Angular Momentum Transport Research Articles

Modeling Photoionized Turbulent Material in the Circumgalactic Medium. III. Effects of Corotation and Magnetic Fields

Abstract Absorption-line measurements of the circumgalactic medium (CGM) display a highly nonuniform distribution of lower ionization state species accompanied by more widespread higher ionization state material. This suggests that the CGM is a dynamic, multiphase medium, such as arises in the presence of turbulence. To better understand this evolution, we perform hydrodynamic and magnetohydrodynamic (MHD) simulations of the CGM surrounding Milky Way–like galaxies. In both cases, the CGM is initially in hydrostatic balance in a 1012 M ⊙ dark matter gravitational potential, and the simulations include rotation in the inner halo and turbulence that decreases radially. They also track ionizations, recombinations, and species-by-species radiative cooling in the presence of the redshift-zero UV background, employing the MAIHEM nonequilibrium chemistry package. We find that after 9 Gyr of evolution, the presence of a magnetic field leads to an overall hotter CGM, with cool gas in the center where magnetic pressure dominates. While the non-MHD run produces more cold clouds overall, we find similar Si iv/O vi and N v/O vi ratios between the MHD and non-MHD runs, which are both very different from their equilibrium values. The non-MHD halo develops cool, low angular momentum filaments above the central disk, in comparison to the MHD run that has more efficient angular momentum transport, especially for the cold gas, which forms a more ordered and extended disk late into its evolution.

Detailed models of interacting short-period massive binary stars

Context. The majority of massive stars are part of binary systems. In about a quarter of these, the companions are so close that mass transfer occurs while they undergo core hydrogen burning, first on the thermal and then on the nuclear timescale. The nuclear timescale mass transfer leads to observational counterparts: the semi-detached so-called massive Algol binaries. These systems may provide urgently needed tests of the physics of mass transfer. However, comprehensive model predictions for these systems are sparse. Aims. We use a large grid of detailed evolutionary models of short-period massive binaries and follow-up population synthesis calculations to derive probability distributions of the observable properties of massive Algols and their descendants. Methods. Our results are based on ∼10 000 binary model sequences calculated with the stellar evolution code MESA, using a metallicity suitable for the Large Magellanic Cloud (LMC), covering initial donor masses between 10 M⊙ and 40 M⊙ and initial orbital periods above 1.4 d. These models include internal differential rotation and magnetic angular momentum transport, non-conservative mass and angular momentum transfer between the binary components, and time-dependent tidal coupling. Results. Our models imply ∼30, or ∼3% of the ∼1000, core hydrogen burning O-star binaries in the LMC to be currently in the semi-detached phase. Our donor models are up to 25 times more luminous than single stars of an identical mass and effective temperature, which agrees with the observed Algols. A comparison of our models with the observed orbital periods and mass ratios implies rather conservative mass transfer in some systems, while a very inefficient one in others. This is generally well reproduced by our spin-dependent mass transfer algorithm, except for the lowest considered masses. The observations reflect the slow increase of the surface nitrogen enrichment of the donors during the semi-detached phase all the way to CNO equilibrium. We also investigate the properties of our models after core hydrogen depletion of the donor star, when these models correspond to Wolf-Rayet or helium+OB star binaries. Conclusions. A dedicated spectroscopic survey of massive Algol systems may allow to derive the dependence of the efficiency of thermal timescale mass transfer on the binary parameters, as well as the efficiency of semiconvective mixing in the stellar interior. This would be a crucial step towards reliable binary models up to the formation of supernovae and compact objects.

Hypercritical Accretion for Black Hole High Spin in Cygnus X-1

Recent observations of AdLIGO and Virgo have shown that the spin measurements in binary black hole (BH) systems are typically small, which is consistent with the predictions by the classical isolated binary evolution channel. In this standard formation channel, the progenitor of the first-born BH is assumed to have efficient angular momentum transport. The BH spins in high-mass X-ray binaries (HMXBs), however, have consistently been found to be extremely high. In order to explain the high BH spins, the inefficient angular momentum transport inside the BH progenitor is required. This requirement, however, is incompatible with the current understanding of conventional efficient angular momentum transport mechanism. We find that this tension can be highly alleviated as long as the hypercritical accretion is allowed. We show that, for a case study of Cygnus X-1, the hypercritical accretion cannot only be a good solution for the inconsistent assumption upon the angular momentum transport within massive stars, but match its other properties reported recently.

