arXiv subjects

Dongsu Ryu

Publications and source records attributed to Dongsu Ryu.

At least 145 records · Page 8Linked to original sources

arXiv1998

Magnetohydrodynamics of Cloud Collisions in a Multi-phase Interstellar Medium

We extend previous studies of the physics of interstellar cloud collisions by beginning investigation of the role of magnetic fields through 2D magnetohydrodynamic (MHD) numerical simulations. We study head-on collisions between equal mass, mildly supersonic diffuse clouds. We include a moderate magnetic field and two limiting field geometries, with the field lines parallel (aligned) and perpendicular (transverse) to the colliding cloud motion. We explore both adiabatic and radiative cases, as well as symmetric and asymmetric ones. We also compute collisions between clouds evolved through prior motion in the intercloud medium and compare with unevolved cases. We find that: In the (i) aligned case, adiabatic collisions, like their HD counterparts, are very disruptive, independent of the cloud symmetry. However, when radiative processes are taken into account, partial coalescence takes place even in the asymmetric case, unlike the HD calculations. In the (ii) transverse case, collisions between initially adjacent unevolved clouds are almost unaffected by magnetic fields. However, the interaction with the magnetized intercloud gas during the pre-collision evolution produces a region of very high magnetic energy in front of the cloud. In collisions between evolved clouds with transverse field geometry, this region acts like a ``bumper'', preventing direct contact between the clouds, and eventually reverses their motion. The ``elasticity'', defined as the ratio of the final to the initial kinetic energy of each cloud, is about 0.5-0.6 in the cases we considered. This behavior is found both in adiabatic and radiative cases.

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arXiv1998

A Divergence-Free Upwind Code for Multidimensional Magnetohydrodynamic Flows

A description is given for preserving ${\bmsy\nabla}\cdot{\vec B}=0$ in a magnetohydrodynamic (MHD) code that employs the upwind, Total Variation Diminishing (TVD) scheme and the Strang-type operator splitting for multi-dimensionality. The method is based on the staggered mesh technique to constrain the transport of magnetic field: the magnetic field components are defined at grid interfaces with their advective fluxes on grid edges, while other quantities are defined at grid centers. The magnetic field at grid centers for the upwind step is calculated by interpolating the values from grid interfaces. The advective fluxes on grid edges for the magnetic field evolution are calculated from the upwind fluxes at grid interfaces. Then, the magnetic field can be maintained with ${\bmsy\nabla}\cdot{\vec B}=0$ exactly, if this is so initially, while the upwind scheme is used for the update of fluid quantities. The correctness of the code is demonstrated through tests comparing numerical solutions either with analytic solutions or with numerical solutions from the code using an explicit divergence-cleaning method. Also the robustness is shown through tests involving realistic astrophysical problems.

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arXiv1998

Where's the Doughnut? LBV bubbles and Aspherical Fast Winds

In this paper we address the issue of the origin of LBV bipolar bubbles. Previous studies have explained the shapes of LBV nebulae, such as $η$ Car, by invoking the interaction of an isotropic fast wind with a previously deposited, slow aspherical wind (a ``slow torus''). In this paper we focus on the opposite scenario where an aspherical fast wind expands into a previously deposited isotropic slow wind. Using high resolution hydrodynamic simulations, which include the effects of radiative cooling, we have completed a series of numerical experiments to test if and how aspherical fast winds effect wind blown bubble morphologies. Our experiments explore a variety of models for the latitudinal variations of fast wind flow parameters. The simulations demonstrate that aspherical fast winds can produce strongly bipolar outflows. In addition the properties of outflows recover some important aspects of LBV bubbles which the previous "slow torus" models can not.

