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Sean P Matt - One of the best experts on this subject based on the ideXlab platform.
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Proceedings of Lowell Observatory (9-13 June 2014) Edited by G. van Belle & H. Harris Numerical aspects of 3D Stellar Winds
2016Co-Authors: Antoine Strugarek, Sean P Matt, A. S. Brun, Victor RévilleAbstract:Abstract. This paper explores and compares the pitfalls of modelling the three-dimensional wind of a spherical star with a cartesian grid. Several numerical methods are compared, using either uniform and stretched grid or adaptative mesh refinement (AMR). An additional numerical complication is added, when an orbiting planet is considered. In this case a rotating frame is added to the model such that the orbiting planet is at rest in the frame of work. The three-dimensional simulations are systematically compared to an equivalent two-dimensional, axisymmetric simulation. The comparative study presented here suggests to limit the rotation rate of the rotating frame below the rotating frame of the star and provides guidelines for further three-dimensional modelling of Stellar Winds in the context of close-in star-planet interactions. 1
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magnetic braking formulation for sun like stars dependence on dipole field strength and rotation rate
The Astrophysical Journal, 2012Co-Authors: Sean P Matt, K B Macgregor, Marc H Pinsonneault, Thomas P GreeneAbstract:We use two-dimensional axisymmetric magnetohydrodynamic simulations to compute steady-state solutions for solar-like Stellar Winds from rotating stars with dipolar magnetic fields. Our parameter study includes 50 simulations covering a wide range of relative magnetic field strengths and rotation rates, extending from the slow- and approaching the fast-magnetic-rotator regimes. Using the simulations to compute the angular momentum loss, we derive a semi-analytic formulation for the external torque on the star that fits all of the simulations to a precision of a few percent. This formula provides a simple method for computing the magnetic braking of Sun-like stars due to magnetized Stellar Winds, which properly includes the dependence on the strength of the magnetic field, mass loss rate, Stellar radius, surface gravity, and spin rate, and which is valid for both slow and fast rotators.
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accretion powered Stellar Winds iii spin equilibrium solutions
The Astrophysical Journal, 2008Co-Authors: Sean P Matt, Ralph E PudritzAbstract:We compare the Stellar wind torque calculated in a previous work (Paper II) to the spin-up and spin-down torques expected to arise from the magnetic interaction between a slowly rotating (~10% of breakup) pre-main-sequence star and its accretion disk. This analysis demonstrates that Stellar Winds can carry off orders of magnitude more angular momentum than can be transferred to the disk, provided that the mass outflow rates are substantially greater than the solar wind. Thus, the equilibrium spin state is simply characterized by a balance between the angular momentum deposited by accretion and that extracted by a Stellar wind. We derive a semianalytic formula for predicting the equilibrium spin rate as a function only of the ratio of w/a and a dimensionless magnetization parameter, Ψ ≡ B2*R2*(avesc)−1, where w is the Stellar wind mass outflow rate, a is the accretion rate, B* is the Stellar surface magnetic field strength, R* is the Stellar radius, and vesc is the surface escape speed. For parameters typical of accreting pre-main-sequence stars, this explains spin rates of ~10% of breakup speed for w/a ~ 0.1. Finally, the assumption that the Stellar wind is driven by a fraction of the accretion power leads to an upper limit to the mass flow ratio of w/a 0.6.
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accretion powered Stellar Winds ii numerical solutions for Stellar wind torques
The Astrophysical Journal, 2008Co-Authors: Sean P Matt, Ralph E PudritzAbstract:In order to explain the slow rotation observed in a large fraction of accreting pre-main-sequence stars (CTTSs), we explore the role of Stellar Winds in torquing down the stars. For this mechanism to be effective, the Stellar Winds need to have relatively high outflow rates, and thus would likely be powered by the accretion process itself. Here, we use numerical magnetohydrodynamical simulations to compute detailed two-dimensional (axisymmetric) Stellar wind solutions, in order to determine the spin-down torque on the star. We discuss wind driving mechanisms and then adopt a Parker-like (thermal pressure driven) wind, modified by rotation, magnetic fields, and enhanced mass-loss rate (relative to the Sun). We explore a range of parameters relevant for CTTSs, including variations in the Stellar mass, radius, spin rate, surface magnetic field strength, mass-loss rate, and wind acceleration rate. We also consider both dipole and quadrupole magnetic field geometries. Our simulations indicate that the Stellar wind torque is of sufficient magnitude to be important for spinning down a "typical" CTTS, for a mass-loss rate of ~10−9 M☉ yr−1. The Winds are wide-angle, self-collimated flows, as expected of magnetic rotator Winds with moderately fast rotation. The cases with quadrupolar field produce a much weaker torque than for a dipole with the same surface field strength, demonstrating that magnetic geometry plays a fundamental role in determining the torque. Cases with varying wind acceleration rate show much smaller variations in the torque, suggesting that the details of the wind driving are less important. We use our computed results to fit a semianalytic formula for the effective Alfven radius in the wind, as well as the torque. This allows for considerable predictive power, and is an improvement over existing approximations.
