arXiv subjects

Eve C. Ostriker

Publications and source records attributed to Eve C. Ostriker.

159 records · Page 9Linked to original sources

arXiv2002

Developing Diagnostics of Molecular Clouds Using Numerical MHD Simulations

An important aspect of astrophysical MHD turbulence research is developing diagnostics to connect simulations with the observable universe. Turbulent systems are by definition structurally complex in all fluid variables (density, velocity, and magnetic field), such that they must be described statistically. By developing and applying diagnostic tools to simulation data, it is possible to interpret empirical laws for the statistical properties of observed systems in terms of fundamental dynamical processes, and to identify and calibrate robust probes of physical parameters that cannot be measured directly. Using several different examples, I describe how structural diagnostic analyses have already yielded significant insights into the nature of turbulent molecular clouds. I review results from several different groups, and discuss directions for future diagnostics to enhance our understanding of cloud structure and constrain models of the evolutionary course that governs star formation.

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arXiv2001

Formation and Fragmentation of Gaseous Spurs in Spiral Galaxies

Intermediate-scale spurs are common in spiral galaxies, but perhaps most distinctively evident in a recent HST image of M51 (Scoville & Rector 2001). We investigate, using time-dependent numerical MHD simulations, how such spurs could form (and subsequently fragment) from the interaction of a gaseous ISM with a stellar spiral arm. We model the gaseous medium as a self-gravitating, magnetized, differentially-rotating, razor-thin disk. The basic flow shocks and compresses as it passes through a local segment of a tightly-wound, trailing stellar spiral arm, modeled as a rigidly-rotating gravitational potential. We first construct 1D profiles for flows with spiral shocks. When the post-shock Toomre parameter Q_sp is sufficiently small, self-gravity is too large for one-dimensional steady solutions to exist. The critical values of Q_sp are 0.8, 0.5, and 0.4 for our models with zero, sub-equipartition, and equipartition magnetic fields, respectively. We then study the growth of self-gravitating perturbations in fully 2D flows, and find that spur-like structures rapidly emerge in our magnetized models. We associate this gravitational instability with the magneto-Jeans mechanism, in which magnetic tension forces oppose the Coriolis forces. The shearing and expanding velocity field shapes the condensed material into spurs as it flows downstream from the arms. Although we find swing amplification can help form spurs when the arm-interarm contrast is moderate, unmagnetized systems that are quasi-axisymmetrically stable are generally also stable to nonaxisymmetric perturbations, suggesting that magnetic effects are essential. In nonlinear stages, the spurs in our models undergo fragmentation to form 4\times 10^6 solar mass clumps, which we suggest could evolve into bright arm/interarm HII regions as seen in spiral galaxies.

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arXiv2001

Suppression of Gravitational Structure Formation by Cosmological Accretion Heating

As increasingly precise information about the spectrum of the cosmic microwave background fluctuations is gathered with balloon and satellite experiments, interest has grown in foreground sources of opacity affecting these observations. One potentially important source is electron scattering produced by a post-recombination luminosity source, which would significantly attenuate the higher harmonics in the spectrum. If such an ionization source exists, then it would also heat the universe, hence increasing the Jeans mass and suppressing early gravitational structure formation. Here we consider the effects of such heating. We concentrate on one type of ionization source: luminosity generated by accretion onto primordial compact objects. We show that if such objects generate enough luminosity to affect the CMB power spectrum, then they would produce enough heat to prevent the formation of 1 sigma collapsed objects until redshifts of about 5, significantly less than the redshift at which baryonic collapse could otherwise occur. Such processes would leave signatures detectable by upcoming instruments such as NGST, SIRTF, and SWIFT.

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arXiv2001

Amplification, Saturation, and Q Thresholds for Runaway: Growth of Self-Gravitating Structures in Models of Magnetized Galactic Gas Disks

We investigate the susceptibility of gaseous, magnetized galactic disks to formation of self-gravitating condensations using two-dimensional, local models. We focus on two issues: (1) determining the threshold condition for gravitational runaway, taking into account nonlinear effects, and (2) distinguishing the magneto-Jeans instability (MJI) that arises under inner-galaxy rotation curves from the modified swing amplification (MSA) that arises under outer-galaxy rotation curves. For axisymmetric density fluctuations, instability is known to require a Toomre parameter Q<1. For nonaxisymmetric fluctuations, any nonzero shear $q \equiv -d\ln Ω/d \ln R$ winds up wavefronts such that in linear theory amplification saturates. Any Q threshold for nonaxisymmetric gravitational runaway must originate from nonlinear effects. We use numerical magnetohydrodynamic simulations to demonstrate the anticipated threshold phenomenon, to analyze the nonlinear processes involved, and to evaluate the critical value $Q_c$ for stabilization. We find $Q_c \sim 1.2-1.4$ for a wide variety of conditions, with the largest values corresponding to nonzero but subthermal mean magnetic fields. Our findings for $Q_c$ are similar to those inferred from thresholds for active star formation in the outer regions of spiral galaxies. MJI is distinct from MSA in that opposition to Coriolis forces by magnetic tension, rather than cooperation of epicyclic motion with kinematic shear, enables nonaxisymmetric density perturbations to grow. We suggest that under low-shear inner-disk conditions, $Q_c$ will be larger than that in outer disks by a factor $\sim (v_A/q c_s)^{1/2}$, where $v_A$ and $c_s$ are the respective Alfven and sound speeds.