Observation of the Orbital Rashba-Edelstein Magnetoresistance.

We report the observation of magnetoresistance (MR) that could originate from the orbital angular momentum (OAM) transport in a permalloy (Py)/oxidized Cu (Cu^{*}) heterostructure: the orbital Rashba-Edelstein magnetoresistance. The angular dependence of the MR depends on the relative angle between the induced OAM and the magnetization in a similar fashion as the spin Hall magnetoresistance. Despite the absence of elements with large spin-orbit coupling, we find a sizable MR ratio, which is in contrast to the conventional spin Hall magnetoresistance which requires heavy elements. Through Py thickness-dependence studies, we conclude another mechanism beyond the conventional spin-based scenario is responsible for the MR observed in Py/Cu^{*} structures-originated in a sizable transport of OAM. Our findings not only suggest the current-induced torques without using any heavy elements via the OAM channel but also provide an important clue towards the microscopic understanding of the role that OAM transport can play for magnetization dynamics.

The Physics of the MHD Disk–Jet Transition in Binary Systems: Jetted Spiral Walls Launched from Disk Spiral Arms

Abstract We present a detailed physical analysis of the jet-launching mechanism of a circumstellar disk that is located in a binary system. Applying 3D resistive magnetohydrodynamics simulations, we investigate the local and global properties of the system, such as angular momentum transport and accretion and ejection mass fluxes. In comparison to previous works, for the first time we have considered the full magnetic torque, the presence of an outflow and thus the angular momentum transport by vertical motion, and the binary torque. We discuss its specific 3D structure and how it is affected by tidal effects. We find that the spiral structure evolving in the disk is launched into the outflow. We propose calling this newly discovered structure a jet spiral wall. These spiral features follow the same time evolution, with the jet spiral somewhat lagging the disk spiral. We find that the vertical transport of angular momentum has a significant role in the total angular momentum budget also in a binary system. The same holds for the magnetic torque; however, the contribution from the ϕderivative of the magnetic pressure and B ϕ B r stresses are small. The gravity torque arising from the time-dependent 3D Roche potential becomes essential, as it constitutes the fundamental cause for all 3D effects appearing in our disk–jet system. Quantitatively, we find that the disk accretion rate in a binary system increases by 20% compared to a disk around a single star. The disk wind mass flux increases by even 50%.

Generation mechanism of solar-like differential rotation in high-resolution simulations

We analyze the simulation result shown in Hotta & Kusano, 2021 in which the solar-like differential rotation is reproduced. The Sun is rotating differentially with the fast equator and the slow pole. It is widely thought that the thermal convection maintains the differential rotation, but recent high-resolution simulations tend to fail to reproduce the fast equator. This fact is an aspect of one of the biggest problems in solar physics called the convective conundrum. Hotta & Kusano, 2021 succeed in reproducing the solar-like differential rotation without using any manipulation with unprecedentedly high-resolution simulation. In this study, we analyze the simulation data to understand the maintenance mechanism of the fast equator. Our analyses lead to conclusions that are summarized as follows. 1. Superequipatition magnetic field is generated by the compression, which can indirectly convert the massive internal energy to magnetic energy. 2. The efficient small-scale energy transport suppresses large-scale convection energy. 3. Non-Taylor--Proudman differential rotation is maintained by the entropy gradient caused by the anisotropic latitudinal energy transport enhanced by the magnetic field. 4. The fast equator is maintained by the meridional flow mainly caused by the Maxwell stress. The Maxwell stress itself also has a role in the angular momentum transport for fast near-surface equator (we call it the Punching ball effect). The fast equator in the simulation is reproduced not due to the low Rossby number regime but due to the strong magnetic field. This study newly finds the role of the magnetic field in the maintenance of differential rotation.

