Abstract
We describe a numerical scheme for magnetohydrodynamics simulations of dust–gas mixture by extending smoothed particle magnetohydrodynamics. We employ the single-species particle approach to ...describe dust–gas mixture with several modifications from the previous studies. We assume that the charged and neutral dust can be treated as single-fluid, that the electromagnetic force acts on the gas, and that that acting on the charged dust is negligible. The validity of these assumptions in the context of protostar formation is not obvious and is extensively evaluated. By investigating the electromagnetic force and electric current with terminal velocity approximation, it is found that as the dust size increases, the contribution of dust to them becomes smaller and negligible. We conclude that our assumption that the electromagnetic force on the dusts is negligible is valid for the dust size with
a
d
≳ 10
μ
m. On the other hand, they do not produce the numerical artifact for the dust
a
d
≲ 10
μ
m in the envelope and disk, where the perfect coupling between gas and dust is realized. However, we also found that our assumptions may break down in outflow (or under an environment with very strong magnetic field and low density) for the dust
a
d
≲ 10
μ
m. We conclude that our assumptions are valid in almost all cases where macroscopic dust dynamics is important in the context of protostar formation. We conduct numerical tests of dusty waves, dusty magnetohydrodynamics shocks, and gravitational collapse of magnetized cloud cores with our simulation code. The results show that our numerical scheme well reproduces the dust dynamics in the magnetized medium.
ABSTRACT The formation process of circumstellar disks is still controversial because of the interplay of complex physical processes that occurs during the gravitational collapse of prestellar cores. ...In this study, we investigate the effect of the Hall current term on the formation of the circumstellar disk using three-dimensional simulations. In our simulations, all non-ideal effects, as well as the radiation transfer, are considered. The size of the disk is significantly affected by a simple difference in the inherent properties of the prestellar core, namely whether the rotation vector and the magnetic field are parallel or anti-parallel. In the former case, only a very small disk ( ) is formed. On the other hand, in the latter case, a massive and large ( ) disk is formed in the early phase of protostar formation. Since the parallel and anti-parallel properties do not readily change, we expect that the parallel and anti-parallel properties are also important in the subsequent disk evolution and the difference between the two cases is maintained or enhanced. This result suggests that the disk size distribution of the Class 0 young stellar objects is bimodal. Thus, the disk evolution can be categorized into two cases and we may call the parallel and anti-parallel systems Ortho-disk and Para-disk, respectively. We also show that the anti-rotating envelopes against the disk rotation appear with a size of . We predict that the anti-rotating envelope will be found in the future observations.
The formation and early evolution of low-mass young stellar objects (YSOs) are investigated using three-dimensional non-ideal magnetohydrodynamics simulations. We investigate the evolution of YSOs up ...to after protostar formation, at which protostellar mass reaches . We particularly focus on the impact of the dust model on the evolution. We found that a circumstellar disk is formed in all simulations, regardless of the dust model. Disk size is approximately 10 au at the protostar formation epoch, and it increases to several tens of au at after protostar formation. The disk mass is comparable to the central protostellar mass, and gravitational instability develops. In simulations with small dust sizes, the warp of the pseudodisk develops after protostar formation. The warp strengthens magnetic braking in the disk and decreases disk size. Ion-neutral drift can occur in the infalling envelope when the typical dust size is and the protostar (plus disk) mass is . The outflow activity is anticorrelated to the dust size, and the strong outflow appears with small dust grains.
The effect of misalignment between the magnetic field and the angular momentum of molecular cloud cores on the angular momentum evolution during the gravitational collapse is investigated by ideal ...and non-ideal MHD simulations. For the non-ideal effect, we consider the ohmic and ambipolar diffusion. Previous studies that considered the misalignment reported qualitatively contradicting results. Magnetic braking was reported as being either strengthened or weakened by misalignment in different studies. We conducted simulations of cloud core collapse by varying the stability parameter (the ratio of the thermal to gravitational energy of the core) with and without including magnetic diffusion. The non-ideal MHD simulations show the central angular momentum of the core, with θ = 0° ( ) being always greater than that with θ = 90° ( ), independently of , meaning that circumstellar disks form more easily in a core with θ = 0°. The ideal MHD simulations, in contrast, show the central angular momentum of the core with θ = 90° being greater than with θ = 0° for small and smaller for large . Inspection of the angular momentum evolution of the fluid elements reveals three mechanisms contributing to the evolution of the angular momentum: (i) magnetic braking in the isothermal collapse phase, (ii) selective accretion of the rapidly (for θ = 90°) or slowly (for θ = 0°) rotating fluid elements to the central region, and (iii) magnetic braking in the first core and the disk. The difference between the ideal and non-ideal simulations arises from the different efficiencies of (iii).