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arXiv2001

Hydrodynamical Simulations of Jet- and Wind-driven Protostellar Outflows

We present two-dimensional hydrodynamical simulations of both jet- and wind-driven models for protostellar outflows in order to make detailed comparisons to the kinematics of observed molecular outflows. Comparing the different simulations with observations, we find that some outflows, e.g., HH 212, show features consistent with the jet-driven model, while others, e.g., VLA 05487, are consistent with the wind-driven model.

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arXiv2001

A ballistic bow shock model for jet-driven protostellar outflow shells

We analyze the dynamics of the shell produced when a bow shock from a collimated jet propagates into the surrounding medium. Under interstellar conditions, the shock is radiative, and a ballistic approximation for the shell flow is appropriate beyond the working surface where the jet impacts its surroundings. The solution is then determined by the ambient and jet densities and velocities and by the momentum impulse applied in the working surface. Using estimates for these impulses (confirmed by separate numerical simulations), we obtain solutions for the shell structure, and for the range of velocities in the shell at any point. We provide predictions for the position-velocity and mass-velocity relations expected for plane-of-sky bow-shock shells, and for the bulk shell properties. In a companion paper, we show that these analytic solutions are in excellent agreement with the results of direct numerical simulations. We argue that classical molecular (CO) outflows cannot be purely jet-driven, because the bow-shock shell solutions are much too elongated compared with observations. Finally, we suggest that the ``spur'' structures seen in position-velocity diagrams of observed molecular outflows are the manifestation of internal bow shocks which may be fit with our simple dynamical models.

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arXiv2001

Hyper- and suspended-accretion states of rotating black holes and the durations of gamma-ray bursts

We analyze the temporal evolution of accretion onto rotating black holes subject to large-scale magnetic torques. Wind torques alone drive a disk towards collapse in a finite time $\sim t_{ff} E_k/E_B$, where $t_{ff}$ is the initial free-fall time and $E_k/E_B$ is the ratio of kinetic-to-poloidal magnetic energy. Additional spin-up torques from a rapidly rotating black hole can arrest the disk's inflow. We associate short/long gamma-ray bursts with hyperaccretion/suspended-accretion onto slowly/rapidly spinning black holes. This model predicts afterglow emission from short bursts, and may be tested by HETE-II.

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arXiv2000

Density, Velocity, and Magnetic Field Structure in Turbulent Molecular Cloud Models

We use 3D numerical MHD simulations to follow the evolution of cold, turbulent, gaseous systems with parameters representing GMC conditions. We study three cloud simulations with varying mean magnetic fields, but identical initial velocity fields. We show that turbulent energy is reduced by a factor two after 0.4-0.8 flow crossing times (2-4 Myr), and that the magnetically supercritical cloud models collapse after ~6 Myr, while the subcritical cloud does not collapse. We compare density, velocity, and magnetic field structure in three sets of snapshots with matched Mach numbers. The volume and column densities are both log-normally distributed, with mean volume density a factor 3-6 times the unperturbed value, but mean column density only a factor 1.1-1.4 times the unperturbed value. We use a binning algorithm to investigate the dependence of kinetic quantities on spatial scale for regions of column density contrast (ROCs). The average velocity dispersion for the ROCs is only weakly correlated with scale, similar to the mean size-linewidth relation for clumps within GMCs. ROCs are often superpositions of spatially unconnected regions that cannot easily be separated using velocity information; the same difficulty may affect observed GMC clumps. We analyze magnetic field structure, and show that in the high density regime, total magnetic field strengths increase with density with logarithmic slope 1/3 -2/3. Mean line-of-sight magnetic field strengths vary widely across a projected cloud, and do not correlate with column density. We compute simulated interstellar polarization maps at varying orientations, and determine that the Chandrasekhar-Fermi formula multiplied by a factor ~0.5 yields a good estimate of the plane-of sky magnetic field strength provided the dispersion in polarization angles is < 25 degrees.