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accretion powered Stellar Winds ii numerical solutions for Stellar wind torques
arXiv: Astrophysics, 2008Co-Authors: Sean P Matt, Ralph E PudritzAbstract:[Abridged] In order to explain the slow rotation observed in a large fraction of accreting pre-main-sequence stars (CTTSs), we explore the role of Stellar Winds in torquing down the stars. For this mechanism to be effective, the Stellar Winds need to have relatively high outflow rates, and thus would likely be powered by the accretion process itself. Here, we use numerical magnetohydrodynamical simulations to compute detailed 2-dimensional (axisymmetric) Stellar wind solutions, in order to determine the spin down torque on the star. We explore a range of parameters relevant for CTTSs, including variations in the Stellar mass, radius, spin rate, surface magnetic field strength, the mass loss rate, and wind acceleration rate. We also consider both dipole and quadrupole magnetic field geometries. Our simulations indicate that the Stellar wind torque is of sufficient magnitude to be important for spinning down a ``typical'' CTTS, for a mass loss rate of $\sim 10^{-9} M_\odot$ yr$^{-1}$. The Winds are wide-angle, self-collimated flows, as expected of magnetic rotator Winds with moderately fast rotation. The cases with quadrupolar field produce a much weaker torque than for a dipole with the same surface field strength, demonstrating that magnetic geometry plays a fundamental role in determining the torque. Cases with varying wind acceleration rate show much smaller variations in the torque suggesting that the details of the wind driving are less important. We use our computed results to fit a semi-analytic formula for the effective Alfv\'en radius in the wind, as well as the torque. This allows for considerable predictive power, and is an improvement over existing approximations.
Ralph E Pudritz - One of the best experts on this subject based on the ideXlab platform.
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accretion powered Stellar Winds iii spin equilibrium solutions
The Astrophysical Journal, 2008Co-Authors: Sean P Matt, Ralph E PudritzAbstract:We compare the Stellar wind torque calculated in a previous work (Paper II) to the spin-up and spin-down torques expected to arise from the magnetic interaction between a slowly rotating (~10% of breakup) pre-main-sequence star and its accretion disk. This analysis demonstrates that Stellar Winds can carry off orders of magnitude more angular momentum than can be transferred to the disk, provided that the mass outflow rates are substantially greater than the solar wind. Thus, the equilibrium spin state is simply characterized by a balance between the angular momentum deposited by accretion and that extracted by a Stellar wind. We derive a semianalytic formula for predicting the equilibrium spin rate as a function only of the ratio of w/a and a dimensionless magnetization parameter, Ψ ≡ B2*R2*(avesc)−1, where w is the Stellar wind mass outflow rate, a is the accretion rate, B* is the Stellar surface magnetic field strength, R* is the Stellar radius, and vesc is the surface escape speed. For parameters typical of accreting pre-main-sequence stars, this explains spin rates of ~10% of breakup speed for w/a ~ 0.1. Finally, the assumption that the Stellar wind is driven by a fraction of the accretion power leads to an upper limit to the mass flow ratio of w/a 0.6.