We investigate the dust structure of gravitationally unstable disks undergoing mass accretion from the envelope, envisioning its application to Class 0/I young stellar objects (YSOs). We find that ...the dust disk quickly settles into a steady state and that, compared to a disk with interstellar medium (ISM) dust-to-gas mass ratio and micron-sized dust, the dust mass in the steady state decreases by a factor of 1/2 to 1/3, and the dust thermal emission decreases by a factor of 1/3 to 1/5. The latter decrease is caused by dust depletion and opacity decrease owing to dust growth. Our results suggest that the masses of gravitationally unstable disks in Class 0/I YSOs are underestimated by a factor of 1/3 to 1/5 when calculated from the dust thermal emission assuming an ISM dust-to-gas mass ratio and micron-sized dust opacity, and that a larger fraction of disks in Class 0/I YSOs is gravitationally unstable than was previously believed. We also investigate the orbital radius within which planetesimals form via coagulation of porous dust aggregates and show that becomes ∼20 au for a gravitationally unstable disk around a solar mass star. Because increases as the gas surface density increases and a gravitationally unstable disk has maximum gas surface density, is the theoretical maximum radius for planetesimal formation. We suggest that planetesimal formation in the Class 0/I phase is preferable to that in the Class II phase because a large amount of dust is supplied by envelope-to-disk accretion.
Fragmentation of protoplanetary discs due to gravitational instabilities is a candidate of a formation mechanism of binary stars, brown dwarfs, and gaseous giant planets. The condition for the ...fragmentation has been thought that the disc cooling time-scale is comparable to its dynamical time-scale. However, some numerical simulations suggest that the fragmentation does not occur even if the cooling time is small enough, or the fragmentation can occur even when the cooling is inefficient. To reveal a realistic condition for fragmentation of self-gravitating discs, we perform two-dimensional numerical simulations that take into account the effect of the irradiation of the central star and radiation cooling of the disc, and precisely investigate the structure of the spiral arms formed in the protoplanetary discs. We show that the Toomre Q parameter in the spiral arms is an essential parameter for fragmentation. The spiral arms fragment only when Q < 0.6 in the spiral arms. We have further confirmed that this fragmentation condition observed in the numerical simulations can be obtained from the linear stability analysis for the self-gravitating spiral arms. These results indicate that the process of fragmentation of protoplanetary discs is divided into two stages: formation of the spiral arms in the discs; and fragmentation of the spiral arm. Our work reduces the condition for the fragmentation of the protoplanetary discs to the condition of the formation of the spiral arm that satisfies Q < 0.6.
We investigate the formation and evolution of a first core, protostar, and circumstellar disc with a three-dimensional non-ideal (including both Ohmic and ambipolar diffusion) radiation ...magnetohydrodynamics simulation. We found that the magnetic flux is largely removed by magnetic diffusion in the first-core phase and that the plasma β of the centre of the first core becomes large, β > 104. Thus, proper treatment of first-core phase is crucial in investigating the formation of protostar and disc. On the other hand, in an ideal simulation, β ∼ 10 at the centre of the first core. The simulations with magnetic diffusion show that the circumstellar disc forms at almost the same time of protostar formation even with a relatively strong initial magnetic field (the value for the initial mass-to-flux ratio of the cloud core relative to the critical value is μ = 4). The disc has a radius of r ∼ 1 AU at the protostar formation epoch. We confirm that the disc is rotationally supported. We also show that the disc is massive (Q ∼ 1) and that gravitational instability may play an important role in the subsequent disc evolution.
