arXiv subjects

Eric G. Blackman

Publications and source records attributed to Eric G. Blackman.

At least 127 records · Page 7Linked to original sources

arXiv2007

Empirical relation between angular momentum transport and thermal-to-magnetic pressure ratio in shearing box simulations

By combining data from different published 3-D simulations of Keplerian shearing boxes unstable to the magnetorotational instability (MRI), we highlight tight anti-correlations between the total effective inferred angular momentum transport parameter, $α_{tot}$, its separate Maxwell and Reynolds contributions $α_{mag}$ and $α_{kin}$, and the kinetic to magnetic pressure ratio $β$, defined with the initial or saturated (when available) thermal pressure. Plots of $Log (α_{kin}), Log (α_{mag})$, and $Log (α_{tot})$ vs $Log (β)$ are well fit by straight lines even as $α_{kin}$, $α_{mag}$,and $α_{tot}$ vary by four orders of magnitude over the simulations included. The ratio $α_{kin}/α_{mag}$ and the product $α_{tot}β$ are quite constant and largely independent of the presence or absence of weak mean fields, the choice of initial and boundary conditions, and the resolution. In short, simulations have more strongly constrained the product $α_{tot}β$ than $α_{tot}$ itself.

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arXiv2007

A Model of Cloud Fragmentation

We present a model in which the supersonic motions observed in molecular clouds are driven by gravitational energy released as large structures fragment into smaller ones. The fragmentation process begins in large molecular clouds, and continues down to fragments of a critical mass, at which gravitational confinement may be replaced by pressure confinement. The power laws that describe the scaling of density and mass, and number spectra and mass spectra of the fragments are given in terms of that of the observed velocity dispersion of the fragments. The results agree reasonably well with observations over the range from several to about a third of a million solar masses.

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arXiv2007

Planetary Nebulae Principles & Paradigms: Binaries, Accretion, Magnetic Fields

Observations suggest that many, if not all, post AGB systems evolve through an aspherical outflow phase. Such outflows require a sufficient engine rotational energy which binaries can provide. Via common envelope evolution, binaries can directly eject equatorial outflows or produce poloidal outflows from magnetized accretion disks around the primary or secondary. We discuss how accretion driven magnetohydrodynamic outflow models all make similar predictions for the outflow power and speed and we distinguish between the launch vs. propagation regimes of such outflows. We suggest that the high velocity bipolar outflows observed in planetary nebulae (PNe) and the lower velocity but higher power bipolar outflows observed in pre-PNe (pPNe) are kinematically consistent with time dependent accretion onto a white dwarf (WD) within a depleting envelope. Since the WD primary core is always present in all post-AGB systems, accretion onto this core is potentially common. Previous work has focused on core accretion from sub-stellar companions, but low mass stellar companions may be more important, and further work is needed.

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arXiv2007

On the role of stochastic Fermi acceleration in setting the dissipation scale of turbulence in the interstellar medium

We consider the dissipation by Fermi acceleration of magnetosonic turbulence in the Reynolds Layer of the interstellar medium. The scale in the cascade at which electron acceleration via stochastic Fermi acceleration (STFA) becomes comparable to further cascade of the turbulence defines the inner scale. For any magnetic turbulent spectra equal to or shallower than Goldreich-Sridhar this turns out to be $\ge 10^{12}$cm, which is much larger than the shortest length scales observed in radio scintillation measurements. While STFA for such spectra then contradict models of scintillation which appeal directly to an extended, continuous turbulent cascade, such a separation of scales is consistent with the recent work of \citet{Boldyrev2} and \citet{Boldyrev3} suggesting that interstellar scintillation may result from the passage of radio waves through the galactic distribution of thin ionized boundary surfaces of HII regions, rather than density variations from cascading turbulence. The presence of STFA dissipation also provides a mechanism for the non-ionizing heat source observed in the Reynolds Layer of the interstellar medium \citep{Reynolds}. STFA accommodates the proper heating power, and the input energy is rapidly thermalized within the low density Reynolds layer plasma.

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arXiv2007

Toward Coupling Flow Driven and Magnetically Driven Dynamos

Most large scale dynamo research for astrophysical rotators focuses on interior flow driven helical dynamos (FDHDs), but larger scale coronal fields most directly influence observations. It is thus important to understand the relationship between coronal and interior fields. Coronal field relaxation is actually a type of magnetically dominated helical dynamo (MDHD). MDHDs also occur in fusion plasma devices where they drive a system toward its relaxed state in response to magnetic helicity injection that otherwise drives the system away from this state. Global scale fields of astrophysical rotators and jets are thus plausibly produced by a direct coupling between an interior FDHD and a coronal MDHD, interfaced by magnetic helicity transport through their mutual boundary. Tracking the magnetic helicity also elucidates how both FDHD and MDHDs evolve and saturate. The utility of magnetic helicity is unhampered by its non-gauge invariance since physical fields can always be recovered.