Finite element numerical technique for magneto-micropolar nanofluid flow filled with chemically reactive casson fluid between parallel plates subjected to rotatory system with electrical and Hall currents

ABSTRACT Steady electro-magnetohydrodynamic flow of a micropolar nanofluid in the attendance of reactive Casson fluid passing through parallel plates influenced by the rotating system with the implementation of Buongiorno nanofluid model is examined in this study. The momentum transport equation is enhanced by incorporating the electric field. In addition, the influence of reactive species has a vital role that is affecting the flow phenomenon in conjunction with a transverse magnetic field. The physical flow problem is modeled in the form of partial differential equations which are then transformed into nonlinear ordinary differential equations by using appropriate similarity functions and then solved numerically by the usage of the finite element method and procured results are visualized graphically. The outcomes for flow rate, microrotation, temperature, concentration, and engineering quantities distributions are shown in terms of graphical presentation. Momentum and angular momentum transport progressively in nature as the Casson parameter grows. Opposite results of microrotational profiles are found for electric currents in comparison with Hall currents. Both thermophoresis and Brownian motion are found to be significant effects in improving heat transportation phenomena in nanofluids. The existing available literature was utilized to test for validation of the numerical findings.

The Three-dimensional Collapse of a Rapidly Rotating 16 M ⊙ Star

Abstract I report on the three-dimensional (3D) hydrodynamic evolution of a rapidly rotating 16 M ⊙ star to iron core collapse. For the first time, I follow the 3D evolution of the angular momentum (AM) distribution in the iron core and convective shell burning regions for the final 10 minutes up to and including gravitational instability and core collapse. In 3D, convective regions show efficient AM transport that leads to an AM profile that differs in shape and magnitude from MESA within a few shell convective turnover timescales. For different progenitor models, such as those with tightly coupled Si/O convective shells, efficient AM transport in 3D simulations could lead to a significantly different AM distribution in the stellar interior affecting estimates of the natal neutron star or black hole spin. The results suggest that 3D AM transport in convective and rotating shell burning regions are critical components in models of massive stars and could qualitatively alter the explosion outcome and inferred compact remnant properties.

On the Angular Momentum Transport Efficiency within the Star Constrained from Gravitational-wave Observations

Abstract The LIGO Scientific Collaboration and Virgo Collaboration (LIGO/Virgo) have recently reported in GWTC-2.1 eight additional candidate events with a probability of astrophysical origin greater than 0.5 in the LIGO/Virgo’s deeper search on O3a running. In GWTC-2.1, the majority of the effective inspiral spins (χ eff) show magnitudes consistent with zero, while two (GW190403-051519 and GW190805-211137) of the eight new events have χ eff > 0 (at 90% credibility). We note that GW190403-051519 was reported with χ eff = 0.70 − 0.27 + 0.15 and mass ratio q = 0.25 − 0.11 + 0.54 . Assuming a uniform prior probability between 0 and 1 for each black hole’s dimensionless spin magnitude, GW190403-051519 was reported with the dimensionless spin of the more massive black hole, χ 1 = 0.92 − 0.22 + 0.07 . This is the fastest black hole ever measured in all current gravitational-wave events. If the immediate progenitor of GW190403-051519 is a close binary system composed of a black hole and a helium star, which can be the natural outcome of the classical isolated binary evolution through the common envelope phase, this extremely high spin challenges, at least in that case, the existence of an efficient angular momentum transport mechanism between the stellar core and the radiative envelope of massive stars, as for instance predicted by the Tayler–Spruit dynamo or its revised version by Fuller et al.