We investigate the dust structure of gravitationally unstable disks undergoing mass accretion from the envelope, envisioning its application to Class 0/I young stellar objects (YSOs). We find that ...the dust disk quickly settles into a steady state and that, compared to a disk with interstellar medium (ISM) dust-to-gas mass ratio and micron-sized dust, the dust mass in the steady state decreases by a factor of 1/2 to 1/3, and the dust thermal emission decreases by a factor of 1/3 to 1/5. The latter decrease is caused by dust depletion and opacity decrease owing to dust growth. Our results suggest that the masses of gravitationally unstable disks in Class 0/I YSOs are underestimated by a factor of 1/3 to 1/5 when calculated from the dust thermal emission assuming an ISM dust-to-gas mass ratio and micron-sized dust opacity, and that a larger fraction of disks in Class 0/I YSOs is gravitationally unstable than was previously believed. We also investigate the orbital radius \({r}_{{\rm{P}}}\) within which planetesimals form via coagulation of porous dust aggregates and show that \({r}_{{\rm{P}}}\) becomes ∼20 au for a gravitationally unstable disk around a solar mass star. Because \({r}_{{\rm{P}}}\) increases as the gas surface density increases and a gravitationally unstable disk has maximum gas surface density, \({r}_{{\rm{P}}}\sim 20\,\mathrm{au}\) is the theoretical maximum radius for planetesimal formation. We suggest that planetesimal formation in the Class 0/I phase is preferable to that in the Class II phase because a large amount of dust is supplied by envelope-to-disk accretion.
We investigate the structure of self-gravitating discs, their fragmentation and the evolution of the fragments (the clumps) using both an analytic approach and three-dimensional radiation ...hydrodynamics simulations starting from molecular cores. The simulations show that non-local radiative transfer determines the disc temperature. We find the disc structure is well described by an analytical model of a quasi-steady self-gravitating disc with radial radiative transfer. Because the radiative process is not local and radiation from the interstellar medium cannot be ignored, the local radiative cooling is not balanced with the viscous heating in a massive disc around a low-mass star. In our simulations, there are cases in which the disc does not fragment even though it satisfies the fragmentation criterion based on disc cooling time (...). This indicates that, at least, the criterion is not a sufficient condition for fragmentation. We determine the parameter range for the host cloud core in which disc fragmentation occurs. In addition, we show that the temperature evolution of the centre of the clump is close to that of typical first cores, and that the minimum initial mass of clumps is about a few Jupiter masses. (ProQuest: ... denotes formulae/symbols omitted.)
In chronic kidney disease stage 5D, diagnostic usefulness of bone mineral density (BMD) in predicting fracture has not been established because of variable results in previous studies. The reason for ...this may be the heterogeneity of underlying pathogenesis of the fracture.
BMD was measured annually and serum biochemistry monthly for 485 hemodialyzed patients from April 2003 to March 2008, and all fractures were recorded.
Forty-six new episodes of any type of fracture and 29 cases of prevalent spine fracture were recorded. Serum bone-specific alkaline phosphatase (b-AP) was a very useful surrogate marker for any type of incident fracture risk area under curve (AUC) = 0.766, P < 0.0001. A significantly greater risk of any type of incident fracture was associated with parathyroid hormone (PTH) levels either <150 pg/mL hazard ratio (HR) = 3.47, P < 0.01 or >300 pg/mL (HR = 5.88, P < 0.0001) compared with 150-300 pg/mL. Receiver-operating characteristic analysis demonstrated a significant predictive power for incident of any type of fracture by BMD at the total hip (AUC = 0.760, P < 0.0001) and other hip regions in females in the lower PTH group (PTH < 204 pg/mL). BMDs at every site but whole body or lumbar spine had significant power to discriminate prevalent spine fracture regardless of gender or PTH.
Hemodialyzed patients with low or high PTH or increased b-AP had a high fracture risk. BMD by Dual Energy X-ray Absorptiometry (DEXA), especially at the total hip region, was useful to predict any type of incident of fracture for females with low PTH or to discriminate prevalent spine fracture for every patient.
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