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arXiv2000

Magnetohydrodynamic Instabilities in Shearing, Rotating, Stratified Winds and Disks

We investigate shear and buoyancy instabilities in radially stratified, magnetized, cylindrical flows, for application to magnetocentrifugally driven winds and to magnetized accretion disks. We identify and study nine principal types of instabilities/overstabilities. When magnetic fields are predominantly toroidal, as in protostellar winds, the system exhibits axisymmetric fundamental (FM) and toroidal resonance modes, axisymmetric and non-axisymmetric toroidal buoyancy (TB) modes, and non-axisymmetric magnetorotational instability (MRI) modes. Winds with sufficiently steep field gradients are unstable to the FM, which promotes narrow dense jets in the centers of wider winds. The TB modes promote small-scale radial mixing. The MRIs have very low growth rates under low-temperature wind conditions. The stabilization of buoyancy modes by shear, and of MRIs by compressibility, may be important in allowing protostellar winds to propagate over vast distances in space. When magnetic fields are predominantly poloidal, as in winds close to their source or in astrophysical disks, the system exhibits axisymmetric Balbus-Hawley (BH), poloidal buoyancy (PB), non-axisymmetric geometric buoyancy (GPB), and poloidal resonance modes. The BH mode has the fastest growth rate. The PB mode promotes radial mixing on small scales. The GPB mode at high m is readily stabilized by shear. We extend previous MRI analyses to focus on compressibility. We introduce a ``coherent wavelet'' technique to derive closed-form expressions for instantaneous instability criteria, growth rates, and net amplification factors for non-axisymmetric MRIs in compressible flows with both poloidal and toroidal fields. We confirm that these are in excellent agreement with the results of shearing-sheet temporal integrations.

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arXiv1998

Kinetic and Structural Evolution of Self-gravitating, Magnetized Clouds: 2.5-Dimensional Simulations of Decaying Turbulence

The molecular component of the Galaxy is comprised of turbulent, magnetized clouds, many of which are self-gravitating and form stars. To understand how these clouds' evolution may depend on their level of turbulence, mean magnetization, and degree of self-gravity, we perform a survey of direct numerical MHD simulations in 2.5 dimensions. Two of our independent survey parameters allow us to model clouds which either meet or fail conditions for magneto-Jeans stability and magnetic criticality. Our third survey parameter allows us to initiate turbulence of either sub- or super-Alfvénic amplitude. We evaluate the times for each cloud model to become gravitationally bound, and measure each model's kinetic energy loss over the fluid flow crossing time. We compare the evolution of density and magnetic field structural morphology, and quantify the differences in the density contrast generated by internal stresses, for models of differing mean magnetization. We find that the values of (c_s/v_A)^2 and L/L_Jeans, but not the initial Mach number v/c_s, determine the time for cloud gravitational binding and collapse. We find, contrary to some previous expectations, less than a factor of two difference between turbulent decay times for models with varying magnetic field strength. In all models, we find turbulent amplification in the magnetic field strength, with the turbulent magnetic energy between 25-60% of the turbulent kinetic energy after one flow crossing time. We find that for non-self-gravitating stages of evolution, the mass-averaged density contrast magnitudes are in the range 0.2-0.5; we note that only the more strongly-magnetized models appear consistent with estimates of clump/interclump density contrasts inferred in Galactic GMCs.

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arXiv1998

Dynamical Friction in a Gaseous Medium

Using time-dependent linear perturbation theory, we evaluate the dynamical friction force on a massive perturber M_p traveling at velocity V through a uniform gaseous medium of density rho_0 and sound speed c_s. This drag force acts in the direction -\hat V, and arises from the gravitational attraction between the perturber and its wake in the ambient medium. For supersonic motion (M=V/c_s>1), the enhanced-density wake is confined to the Mach cone trailing the perturber; for subsonic motion (M<1), the wake is confined to a sphere of radius c_s t centered a distance V t behind the perturber. Inside the wake, surfaces of constant density are hyperboloids or oblate spheroids for supersonic or subsonic perturbers, respectively, with the density maximal nearest the perturber. The dynamical drag force has the form F_df= - I 4π(G M_p)^2ρ_0/V^2. We evaluate I analytically; its limits are I\to M^3/3 for M<<1 and I\to ln(Vt/r_min) for M>>1. We compare our results to the Chandrasekhar formula for dynamical friction in a collisionless medium, noting that the gaseous drag is generally more efficient when M>1 but less efficient when M<1. To allow simple estimates of orbit evolution in a gaseous protogalaxy or proto-star cluster, we use our formulae to evaluate the decay times of a (supersonic) perturber on a near-circular orbit in an isothermal ρ\propto r^{-2} halo, and of a (subsonic) perturber on a near-circular orbit in a constant-density core. We also mention the relevance of our calculations to protoplanet migration in a circumstellar nebula.

