Results Of Three-dimensional Hydrodynamical Simulations Research Articles

Delivery of Dust Particles from Protoplanetary Disks onto Circumplanetary Disks of Giant Planets

The principal regular satellites of gas giants are thought to be formed by the accumulation of solid materials in circumplanetary disks (CPDs). While there has been significant progress in the study of satellite formation in CPDs, details of the supply of satellite building blocks to CPDs remain unclear. We perform the orbital integration of solid particles in the protoplanetary disk (PPD) approaching a planet, considering the gas drag force by using the results of three-dimensional hydrodynamical simulations of a local region around the planet. We investigate the planetary mass dependence of the capture positions and the capture rates of dust particles accreting onto the CPD. We also examine the degree of dust retention in the accreting gas onto the CPD, which is important for determining the ratio of the dust-to-gas inflow rates, a key parameter in satellite formation. We find that the degree of dust retention increases with increasing planetary mass for a given dust scale height in the PPD. In the case of a small planet (M p = 0.2M Jup), most particles with insufficient initial altitudes in the PPD are isolated from the gas in the accreting region. On the other hand, in the case of a massive planet (M p = 1M Jup), dust particles can be coupled to the vertically accreting gas, even when the dust scale height is about 10%–30% of the gas scale height. The results of this study can be used for models of dust delivery and satellite formation in the CPDs of gas giants of various masses, including exoplanets.

Wind accretion in Cygnus X-1

Context. Cygnus X-1 is a black hole X-ray binary system in which the black hole captures and accretes gas from the strong stellar wind emitted by its supergiant O9.7 companion star. The irradiation of the supergiant star essentially determines the flow properties of the stellar wind and the X-ray luminosity from the system. The results of three-dimensional hydrodynamical simulations of wind-fed X-ray binary systems reported in recent work reveal that the ionizing feedback of the X-ray irradiation leads to the existence of two stable states with either a soft or a hard spectrum. Aims. We discuss the observed radiation of Cygnus X-1 in the soft and hard state in the context of mass flow in the corona and disk, as predicted by the recent application of a condensation model. Methods. The rates of gas condensation from the corona to the disk for Cygnus X-1 are determined, and the spectra of the hard and soft radiation are computed. The theoretical results are compared with the MAXI observations of Cygnus X-1 from 2009 to 2018. In particular, we evaluate the hardness-intensity diagrams (HIDs) for its ten episodes of soft and hard states which show that Cygnus X-1 is distinct in its spectral changes as compared to those found in the HIDs of low-mass X-ray binaries. Results. The theoretically derived values of photon counts and hardness are in approximate agreement with the observed data in the HID. However, the scatter in the diagram is not reproduced. Improved agreement could result from variations in the viscosity associated with clumping in the stellar wind and corresponding changes of the magnetic fields in the disk. The observed dipping events in the hard state may also contribute to the scatter and to a harder spectrum than predicted by the model.

Resonances in retrograde circumbinary discs

We analyse the interaction of an eccentric binary with a circular coplanar circumbinary disc that rotates in a retrograde sense with respect to the binary. In the circular binary case, no Lindblad resonances lie within the disc and no Lindblad resonant torques are produced, as was previously known. By analytic means, we show that when the binary orbit is eccentric, there exist components of the gravitational potential of the binary which rotate in a retrograde sense to the binary orbit and so rotate progradely with respect to this disc, allowing a resonant interaction to occur between the binary and the disc. The resulting resonant torques distinctly alter the disc response from the circular binary case. We describe results of three-dimensional hydrodynamic simulations to explore this effect and categorize the response of the disc in terms of modes whose strengths vary as a function of binary mass ratio and eccentricity. These mode strengths are weak compared to the largest mode strengths expected in the prograde case where the binary and disc rotate in the same sense. However, for sufficiently high binary eccentricity, resonant torques open a gap in a retrograde circumbinary disc, while permitting gas inflow on to the binary via gas streams. The inflow results in a time varying accretion rate on to the binary that is modulated over the binary orbital period, as was previously found to occur in the prograde case.

Hydrodynamic states in a wind-driven microtidal estuary (Alfacs Bay)

A conceptual model of the physical behavior of a shallow (6m deep) micro-tidal estuary (Alfacs Bay) is proposed, based on the interpretation of a field data set, and subsequently tested against the results of three-dimensional hydrodynamic simulations. At seasonal timescales, the buoyancy associated with freshwater inflows dominates the tidal forcing, yielding a strongly stratified two-layered system, with the surface and the bottom layers flowing in opposite directions (classical estuarine circulation). Wind controls the physical behavior of the bay at shorter (days to weeks) timescales. Three scenarios or states have been defined, depending on the strength of stratification relative to the predominant direction and magnitude of the wind forcing, parameterized through the Wedderburn number, W. For weak winds (scenario 1), with W≫1, mixing occurs as a consequence of stirring and convective cooling, and the mixed layer deepens slowly. For strong winds, with W≪1/2, mixing is fast and is largely driven by shear at the pycnocline. Two scenarios are further identified for W≪1/2 depending on the directionality of the winds: one for persistent NW winds (scenario 2) and another for diurnal SW winds (scenario 3). In scenario 2, the water is pushed laterally, overturning the stratification and generating transverse density gradients. In scenario 3, the estuarine circulation is weakened and even reversed, yielding strong longitudinal density gradients. The bay waters relax quickly (within 10–18h) back to the original state, after the wind ceases, as a result of the horizontal density gradients developed under wind forcing. Bay-sea exchange rates are shown to decrease significantly in the low W scenarios, the magnitudes of these changes being largely dependent on wind direction. These scenarios have important implications for the ecology of the bay, including the occurrence of phenomena such as harmful algal blooms.

