arXiv subjects

S. Okuzumi

Publications and source records attributed to S. Okuzumi.

3 recordsLinked to original sources

arXiv2018

Does misalignment between magnetic field and angular momentum enhance or suppress circumstellar disk formation?

The effect of misalignment between the magnetic field $\magB$ and the angular momentum $\Jang$ 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 $α$ (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 $θ=0^\circ$ ($\Jang \parallel \magB$) being always greater than that with $θ=90^\circ$ ($\Jang \perp \magB$), independently of $α$, meaning that circumstellar disks form more easily form in a core with $θ=0^\circ$. The ideal MHD simulations, in contrast, show the the central angular momentum of the core with $θ=90^\circ$ being greater than with $θ=0^\circ$ for small $α$, and is smaller for large $α$. 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 $θ=90^\circ$ ) or slowly (for $θ=0^\circ$) 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).

astro-ph.SR↗

arXiv2017

Apparent disk-mass reduction and planetesimal formation in gravitationally unstable disks in Class 0/I YSOs

We investigate the dust structure of gravitationally unstable disks undergoing mass accretion from the envelope envisioning the 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 the 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 $\sim 20$ 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 a maximum gas surface density, $r_{\rm P}\sim 20$ AU is the theoretical maximum radius. We suggest that planetesimals formation in the Class 0/I phase is preferable to that in the Class II phase because large gas surface density is expected and large amount of dust is supplied by envelope-to-disk accretion.

astro-ph.SR↗

arXiv2015

Bimodality of circumstellar disk evolution induced by Hall current

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 (< 1 AU) is formed. On the other hand, in the latter case, a massive and large (> 20 AU) 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 as Ortho-disk and Para-disk, respectively. We also show that the anti-rotating envelopes against the disk-rotation appear with a size of 200 AU. We predict that the anti-rotating envelope will be found in the future observations.

astro-ph.SR↗