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accretion powered Stellar Winds ii numerical solutions for Stellar wind torques
The Astrophysical Journal, 2008Co-Authors: Sean P Matt, Ralph E PudritzAbstract:In order to explain the slow rotation observed in a large fraction of accreting pre-main-sequence stars (CTTSs), we explore the role of Stellar Winds in torquing down the stars. For this mechanism to be effective, the Stellar Winds need to have relatively high outflow rates, and thus would likely be powered by the accretion process itself. Here, we use numerical magnetohydrodynamical simulations to compute detailed two-dimensional (axisymmetric) Stellar wind solutions, in order to determine the spin-down torque on the star. We discuss wind driving mechanisms and then adopt a Parker-like (thermal pressure driven) wind, modified by rotation, magnetic fields, and enhanced mass-loss rate (relative to the Sun). We explore a range of parameters relevant for CTTSs, including variations in the Stellar mass, radius, spin rate, surface magnetic field strength, mass-loss rate, and wind acceleration rate. We also consider both dipole and quadrupole magnetic field geometries. Our simulations indicate that the Stellar wind torque is of sufficient magnitude to be important for spinning down a "typical" CTTS, for a mass-loss rate of ~10−9 M☉ yr−1. The Winds are wide-angle, self-collimated flows, as expected of magnetic rotator Winds with moderately fast rotation. The cases with quadrupolar field produce a much weaker torque than for a dipole with the same surface field strength, demonstrating that magnetic geometry plays a fundamental role in determining the torque. Cases with varying wind acceleration rate show much smaller variations in the torque, suggesting that the details of the wind driving are less important. We use our computed results to fit a semianalytic formula for the effective Alfven radius in the wind, as well as the torque. This allows for considerable predictive power, and is an improvement over existing approximations.
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accretion powered Stellar Winds ii numerical solutions for Stellar wind torques
arXiv: Astrophysics, 2008Co-Authors: Sean P Matt, Ralph E PudritzAbstract:[Abridged] In order to explain the slow rotation observed in a large fraction of accreting pre-main-sequence stars (CTTSs), we explore the role of Stellar Winds in torquing down the stars. For this mechanism to be effective, the Stellar Winds need to have relatively high outflow rates, and thus would likely be powered by the accretion process itself. Here, we use numerical magnetohydrodynamical simulations to compute detailed 2-dimensional (axisymmetric) Stellar wind solutions, in order to determine the spin down torque on the star. We explore a range of parameters relevant for CTTSs, including variations in the Stellar mass, radius, spin rate, surface magnetic field strength, the mass loss rate, and wind acceleration rate. We also consider both dipole and quadrupole magnetic field geometries. Our simulations indicate that the Stellar wind torque is of sufficient magnitude to be important for spinning down a ``typical'' CTTS, for a mass loss rate of $\sim 10^{-9} M_\odot$ yr$^{-1}$. The Winds are wide-angle, self-collimated flows, as expected of magnetic rotator Winds with moderately fast rotation. The cases with quadrupolar field produce a much weaker torque than for a dipole with the same surface field strength, demonstrating that magnetic geometry plays a fundamental role in determining the torque. Cases with varying wind acceleration rate show much smaller variations in the torque suggesting that the details of the wind driving are less important. We use our computed results to fit a semi-analytic formula for the effective Alfv\'en radius in the wind, as well as the torque. This allows for considerable predictive power, and is an improvement over existing approximations.
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accretion powered Stellar Winds iii spin equilibrium solutions
arXiv: Astrophysics, 2008Co-Authors: Sean P Matt, Ralph E PudritzAbstract:We compare the Stellar wind torque calculated in a previous work (Paper II) to the spin-up and spin-down torques expected to arise from the magnetic interaction between a slowly rotating ($\sim 10$% of breakup) pre-main-sequence star and its accretion disk. This analysis demonstrates that Stellar Winds can carry off orders of magnitude more angular momentum than can be transferred to the disk, provided that the mass outflow rates are greater than the solar wind. Thus, the equilibrium spin state is simply characterized by a balance between the angular momentum deposited by accretion and that extracted by a Stellar wind. We derive a semi-analytic formula for predicting the equilibrium spin rate as a function only of the ratio of $\dot M_{\rm w} / \dot M_{\rm a}$ and a dimensionless magnetization parameter, $\Psi \equiv B_*^2 R_*^2 (\dot M_{\rm a} v_{\rm esc})^{-1}$, where $\dot M_{\rm w}$ is the Stellar wind mass outflow rate, $\dot M_{\rm a}$ the accretion rate, $B_*$ the Stellar surface magnetic field strength, $R_*$ the Stellar radius, and $v_{\rm esc}$ the surface escape speed. For parameters typical of accreting pre-main-sequence stars, this explains spin rates of $\sim 10$% of breakup speed for $\dot M_{\rm w} / \dot M_{\rm a} \sim 0.1$. Finally, the assumption that the Stellar wind is driven by a fraction of the accretion power leads to an upper limit to the mass flow ratio of $\dot M_{\rm w} / \dot M_{\rm a} \la 0.6$.