Aspect-Ratio Dependence of Heat and Angular Momentum Transport in Turbulent Taylor-Couette Flows with Axial Thermal Forcing

Aspect-Ratio Dependence of Heat and Angular Momentum Transport in Turbulent Taylor-Couette Flows with Axial Thermal Forcing

Probing the progenitors of spinning binary black-hole mergers with long gamma-ray bursts

Long-duration gamma-ray bursts are thought to be associated with the core-collapse of massive, rapidly spinning stars and the formation of black holes. However, efficient angular momentum transport in stellar interiors, currently supported by asteroseismic and gravitational-wave constraints, leads to predominantly slowly-spinning stellar cores. Here, we report on binary stellar evolution and population synthesis calculations, showing that tidal interactions in close binaries not only can explain the observed subpopulation of spinning, merging binary black holes but also lead to long gamma-ray bursts at the time of black-hole formation. Given our model calibration against the distribution of isotropic-equivalent energies of luminous long gamma-ray bursts, we find that ≈10% of the GWTC-2 reported binary black holes had a luminous long gamma-ray burst associated with their formation, with GW190517 and GW190719 having a probability of ≈85% and ≈60%, respectively, being among them. Moreover, given an assumption about their average beaming fraction, our model predicts the rate density of long gamma-ray bursts, as a function of redshift, originating from this channel. For a constant beaming fraction fB ∼ 0.05 our model predicts a rate density comparable to the observed one, throughout the redshift range, while, at redshift z ∈ [0, 2.5], a tentative comparison with the metallicity distribution of observed LGRB host galaxies implies that between 20% to 85% of the observed long gamma-ray bursts may originate from progenitors of merging binary black holes. The proposed link between a potentially significant fraction of observed, luminous long gamma-ray bursts and the progenitors of spinning binary black-hole mergers allows us to probe the latter well outside the horizon of current-generation gravitational wave observatories, and out to cosmological distances.

Long-Term Effect of Barotropic Instability across the Moat in Double-Eyewall Tropical Cyclone–Like Vortices in Forced and Unforced Shallow-Water Models

Abstract Secondary eyewall formation and the ensuing eyewall replacement cycles may take place in mature tropical cyclones (TCs) during part of their lifetime. A better understanding of the underlying dynamics is beneficial to improving the prediction of TC intensity and structure. Previous studies suggested that the barotropic instability (BI) across the moat (aka type-2 BI) can make a substantial contribution to the inner-eyewall decay through the associated eddy radial transport of absolute angular momentum (AAM). Simultaneously, the type-2 BI can also increase the AAM of the outer eyewall. While the previous studies focused on the early stage of the type-2 BI, this paper explores the long-term effect of the type-2 BI and the underlying processes in forced and unforced shallow-water experiments. Under the long-term effect, it will be shown that the inner eyewalls repeatedly weaken and strengthen (while the order is reversed for the outer eyewalls). Sensitivity tests are conducted to examine the sensitivity of the long-term effect of the type-2 BI to different vortex parameters and the strength of the parameterized diabatic heating. Implication of the long-term effect for the intensity changes of the inner and outer eyewalls of real TCs are also discussed.

On the Dynamics of Overshooting Convection in Spherical Shells: Effect of Density Stratification and Rotation.

Overshooting of turbulent motions from convective regions into adjacent stably stratified zones plays a significant role in stellar interior dynamics, as this process may lead to mixing of chemical species and contribute to the transport of angular momentum and magnetic fields. We present a series of fully nonlinear, three-dimensional (3D) anelastic simulations of overshooting convection in a spherical shell that are focused on the dependence of the overshooting dynamics on the density stratification and the rotation, both key ingredients in stars that however have not been studied systematically together via global simulations. We demonstrate that the overshoot lengthscale is not simply a monotonic function of the density stratification in the convective region, but instead it depends on the ratio of the density stratifications in the two zones. Additionally, we find that the overshoot lengthscale decreases with decreasing Rossby number Ro and scales as Ro0.23 while it also depends on latitude with higher Rossby cases leading to a weaker latitudinal variation. We examine the mean flows arising due to rotation and find that they extend beyond the base of the convection zone into the stable region. Our findings may provide a better understanding of the dynamical interaction between stellar convective and radiative regions, and motivate future studies particularly related to the solar tachocline and the implications of its overlapping with the overshoot region.