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arXiv1998

Cosmic Magnetic Fields in Large Scale Filaments and Sheets

We consider the possibility that cosmic magnetic field, instead of being uniformly distributed, is strongly correlated with the large scale structure of the universe. Then, the observed rotational measure of extra-galactic radio sources would be caused mostly by the clumpy magnetic field in cosmological filaments/sheets rather than by a uniform magnetic field, which was often assumed in previous studies. As a model for the inhomogeneity of the cosmological magnetic field, we adopt a cosmological hydrodynamic simulation, where the field is passively included, and can approximately represent the real field distribution with an arbitrary normalization for the field strength. Then, we derive an upper limit of the magnetic field strength by comparing the observed limit of rotational measure with the rotational measure expected from the magnetic field geometry in the simulated model universe. The resulting upper limit to the magnetic field in filaments and sheets is ${\bar B}_{fs} \la 1 μG$ which is $\sim10^3$ times higher than the previously quoted values. This value is close to, but larger than, the equipartition magnetic field strength in filaments and sheets. The amplification mechanism of the magnetic field to the above strength is uncertain. The implications of such a strength of the cosmic magnetic field are discussed.

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arXiv1998

Hydrodynamics of Cloud Collisions in 2D: The Fate of Clouds in a Multi-phase Medium

We have studied head-on collisions between equal-mass, mildly supersonic (Mach number 1.5) HI clouds, in a standard Two-phase ISM (T_cl = 74 K, n_cl = 22 cm^-3, χ= 100). We explore the role of various factors, including the radiative cooling parameter η= t_rad/t_coll (t_coll=R_c/v_c), evolutionary modifications on the cloud structure (by colliding clouds ``evolved'' through independent motion within the intercloud medium (WIM)), and the symmetry of the problem (by colliding initially identical clouds, evolved to different ages before impact). The presence of bow shocks and ram pressure from material in the cloud wake, developed during such evolution through the WIM, significantly alters these interactions with respect to the standard case of non-evolved clouds. In general, in our adiabatic collisions the clouds are disrupted and convert their gas into a few low density contrast clumps. By contrast, for symmetric radiative cases we find that the two clouds coalesce, with almost all the initial kinetic energy radiated away. On the other hand, for both adiabatic and radiative collisions, asymmetric collisions have a much greater tendency to disrupt the two clouds. Fragmentation of the clouds may occur, and instabilities are in general enhanced. In addition, radiative cooling is less efficient in our asymmetric interactions, so that those parts of the clouds that initially seem to merge are more likely to re-expand and fade into the WIM. Since the majority of real cloud collisions should be asymmetric for one reason or another, we conclude that most gasdynamical diffuse cloud collisions will be disruptive, at least in the absence of significant self-gravity or of a significant magnetic field.

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arXiv1997

Baryonic fraction in the cold plus hot dark matter universe

We report a study to constrain the fraction of baryonic matter in the cold plus hot dark matter (CHDM) universe by numerical simulations which include the hydrodynamics of baryonic matter as well as the particle dynamics of dark matter. Spatially flat, COBE-normalized CHDM models with the fraction of hot component $Ω_h\leq0.2$ are considered. We show that the models with $h/n/Ω_h=0.5/0.9/0.1$ and $0.5/0.9/0.2$ give a linear power spectrum which agrees well with observations. Here, $h$ is the Hubble constant in unit of $100~km/s/Mpc$ and $n$ is the spectral index of the initial power spectrum. Then, for the models with $h/n/Ω_h=0.5/0.9/0.2$ and baryonic fraction $Ω_b=0.05$ and 0.1 we calculate the properties of X-ray clusters, such as luminosity function, temperature distribution function, luminosity-temperature relation, histogram of gas to total mass ratio, and change of average temperature with redshift $z$. Comparison with the observed data of X-ray clusters indicates that the model with $Ω_b=0.05$ is preferred. The COBE-normalized CHDM model with $Ω_b>0.1$ may be ruled out by the present work, since it produces too many X-ray bright clusters.

