arXiv subjects

James M. Stone

Publications and source records attributed to James M. Stone.

At least 73 records · Page 4Linked to original sources

arXiv2019

A Resolution Study of Magnetically Arrested Disks

We investigate numerical convergence in simulations of magnetically arrested disks around spinning black holes. Using the general-relativistic magnetohydrodynamics code Athena++, we study the same system at four resolutions (up to an effective 512 by 256 by 512 cells) and with two different spatial reconstruction algorithms. The accretion rate and general large-scale structure of the flow agree across the simulations. This includes the amount of magnetic flux accumulated in the saturated state and the ensuing suppression of the magnetorotational instability from the strong field. The energy of the jet and the efficiency with which spin energy is extracted via the Blandford-Znajek process also show convergence. However the spatial structure of the jet shows variation across the set of grids employed, as do the Lorentz factors. Small-scale features of the turbulence, as measured by correlation lengths, are not fully converged. Despite convergence of a number of aspects of the flow, modeling of synchrotron emission shows that variability is not converged and decreases with increasing resolution even at our highest resolutions.

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arXiv2019

Hydrodynamic Torques in Circumbinary Accretion Disks

Gaseous disks have been proposed as a mechanism for facilitating mergers of binary black holes. We explore circumbinary disk systems to determine the evolution of the central binary. To do so, we perform 3D, hydrodynamic, locally isothermal simulations of circumbinary disks on a Cartesian grid. We focus on binaries of equal mass ratios on fixed circular orbits. To investigate the orbital evolution of the binary, we examine the various torques exerted on the system. For the case where the disk plane and binary orbital plane are aligned, we find that the total torque is positive so that the semi-major axis of the binary increases. For the misaligned case, we run simulations with the binary orbital plane and disk midplane misaligned by 45\degree and find the same results - the binary grows. The timescale for the circumbinary disk to realign to the plane of the binary is consistent with the global viscous timescale of the disk.

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arXiv2019

Magnetohydrodynamic-Particle-in-Cell Simulations of the Cosmic-Ray Streaming Instability: Linear Growth and Quasi-linear Evolution

The gyro-resonant cosmic-ray (CR) streaming instability is believed to play a crucial role in CR transport, leading to growth of Alfvén waves at small scales that scatter CRs, and impacts the interaction of CRs with the ISM on large scales. However, extreme scale separation ($λ\ll \rm pc$), low cosmic ray number density ($n_{\rm CR}/n_{\rm ISM} \sim 10^{-9}$), and weak CR anisotropy ($\sim v_A/c$) pose strong challenges for proper numerical studies of this instability on a microphysical level. Employing the recently developed magnetohydrodynamic-particle-in-cell (MHD-PIC) method, which has unique advantages to alleviate these issues, we conduct one-dimensional simulations that quantitatively demonstrate the growth and saturation of the instability in the parameter regime consistent with realistic CR streaming in the large-scale ISM. Our implementation of the $δf$ method dramatically reduces Poisson noise and enables us to accurately capture wave growth over a broad spectrum, equally shared between left and right handed Alfvén modes. We are also able to accurately follow the quasi-linear diffusion of CRs subsequent to wave growth, which is achieved by employing phase randomization across periodic boundaries. Full isotropization of the CRs in the wave frame requires pitch angles of most CRs to efficiently cross $90^\circ$, and can be captured in simulations with relatively high wave amplitude and/or high spatial resolution. We attribute this crossing to non-linear wave-particle interaction (rather than mirror reflection) by investigating individual CR trajectories. We anticipate our methodology will open up opportunities for future investigations that incorporate additional physics.