GRAVITATIONAL DRAG ON A POINT MASS IN HYPERSONIC MOTION WITHIN A GAUSSIAN DISK

We develop an analytical model for the accretion and gravitational drag on a point mass that moves hypersonically in the midplane of a gaseous disk with a Gaussian vertical density stratification. Such a model is of interest for studying the interaction between a planet and a protoplanetary disk, as well as the dynamical decay of massive black holes in galactic nuclei. The model considers that the flow is ballistic, and gives fully analytical expressions for both the accretion rate onto the point mass, and the gravitational drag it suffers. The expressions are further simplified by taking the limits of a thick, and of a thin disk. The results for the thick disk reduce correctly to those for a uniform density environment (Cant\'o et al. 2011). We find that for a thin disk (small vertical scaleheight compared to the gravitational radius) the accretion rate is proportional to the mass of the moving object and to the surface density of the disk, while the drag force is independent of the velocity of the object. The gravitational deceleration of the hypersonic perturber in a thin disk was found to be independent of its parameters (i.e. mass or velocity) and depends only on the surface mass density of the disk. The predictions of the model are compared to the results of three-dimensional hydrodynamical simulations, with a reasonable agreement.

SPECTRUM ENERGY DISTRIBUTION AND SUBMILLIMETER IMAGE OF A ROTATING FIRST CORE

The first core is a hydrostatic object which is formed just before the stellar core formation and is one of the most important objects needed to understand the star formation process. However, it has not been observed yet. In this paper, we studied observational characteristics of the first core through radiative transfer calculation. We adopted the rotating first core model from the results of three-dimensional hydrodynamical simulations with a barotropic equation of state. The rotating first core has a radius of ~10 AU, a thickness of ~1 AU, and a mass of ~0.1-0.01M ?. The first core is covered with a thin (~0.001 AU) accretion shock layer and radiates mainly in the mid-infrared (mid-IR; a few ?m <? < 50 ?m). However, most mid-IR radiations are absorbed by the infalling envelope and this heating forms a 10 AU scale warm region (20-50 K) above the first core. As a result, the rotating first core is observed as an object that has cold spectral energy distribution (SED; the radiation of 12 ?m or 24 ?m is much darker than that of 70 ?m) and low luminosity (~0.1-0.01 L ?). The peak of SED corresponds to the blackbody radiation of 20-30?K. The first core increases the luminosity and temperature gradually. We show that it is possible to identify the rotating first core with the ALMA in full operation phase. In the face-on view, the dust thermal emissions from the first core are characterized by a central bright core with a radius of a few AU and a low surface brightness disk of 10-20 AU scale, in which a weak spiral structure is seen with a 001 resolution if the object is in a nearby star-forming region with a distance of d = 150 pc. Another way to identify this object is to detect submillimeter OH lines from the shocked layer.

Mass exchange in close binaries on the dynamical time scale during the stage of contracting onto the main sequence

We consider mass exchange in close binaries containing low-mass, fully convective components on the dynamical time scale. We present the results of three-dimensional hydrodynamical simulations of this process in close binaries contracting onto the main sequence (binary pre-MS stars). Our results suggest that some systems with superplanets could be formed by this process, which could be considered a special type of binary star. We have determined the ranges of the relative donor masses that allow the merger of the binary components and the formation of systems with superplanets or the survival of the binary during the mass exchange. These process should result in a deficit of binaries with similar component masses.

Three-dimensional simulations of type Ia supernovae

We present the results of three-dimensional hydrodynamical simulations of the subsonic thermonuclear burning phase in type Ia supernovae. The burning front model contains no adjustable parameters so that variations of the explosion outcome can be linked directly to changes in the initial conditions. In particular, we investigate the influence of the initial flame geometry on the explosion energy and find that it appears to be weaker than in 2D. Most importantly, our models predict global properties such as the produced nickel masses and ejecta velocities within their observed ranges without any fine tuning.