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arXiv1997

The supergalactic structure and the origin of the highest energy cosmic rays

The recent discoveries of several reliable events of high energy cosmic rays at an energy above 10^20 eV raise questions about their path through the nearby universe. The two analyses of the Haverah Park data set and the Akeno data set appear to have an inconsistent pattern. Both data sets showed some measure of a correlation with the supergalactic plane, the locus of cosmologically nearby galaxies, radio galaxies and clusters of galaxies. We need a reasonable model of the true intergalactic magnetic field. Using recent cosmological simulations of structure formation in the universe, we estimate the magnetic fields which correspond to the upper limits in the Rotation Measure to distant radio sources. We estimate that the magnetic field strength in supergalactic sheets and filaments may be in the range of 0.1 to 1 muG. If such strengths are realized inside our Local Supercluster, this opens up the possibility to focus charged particles in the direction perpendicular to the supergalatic plane, analoguously but in the opposite direction to solar wind modulation. If focusing exists, it means that for all particles captured into the sheets, the dilution with distance d is 1/d instead of 1/d^2. This means in effect, that we may see sources to much larger distances than expected sofar. This effect is relevant only for energies for which the possible distances are smaller than the void scale of the cosmological galaxy distribution, in the range possibly up to 100 Mpc.

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arXiv1997

Cosmic Structure of Magnetic Fields

The simulations of the formation of cosmological structure allows to determine the spatial inhomogeneity of cosmic magnetic fields. Such simulations, however, do not give an absolute number for the strength of the magnetic field due to insufficient spatial resolution. Combining these simulations with observations of the Rotation Measure to distant radio sources allows then to deduce upper limits for the strength of the magnetic field. These upper limits are of order 0.2 - 2 muG along the filaments and sheets of the galaxy distribution. In one case, the sheet outside the Coma cluster, there is a definitive estimate of the strength of the magnetic field consistent with this range. Such estimates are almost three orders of magnitude higher than hitherto assumed usually. High energy cosmic ray particles can be either focussed or strongly scattered in such magnetic filaments and sheets, depending on the initial transverse momentum. The cosmological background in radio and X-ray wavelengths will have contributions from these intergalactic filaments and sheets, should the magnetic fields really be as high as 0.2 - 2 muG.

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arXiv1997

The Parker Instability under a Linear Gravity

A linear stability analysis has been done to a magnetized disk under a linear gravity. We have reduced the linearized perturbation equations to a second-order differential equation which resembles the Schrödinger equation with the potential of a harmonic oscillator. Depending on the signs of energy and potential terms, eigensolutions can be classified into ``continuum'' and ``discrete'' families. When magnetic field is ignored, the continuum family is identified as the convective mode, while the discrete family as acoustic-gravity waves. If the effective adiabatic index $γ$ is less than unity, the former develops into the convective instability. When a magnetic field is included, the continuum and discrete families further branch into several solutions with different characters. The continuum family is divided into two modes: one is the original Parker mode, which is a slow MHD mode modulated by the gravity, and the other is a stable Alfvén mode. The Parker modes can be either stable or unstable depending on $γ$. When $γ$ is smaller than a critical value $γ_{cr}$, the Parker mode becomes unstable. The discrete family is divided into three modes: a stable fast MHD mode modulated by the gravity, a stable slow MHD mode modulated by the gravity, and an unstable mode which is also attributed to a slow MHD mode. The unstable discrete mode does not always exist. Even though the unstable discrete mode exists, the Parker mode dominates it if the Parker mode is unstable. However, if $γ\ge γ_{cr}$, the discrete mode could be the only unstable one. When $γ$ is equal $γ_{cr}$, the minimum growth time of the unstable discrete mode is $1.3 \times 10^8$ years with a corresponding length scale of 2.4 kpc. It is suggestive that the corrugatory features seen in the Galaxy and external galaxies are related to the unstable discrete mode.