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arXiv2018

Polygram Stars: Resonant Tidal Excitation of Fundamental Oscillation Modes in Asynchronous Stellar Coalescence

The prevalence of binary stars at close separations implies that many of these systems will interact or merge during the binary's lifetime. This paper presents hydrodynamic simulations of the scenario of binary coalescence through unstable mass transfer, which drives the pair to closer separations. When the donor star does not rotate synchronously with respect to the orbit, dynamical tidal waves are excited in its envelope. We show that resonance crossings with high azimuthal-order $(m\sim3$ to $6$) fundamental modes induce a visible "polygram" distortion to the star. As the binary orbit tightens, the system sweeps through resonance with modes of decreasing azimuthal order, which are selectively excited. We compare our hydrodynamic simulations to predictions from linear theory of resonant mode excitation. The linear theory provides an estimate of mode amplitudes to within a factor of two, even as the oscillations become quite non-linear as the stars coalesce. We estimate that a wave with 10% radial amplitude generates approximately 1% photometric variability; this may be detectible if such a binary coalescence is caught in action by future photometric all-sky surveys.

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arXiv2018

Accretion of Magnetized Stellar Winds in the Galactic Centre: Implications for Sgr A* and PSR J1745-2900

The observed rotation measures (RMs) towards the galactic centre magnetar and towards Sagittarius A* provide a strong constraint on MHD models of the galactic centre accretion flow, probing distances from the black hole separated by many orders of magnitude. We show, using 3D simulations of accretion via magnetized stellar winds of the Wolf-Rayet stars orbiting the black hole, that the large, time-variable RM observed for the pulsar PSR J1745-2900 can be explained by magnetized wind-wind shocks of nearby stars in the clockwise stellar disc. In the same simulation, both the total X-ray luminosity integrated over 2-10$''$, the time variability of the magnetar's dispersion measure, and the RM towards Sagittarius A* are consistent with observations. We argue that (in order for the large RM of the pulsar to not be a priori unlikely) the pulsar should be on an orbit that keeps it near the clockwise disc of stars. We present a 2D RM map of the central 1/2 parsec of the galactic centre that can be used to test our models. Our simulations predict that Sgr A* is typically accreting a significantly ordered magnetic field that ultimately could result in a strongly magnetized flow with flux threading the horizon at $\sim$ 10$\%$ of the magnetically arrested limit.

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arXiv2018

Bound Outflows, Unbound Ejecta, and the Shaping of Bipolar Remnants during Stellar Coalescence

Recent observations have revealed that the remnants of stellar-coalescence transients are bipolar. This raises the questions of how these bipolar morphologies arise and what they teach us about the mechanisms of mass ejection during stellar mergers and common-envelope phases. In this paper, we analyze hydrodynamic simulations of the lead-in to binary coalescence, a phase of unstable Roche lobe overflow that takes the binary from the Roche limit separation to the engulfment of the more compact accretor within the envelope of the extended donor. As mass transfer runs away at increasing rates, gas trails away from the binary. Contrary to previous expectations, early mass loss from the system remains bound to the binary and forms a circumbinary torus. Later ejecta, generated as the accretor grazes the surface of the donor, have very different morphologies and are unbound. These two components of mass loss from the binary interact as later, higher-velocity ejecta collide with the circumbinary torus formed by earlier mass loss. Unbound ejecta are redirected toward the poles, and escaping material creates a bipolar outflow. Our findings show that the transition from bound to unbound ejecta from coalescing binaries can explain the bipolar nature of their remnants, with implications for our understanding of the origin of bipolar remnants of stellar-coalescence transients and, perhaps, some preplanetary nebulae.

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arXiv2018

Theoretical X-ray light curves of young SNe II: the example of SN 2013ej

The X-ray signal from hydrogen-rich supernovae (SNe II) in the first tens to hundreds of days after the shock breakout encodes important information about the circumstellar material (CSM) surrounding their progenitors before explosion. In this study, we describe a way to generate the SN II X-ray light curves from hydrodynamical simulations performed with the code Athena++, using the X-ray package XSPEC. In addition, we employ a radiation diffusion hydrodynamic code SNEC for generating the optical light curves in different bands. In this numerical setup, we model the X-ray and optical emission from a set of progenitor models, consisting of either two (red supergiant + low density steady wind), or three (red supergiant + dense CSM + low density steady wind) components. We vary the density in the wind and the slope in the CSM to see how these parameters influence the resulting X-ray and optical light curves. Among our models, we identify one that is able to roughly reproduce both optical and X-ray data of the well observed SN 2013ej. In order to achieve this, the slope of the dense CSM in this model should be steeper than the one of a steady wind ($ρ\propto r^{-2}$), and closer to $ρ\propto r^{-5}$. On the other hand, we show that too steep and extended CSM profiles may produce excessive X-ray emission in the first few tens of days, up to a few orders of magnitude larger than observed. We conclude that ability to reproduce the observed X-ray signal from SNe~II together with their optical light curves is crucial in establishing the validity of different CSM models.

