 5. As the fraction of support by radiation pressure increases, the disk becomes more prone to fragmentation, with a reduced (increased) critical value of α (Ωt cool). The effect is already significant when the radiation pressure exceeds 10% of the gas pressure, while fully radiation-pressure-dominated disks fragment at t cool ≲ 50 Ω−1. The latter translates to a maximum turbulence level α ≲ 0.02, comparable to that generated by magnetorotational instability. Our results suggest that gravitationally unstable (Q T ∼ 1) outer regions of AGN disks with significant radiation pressure (likely for high/near-Eddington accretion rates) should always fragment into stars, and perhaps black holes.

Asteroseismology of evolved stars to constrain the internal transport of angular momentum

Context. Asteroseismic measurements of the internal rotation rate in evolved stars pointed to a lack of angular momentum (AM) transport in stellar evolution models. Several physical processes in addition to hydrodynamical ones were proposed as candidates for the missing mechanism. Nonetheless, no current candidate can satisfy all the constraints provided by asteroseismology. Aims. We revisit the role of a candidate process whose efficiency scales with the contrast between the rotation rate of the core and the surface. This process was proposed in previous works to be related to the azimuthal magneto-rotational instability. Methods. We computed stellar evolution models of low- and intermediate-mass stars with the parametric formulation of AM transport proposed in previous works for this instability until the end of the core-helium burning for low- and intermediate-mass stars, and compare our results to the latest asteroseismic constraints available in the post-main sequence phase. Results. Both hydrogen-shell-burning stars in the red-giant branch and core-helium-burning stars of low- and intermediate-mass in the mass range 1 M⊙ ≲ M ≲ 2.5 M⊙ can be simultaneously reproduced by this kind of parametrisation. Conclusions. Given the current constraints from asteroseismology, the core rotation rate of post-main sequence stars seems to be well explained by a process whose efficiency is regulated by the internal degree of differential rotation in radiative zones.