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arXiv1997

Numerical Simulations of Rotating Accretion Flows near a Black Hole

We present time-dependent solutions of thin, supersonic accretion flows near a black hole and compare them with analytical solutions. Such flows of inviscid, adiabatic gas are characterized by the specific angular momentum and the specific energy. We confirm that for a wide range of above parameters a stable standing shock wave with a vortex inside it forms close to the black hole. Apart from steady state solutions, we show the existence of non-steady solutions for thin accretion flows where the accretion shock is destroyed and re-generated periodically. The unstable behavior should be caused by dynamically induced instabilities, since inviscid, adiabatic gas is considered. We discuss possible relevance of the periodic behavior on quasi-periodic oscillations (QPOs) observed in galactic and extragalactic black hole candidates.

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arXiv1997

Cosmic Shock Waves on Large Scales of the Universe

In the standard theory of the large scale structure formation, matter accretes onto high density perturbations via gravitational instability. Collisionless dark matter forms caustics around such structures, while collisional baryonic matter forms accretion shocks which then halt and heat the infalling gas. Here, we discuss the characteristics, roles, and observational consequences of these accretion shocks.

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arXiv1997

The MHD Kelvin-Helmholtz Instability II: The Roles of Weak and Oblique Fields in Planar Flows

We have carried out high resolution MHD simulations of the nonlinear evolution of Kelvin-Helmholtz unstable flows in 2 1/2 dimensions. The modeled flows and fields were initially uniform except for a thin shear layer with a hyperbolic tangent velocity profile and a small, normal mode perturbation. The calculations consider periodic sections of flows containing magnetic fields parallel to the shear layer, but projecting over a full range of angles with respect to the flow vectors. They are intended as preparation for fully 3D calculations and to address two specific questions raised in earlier work: 1) What role, if any, does the orientation of the field play in nonlinear evolution of the MHD Kelvin-Helmholtz instability in 2 1/2 D. 2) Given that the field is too weak to stabilize against a linear perturbation of the flow, how does the nonlinear evolution of the instability depend on strength of the field. The magnetic field component in the third direction contributes only through minor pressure contributions, so the flows are essentially 2D. Even a very weak field can significantly enhance the rate of energy dissipation. In all of the cases we studied magnetic field amplification by stretching in the vortex is limited by tearing mode, ``fast'' reconnection events that isolate and then destroy magnetic flux islands within the vortex and relax the fields outside the vortex. If the magnetic tension developed prior to reconnection is comparable to Reynolds stresses in the flow, that flow is reorganized during reconnection. Otherwise, the primary influence on the plasma is generation of entropy. The effective expulsion of flux from the vortex is very similar to that shown by Weiss for passive fields in idealized vortices with large magnetic Reynolds numbers. We demonstrated that this

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arXiv1996

Accreting Matter around Clusters of Galaxies: One-Dimensional Considerations

During the formation of the large scale structure of the Universe, matter accretes onto high density peaks. Accreting collisionless dark matter (DM) forms caustics around them, while accreting collisional baryonic matter (BM) forms accretion shocks. The properties of the accreting matter depend upon the power spectrum of the initial perturbations on a given scale as well as the background expansion in a given cosmological model. In this paper, we have calculated the accretion of DM particles in one-dimensional spherical geometry under various cosmological models including the Einstein-de Sitter universe, the open universe with $Ω_o<1$, and the flat universe with $Ω_Λ=1-Ω_o$. A density parameter in the range $0.1\le Ω_o \le 1$ has been considered. The initial perturbation characterized by a point mass at the origin has been considered. Since the accretion shock of BM is expected to form close to the first caustic of DM, the properties of the accreting BM are common with those of the DM. Hence, the accretion calculations with DM particles have been used to find the position and velocity of the accretion shock and the cluster mass inside it. The average temperature of BM has been estimated by adopting simplifying assumptions. The velocity of the accreting BM around clusters of a given temperature is smaller in a universe with smaller $Ω_o$, but only by up to $\sim24\%$ in the models with $0.1\le Ω_o \le 1$. Thus, it would be difficult to use that quantity to discriminate among the cosmological models. However, the accretion velocity around clusters of a given mass or a given radius depends more sensitively on the cosmological models. It is smaller in a universe with smaller $Ω_o$ by up to $\sim41\%$ and $\sim65\%$, respectively. So, it can provide a better