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arXiv2018

MHD instabilities in accretion disks and their implications in driving fast magnetic reconnection

Magnetohydrodynamic instabilities play an important role in accretion disks systems. Besides the well-known effects of the magnetorotational instability (MRI), the Parker-Rayleigh-Taylor instability (PRTI) also arises as an important mechanism to help in the formation of the coronal region around an accretion disk and in the production of magnetic reconnection events similar to those occurring in the solar corona. In this work, we have performed three-dimensional magnetohydrodynamical (3D-MHD) shearing-box numerical simulations of accretion disks with an initial stratified density distribution and a strong azimuthal magnetic field with a ratio between the thermal and magnetic pressures of the order of unity. This study aimed at verifying the role of these instabilities in driving fast magnetic reconnection in turbulent accretion disk/corona systems. As we expected, the simulations showed an initial formation of large-scale magnetic loops due to the PRTI followed by the development of a nearly steady-state turbulence driven by both instabilities. In this turbulent environment, we have employed an algorithm to identify the presence of current sheets produced by the encounter of magnetic flux ropes of opposite polarity in the turbulent regions of both the corona and the disk. We computed the magnetic reconnection rates in these locations obtaining average reconnection velocities in Alfvén speed units of the order of $0.13 \pm 0.09$ in the accretion disk and $0.17 \pm 0.10$ in the coronal region (with mean peak values of order of $0.2$), which are consistent with the predictions of the theory of turbulence-induced fast reconnection.

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arXiv2018

Runaway Coalescence at the Onset of Common Envelope Episodes

Luminous red nova transients, presumably from stellar coalescence, exhibit long-term precursor emission over hundreds of binary orbits, leading to impulsive outbursts with durations similar to a single orbital period. In an effort to understand these signatures, we present and analyze a hydrodynamic model of unstable mass transfer from a giant-star donor onto a more compact accretor in a binary system. Our simulation begins with mass transfer at the Roche limit separation and traces a phase of runaway decay leading up to the plunge of the accretor within the envelope of the donor. We characterize the fluxes of mass and angular momentum through the system and show that the orbital evolution can be reconstructed from measurements of these quantities. The morphology of outflow from the binary changes significantly as the binary orbit tightens. At wide separations, a thin stream of relatively high-entropy gas trails from the outer Lagrange points. As the orbit tightens, the orbital motion desynchronizes from the donor's rotation, and low-entropy ejecta trace a broad fan of largely ballistic trajectories. An order-of-magnitude increase in mass ejection rate accompanies the plunge of the accretor with the envelope of the donor. We argue that this transition marks the precursor-to-outburst transition observed in stellar coalescence transients.

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arXiv2018

Hydrodynamic Simulations of the Inner Accretion Flow of Sagittarius A* Fueled By Stellar Winds

We present Athena++ grid-based, hydrodynamic simulations of accretion onto Sagittarius A* via the stellar winds of the $\sim 30$ Wolf-Rayet stars within the central parsec of the galactic center. These simulations span $\sim$ 4 orders of magnitude in radius, reaching all the way down to 300 gravitational radii of the black hole, $\sim 32$ times further in than in previous work. We reproduce reasonably well the diffuse thermal X-ray emission observed by Chandra in the central parsec. The resulting accretion flow at small radii is a superposition of two components: 1) a moderately unbound, sub-Keplerian, thick, pressure-supported disc that is at most (but not all) times aligned with the clockwise stellar disc, and 2) a bound, low-angular momentum inflow that proceeds primarily along the southern pole of the disc. We interpret this structure as a natural consequence of a few of the innermost stellar winds dominating accretion, which produces a flow with a broad distribution of angular momentum. Including the star S2 in the simulation has a negligible effect on the flow structure. Extrapolating our results from simulations with different inner radii, we find an accretion rate of $\sim$ a few $\times 10^{-8} M_\odot$/yr at the horizon scale, consistent with constraints based on modeling the observed emission of Sgr A*. The flow structure found here can be used as more realistic initial conditions for horizon scale simulations of Sgr A*.

