共查询到20条相似文献，搜索用时 31 毫秒
1.
The effect of artificial viscosity on a hydrodynamic simulation of stellar radial pulsation is examined for the purpose of studying the dependence of the modal coupling on the sharpness of the shock front. The model used in our study is a 2.5-d first-overtone pulsator in the Cepheid instability strip. By increasing the parameters in the von Neumann–Richtmyer formula of artificial viscosity, we obtained a low growth rate together with a small pulsation amplitude. The time-scale related to pulsation mode-switching is also increased. 相似文献
2.
We report on an application of gas-kinetic BGK scheme to the computation of turbulent compressible convection in the stellar interior. After incorporating the Sub-grid Scale (SGS) turbulence model into the BGK scheme, we tested the effects of numerical parameters on the quantitative relationships among the thermodynamic variables, their fluctuations and correlations in a very deep, initially gravity-stratified stellar atmosphere. Comparison indicates that the thermal properties and dynamic properties are dominated by different aspects of numerical models separately. An adjustable Deardorff constant in the SGS model cμ = 0.25 and an amplitude of artificial viscosity in the gas-kinetic BGK scheme C2 = 0 are appropriate for the current study. We also calculated the density-weighted auto-and cross-correlation functions in Xiong's turbulent stellar convection theory based on which the gradient type of models of the non-local transport and the anisotropy of the turbulence were preliminarily studied. No universal relations or con-stant parameters were found for these models. 相似文献
3.
Annabel Cartwright Dimitris Stamatellos Anthony P. Whitworth 《Monthly notices of the Royal Astronomical Society》2009,395(4):2373-2380
A simple algorithm is presented which generates a lattice-like, regular spacing of smoothed particle hydrodynamics (SPH) particles in discs, with any prescribed density gradient. Using this for comparison we demonstrate the effect of Poisson noise on SPH estimates of density, pressure and viscous forces when calculated using randomly distributed particles. The standard deviation of density and pressure is typically increased by greater than an order of magnitude. In a disc with a Keplerian velocity profile, the effectiveness of the Balsara switch in reducing the shear component of SPH artificial viscosity is greatly enhanced when the particles are properly spaced, reducing the magnitude of viscosity by two orders of magnitude. Noise problems are exacerbated, not removed, by increasing the numbers of SPH particles, if the number of neighbours used is kept constant. However, comparison of the evolution of a disc created using perfectly spaced particles and a disc with identical parameters but randomly placed particles, reveals very similar results. Although there are subtle differences in the evolution, and the smooth disc takes longer to begin developing structure, in both cases the identical number of objects is created by gravitational collapse. SPH disc simulations do not depend on initial density disturbances to evolve objects by gravitational collapse, which gives added credence to the validity of the results. It also appears that complicated disc settling procedures are unnecessary. 相似文献
4.
E. Brannigan M. Takami A. Chrysostomou J. Bailey 《Monthly notices of the Royal Astronomical Society》2006,367(1):315-322
We demonstrate that artificial bipolar structure can be detected using spectro-astrometry when the point spread function (PSF) of a point source suffers distortion in a relatively wide slit. Spectro-astrometry is a technique which allows us to probe the spatial structure of astronomical sources on milliarcsec (mas) scales making it possible to detect close binaries and to study the geometry and kinematics of outflowing gas on scales much smaller than the seeing or the diffraction limit of the telescope. It is demonstrated that a distorted PSF, caused by tracking errors of the telescope or unstable active optics during an exposure, can induce artificial signals which may be misinterpreted as a real spectro-astrometric signal. Using simulations, we show that these may be minimized by using a narrow slit relative to the seeing. Spectra should be obtained at antiparallel slit position angles (e.g. 0° and 180°) for comparison in order to allow artificial signatures to be identified. 相似文献
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Rebecca G. Martin J. E. Pringle Christopher A. Tout 《Monthly notices of the Royal Astronomical Society》2007,381(4):1617-1624
We consider the shape of an accretion disc whose outer regions are misaligned with the spin axis of a central black hole and calculate the steady state form of the warped disc in the case where the viscosity and surface densities are power laws in the distance from the central black hole. We discuss the shape of the resulting disc in both the frame of the black hole and that of the outer disc. We note that some parts of the disc and also any companion star maybe shadowed from the central regions by the warp. We compute the torque on the black hole caused by the Lense–Thirring precession, and hence compute the alignment and precession time-scales. We generalize the case with viscosity and hence surface density independent of radius to more realistic density distributions for which the surface density is a decreasing function of radius. We find that the alignment time-scale does not change greatly but the precession time-scale is more sensitive. We also determine the effect on this time-scale if we truncate the disc. For a given truncation radius, the time-scales are less affected for more sharply falling density distributions. 相似文献
7.