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arXiv1996

The Protogalactic Origin for Cosmic Magnetic Fields

It is demonstrated that strong magnetic fields are produced from a zero initial magnetic field during the pregalactic era, when galaxies are first forming. Their development proceeds in three phases. In the first phase, weak magnetic fields are created by the Biermann battery mechanism, acting in shocked parts of the intergalactic medium where caustics form and intersect. In the second phase, these weak magnetic fields are amplified to strong magnetic fields by the Kolmogoroff turbulence endemic to gravitational structure formation of galaxies. During this second phase, the magnetic fields reach saturation with the turbulent power, but they are coherent only on the scale of the smallest eddy. In the third phase, the magnetic field strength increases to equipartition with the turbulent energy, and the coherence length of the magnetic fields increases to the scale of the largest turbulent eddy, comparable to the scale of the entire galaxy. The resulting magnetic field represents a galactic magnetic field of primordial origin. No further dynamo action is necessary, after the galaxy forms, to explain the origin of magnetic fields. However, the magnetic field may be altered by dynamo action once the galaxy and the galactic disk have formed. It is first shown by direct numerical simulations, that thermoelectric currentsassociated with the Biermann battery, build the field up from zero to $10^{-21}$ G in the regions about to collapse into galaxies, by $z\sim3$. For weak fields, in the absence of dissipation, the cyclotron frequency ${\bf ω_{cyc}}=e{\bf B } /m_H c $ and $ {\bf ω}/(1+ χ)$, where ${\bf ω= \nabla \times v }$ is the vorticity and $χ$ is the degree of ionization, satisfy the same equations, and initial conditions ${\bf ω_{cyc}=ω}=0$, so that, ${\bf ω_{cyc}}({\bf

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arXiv1996

Zero Energy Rotating Accretion Flows near a Black Hole

We characterize the nature of thin, axisymmetric, inviscid, accretion flows of cold adiabatic gas with zero specific energy in the vicinity of a black hole by the specific angular momentum. Using two-dimensional hydrodynamic simulations in cylindrical geometry, we present various regimes in which the accretion flows behave distinctly differently. When the flow has a small angular momentum $(λ\lsimλ_b)$, most of the material is accreted into the black hole forming a quasi-spherical flow or a simple disk-like structure around it. When the flow has a large angular momentum (typically, larger than the marginally bound value, $λ\gsimλ_{mb}$), almost no accretion into the black hole occurs. Instead, the flow produces a stable standing shock with one or more vortices behind it and is deflected away at the shock as a conical outgoing wind of higher entropy. If the flow has an angular momentum somewhat smaller than $λ_{mb}$ $(λ_{u}\lsimλ\lsimλ_{mb})$, a fraction (typically, $5-10$\%) of the incoming material is accreted into the black hole, but the the flow structure formed is similar to that as for $λ\gsimλ_{mb}$. Some of the deflected material is accreted back into the black hole, while the rest is blown away as an outgoing wind. These two cases with $λ\gsimλ_u$ correspond those studied in the previous works by Molteni, Lanzafame, \& Chakrabarti (1994) and Ryu \etal (1995). However, the flow with an angular momentum close to the marginally stable value $(λ_{ms})$ is found to be unstable. More specifically, if $λ_b\lsimλ\simλ_{ms}\lsimλ_u$, the flow displays a distinct periodicity in the sense that the inner part of the disk is built and

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arXiv1996

The Magnetohydrodynamics of Supersonic Gas Clouds: MHD Cosmic Bullets and Wind-Swept Clumps