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arXiv2007

PPN as Explosions: Bullets vs Jets and Nebular Shaping

Many proto-planetary nebulae (PPN) appear as narrow collimated structures sometimes showing multiple, roughly aligned lobes. In addition, many PPN flows have been shown to have short acceleration times. In this paper we explore whether jet or ``bullet'' (a massive clump) models fit the observations of individual collimated lobes adequately by comparing simulations of both radiatively cooled (stable) jets and bullets. We find that the clump model is somewhat favored over jets because (1) it leads to greater collimation of outflows (2) it accounts better and more naturally for ring-like structures observed in the PPN CRL 618, and (3) it is more successful in reproducing the Hubble-flow character of observed kinematics in some PPN. In addition, bullets naturally account for observed multipolar flows, since the likely MHD launch mechanisms required to drive outflows make multiple non-aligned jets unlikely. Thus we argue that PPN outflows may be driven by explosive MHD launch mechanisms such as those discussed in the context of supernovae (SNe) and gamma-ray bursts(GRB).

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arXiv2007

Particle-in-cell simulations of fast collisionless reconnection in gamma-ray burst outflows

We present preliminary results of particle-in-cell simulations of collisionless magnetic reconnection for conditions that apply to magnetically dominated pair plasma jets such as those in gamma-ray bursts. We find similar behaviour to previous authors but with reduced reconnection efficiency. This results because we include the full electromagnetic field dynamically and allow electric field fluctuations to grow. Although weak, these fluctuations impede early x-point formation by periodically decelerating and even reversing the inflow.

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arXiv2006

Understanding the saturation of proton-driven Weibel instabilities and implications for astrophysics

The linear growth rate and saturation level of magnetic fields for Weibel instabilities driven by ion temperature anisotropy, defined as $α=(T_\perp/T_\parallel)-1$ where $T_\perp$ and $T_\parallel$ are ion temperatures perpendicular and parallel to the wave vector, are derived in the small $α$-limit. It is shown that the ratio of the saturated magnetic energy to the initial ion energy scales as the fourth power of the electron to ion mass ratio, $m/M$, for an initially unmagnetized plasma with $α\leq M/m$. Particle-in-cell simulations confirm the mass scaling and also show that the electron energy gain is of the same order of magnitude as the magnetic field energy. This implies that the Weibel instabilities cannot provide a faster-than-Coulomb collisionless mechanism to equilibrate ion-electron plasmas with ions initially much hotter than electrons, a key component in low-luminosity astrophysical accretion flows. The results here also show that the large $α$-limit formulae used in the study of magnetic field generation in collisionless shocks are only valid if $α\geq M/m$.

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arXiv2006

Outflow Driven Cavities: Numerical Simulations of Intermediaries of Protostellar Turbulence

We investigate the evolution of fossil cavities produced by extinct YSO jets and wide angle outflows. Fossil cavities are ellipsoidal or cylindrical shells of swept-up of ambient (molecular cloud) material moving at low velocities. The cavities form when the momentum in a YSO jet or wide angle outflow decays in time allowing the bowshock or swept-up shell to decelerate to velocities near the turbulent speed in the cloud. It has been suggested in previous studies that cavities provide efficient coupling between the jets/outflows and the cloud and, as such, are the agents by which cloud turbulence is can be re-energized. In this paper we carry forward a series of numerical simulations of jets and outflows whose momentum flux decrease in time. We compare simulations with decaying momentum fluxes to those with constant flux. We show that decaying flux models exhibit deceleration of the outflow head and backfilling via expansion off of the cavity walls. They also have lower density contrast, are longer lived and wider than their continuously driven counterparts. The simulations recover the basic properties of observed fossil cavities. In addition, we provide synthetic observations in terms of P-V diagrams which demonstrate that fossil cavities form both jets and wide angle outflows are characterized by linear "Hubble-law" expansions patterns superimposed on "spur" patterns indicative of the head of a bow shock.