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arXiv2018

Internal shocks in microquasar jets with a continuous Lorentz factor modulation

We perform relativistic hydrodynamic simulations of internal shocks formed in microquasar jets by continuous variation of the bulk Lorentz factor, in order to investigate the internal shock model. We consider one-, two-, and flicker noise 20-mode variability. We observe emergence of a forward-reverse shock structure for each peak of the Lorentz factor modulation. The high pressure in the shocked layer launches powerful outflows perpendicular to the jet beam into the ambient medium. These outflows dominate the details of the jet's kinetic energy thermalization. They are responsible for mixing between the jet and surrounding medium and generate powerful shocks in the latter. These results do not concur with the popular picture of well-defined internal shells depositing energy as they collide within the confines of the jet, in fact collisions between internal shells themselves are quite rare in our continuous formulation of the problem. For each of our simulations, we calculate the internal energy deposited in the system, the "efficiency" of this deposition (defined as the ratio of internal to total flow energy), and the maximum temperature reached in order to make connections to emission mechanisms. We probe the dependence of these diagnostics on the Lorentz factor variation amplitudes, modulation frequencies, as well as the initial density ratio between the jet and the ambient medium.

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arXiv2018

Global Evolution of an Accretion Disk with Net Vertical Field: Coronal Accretion, Flux Transport, and Disk Winds

We report new global ideal MHD simulations for thin accretion disks (with thermal scale height H/R=0.1 and 0.05) threaded by net vertical magnetic fields. Our computations span three orders of magnitude in radius, extend all the way to the pole, and are evolved for more than one viscous time over the inner decade in radius. Static mesh refinement is used to properly resolve MRI. We find that:(1) inward accretion occurs mostly in the upper magnetically dominated regions of the disk, similar to the predictions from some previous analytical work and the "coronal accretion" in previous GRMHD simulations. Rapid inflow in the upper layers combined with slow outflow at the midplane creates strong $Rϕ$ and $zϕ$ stresses in the mean field; the vertically integrated $α\sim 0.5-1$ when the initial field has $β_{0}=10^3$ at the midplane. (2) A quasi-static global field geometry is established in which flux transport by inflows at the surface is balanced by turbulent diffusion. The field is strongly pinched inwards at the surface. A steady-state advection-diffusion model, with turbulent magnetic Prandtl number of order unity, reproduces this geometry well. (3) Weak unsteady disk winds are launched at $z/R\sim1$ with the Alfven radius $R_{A}/R_{0}\sim3$. Although the wind is episodic, the time averaged properties are well described by steady wind theory. Wind is not efficient at transporting angular momentum. Even with $β_{0}=10^3$, only 5% of the angular momentum transport is driven by torque from the wind, and the wind mass flux from the inner decade of radius is only $\sim$ 0.4% of the mass accretion rate. With weaker fields or thinner disks, the wind contributes even less. (4) Most of the disk accretion is driven by the $Rϕ$ stress from the MRI and global magnetic fields. Our simulations have many applications to astrophysical accretion disk systems.