With the high signal-to-noise ratio spectra, we obtained Si abundances of 22 extrasolar planet host stars, and discussed some
constraints on the planet formation. Using our silicon abundance results and other authors’ Si abundance studies about planets-harboring
stars, we investigated the correlation between the dynamical properties and the silicon abundance. We propose a hypothesis
that higher primordial metallicity in the host stars’ birth cloud with higher abundance of Si will make the cloud more sticky
to bypass the time scale restriction in planet formation and easier to form the planets. 相似文献
8.
M. R. Truss J. R. Murray G. A. Wynn R. G. Edgar 《Monthly notices of the Royal Astronomical Society》2000,319(2):467-476
We present a numerical scheme for the evolution of an accretion disc through a dwarf nova outburst. We introduce a time-varying artificial viscosity into an existing smoothed particle hydrodynamics code optimized for two- and three-dimensional simulations of accretion discs. The technique gives rise to coherent outbursts and can easily be adapted to include a complete treatment of thermodynamics. We apply a two-dimensional isothermal scheme to the system SS Cygni, and present a wide range of observationally testable results. 相似文献
9.
Adrianne D. Slyz Thilo Kranz † Hans-Walter Rix 《Monthly notices of the Royal Astronomical Society》2003,346(4):1162-1178
We study how well the complex gas velocity fields induced by massive spiral arms are modelled by the hydrodynamical simulations that we used recently to constrain the dark matter fraction in nearby spiral galaxies. More specifically, we explore the dependence of the positions and amplitudes of features in the gas flow on the temperature of the interstellar medium (assumed to behave as a one-component isothermal fluid), the non-axisymmetric disc contribution to the galactic potential, the pattern speed Ωp , and finally the numerical resolution of the simulation. We argue that, after constraining the pattern speed reasonably well by matching the simulations to the observed spiral arm morphology, the amplitude of the non-axisymmetric perturbation (the disc fraction) is left as the primary parameter determining the gas dynamics. However, owing to the sensitivity of the positions of the shocks to modelling parameters, one has to be cautious when quantitatively comparing the simulations to observations. In particular, we show that a global least-squares analysis is not the optimal method for distinguishing different models, as it tends to slightly favour low disc fraction models. Nevertheless, we conclude that, given observational data of reasonably high spatial resolution and an accurate shock-resolving hydro-code, this method tightly constrains the dark matter content within spiral galaxies. We further argue that, even if the perturbations induced by spiral arms are weaker than those of strong bars, they are better suited for this kind of analysis because the spiral arms extend to larger radii where effects like inflows due to numerical viscosity and morphological dependence on gas sound speed are less of a concern than they are in the centres of discs. 相似文献
10.
On corotation torques, horseshoe drag and the possibility of sustained stalled or outward protoplanetary migration 总被引：1，自引：0，他引：1
S.-J. Paardekooper J. C. B. Papaloizou 《Monthly notices of the Royal Astronomical Society》2009,394(4):2283-2296
We study the torque on low-mass protoplanets on fixed circular orbits, embedded in a protoplanetary disc in the isothermal limit. We consider a wide range of surface density distributions including cases where the surface density increases smoothly outwards. We perform both linear disc response calculations and non-linear numerical simulations. We consider a large range of viscosities, including the inviscid limit, as well as a range of protoplanet mass ratios, with special emphasis on the co-orbital region and the corotation torque acting between disc and protoplanet.
For low-mass protoplanets and large viscosity, the corotation torque behaves as expected from linear theory. However, when the viscosity becomes small enough to enable horseshoe turns to occur, the linear corotation torque exists only temporarily after insertion of a planet into the disc, being replaced by the horseshoe drag first discussed by Ward. This happens after a time that is equal to the horseshoe libration period reduced by a factor amounting to about twice the disc aspect ratio. This torque scales with the radial gradient of specific vorticity, as does the linear torque, but we find it to be many times larger. If the viscosity is large enough for viscous diffusion across the co-orbital region to occur within a libration period, we find that the horseshoe drag may be sustained. If not, the corotation torque saturates leaving only the linear Lindblad torques. As the magnitude of the non-linear co-orbital torque (horseshoe drag) is always found to be larger than the linear torque, we find that the sign of the total torque may change even for mildly positive surface density gradients. In combination with a kinematic viscosity large enough to keep the torque from saturating, strong sustained deviations from linear theory and outward or stalled migration may occur in such cases. 相似文献
For low-mass protoplanets and large viscosity, the corotation torque behaves as expected from linear theory. However, when the viscosity becomes small enough to enable horseshoe turns to occur, the linear corotation torque exists only temporarily after insertion of a planet into the disc, being replaced by the horseshoe drag first discussed by Ward. This happens after a time that is equal to the horseshoe libration period reduced by a factor amounting to about twice the disc aspect ratio. This torque scales with the radial gradient of specific vorticity, as does the linear torque, but we find it to be many times larger. If the viscosity is large enough for viscous diffusion across the co-orbital region to occur within a libration period, we find that the horseshoe drag may be sustained. If not, the corotation torque saturates leaving only the linear Lindblad torques. As the magnitude of the non-linear co-orbital torque (horseshoe drag) is always found to be larger than the linear torque, we find that the sign of the total torque may change even for mildly positive surface density gradients. In combination with a kinematic viscosity large enough to keep the torque from saturating, strong sustained deviations from linear theory and outward or stalled migration may occur in such cases. 相似文献
11.