We report an extensive set of two-dimensional MHD simulations exploring the role and evolution of magnetic fields in the dynamics of supersonic plasma clumps. We examine the influence of both ambient field strength and orientation on the problem. Of those two characteristics, field orientation is far more important in the cases we have considered with $β_0 = p_g/p_b \ge 1$. That is due to the geometry-sensitivity of field stretching/amplification from large-scale shearing motions around the bullet. When the ambient magnetic field is transverse to the bullet motion, even a very modest field, well below equipartition strength, can be amplified by field line stretching around the bullet within a couple of bullet crushing times so that Maxwell stresses become comparable to the ram pressure associated with the bullet motion. The possibility is discussed that those situations might lead to large, induced electric potentials capable of accelerating charged particles. When the ambient field is aligned to the bullet motion, on the other hand, reconnection- prone topologies develop that shorten the stretched field and release much of the excess energy it contains. In this geometry, the Maxwell stresses on the bullet never approach the ram pressure level. In both cases, however, the presence of a field with even moderate initial strength acts to help the flow realign itself around the bullet into a smoother, more laminar form. That reduces bullet fragmentation tendencies caused by destructive instabilities. Eddies seem less effective at field amplification than flows around the bullet, because fields within eddies tend to be expelled to the eddy perimeters. Similar effects cause the magnetic field within the bullet itself to be reduced below its initial value over time.

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arXiv1996

Numerical Simulations of Standing Shocks in Accretion Flows around Black Holes: A Comparative Study

We compare the results of numerical simulations of thin and quasi-spherical (thick) accretion flows with existing analytical solutions. We use a Lagrangian code based on the Smooth Particle Hydrodynamics (SPH) scheme and an Eulerian finite difference code based on the Total Variation Diminishing (TVD) scheme. In one-dimensional thin flows, the results of the simulations, with or without shocks, agree very well with each other and with analytical solutions. In two-dimensional thick flows, the general features, namely the locations and strengths of centrifugal and turbulent pressure supported shocks, centrifugal barriers, and the funnel walls which are expected from analytical models, agree very well, though the details vary. Generally speaking, the locations of the shocks may be better obtained by SPH since the angular momentum is strictly preserved in SPH, but the shocks themselves are better resolved by TVD. The agreement of these code test results with analytical solutions provides us with confidence to apply these codes to more complex problems which we will discuss elsewhere.

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arXiv1995

Supersonic Collisions between Two Gas Streams

A star around a massive black hole can be disrupted tidally by the gravity of the black hole. Then, its debris may form a precessing stream which may even collide with itself. In order to understand the dynamical effects of the stream-stream collision on the eventual accretion of the stellar debris onto the black hole, we have studied how gas flow behaves when the outgoing stream collides supersonically with the incoming stream. We have investigated the problem analytically with one-dimensional plane-parallel streams and numerically with more realistic three-dimensional streams. A shock formed around the contact surface converts the bulk of the orbital streaming kinetic energy into thermal energy. In three-dimensional simulations, the accumulated hot post-shock gas then expands adiabatically and drives another shock into the low density ambient region. Through this expansion, thermal energy is converted back to the kinetic energy associated with the expanding motion. Thus, in the end, only a small fraction of the orbital kinetic energy is actually converted to the thermal energy, while most of it is transferred to the kinetic energy of the expanding gas. Nevertheless the collision is effective in circularizing the debris orbit, because the shock efficiently transforms the ordered motion of the streams into the expanding motion in directions perpendicular to the streams. The circularization efficiency decreases, if two colliding streams have a large ratio of cross sections and a large density contrast. But even in such cases, the main shock extends beyond the overlapping contact surface and the high pressure region behind the shock keeps the stream of the larger cross section from passing freely. Thus the stream-stream collisions are still expected to circularize the stellar debris rather efficiently, unless the ratio

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