Evolution of Rotating Molecular Cloud Core with Oblique Magnetic Field

Abstract

We studied the collapse of rotating molecular cloud cores with inclined magnetic fields, based on three-dimensional numerical simulations. The numerical simulations start from a rotating Bonnor-Ebert isothermal cloud in a uniform magnetic field. The magnetic field is initially taken to be inclined from the rotation axis. As the cloud collapses, the magnetic field and rotation axis change their directions. When the rotation is slow and the magnetic field is relatively strong, the direction of the rotation axis changes to align with the magnetic field, as shown earlier by Matsumoto & Tomisaka. When the magnetic field is weak and the rotation is relatively fast, the magnetic field inclines to become perpendicular to the rotation axis. In other words, the evolution of the magnetic field and rotation axis depends on the relative strength of the rotation and magnetic field. Magnetic braking acts to align the rotation axis and magnetic field, while the rotation causes the magnetic field to incline through dynamo action. The latter effect dominates the former when the ratio of the angular velocity to the magnetic field is larger than a critical value 0 /B 0 > 0:39G 1 / 2 c À1 s , where B 0 , 0 , G, and c s denote the initial magnetic field, initial angular velocity, gravitational constant, and sound speed, respectively. When the rotation is relatively strong, the collapsing cloud forms a disk perpendicular to the rotation axis and the magnetic field becomes nearly parallel to the disk surface in the high-density region. A spiral structure appears due to the rotation and the wound up magnetic field in the disk.

Protostellar Collapse of Magneto-Turbulent Cloud Cores: Shape During Collapse and Outflow Formation

“…The magnetic field lines are wound up by the rotation of the axes oriented perpendicular to the field. A similar magnetic field morphology was reported by Machida et al (2006) for inclined rotators (see their Figure 13). However, they did not confirm this type of outflow in their simulations because they could not follow the evolution for a sufficiently long period.…”

“…On the other hand, simulations carried out to date on the formation of the Larson first core (Larson 1969) have taken account of rotation and magnetic fields but not turbulence, both for aligned rotators (Tomisaka 2002;Machida et al 2004;Banerjee & Pudritz 2006;Commerçon et al 2010;Tomida et al 2010) and inclined rotators Machida et al 2006;Hennebelle & Ciardi 2009). In these studies, the rotation speed and rotation axis are explicitly assumed as initial conditions, and the origin of the rotation has not been addressed.…”

The First Jets in the Universe: Protostellar Jets from the First Stars

“…The box size of the coarsest grid is chosen to l p 1 be . A new, finer grid is generated whenever the minimum n c the cloud collapses non-homologously into a self-similar disklike structure (Machida et al 2005a(Machida et al , 2006b n Ӎ 10 cm c of the collapse and magnetic braking: for stronger magnetic field, the magnetic braking is more effective and the growth rate of angular velocity tends to be smaller. Since we assume rapid (slow) rotation for strongly (weakly) magnetized clouds in this paper, the angular velocities tend to converge to the unique scaling law as the collapse proceeds.…”

Variability of the NGC 1333 IRAS 4A Outflow: Molecular Hydrogen and Silicon Monoxide Images

“…In such systems, changes of outflow direction may be observable. The evolution of the field direction with respect to the rotation axis may depend on the relative strength between the rotation and the magnetic fields (Machida et al 2006). …”