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Specific Angular Momentum (specific + angular_momentum)
Selected AbstractsThermal effects of circumplanetary disc formation around proto-gas giant planetsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2009M. N. Machida ABSTRACT The formation of a circumplanetary disc and accretion of angular momentum on to a protoplanetary system are investigated using three-dimensional hydrodynamical simulations. The local region around a protoplanet in a protoplanetary disc is considered with sufficient spatial resolution: the region from outside the Hill sphere to the Jovian radius is covered by the nested-grid method. To investigate the thermal effects of the circumplanetary disc, various equations of state are adopted. Large thermal energy around the protoplanet slightly changes the structure of the circumplanetary disc. Compared with a model adopting an isothermal equation of state, in a model with an adiabatic equation of state, the protoplanet's gas envelope extends farther, and a slightly thick disc appears near the protoplanet. However, different equations of state do not affect the acquisition process of angular momentum for the protoplanetary system. Thus, the specific angular momentum acquired by the system is fitted as a function only of the protoplanet's mass. A large fraction of the total angular momentum contributes to the formation of the circumplanetary disc. The disc forms only in a compact region in very close proximity to the protoplanet. Adapting the results to the Solar system, the proto-Jupiter and Saturn have compact discs in the region of r < 21rJup(r < 0.028 rH,Jup) and r < 66rSat(r < 0.061rH,Sat), respectively, where rJup(rH,Jup) and rSat(rH,Sat) are the Jovian and Saturnian (Hill) radius, respectively. The surface density has a peak in these regions due to the balance between centrifugal force and gravity of the protoplanet. The size of these discs corresponds well to the outermost orbit of regular satellites around Jupiter and Saturn. Regular satellites may form in such compact discs around proto-gas giant planets. [source] Dynamical response to supernova-induced gas removal in spiral galaxies with dark matter haloMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008Hiroko Koyama ABSTRACT We investigate the dynamical response, in terms of disc size and rotation velocity, to mass loss by supernovae in the evolution of spiral galaxies. A thin baryonic disc having the Kuzmin density profile embedded in a spherical dark matter halo having a density profile proposed by Navarro, Frenk & White is considered. For the purpose of comparison, we also consider the homogeneous and r,1 profiles for dark matter in a truncated spherical halo. Assuming for simplicity that the dark matter distribution is not affected by mass-loss from discs and the change of baryonic disc matter distribution is homologous, we evaluate the effects of dynamical response in the resulting discs. We found that the dynamical response only for an adiabatic approximation of mass-loss can simultaneously account for the rotation velocity and disc size as observed particularly in dwarf spiral galaxies, thus reproducing the Tully,Fisher relation and the size versus magnitude relation over the full range of magnitude. Furthermore, we found that the mean specific angular momentum in discs after the mass-loss becomes larger than that before the mass-loss, suggesting that the mass-loss would most likely occur from the central disc region where the specific angular momentum is low. [source] Bulges versus discs: the evolution of angular momentum in cosmological simulations of galaxy formationMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008Jesus Zavala ABSTRACT We investigate the evolution of angular momentum in simulations of galaxy formation in a cold dark matter universe. We analyse two model galaxies generated in the N -body/hydrodynamic simulations of Okamoto et al. Starting from identical initial conditions, but using different assumptions for the baryonic physics, one of the simulations produced a bulge-dominated galaxy and the other one a disc-dominated galaxy. The main difference is the treatment of star formation and feedback, both of which were designed to be more efficient in the disc-dominated object. We find that the specific angular momentum of the disc-dominated galaxy tracks the evolution of the angular momentum of the dark matter halo very closely: the angular momentum grows as predicted by linear theory until the epoch of maximum expansion and remains constant thereafter. By contrast, the evolution of the angular momentum of the bulge-dominated galaxy resembles that of the central, most bound halo material: it also grows at first according to linear theory, but 90 per cent of it is rapidly lost as pre-galactic fragments, into which gas had cooled efficiently, merge, transferring their orbital angular momentum to the outer halo by tidal effects. The disc-dominated galaxy avoids this fate because the strong feedback reheats the gas, which accumulates in an extended hot reservoir and only begins to cool once the merging activity has subsided. Our analysis lends strong support to the classical