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arXiv2018

Three-Dimensional Disk-Satellite Interaction: Torques, Migration, and Observational Signatures

The interaction of a satellite with a gaseous disk results in the excitation of spiral density waves which remove angular momentum from the orbit. In addition, if the orbit is not coplanar with the disk, three-dimensional effects will excite bending and eccentricity waves. We perform three-dimensional hydrodynamic simulations to study nonlinear disk-satellite interaction in inviscid protoplanetary disks for a variety of orbital inclinations from $0^\circ$ to $180^\circ$. It is well known that three-dimensional effects are important even for zero inclination. In this work we (1) show that for planets with small inclinations (as in the Solar system), effects such as the total torque and migration rate strongly depend on the inclination and are significantly different (about 2.5 times smaller) from the two-dimensional case, (2) give formulae for the migration rate, inclination damping, and precession rate of planets with different inclination angles in disk with different scale heights, and (3) present the observational signatures of a planet on an inclined orbit with respect to the protoplanetary disk. For misaligned planets we find good agreement with linear theory in the limit of small inclinations, and with dynamical friction estimates for intermediate inclinations. We find that in the latter case, the dynamical friction force is not parallel to the relative planetary velocity. Overall, the derived formulae will be important for studying exoplanets with obliquity.

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arXiv2018

Dusty Cloud Acceleration by Radiation Pressure in Rapidly Star-Forming Galaxies

We perform two-dimensional and three-dimensional radiation hydrodynamic simulations to study cold clouds accelerated by radiation pressure on dust in the environment of rapidly star-forming galaxies dominated by infrared flux. We utilize the reduced speed of light approximation to solve the frequency-averaged, time-dependent radiative transfer equation. We find that radiation pressure is capable of accelerating the clouds to hundreds of kilometers per second while remaining dense and cold, consistent with observations. We compare these results to simulations where acceleration is provided by entrainment in a hot wind, where the momentum injection of the hot flow is comparable to the momentum in the radiation field. We find that the survival time of the cloud accelerated by the radiation field is significantly longer than that of a cloud entrained in a hot outflow. We show that the dynamics of the irradiated cloud depends on the initial optical depth, temperature of the cloud, and the intensity of the flux. Additionally, gas pressure from the background may limit cloud acceleration if the density ratio between the cloud and background is $\lesssim 10^{2}$. In general, a 10 pc-scale optically thin cloud forms a pancake structure elongated perpendicular to the direction of motion, while optically thick clouds form a filamentary structure elongated parallel to the direction of motion. The details of accelerated cloud morphology and geometry can also be affected by other factors, such as the cloud lengthscale, the reduced speed of light approximation, spatial resolution, initial cloud structure, and the dimensionality of the run, but these have relatively little affect on the cloud velocity or survival time.

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arXiv2017

Magnetized Reverse Shock: Density-fluctuation-induced Field Distortion, Polarization Degree Reduction, and Application to GRBs

The early optical afterglow emission of several gamma-ray bursts (GRBs) shows a high linear polarization degree (PD) of tens of percent, suggesting an ordered magnetic field in the emission region. The light curves are consistent with being of a reverse shock (RS) origin. However, the magnetization parameter, $σ$, of the outflow is unknown. If $σ$ is too small, an ordered field in the RS may be quickly randomized due to turbulence driven by various perturbations so that the PD may not be as high as observed. Here we use the "Athena++" relativistic MHD code to simulate a relativistic jet with an ordered magnetic field propagating into a clumpy ambient medium, with a focus on how density fluctuations may distort the ordered magnetic field and reduce PD in the RS emission for different $σ$ values. For a given density fluctuation, we discover a clear power-law relationship between the relative PD reduction and the $σ$ value of the outflow. Such a relation may be applied to estimate $σ$ of the GRB outflows using the polarization data of early afterglows.

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arXiv2017

Global MHD Simulations of Accretion Disks in Cataclysmic Variables (CVs): II The Relative Importance of MRI and Spiral Shocks

