Magnetic Fields and Rotations of Protostars

The evolution of the magnetic field and angular momentum in the collapsing cloud core is studied using three-dimensional resistive MHD nested grid simulations. Starting with a Bonnor-Ebert isothermal cloud rotating in a uniform magnetic field, we calculate the cloud evolution from the molecular cloud core (nc ≃ 10 4 cm, r = 4.6× 10 AU) to the stellar core (nc ≃ 10 22 cm, r ∼ 1R⊙), where nc and r denote the central density and radius of each object, respectively. The magnetic field strengths at the center of the clouds converge to a certain value as the clouds collapse for nc . 10 12 cm, when the clouds have the same angular momenta but different strengths of the magnetic fields at the initial state. For 10 . nc . 10 16 cm, Ohmic dissipation considerably removes the magnetic field from the collapsing cloud core, and the magnetic field lines, which are strongly twisted for nc . 10 12 cm, is de-collimated. The magnetic field lines are twisted and amplified again for nc & 10 16 cm, because the magnetic field is recoupled with the warm gas for nc & 10 16 cm. Finally, protostars at their formation epoch (nc ≃ 10 21 cm) have ∼0.1–1 kG of the magnetic fields, which are comparable to observations. The magnetic field strength of protostar slightly depends on the angular momentum of the host cloud. The protostar formed from the slowly rotating cloud core has a stronger magnetic field. The evolution of the angular momentum is closely related to the evolution of the magnetic field. The angular momentum in the collapsing cloud is removed by the magnetic effect (magnetic braking, outflow and jet). The formed protostars have 0.1–2 days of the rotation period at their formation epoch, which are slightly shorter than the observation. This indicates that the further removal mechanism of the angular momentum such as interaction between the protostar and disk, wind gas or jet is important in further evolution of the protostar. Subject headings: ISM: clouds: ISM: magnetic fields—MHD—stars: formation— stars: rotation Department of Physics, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan; [email protected], [email protected] Faculty of Humanity and Environment, Hosei University, Fujimi, Chiyoda-ku, Tokyo 102-8160, Japan; [email protected]