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the nature of Stellar Winds in the star disk interaction
arXiv: Astrophysics, 2007Co-Authors: Sean P Matt, Ralph E PudritzAbstract:Stellar Winds may be important for angular momentum transport from accreting T Tauri stars, but the nature of these Winds is still not well-constrained. We present some simulation results for hypothetical, hot (~1e6 K) coronal Winds from T Tauri stars, and we calculate the expected emission properties. For the high mass loss rates required to solve the angular momentum problem, we find that the radiative losses will be much greater than can be powered by the accretion process. We place an upper limit to the mass loss rate from accretion-powered coronal Winds of ~1e-11 solar masses per year. We conclude that accretion powered Stellar Winds are still a promising scenario for solving the Stellar angular momentum problem, but the Winds must be cool (~1e4 K) and thus are not driven by thermal pressure.
Eliot Quataert - One of the best experts on this subject based on the ideXlab platform.
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ab initio horizon scale simulations of magnetically arrested accretion in sagittarius a fed by Stellar Winds
The Astrophysical Journal, 2020Co-Authors: Sean M Ressler, Eliot Quataert, Christopher J White, James M StoneAbstract:We present 3D general relativistic magnetohydrodynamic (GRMHD) simulations of the accretion flow surrounding Sagittarius A* that are initialized using larger-scale MHD simulations of the $\sim$ 30 Wolf--Rayet (WR) Stellar Winds in the Galactic center. The properties of the resulting accretion flow on horizon scales are set not by ad hoc initial conditions but by the observationally constrained properties of the WR Winds with limited free parameters. For this initial study we assume a non-spinning black hole. Our simulations naturally produce a $\sim 10^{-8} M_\odot$ yr$^{-1}$ accretion rate, consistent with previous phenomenological estimates. We find that a magnetically arrested flow is formed by the continuous accretion of coherent magnetic field being fed from large radii. Near the event horizon, the magnetic field is so strong that it tilts the gas with respect to the initial angular momentum and concentrates the originally quasi-spherical flow to a narrow disk-like structure. We also present 230 GHz images calculated from our simulations where the inclination angle and physical accretion rate are not free parameters but are determined by the properties of the WR Stellar Winds. The image morphology is highly time variable. Linear polarization on horizon scales is coherent with weak internal Faraday rotation.
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the surprisingly small impact of magnetic fields on the inner accretion flow of sagittarius a fueled by Stellar Winds
Monthly Notices of the Royal Astronomical Society, 2020Co-Authors: Sean M Ressler, Eliot Quataert, James M StoneAbstract:We study the flow structure in 3D magnetohydrodynamic (MHD) simulations of accretion onto Sagittarius A* via the magnetized Winds of the orbiting Wolf-Rayet stars. These simulations cover over 3 orders of magnitude in radius to reach $\approx$ 300 gravitational radii, with only one poorly constrained parameter (the magnetic field in the Stellar Winds). Even for Winds with relatively weak magnetic fields (e.g., plasma $\beta$ $\sim$ $10^6$), flux freezing/compression in the inflowing gas amplifies the field to $\beta$ $\sim$ few well before it reaches the event horizon. Overall, the dynamics, accretion rate, and spherically averaged flow profiles (e.g., density, velocity) in our MHD simulations are remarkably similar to analogous hydrodynamic simulations. We attribute this to the broad distribution of angular momentum provided by the Stellar Winds, which sources accretion even absent much angular momentum transport. We find that the magneto-rotational instability is not important because of i) strong magnetic fields that are amplified by flux freezing/compression, and ii) the rapid inflow/outflow times of the gas and inefficient radiative cooling preclude circularization. The primary effect of magnetic fields is that they drive a polar outflow that is absent in hydrodynamics. The dynamical state of the accretion flow found in our simulations is unlike the rotationally supported tori used as initial conditions in horizon scale simulations, which could have implications for models being used to interpret Event Horizon Telescope and GRAVITY observations of Sgr A*.