Asteroseismogyrometry of low-mass red giants

Context. During the past 10 years, the unprecedented quality and frequency resolution of asteroseismic data provided by space photometry have revolutionised the study of red-giant stars providing us with the possibility to probe the interior of thousands of these targets. Aims. Our aim is to present an asteroseismic tool which allows one to determine the total angular momentum of stars, without an a priori inference of their internal rotational profile. Methods. We adopted the asteroseismic inversion technique developed for the case of the Sun and adapted it to red giants. The method was tested assuming different artificial sets of data, also including modes with harmonic degree l ≥ 2. Results. We estimate with an accuracy of 14.5% the total angular momentum of the red-giant star KIC 4448777 observed by Kepler during the first four consecutive years of operation. Conclusions. Our results indicate that the measurement of the total angular momentum of red-giant stars can be determined with a fairly high precision by means of asteroseismology by using a small set of rotational splittings of only dipolar modes; they also show that our method, based on observations of stellar pulsations, provides a powerful mean for testing and modelling the transport of angular momentum in stars.

Model of Angular Momentum Transport at the Protoplane-tary Disk Evolution and Disk Surface Density

We consider how the tidal effect of a protoplanetary disk interaction can be incorporated into calculations of its viscous evolution. The evolution of the disk occurs under the action of both internal viscous torques and external torques resulting from the presence of one or more embedded planets. The planets migrate under the effect of their tidal interaction with the disk (in the type-II migration regime). Torques on a planet are caused by its gravitational interaction with the density waves which occupy the Lindblad resonances in the disk. Our model simplifies the functional form of the rate of injection of the angular momentum Λ(r) to construct and solve the evolution equation for a disk and an embedded protoplanet. The functional Λ(r) depends on the tidal dissipation distribution in the disk which is concentrated in a vicinity of the protoplanet’s orbit. We have found an analytic solution for the disk surface density.

Secular evolution of MHD wind-driven discs: analytical solutions in the expanded α-framework

ABSTRACT The evolution of protoplanetary discs and the related process of planet formation is regulated by angular momentum transport and mass-loss processes. Over the past decade, the paradigm of viscosity has been challenged and MHD disc winds appear as a compelling scenario to account for disc accretion. In this work, we aim to construct the equivalent of the widely used analytical description of viscous evolution for the MHD wind case. The transport of angular momentum and mass induced by the wind is parametrized by an α-like parameter and by the magnetic lever arm parameter λ. Extensions of the paradigmatic Lynden-Bell and Pringle similarity solutions to the wind case are presented. We show that wind-driven accretion leads to a steeper decrease in the disc mass and accretion rate than in viscous models due to the absence of disc spreading. If the decline of the magnetic field strength is slower than that of the gas surface density, the disc is dispersed after a finite time. The evolution of the disc in the $\dot{M}_*-M_{D}$ plane is sensitive to the wind and turbulence parameters. A disc population evolving under the action of winds can exhibit a correlation between $\dot{M}_*$ and MD depending on the initial conditions. The simplified framework proposed in this work opens to a new avenue to test the effectiveness of wind-driven accretion from the observed disc demographics and constitutes an important step to include wind-driven accretion in planet population synthesis models.

Contribution to the angular momentum transport paradigm for accretion disks

We analyze the stationary configuration of a thin axisymmetric stellar accretion disk, neglecting non-linear terms in the plasma poloidal velocity components. We set up the Grad-Shafranov equation for the system, including the plasma differential rotation (according to the so-called co-rotation theorem). Then, we study the small scale backreaction of the disk to the central body magnetic field and we calculate the resulting radial infalling velocity. We show that the small scale radial oscillation of the perturbed magnetic surface is associated to the emergence of relevant toroidal current densities, able to balance the Ohm law even in the presence of quasi-ideal values of the plasma resistivity. The contribution to the infalling velocity of the averaged backreaction contrasts accretion, but it remains negligible as far as the induced magnetic field is small compared to that of the central body.