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arXiv2006

Planetesimal growth in turbulent discs before the onset of gravitational instability

It is difficult to imagine a planet formation model that does not at some stage include a gravitationally unstable disc. Initially unstable gas-dust discs may form planets directly, but the high surface density required has motivated the alternative that gravitational instability occurs in a dust sub-layer only after grains have grown large enough by electrostatic sticking. Although such growth up to the instability stage is efficient for laminar discs, concern has mounted as to whether realistic disc turbulence catastrophically increases the settling time, thereby requiring additional processes to facilitate planet formation on the needed time scales. To evaluate this concern, we develop a model for grain growth that accounts for the influence of turbulence on the collisional velocity of grains and on the scale height of the dust layer. The relative effect on these quantities depends on the grain size. The model produces a disc-radius dependent time scale to reach a gravitationally unstable phase of planet formation. For a range of dust sticking and disc parameters, we find that for viscosity parameters $α\le 10^{-3}$, this time scale is short enough over a significant range in radii $R$ that turbulence does not catastrophically slow the early phases of planet formation, even in the absence of agglomeration enhancement agents like vortices.

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arXiv2006

Astrophysical Explosions Driven by a Rotating, Magnetized, Gravitating Sphere

We present the results of a numerical magnetohydrodynamic simulation that demonstrates a mechanism by which magnetic fields tap rotational energy of a stellar core and expel the envelope. Our numerical setup, designed to focus on the basic physics of the outflow mechanism, consists of a solid, gravitating sphere, which may represent the compact core of a star, surrounded by an initially hydrostatic envelope of ionized gas. The core is threaded by a dipolar magnetic field that also permeates the envelope. At the start of the simulation, the core begins to rotate at 10% of the escape speed. The magnetic field is sufficiently strong to drive a magneto-rotational explosion, whereby the entire envelope is expelled, confirming the expectation of analytical models. Furthermore, the dipolar nature of the field results in an explosion that is enhanced simultaneously along the rotation axis (a jet) and along the magnetic equator. While the initial condition is simplified, the simulation approximates circumstances that may arise in astrophysical objects such as Type II supernovae, gamma ray bursts, and proto-planetary nebulae.

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arXiv2006

Distinguishing Propagation vs. Launch Physics of Astrophysical Jets and the Role of Experiments

The absence of other viable momentum sources for collimated flows leads to the likelihood that magnetic fields play a fundamental role in jet launch and/or collimation in astrophysical jets. To best understand the physics of jets, it is useful to distinguish between the launch region where the jet is accelerated and the larger scales where the jet propagates as a collimated structure. Observations presently resolve jet propagation, but not the launch region. Simulations typically probe the launch and propagation regions separately, but not both together. Here, I identify some of the physics of jet launch vs. propagation and what laboratory jet experiments to date have probed. Reproducing an astrophysical jet in the lab is unrealistic, so maximizing the benefit of the experiments requires clarifying the astrophysical connection.

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arXiv2006

AGN Jet Mass Loading and Truncation by Stellar Winds

Active Galactic Nuclei can produce extremely powerful jets. While tightly collimated, the scale of these jets and the stellar density at galactic centers implies that there will be many jet/star interactions, which can mass-load the jet through stellar winds. Previous work employed modest wind mass outflow rates, but this does not apply when mass loading is provided by a small number of high mass-loss stars. We construct a framework for jet mass-loading by stellar winds for a broader spectrum of wind mass-loss rates than has been previously considered. Given the observed stellar mass distributions in galactic centers, we find that even highly efficient (0.1 Eddington luminosity) jets from supermassive black holes of masses $M_{BH} \la 10^4M_{\odot}$ are rapidly mass loaded and quenched by stellar winds. For $10^4 M_{\odot}<M_{BH}<10^8 M_{\odot}$, the quenching length of highly efficient jets is independent of the jet's mechanical luminosity. Stellar wind mass-loading is unable to quench efficient jets from more massive engines, but can account for the observed truncation of the inefficient M87 jet, and implies a baryon dominated composition on scales $\ga 2$ kpc therein even if the jet is initially pair plasma dominated.

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arXiv2006

Laboratory Plasma Dynamos, Astrophysical Dynamos, and Magnetic Helicity Evolution

The term ``dynamo'' means different things to the laboratory fusion plasma and astrophysical plasma communities. To alleviate the resulting confusion and to facilitate interdisciplinary progress, we pinpoint conceptual differences and similarities between laboratory plasma dynamos and astrophysical dynamos. We can divide dynamos into three types: 1. magnetically dominated helical dynamos which sustain a large scale magnetic field against resistive decay and drive the magnetic geometry toward the lowest energy state, 2. flow-driven helical dynamos which amplify or sustain large scale magnetic fields in an otherwise turbulent flow, and 3. flow-driven nonhelical dynamos which amplify fields on scales at or below the driving turbulence. We discuss how all three types occur in astrophysics whereas plasma confinement device dynamos are of the first type. Type 3 dynamos requires no magnetic or kinetic helicity of any kind. Focusing on type 1 and 2 dynamos, we show how different limits of a unified set of equations for magnetic helicity evolution reveal both types. We explicitly describe a steady-state example of a type 1 dynamo, and three examples of type 2 dynamos: (i) closed volume and time dependent; (ii) steady-state with open boundaries; (iii) time dependent with open boundaries.