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accretion of magnetized Stellar Winds in the galactic centre implications for sgr a and psr j1745 2900
Monthly Notices of the Royal Astronomical Society: Letters, 2019Co-Authors: Sean M Ressler, Eliot Quataert, James M StoneAbstract: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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hydrodynamic simulations of the inner accretion flow of sagittarius a fuelled by Stellar Winds
Monthly Notices of the Royal Astronomical Society, 2018Co-Authors: Sean M Ressler, Eliot Quataert, James M StoneAbstract: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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super eddington Stellar Winds unifying radiative enthalpy versus flux driven models
Monthly Notices of the Royal Astronomical Society, 2017Co-Authors: Eliot Quataert, Stanley P Owocki, R H D TownsendAbstract:We derive semi-analytic solutions for optically thick, super-Eddington Stellar Winds, induced by an assumed steady energy addition $\Delta {\dot E}$ concentrated around a near-surface heating radius $R$ in a massive star of central luminosity $L_\ast$. We show that obtaining steady wind solutions requires both that the resulting total luminosity $L_o = L_\ast + \Delta {\dot E}$ exceed the Eddington luminosity, $\Gamma_o \equiv L_o/L_{Edd} > 1$, and that the induced mass loss rate be such that the "photon-tiring" parameter $m \equiv {\dot M} GM/R L_o \le 1-1/\Gamma_o$, ensuring the luminosity is sufficient to overcome the gravitational potential $GM/R$. Our analysis unifies previous super-Eddington wind models that either: (1) assumed a direct radiative flux-driving without accounting for the advection of radiative enthalpy that can become important in such an optically thick flow; or (2) assumed that such super-Eddington outflows are adiabatic, neglecting the effects of the diffusive radiative flux. We show that these distinct models become applicable in the asymptotic limits of small vs. large values of $m \Gamma_o $, respectively. By solving the coupled differential equations for radiative diffusion and wind momentum, we obtain general solutions that effectively bridge the behaviours of these limiting models. Two key scaling results are for the terminal wind speed to escape speed, which is found to vary as $v_\infty^2/v_{esc}^2 = \Gamma_o/(1+m \Gamma_o) -1$, and for the final observed luminosity $L_{ obs}$, which for all allowed steady-solutions with $m L_{Edd}$. Our super-Eddington wind solutions have potential applicability for modeling phases of eruptive mass loss from massive stars, classical novae, and the remnants of Stellar mergers.
Stanley P Owocki - One of the best experts on this subject based on the ideXlab platform.
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radiation transport through super eddington Stellar Winds
pas8, 2018Co-Authors: J A Guzik, Chris L Fryer, Todd Urbatsch, Stanley P OwockiAbstract:We present results of simulations to assess the feasibility of modeling outflows from massive stars using the Los Alamos 3-D radiation hydrodynamics code Cassio developed for inertial confinement fusion (ICF) applications. We find that a 1-D Stellar envelope simulation relaxes into hydrostatic equilibrium using computing resources that would make the simulation tractable in 2-D. We summarize next steps to include more physics fidelity and model the response to a large and abrupt energy deposition at the base of the envelope.
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super eddington Stellar Winds unifying radiative enthalpy versus flux driven models
Monthly Notices of the Royal Astronomical Society, 2017Co-Authors: Eliot Quataert, Stanley P Owocki, R H D TownsendAbstract:We derive semi-analytic solutions for optically thick, super-Eddington Stellar Winds, induced by an assumed steady energy addition $\Delta {\dot E}$ concentrated around a near-surface heating radius $R$ in a massive star of central luminosity $L_\ast$. We show that obtaining steady wind solutions requires both that the resulting total luminosity $L_o = L_\ast + \Delta {\dot E}$ exceed the Eddington luminosity, $\Gamma_o \equiv L_o/L_{Edd} > 1$, and that the induced mass loss rate be such that the "photon-tiring" parameter $m \equiv {\dot M} GM/R L_o \le 1-1/\Gamma_o$, ensuring the luminosity is sufficient to overcome the gravitational potential $GM/R$. Our analysis unifies previous super-Eddington wind models that either: (1) assumed a direct radiative flux-driving without accounting for the advection of radiative enthalpy that can become important in such an optically thick flow; or (2) assumed that such super-Eddington outflows are adiabatic, neglecting the effects of the diffusive radiative flux. We show that these distinct models become applicable in the asymptotic limits of small vs. large values of $m \Gamma_o $, respectively. By solving the coupled differential equations for radiative diffusion and wind momentum, we obtain general solutions that effectively bridge the behaviours of these limiting models. Two key scaling results are for the terminal wind speed to escape speed, which is found to vary as $v_\infty^2/v_{esc}^2 = \Gamma_o/(1+m \Gamma_o) -1$, and for the final observed luminosity $L_{ obs}$, which for all allowed steady-solutions with $m L_{Edd}$. Our super-Eddington wind solutions have potential applicability for modeling phases of eruptive mass loss from massive stars, classical novae, and the remnants of Stellar mergers.