Differential rotation in a 3D simulation of oxygen shell burning

ABSTRACTWe study differential rotation in late-stage shell convection in a 3D hydrodynamic simulation of a rapidly rotating $16\, {\rm M}_{\odot }$ helium star with a particular focus on the convective oxygen shell. We find that the oxygen shell develops a quasi-stationary pattern of differential rotation that is described neither by uniform angular velocity as assumed in current stellar evolution models of supernova progenitors, nor by uniform specific angular momentum. Instead, the oxygen shell develops a positive angular velocity gradient with faster rotation at the equator than at the pole by tens of per cent. We show that the angular momentum transport inside the convection zone is not adequately captured by a diffusive mixing-length flux proportional to the angular velocity or angular momentum gradient. Zonal flow averages reveal stable large-scale meridional flow and an entropy deficit near the equator that mirrors the patterns in the angular velocity. The structure of the flow is reminiscent of simulations of stellar surface convection zones and the differential rotation of the Sun, suggesting that similar effects are involved; future simulations will need to address in more detail how the interplay of buoyancy, inertial forces, and turbulent stresses shapes differential rotation during late-stage convection in massive stars. If convective regions develop positive angular velocity gradients, angular momentum could be shuffled out of the core region more efficiently, potentially making the formation of millisecond magnetars more difficult. Our findings have implications for neutron star birth spin periods and supernova explosion scenarios that involve rapid core rotation.

The Implications of High Black Hole Spins for the Origin of Binary Black Hole Mergers

Abstract The LIGO–Virgo collaboration has reported 50 black hole–black hole (BH–BH) mergers and 8 candidates recovered from digging deeper into the detector noise. The majority of these mergers have low effective spins pointing toward low BH spins and efficient angular momentum (AM) transport in massive stars as proposed by several models (e.g., the Tayler–Spruit dynamo). However, out of these 58 mergers, 7 are consistent with having high effective-spin parameter (χ eff > 0.3). Additionally, two events seem to have high effective spins sourced from the spin of the primary (more massive) BH. These particular observations could be used to discriminate between the isolated binary and dynamical formation channels. It might seem that high BH spins point to a dynamical origin if AM in stars is efficient and forms low-spinning BHs. In such a case dynamical formation is required to produce second and third generations of BH–BH mergers with typically high spinning BHs. Here we show, however, that isolated binary BH–BH formation naturally reproduces such highly spinning BHs. Our models start with efficient AM in massive stars that is needed to reproduce the majority of BH–BH mergers with low effective spins. Later, some of the binaries are subject to a tidal spin-up allowing the formation of a moderate fraction (∼10%) of BH–BH mergers with high effective spins (χ eff ≳ 0.4–0.5). In addition, isolated binary evolution can produce a small fraction of BH–BH mergers with almost maximally spinning primary BHs. Therefore, the formation scenario of these atypical BH–BH mergers remains to be found.

Boundary layers of accretion discs: Discovery of vortex-driven modes and other waves

ABSTRACT Disc accretion on to weakly magnetized objects possessing a material surface must proceed via the so-called boundary layer (BL) – a region at the inner edge of the disc, in which the velocity of accreting material abruptly decreases from its Keplerian value. Supersonic shear arising in the BL is known to be conducive to excitation of acoustic waves that propagate into both the accretor and the disc, enabling angular momentum and mass transport across the BL. We carry out a numerical exploration of different wave modes that operate near the BL, focusing on their morphological characteristics in the innermost parts of accretion disc. Using a large suite of simulations covering a broad range of Mach numbers (of the supersonic shear flow in the BL), we provide accurate characterization of the different types of modes, verifying their properties against analytical results, when available. We discover new types of modes, in particular, global spiral density waves launched by vortices forming in the disc near the BL as a result of the Rossby wave instability; this instability is triggered by the vortensity production in that region caused by the non-linear damping of acoustic waves. Azimuthal wavenumbers of the dominant modes that we observe appear to increase monotonically with the Mach number of the runs, but a particular mix of modes found in a simulation is mildly stochastic. Our results provide a basis for better understanding of the angular momentum and mass transport across the BL as well as the emission variability in accreting objects.