We perform global three-dimensional MHD simulations of unstratified accretion disks in cataclysmic variables (CVs). By including mass inflow via an accretion stream, we are able to evolve the disk to a steady state. We investigate the relative importance of spiral shocks and the magnetorotational instability (MRI) in driving angular momentum transport and how each depend on the geometry and strength of seed magnetic field and the Mach number of the disk (where Mach number is ratio of the azimuthal velocity and the sound speed of gas). We use a locally isothermal equation of state and adopt temperature profiles that are consistent with CV disk observations. Our results indicate that the relative importance of spiral shocks and MRI in driving angular momentum transport is controlled by the gas Mach number and the seed magnetic field strength. MRI and spiral shocks provide comparable efficiency of angular momentum transport when the disk Mach number is around 10 and the seed magnetic field has plasma $β=400$ (where $β$ is ratio of gas pressure and magnetic pressure). The MRI dominates whenever the seed field strength, or the disk Mach number, is increased. Among all of our simulations, the effective viscosity parameter $α_{eff} \sim 0.016-0.1$ after MRI saturates and the disk reaches steady state. Larger values of $α_{eff}$ are favored when the seed magnetic field has vertical components or the flow has stronger magnetization ($1/β$). Our models all indicate that the role of MRI in driving angular momentum transport thus mass accretion in CV disks is indispensable, especially in cool disks with weak spiral shocks.

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arXiv2016

Shock-driven Accretion in Circumplanetary Disks: Observables and Satellite Formation

Circumplanetary disks (CPDs) control the growth of planets, supply material for satellites to form, and provide observational signatures of young forming planets. We have carried out two dimensional hydrodynamical simulations with radiative cooling to study CPDs, and suggested a new mechanism to drive the disk accretion. Two spiral shocks are present in CPDs, excited by the central star. We find that spiral shocks can at least contribute to, if not dominate the angular momentum transport and energy dissipation in CPDs. Meanwhile, dissipation and heating by spiral shocks have a positive feedback on shock-driven accretion itself. As the disk is heated up by spiral shocks, the shocks become more open, leading to more efficient angular momentum transport. This shock driven accretion is, on the other hand, unsteady on a timescale of months/years due to production and destruction of vortices in disks. After being averaged over time, a quasi-steady accretion is reached from the planet's Hill radius all the way to the planet surface, and the disk $α$-coefficient characterizing angular momentum transport due to spiral shocks is $\sim$0.001-0.02. The disk surface density ranges from 10 to 1000 g cm$^{-2}$ in our simulations, which is at least 3 orders of magnitude smaller than the "minimum mass sub-nebula" model used to study satellite formation; instead it is more consistent with the "gas-starved" satellite formation model. Finally, we calculate the millimeter flux emitted by CPDs at ALMA and EVLA wavelength bands and predict the flux for several recently discovered CPD candidates, which suggests that ALMA is capable of discovering these accreting CPDs.

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arXiv2016

Hall-effect Mediated Magnetic Flux Transport in Protoplanetary Disks

The global evolution of protoplanetary disks (PPDs) has recently been shown to be largely controlled by the amount of poloidal magnetic flux threading the disk, which is further controlled by the poorly understood process of magnetic flux transport. In weakly ionized gas as in PPDs, magnetic flux is largely frozen in the electron fluid, except when resistivity is large. When the disk is largely laminar, we show that the relative drift between the electrons and ions (the Hall-drift), and the ions and neutral fluids (ambipolar-drift) can play a dominant role on the transport of magnetic flux. Using two-dimensional simulations that incorporate the Hall effect and ambipolar diffusion (AD) with prescribed diffusivities, we show that when large-scale poloidal field is aligned with disk rotation, the Hall effect rapidly drags magnetic flux inward at the midplane region, while it slowly pushes flux outward above/below the midplane. This leads to a highly radially elongated field configuration as a global manifestation of the Hall-shear instability. This field configuration further promotes rapid outward flux transport by AD at the midplane, leading to instability saturation. In quasi-steady state, magnetic flux is transported outward at approximately the same rate at all heights, and the rate is comparable to the Hall-free case. For anti-aligned field polarity, the Hall effect consistently transports magnetic flux outward, leading to a largely vertical field configuration in the midplane region. The field lines in the upper layer first bend radially inward and then outward to launch a disk wind. Overall, the net rate of outward flux transport is about twice faster than the aligned case. In addition, the rate of flux transport increases with increasing disk magnetization. The absolute rate of transport is sensitive to disk microphysics which remains to be explored in future studies.

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