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arXiv1998

Dissipation in Compressible MHD Turbulence

We report results of a three dimensional, high resolution (up to 512^3) numerical investigation of supersonic compressible magnetohydrodynamic turbulence. We consider both forced and decaying turbulence. The model parameters are appropriate to conditions found in Galactic molecular clouds. We find that the dissipation time of turbulence is of order the flow crossing time or smaller, even in the presence of strong magnetic fields. About half the dissipation occurs in shocks. Weak magnetic fields are amplified and tangled by the turbulence, while strong fields remain well ordered.

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arXiv1997

Theory of Protostellar Accretion

This paper reviews current theoretical work on the various stages of accretion in protostars, and the relationship of these ideal stages to the spectral classes of observed protostellar systems. I discuss scaling relationships that have been obtained for models of pre-stellar cores as they evolve by ambipolar diffusion toward a central singularity, and expectations for the dynamical evolution as the cores collapse radially to form a rotating disk. I summarize work that suggests accretion in T Tauri systems may be limited by ionization to disk surface layers, and implications for the variation of the disk accretion rate with radius. Finally, I describe models for the asymptotic structure of free magnetocentrifugal winds, and show that the constant-density surfaces in these winds may be strongly collimated even when the streamlines are not.

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arXiv1997

Self-Similar Magnetocentrifugal Disk Winds with Cylindrical Asymptotics

We construct a two-parameter family of models for self-collimated, radially self-similar magnetized outflows from accretion disks. A flow at zero initial poloidal speed leaves the surface of a rotating disk and is accelerated and redirected toward the pole by helical magnetic fields threading the disk. At large distances from the disk, the flow streamlines asymptote to wrap around the surfaces of nested cylinders. In constrast to previous disk wind modeling, we have explicitly implemented the cylindrical asymptotic boundary condition to examine the consequences for flow dynamics. The solutions are characterized by the logarithmic gradient of the magnetic field strength and the ratios between the footpoint radius R_0 and asymptotic radius R_1 of streamlines; the Alfven radius must be found as an eigenvalue. Cylindrical solutions require the magnetic field to drop less steeply than 1/R. We find that the asymptotic poloidal speed on any streamline is typically just a few tenths of the Kepler speed at the corresponding disk footpoint. The asymptotic toroidal Alfven speed is, however, a few times the footpoint Kepler speed. We discuss the implications of the models for interpretations of observed optical jets and molecular outflows from young stellar systems. We suggest that the difficulty of achieving strong collimation in vector velocity simultaneously with a final speed comparable to the disk rotation rate argues against isolated jets and in favor of models with broader winds.

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arXiv1996

Can Nonlinear Hydromagnetic Waves Support a Self-Gravitating Cloud?

Using self-consistent magnetohydrodynamic (MHD) simulations, we explore the hypothesis that nonlinear MHD waves dominate the internal dynamics of galactic molecular clouds. We employ an isothermal equation of state and allow for self-gravity. We adopt ``slab-symmetry,'' which permits motions $\bf v_\perp$ and fields $\bf B_\perp$ perpendicular to the mean field, but permits gradients only parallel to the mean field. The Alfvén speed $v_A$ exceeds the sound speed $c_s$ by a factor $3-30$. We simulate the free decay of a spectrum of Alfvén waves, with and without self-gravity. We also perform simulations with and without self-gravity that include small-scale stochastic forcing. Our major results are as follows: (1) We confirm that fluctuating transverse fields inhibit the mean-field collapse of clouds when the energy in Alfvén- like disturbances remains comparable to the cloud's gravitational binding energy. (2) We characterize the turbulent energy spectrum and density structure in magnetically-dominated clouds. The spectra evolve to approximately $v_{\perp,\,k}^2\approx B_{\perp,\,k}^2/4πρ\propto k^{-s}$ with $s\sim 2$, i.e. approximately consistent with a ``linewidth-size'' relation $σ_v(R) \propto R^{1/2}$. The simulations show large density contrasts, with high density regions confined in part by the fluctuating magnetic fields. (3) We evaluate the input power required to offset dissipation through shocks, as a function of $c_s/v_A$, the velocity dispersion $σ_v$, and the scale $λ$ of the forcing. In equilibrium, the volume dissipation rate is $5.5(c_s/v_a)^{1/2} (λ/L)^{-1/2}\times ρσ_v^3/L$, for a cloud of linear size $L$ and density $ρ$. (4) Somewhat speculatively, we apply our results to a ``typical'' molecular cloud. The mechanical power input required

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