M. D'Onofrio 《Monthly notices of the Royal Astronomical Society》2001,326(4):1508-1516
This paper is the first of two dedicated to the application of a two-dimensional (2D) fit to the light distribution of 73 early-type galaxies belonging to the Virgo and Fornax clusters, a sample of which is 80 per cent complete, volume and magnitude limited down to M B =−17.3 , and highly homogeneous. Two empirical models of the surface-brightness distribution of the early-type galaxies have been tested: the first uses a single spheroid represented by the r 1/ n Sersic law, while the second is characterized by two components, 'bulge' and 'disc', by means of the ( r 1/ n +exp) laws. The χ 2 fitting routine ( minuit ), used to fit the 2D light distribution of real galaxies, is tested here on artificial galaxy images that were built with structural parameters chosen randomly from a large fixed interval. In this paper we present the properties of the two models, describe the minimization technique, and discuss the results of the tests of the 2D fit obtained from simulated artificial galaxies. By exploiting these tests we were able better to constrain the errors affecting the structural parameters, and the strategies that should be adopted to get the best fit. 相似文献
12.
G. Lanzafame 《Astronomische Nachrichten》2009,330(8):843-850
Inflow kinematics at the inner Lagrangian point L1, gas compressibility, and physical turbulent viscosity play a fundamental role on accretion disc dynamics and structure in a close binary (CB). Physical viscosity supports the accretion disc development inside the primary gravitational potential well, developing the gas radial transport, converting mechanical energy into heat. The Stellar‐Mass‐Ratio (SMR) between the compact primary and the secondary star (M1/M2) is also effective, not only in the location of the inner Lagrangian point, but also in the angular kinematics of the mass transfer and in the geometry ofthe gravitational potential wells. In this work we pay attention in particular to the role ofthe SMR, evaluating boundaries, separating theoretical domains in compressibility‐viscosity graphs where physical conditions allow a well‐bound disc development, as a function ofmass transfer kinematic conditions. In such domains, the lower is the gas compressibility (the higher the polytropic index γ), the higher is the physical viscosity (α) requested. In this work, we show how the boundaries of such domains vary as a function of M1/M2. Conclusions as far as dwarf novae outbursts are concerned, induced by mass transfer rate variations, are also reported. The smaller M1/M2, the shorter the duration of the active‐to‐quiet and vice‐versa transitional phases. Time‐scales are of the order of outburst duration of SU Uma, OY Car, Z Cha and SS Cyg‐like objects. Moreover, conclusions as far as active‐quiet‐active phenomena in a CB, according to viscous‐thermal instabilities, in accordance to such domains, are also reported (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
13.
Na-Na Pan Xiao-Ping Zheng Jia-Rong Li 《Monthly notices of the Royal Astronomical Society》2006,371(3):1359-1366
We calculate the coefficient of bulk viscosity by considering the non-leptonic weak interactions in the cores of hybrid stars with both hyperons and quarks. We first determine the dependence of the production rate of neutrons on the reaction rate of quarks in the non-leptonic processes, that is, Γ n = K s Γ s +ΓΛ + 2ΓΣ− . The conversion rate, K s , in our scenario is a complicated function of baryon number density. We also consider the medium effect of quark matter on bulk viscosity. Using these results, we estimate the limiting rotation of the hybrid stars, which may suppress the r-mode instability more effectively. Hybrid stars should be the candidates for the extremely rapid rotators. 相似文献
14.
D. García-Senz A. Relaño R. M. Cabezón E. Bravo 《Monthly notices of the Royal Astronomical Society》2009,392(1):346-360
The axisymmetric form of the hydrodynamic equations within the smoothed particle hydrodynamics (SPH) formalism is presented and checked using idealized scenarios taken from astrophysics (free fall collapse, implosion and further pulsation of a Sun-like star), gas dynamics (wall heating problem, collision of two streams of gas) and inertial confinement fusion (ablative implosion of a small capsule). New material concerning the standard SPH formalism is given. That includes the numerical handling of those mass points which move close to the singularity axis, more accurate expressions for the artificial viscosity and the heat conduction term and an easy way to incorporate self-gravity in the simulations. The algorithm developed to compute gravity does not rely in any sort of grid, leading to a numerical scheme totally compatible with the Lagrangian nature of the SPH equations. 相似文献
15.