theory of disc formation whereby tidally torqued gas is accreted into the centre of the halo conserving its angular momentum. [source] Dynamics of oscillating relativistic tori around Kerr black holesMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2005Olindo Zanotti ABSTRACT We present a comprehensive numerical study of the dynamics of relativistic axisymmetric accretion tori with a power-law distribution of specific angular momentum orbiting in the background space,time of a Kerr black hole. By combining general relativistic hydrodynamics simulations with a linear perturbative approach we investigate the main dynamical properties of these objects over a large parameter space. The astrophysical implications of our results extend and improve two interesting results that have been recently reported in the literature. First, the induced quasi-periodic variation of the mass quadrupole moment makes relativistic tori of nuclear matter densities, as those formed during the last stages of binary neutron star mergers, promising sources of gravitational radiation, potentially detectable by interferometric instruments. Secondly, p-mode oscillations in relativistic tori of low rest-mass densities could be used to explain high-frequency quasi-periodic oscillations observed in X-ray binaries containing a black hole candidate under conditions more generic than those considered so far. [source] The Tully,Fisher relation and its implications for the halo density profile and self-interacting dark matterMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2000H. J. Mo We show that the Tully,Fisher relation observed for spiral galaxies can be explained in the current scenario of galaxy formation without invoking subtle assumptions, provided that galactic-sized dark haloes have low concentrations which do not change significantly with halo circular velocity. This conclusion does not depend significantly on whether haloes have cuspy or flat profiles in the inner region. In such a system, both the disc and the halo may contribute significantly to the maximum rotation of the disc, and the gravitational interaction between the disc and halo components leads to a tight relation between the disc mass and maximum rotation velocity. The model can therefore be tested by studying the Tully,Fisher zero points for galaxies with different disc mass-to-light ratios. With model parameters (such as the ratio between disc and halo mass, the specific angular momentum of disc material, disc formation time) chosen in plausible ranges, the model can well accommodate the zero-point, slope and scatter of the observed Tully,Fisher relation, as well as the observed large range of disc surface densities and sizes. In particular, the model predicts that low surface brightness disc galaxies obey a Tully,Fisher relation very similar to that of normal discs, if the disc mass-to-light ratio is properly taken into account. About half of the gravitational force at maximum rotation comes from the disc component for normal discs, while the disc contribution is lower for galaxies with a lower surface density. The halo profile required by the Tully,Fisher relation is as concentrated as that required by the observed rotation curves of faint discs, but less concentrated than that given by current simulations of cold dark matter (CDM) models. We discuss the implication of such profiles for structure formation in the Universe and for the properties of dark matter. Our results cannot be explained by some of the recent proposals for resolving the conflict between conventional CDM models and the observed rotation-curve shapes of faint galaxies. If dark matter self-interaction (either scattering or annihilation) is responsible for the shallow profile, the observed Tully,Fisher relation requires the interaction cross-section ,X to satisfy ,,X|v|,/mX,10,16 cm3 s,1 GeV,1, where mX is the mass of a dark matter particle. [source] Thin discs, thick dwarfs and the effects of stellar feedbackMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2010R. Sánchez-Janssen ABSTRACT We investigate the role of stellar mass in shaping the intrinsic thickness of galaxy discs by determining the probability distribution of apparent axial ratios (b/a) for two different samples that probe the faint end of the galaxy luminosity function. We find that the b/a distribution has a characteristic ,U-shape' and identify a limiting mass M*, 2 × 109 M, below which low-mass galaxies start to be systematically thicker. This tendency holds for very faint (MB,,8) dwarfs in the local volume, which are essentially spheroidal systems. We argue that galaxy shape is the result of the complex interplay between mass, specific angular momentum and stellar feedback effects. Thus, the increasing importance of turbulent motions in lower mass galaxies leads to the formation of thicker systems, a result supported by the latest hydrodynamical simulations of dwarf galaxy formation and other theoretical expectations. We discuss several implications of this finding, including the formation of bars in faint galaxies, the deprojection of H i line profiles and simulations of environmental effects on the dwarf galaxy population. [source] | ||||||||||