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Register a new userThe Effect of Misalignment between Rotation Axis and Magnetic Field on Circumstellar Disk
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The formation of circumstellar disks is investigated using three-dimensional resistive magnetohydrodynamic simulations, in which the initial prestellar cloud has a misaligned rotation axis with respect to the magnetic field. We examine the effects of (i) the initial angle difference between the global magnetic field and the cloud rotation axis ($theta_0$) and (ii) the ratio of the thermal to gravitational energy ($alpha_0$). We study $16$ models in total and calculate the cloud evolution until $sim ! 5000$ yr after protostar formation. Our simulation results indicate that an initial non-zero $theta_0$ ($> 0$) promotes the disk formation but tends to suppress the outflow driving, for models that are moderately gravitationally unstable, $alpha_0 lesssim 1$. In these models, a large-sized rotationally-supported disk forms and a weak outflow appears, in contrast to a smaller disk and strong outflow in the aligned case ($theta_0 = 0$). Furthermore, we find that when the initial cloud is highly unstable with small $alpha_0$, the initial angle difference $theta_0$ does not significantly affect the disk formation and outflow driving.
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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 $alpha$ (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 $theta=0^circ$ ($Jang parallel magB$) being always greater than that with $theta=90^circ$ ($Jang perp magB$), independently of $alpha$, meaning that circumstellar disks form more easily form in a core with $theta=0^circ$. The ideal MHD simulations, in contrast, show the the central angular momentum of the core with $theta=90^circ$ being greater than with $theta=0^circ$ for small $alpha$, and is smaller for large $alpha$. 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 $theta=90^circ$ ) or slowly (for $theta=0^circ$) 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).
Stars form in dense cores of molecular clouds that are observed to be significantly magnetized. In the simplest case of a laminar (non-turbulent) core with the magnetic field aligned with the rotation axis, both analytic considerations and numerical simulations have shown that the formation of a large, $10^2au$-scale, rotationally supported protostellar disk is suppressed by magnetic braking in the ideal MHD limit for a realistic level of core magnetization. This theoretical difficulty in forming protostellar disks is termed magnetic braking catastrophe. A possible resolution to this problem, proposed by citeauthor{HennebelleCiardi2009} and citeauthor{Joos+2012}, is that misalignment between the magnetic field and rotation axis may weaken the magnetic braking enough to enable disk formation. We evaluate this possibility quantitatively through numerical simulations. We confirm the basic result of citeauthor{Joos+2012} that the misalignment is indeed conducive to disk formation. In relatively weakly magnetized cores with dimensionless mass-to-flux ratio $gtrsim 5$, it enabled the formation of rotationally supported disks that would otherwise be suppressed if the magnetic field and rotation axis are aligned. For more strongly magnetized cores, disk formation remains suppressed, however, even for the maximum tilt angle of $90degree$. If dense cores are as strongly magnetized as indicated by OH Zeeman observations (with a mean dimensionless mass-to-flux ratio $sim 2$), it would be difficult for the misalignment alone to enable disk formation in the majority of them. We conclude that, while beneficial to disk formation, especially for the relatively weak field case, the misalignment does not completely solve the problem of catastrophic magnetic braking in general.
The majority of solar-type stars reside in multiple systems, especially binaries. They form in dense cores of molecular clouds that are observed to be significantly magnetized. Our previous study shows that magnetic braking can tighten the binary separation during the protostellar mass accretion phase by removing the angular momentum of the accreting material. Recent numerical calculations of single star formation have shown that misalignment between the magnetic field and rotation axis may weaken both magnetic braking and the associated magnetically driven outflows. These two effects allow for disk formation even in strongly magnetized cores. Here we investigate the effects of magnetic field misalignment on the properties of protobinaries. Somewhat surprisingly, the misaligned magnetic field is more efficient at tightening the binary orbit compared to the aligned field. The main reason is that the misalignment weakens the magnetically-driven outflow, which allows more material to accrete onto the binary. Even though the specific angular momentum of this inner material is higher than in the aligned case, it is insufficient to compensate for the additional mass. A corollary of this result is that a weaker field is required to achieve the same degree of inward migration when the field is tilted relative to the rotation axis. Large field misalignment also helps to produce rotationally-supported circumbinary disks even for relatively strong magnetic fields, by weakening the magnetically-dominated structure close to the binary. Our result may provide an explanation for the circumbinary disks detected in recent SMA and ALMA observations.
We present results of 1.3 mm dust polarization observations toward 16 nearby, low-mass protostars, mapped with ~2.5 resolution at CARMA. The results show that magnetic fields in protostellar cores on scales of ~1000 AU are not tightly aligned with outflows from the protostars. Rather, the data are consistent with scenarios where outflows and magnetic fields are preferentially misaligned (perpendicular), or where they are randomly aligned. If one assumes that outflows emerge along the rotation axes of circumstellar disks, and that the outflows have not disrupted the fields in the surrounding material, then our results imply that the disks are not aligned with the fields in the cores from which they formed.
We studied the solar surface flows (differential rotation and meridional circulation) using a magnetic element feature tracking technique by which the surface velocity is obtained using magnetic field data. We used the line-of-sight magnetograms obtained by the Helioseismic and Magnetic Imager aboard the Solar Dynamics Observatory from 01 May 2010 to 16 August 2017 (Carrington rotations 2096 to 2193) and tracked the magnetic element features every hour. Using our method, we estimated the differential rotation velocity profile. We found rotation velocities of $sim$ 30 and -170 m s$^{-1}$ at latitudes of 0$^{circ}$ and 60$^{circ}$ in the Carrington rotation frame, respectively. Our results are consistent with previous results obtained by other methods, such as direct Doppler, time-distance helioseismology, or cross correlation analyses. We also estimated the meridional circulation velocity profile and found that it peaked at $sim$12 m s$^{-1}$ at a latitude of 45$^{circ}$, which is also consistent with previous results. The dependence of the surface flow velocity on the magnetic field strength was also studied. In our analysis, the magnetic elements having stronger and weaker magnetic fields largely represent the characteristics of the active region remnants and solar magnetic networks, respectively. We found that magnetic elements having a strong (weak) magnetic field show faster (slower) rotation speed. On the other hand, magnetic elements having a strong (weak) magnetic field show slower (faster) meridional circulation velocity. These results might be related to the Suns internal dynamics.
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