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dynamical simulations of magnetically channelled line driven Stellar Winds ii the effects of field aligned rotation
Monthly Notices of the Royal Astronomical Society, 2008Co-Authors: Stanley P Owocki, A Uddoula, R H D TownsendAbstract:Building upon our previous MHD simulation study of magnetic channeling in radiatively driven Stellar Winds, we examine here the additional dynamical eects of Stellar rotation in the (still) 2-D axisymmetric case of an aligned dipole surface eld. In addition to the magnetic connement parameter introduced in Paper I, we characterize the Stellar rotation in terms of a parameter W Vrot=Vorb (the ratio of the equatorial surface rotation speed to orbital speed), examining specically models with moderately strong rotation W = 0.25 and 0.5, and comparing these to analogous non-rotating cases. Dening the associated Alfv en radius RA 1=4 R and Kepler
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dynamical simulations of magnetically channeled line driven Stellar Winds ii the effects of field aligned rotation
arXiv: Astrophysics, 2007Co-Authors: A Uddoula, Stanley P Owocki, R H D TownsendAbstract:Building upon our previous MHD simulation study of magnetic channeling in radiatively driven Stellar Winds, we examine here the additional dynamical effects of Stellar {\em rotation} in the (still) 2-D axisymmetric case of an aligned dipole surface field. In addition to the magnetic confinement parameter $\eta_{\ast}$ introduced in Paper I, we characterize the Stellar rotation in terms of a parameter $W \equiv V_{\rm{rot}}/V_{\rm{orb}}$ (the ratio of the equatorial surface rotation speed to orbital speed), examining specifically models with moderately strong rotation $W =$ 0.25 and 0.5, and comparing these to analogous non-rotating cases. Defining the associated Alfv\'{e}n radius $R_{\rm{A}} \approx \eta_{\ast}^{1/4} \Rstar$ and Kepler corotation radius $R_{\rm{K}} \approx W^{-2/3} \Rstar$, we find rotation effects are weak for models with $R_{\rm{A}} < R_{\rm{K}}$, but can be substantial and even dominant for models with $R_{\rm{A}} \gtwig R_{\rm{K}}$. In particular, by extending our simulations to magnetic confinement parameters (up to $\eta_{\ast} = 1000$) that are well above those ($\eta_{\ast} = 10$) considered in Paper I, we are able to study cases with $R_{\rm{A}} \gg R_{\rm{K}}$; we find that these do indeed show clear formation of the {\em rigid-body} disk predicted in previous analytic models, with however a rather complex, dynamic behavior characterized by both episodes of downward infall and outward breakout that limit the buildup of disk mass. Overall, the results provide an intriguing glimpse into the complex interplay between rotation and magnetic confinement, and form the basis for a full MHD description of the rigid-body disks expected in strongly magnetic Bp stars like $\sigma$ Ori E.
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centrifugal breakout of magnetically confined line driven Stellar Winds
arXiv: Astrophysics, 2006Co-Authors: R H D Townsend, A Uddoula, Stanley P OwockiAbstract:We present 2D MHD simulations of the radiatively driven outflow from a rotating hot star with a dipole magnetic field aligned with the star's rotation axis. We focus primarily on a model with moderately rapid rotation (half the critical value), and also a large magnetic confinement parameter, $\eta_{\ast} \equiv B_{\ast}^2 R_{\ast}^{2} / \dot{M} V_{\infty} = 600$. The magnetic field channels and torques the wind outflow into an equatorial, rigidly rotating disk extending from near the Kepler corotation radius outwards. Even with fine-tuning at lower magnetic confinement, none of the MHD models produce a stable Keplerian disk. Instead, material below the Kepler radius falls back on to the Stellar surface, while the strong centrifugal force on material beyond the corotation escape radius stretches the magnetic loops outwards, leading to episodic breakout of mass when the field reconnects. The associated dissipation of magnetic energy heats material to temperatures of nearly $10^{8}$K, high enough to emit hard (several keV) X-rays. Such \emph{centrifugal mass ejection} represents a novel mechanism for driving magnetic reconnection, and seems a very promising basis for modeling X-ray flares recently observed in rotating magnetic Bp stars like $\sigma$ Ori E.