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arXiv2005

Protostellar Jets and Turbulence in Molecular Clouds: The Role of Interactions

We present a series of numerical studies of the interaction of colliding radiative, hydrodynamic young stellar outflows. We study the effect of the collision impact parameter on the acceleration of ambient material and the degree to which the flow is isotropized by the collision as a mechanism for driving turbulence in the parent molecular cloud. Our results indicate that the high degrees of compression of outflow material, achieved through radiative shocks near the vertex of the interaction, prevents the redirected outflow from spraying over a large spatial region. Furthermore, the collision reduces the redirected outflow's ability to entrain and impart momentum into the ambient cloud. Consideration of the probabilities of outflow collisions leads us to conclude that individual low velocity fossil outflows are the principle coupling between outflows and the cloud.

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arXiv2005

The shock reprocessing model of electron acceleration in impulsive solar flares

We propose a new two-stage model for acceleration of electrons in solar flares. In the first stage, electrons are accelerated stochastically in a post-reconnection turbulent downflow. The second stage is the reprocessing of a subset of these electrons as they pass through a weakly compressive fast shock above the apex of the closed flare loop on their way to the chromosphere. We call this the "shock reprocessing" model. The model reproduces the energy dependent arrival time delays observed for both the pulsed and smooth components of impulsive solar flare x-rays with physically reasonable parameters for the downflow region. The model also predicts an emission site above the loop-top, as seen in the Masuda flare. The loop-top source distinguishes the shock reprocessing model from previous models. The model makes testable predictions for the energy dependence of footpoint pulse strengths and the location and spectrum of the loop-top emission, and can account for the observed soft-hard-soft trend in the spectral evolution of footpoint emission. Our model highlights the concept that reconnection is an acceleration environment rather than a single process. Which combination of processes operate may depend on the initial conditions that determine, for example, whether the reconnection downflow is turbulent. The shock reprocessing model comprises one such combination.

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arXiv2005

Extracting Rotational Energy in Supernova Progenitors: Transient Poynting Flux Growth vs. Turbulent Dissipation

Observational evidence for anisotropy in supernovae (SN) may signal the importance of angular momentum and differential rotation in the progenitors. Free energy in differential rotation and rotation can be extracted magnetically or via turbulent dissipation. The importance that magnetohydrodyamic jets and coronae may play in driving SN motivates understanding large scale dynamos in SN progenitors. We develop a dynamical large scale interface dynamo model in which the differential rotation and rotation deplete both through Poynting flux and turbulent diffusion. We apply the model to a differentially rotating core surrounded by a convection zone of a SN progenitor from a initial 15$M_\odot$ star. Unlike the Sun, the dynamo is transient because the differential rotation is primarily due to the initial collapse. Up to $\sim 10^{51}$erg can be drained into time-integrated Poynting flux and heat, the relative fraction of which depends on the relative amount of turbulence in the shear layer vs. convection zone and the fraction of the shear layer into which the magnetic field penetrates. Both sinks can help facilitate explosions and could lead to different levels of anisotropy and pulsar kicks. In all cases, the poloidal magnetic field is much weaker than the toroidal field, and the Poynting flux is lower than previous estimates which invoke the magnitude of the total magnetic energy. A signature of a large scale dynamo is that the oscillation of the associated Poynting flux on $\sim 1$ sec time scales, implying the same for the energy delivery to a SN.

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arXiv2005

On particle acceleration and trapping by Poynting flux dominated flows

Using particle-in-cell (PIC) simulations, we study the evolution of a strongly magnetized plasma slab propagating into a finite density ambient medium. Like previous work, we find that the slab breaks into discrete magnetic pulses. The subsequent evolution is consistent with diamagnetic relativistic pulse acceleration of \cite{liangetal2003}. Unlike previous work, we use the actual electron to proton mass ratio and focus on understanding trapping vs. transmission of the ambient plasma by the pulses and on the particle acceleration spectra. We find that the accelerated electron distribution internal to the slab develops a double-power law. We predict that emission from reflected/trapped external electrons will peak after that of the internal electrons. We also find that the thin discrete pulses trap ambient electrons but allow protons to pass through, resulting in less drag on the pulse than in the case of trapping of both species. Poynting flux dominated scenarios have been proposed as the driver of relativistic outflows and particle acceleration in the most powerful astrophysical jets.

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