Newtonian hydrodynamics of the coalescence of black holes with neutron stars - IV. Irrotational binaries with a soft equation of state

We present the results of three-dimensional hydrodynamical simulations of the final stages of in-spiral in a black hole–neutron star binary, when the separation is comparable to the stellar radius. We use a Newtonian smooth particle hydrodynamics (SPH) code to model the evolution of the system, and take the neutron star to be a polytrope with a soft (adiabatic indices and equation of state and the black hole to be a Newtonian point mass. The only non-Newtonian effect we include is a gravitational radiation back reaction force, computed in the quadrupole approximation for point masses. We use irrotational binaries as initial conditions for our dynamical simulations, which are begun when the system is on the verge of initiating mass transfer and followed for approximately 23 ms. For all the cases studied we find that the star is disrupted on a dynamical time-scale, and forms a massive accretion torus around the spinning (Kerr) black hole. The rotation axis is clear of baryons (less than 10−5 M⊙ within 10°) to an extent that would not preclude the formation of a relativistic fireball capable of powering a cosmological gamma-ray burst. Some mass (the specific amount is sensitive to the stiffness of the equation of state) may be dynamically ejected from the system during the coalescence and could undergo r-process nucleosynthesis. We calculate the waveforms, luminosities and energy spectra of the gravitational radiation signal, and show how they reflect the global outcome of the coalescence process.

On the Stability and Evolution of Isolated Bok Globules

We present the results of three-dimensional hydrodynamic simulations of evolving isolated low-mass clouds and Bok globules, where the interstellar radiation field plays an important role in the chemical and thermal evolution. We consider two classes of cloud models: (1) clouds that are initially supported against gravitational collapse by thermal pressure alone, and (2) clouds that are initially supported by a mildly supersonic, complex internal velocity field (turbulence). The models are based on our earlier work with a smoothed particle hydrodynamics code, but upgraded to include a larger chemical network, refined chemical and dust properties, and different boundary conditions. The chemical network predicts the abundances of several key tracers of cloud structure and evolution, including C+, C I, and CO. There are two main purposes of this work. The first is to calculate the effective Jeans masses of isolated and externally heated clouds under a range of initial conditions, in order to delineate the physical parameters necessary for gravitational collapse and star formation to occur. The second is to calculate density, temperature, and chemical species profiles for comparison with observations. We consider clouds with masses in the range 8 ≤ M ≤ 70 M☉, radii in the range 0.34 ≤ R ≤ 1.8 pc, and initial number densities in the range 50 ≤ n ≤ 1000 cm-3, corresponding to low-mass Bok globules. We examine the evolution of both uniform-density and centrally condensed clouds, and clouds with and without a turbulent velocity field. The main results of our calculations are: 1. Clouds that proved to be gravitationally unstable collapsed to form cold, dense molecular cores, surrounded by warm, thermally supported, tenuous halos in which the trace species were in ionic or atomic form. 2. The evolution of the thermally supported clouds is driven in the first instance by a pressure gradient through the cloud that arises because of the attenuation of the interstellar radiation field. Subsequent thermal evolution leads to cooling of the gas, which can induce gravitational instability. 3. Initially turbulent clouds evolve through the dissipation of their internal kinetic energy and then follow evolutionary paths similar to those of the thermally supported clouds. The effect of the turbulence is to delay the collapse of the clouds until the turbulence decays, which occurs on a rapid timescale through shock dissipation, and to increase the stability of the cloud models by a small amount. 4. The collapsing dense cores that arise in the simulations have masses in the range 3 M 20 M☉, radii in the range 0.1 R 0.2 pc, and temperatures in the range 8 T 12 K. These align closely with the observationally derived properties of Bok globule cores. 5. The characteristics of the collapsing dense cores are similar to those of collapsing isothermal spheres, since the gas evolves toward a constant temperature of 10 K before collapse ensues, because of gas-dust thermal coupling.

Accretion Disk Evolution with Wind Infall. II. Results of Three‐dimensional Hydrodynamical Simulations with an Illustrative Application to Sagittarius A*

In the first paper of this series, using analytic tools, we examined how the evolution and structure of a massive accretion disk may be influenced significantly by the deposition of mass and angular momentum by an infalling Bondi-Hoyle wind. Such a mass influx impacts the long-term behavior of the disk by providing additional sources of viscosity and heating. In this paper, we make a significant improvement over this earlier work by incorporating the results of three-dimensional hydrodynamical simulations of the large-scale accretion from an ambient medium into the disk evolution equations developed previously. We discuss in detail two models, one with the axis of the disk parallel to and the second with the axis oriented perpendicular to the large scale Bondi-Hoyle flow. We find that the mass inflow rate onto the disk within logarithmic annuli is roughly constant with radius and that the impacting wind carries much less specific angular momentum than Keplerian. We also find, in general, that the infrared spectrum of a wind-fed disk system is steeper than that of a Shakura-Sunyaev configuration, due mainly to the dissipation of the wind's kinetic energy at the disk's surface. In applying our results to the Galactic center black hole candidate Sgr A*, accreting from nearby stellar winds, we demonstrate that a high wind inflow rate of the order of 10-4 M☉ yr-1 cannot be incorporated into a fossil disk without a significant dissipation of kinetic energy at all radii. Such a high dissipation would violate current infrared and near-infrared limits on the observed spectrum of Sgr A*.