James M Stone - One of the best experts on this subject based on the ideXlab platform.
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ab initio horizon scale simulations of magnetically arrested accretion in sagittarius a fed by Stellar Winds
The Astrophysical Journal, 2020Co-Authors: Sean M Ressler, Eliot Quataert, Christopher J White, James M StoneAbstract:We present 3D general relativistic magnetohydrodynamic (GRMHD) simulations of the accretion flow surrounding Sagittarius A* that are initialized using larger-scale MHD simulations of the $\sim$ 30 Wolf--Rayet (WR) Stellar Winds in the Galactic center. The properties of the resulting accretion flow on horizon scales are set not by ad hoc initial conditions but by the observationally constrained properties of the WR Winds with limited free parameters. For this initial study we assume a non-spinning black hole. Our simulations naturally produce a $\sim 10^{-8} M_\odot$ yr$^{-1}$ accretion rate, consistent with previous phenomenological estimates. We find that a magnetically arrested flow is formed by the continuous accretion of coherent magnetic field being fed from large radii. Near the event horizon, the magnetic field is so strong that it tilts the gas with respect to the initial angular momentum and concentrates the originally quasi-spherical flow to a narrow disk-like structure. We also present 230 GHz images calculated from our simulations where the inclination angle and physical accretion rate are not free parameters but are determined by the properties of the WR Stellar Winds. The image morphology is highly time variable. Linear polarization on horizon scales is coherent with weak internal Faraday rotation.
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the surprisingly small impact of magnetic fields on the inner accretion flow of sagittarius a fueled by Stellar Winds
Monthly Notices of the Royal Astronomical Society, 2020Co-Authors: Sean M Ressler, Eliot Quataert, James M StoneAbstract:We study the flow structure in 3D magnetohydrodynamic (MHD) simulations of accretion onto Sagittarius A* via the magnetized Winds of the orbiting Wolf-Rayet stars. These simulations cover over 3 orders of magnitude in radius to reach $\approx$ 300 gravitational radii, with only one poorly constrained parameter (the magnetic field in the Stellar Winds). Even for Winds with relatively weak magnetic fields (e.g., plasma $\beta$ $\sim$ $10^6$), flux freezing/compression in the inflowing gas amplifies the field to $\beta$ $\sim$ few well before it reaches the event horizon. Overall, the dynamics, accretion rate, and spherically averaged flow profiles (e.g., density, velocity) in our MHD simulations are remarkably similar to analogous hydrodynamic simulations. We attribute this to the broad distribution of angular momentum provided by the Stellar Winds, which sources accretion even absent much angular momentum transport. We find that the magneto-rotational instability is not important because of i) strong magnetic fields that are amplified by flux freezing/compression, and ii) the rapid inflow/outflow times of the gas and inefficient radiative cooling preclude circularization. The primary effect of magnetic fields is that they drive a polar outflow that is absent in hydrodynamics. The dynamical state of the accretion flow found in our simulations is unlike the rotationally supported tori used as initial conditions in horizon scale simulations, which could have implications for models being used to interpret Event Horizon Telescope and GRAVITY observations of Sgr A*.
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accretion of magnetized Stellar Winds in the galactic centre implications for sgr a and psr j1745 2900
Monthly Notices of the Royal Astronomical Society: Letters, 2019Co-Authors: Sean M Ressler, Eliot Quataert, James M StoneAbstract: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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hydrodynamic simulations of the inner accretion flow of sagittarius a fuelled by Stellar Winds
Monthly Notices of the Royal Astronomical Society, 2018Co-Authors: Sean M Ressler, Eliot Quataert, James M StoneAbstract: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*.