S. B. Potter P. J. Hakala & Mark Cropper 《Monthly notices of the Royal Astronomical Society》1998,297(4):1261-1268
Modelling the polarized cyclotron emission from magnetic cataclysmic variables has been a pivotal technique for determining the structure of the accretion zones on the white dwarf. To date, model solutions have been obtained from trial fits to the intensity and polarization data, which have been constructed from emission regions (for example arcs and spots) put in by hand. These models were all inferred indirectly from arguments based on the polarization and X-ray light curves. We present a more analytical and objective technique using optimization by a genetic algorithm, Tikhonov regularization and Powell's method that robustly models the details of polarized emission. To demonstrate the success of this technique, we show the results of several simulations in which we calculated the intensity and polarization curves from arbitrarily shaped emission regions on the surface of a sphere and then applied our code to these curves to recover the original test data. We also show how adding artificial noise affects the outcome of the optimization technique. 相似文献
16.
Yang Wang Christian R. Kaiser 《Monthly notices of the Royal Astronomical Society》2008,388(2):677-696
By combining a model for the evolution of the radio luminosity of an individual source with the radio luminosity function, we perform a multidimensional Monte Carlo simulation to investigate the cosmological evolution of the Fanaroff–Riley type II (FR II) radio galaxy population by generating large artificial samples. The properties of FR II sources are required to evolve with redshift in the artificial samples to fit the observations. Either the maximum jet age or the maximum density of the jet environment or both evolve with redshift. We also study the distribution of FR II source properties as a function of redshift. From currently available data we cannot constrain the shape of the distribution of environment density or age, but jet power is found to follow a power-law distribution with an exponent of approximately −2. This power-law slope does not change with redshift out to z = 0.6 . We also find the distribution of the pressure in the lobes of FR II sources to evolve with redshift up to z ∼ 1.2 . 相似文献
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18.
Pulsar slow glitches in a solid quark star model 总被引：1，自引：0，他引：1
A series of five unusual slow glitches of the radio pulsar B1822–09 (PSR J1825–0935) was observed between 1995 and 2005. This is a phenomenon that is understood in a solid quark star model, and reasonable parameters for slow glitches are given in this paper. We propose that, because of increasing shear stress as the pulsar spins down, a slow glitch may occur, beginning with the collapse of a superficial layer of the quark star. This layer of material turns to viscous fluid at first, the viscosity of which helps to deplete the energy released from both the accumulated elastic energy and the gravitation potential. There is then a slow glitch. Numerical calculations show that the slow glitches that have been observed could be reproduced if the effective coefficient of viscosity is ∼102 cm2 s−1 and the initial velocity of the superficial layer is of the order of 10−10 cm s−1 in the coordinate rotating frame of the star. 相似文献
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Ji&#;í Krti&#;ka Ji&#;í Kubát 《Monthly notices of the Royal Astronomical Society》2009,394(4):2065-2079
We study the influence of X-rays on the wind structure of selected O stars. For this purpose we use our non-local thermodynamic equilibrium (NLTE) wind code with inclusion of additional artificial source of X-rays, assumed to originate in the wind shocks.
We show that the influence of shock X-ray emission on wind mass-loss rate is relatively small. Wind terminal velocity may be slightly influenced by the presence of strong X-ray sources, especially for stars cooler than Teff ≲ 35 000 K .
We discuss the origin of the LX / L ∼ 10−7 relation. For stars with thick wind this relation can be explained assuming that the cooling time depends on wind density. Stars with optically thin winds exhibiting the 'weak wind problem' display enhanced X-ray emission which may be connected with large shock cooling length. We propose that this effect can explain the 'weak wind problem'.
Inclusion of X-rays leads to a better agreement of the model ionization structure with observations. However, we do not find any significant influence of X-rays on P v ionization fraction implying that the presence of X-rays cannot explain the P v problem.
We study the implications of modified ionization equilibrium due to shock emission on the line transfer in the X-ray region. We conclude that the X-ray line profiles of helium-like ions may be affected by the line absorption within the cool wind. 相似文献
We show that the influence of shock X-ray emission on wind mass-loss rate is relatively small. Wind terminal velocity may be slightly influenced by the presence of strong X-ray sources, especially for stars cooler than T
We discuss the origin of the L
Inclusion of X-rays leads to a better agreement of the model ionization structure with observations. However, we do not find any significant influence of X-rays on P v ionization fraction implying that the presence of X-rays cannot explain the P v problem.
We study the implications of modified ionization equilibrium due to shock emission on the line transfer in the X-ray region. We conclude that the X-ray line profiles of helium-like ions may be affected by the line absorption within the cool wind. 相似文献