The early evolution of the magnetic field and angular momentum of newly formed protostars are 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 ≃ 104 cm-3 and r = 4.6 × 105AU, where nc and r are the central density and radius, respectively) to the stellar core (nc ≃ 1022 cm-3;r ∼ 1 R⊙). The magnetic field strengths at the centers of clouds with the same initial angular momentum but different magnetic field strengths converge to a certain value as the clouds collapse for nc ≲ 1012 cm-3. For 1012 cm-3 ≲ nc ≲ 1016 cm-3, ohmic dissipation largely removes the magnetic field from a collapsing cloud core, and the magnetic field lines, which are strongly twisted for nc ≲ 1012 cm-3, are decollimated. The magnetic field lines are twisted and amplified again for nc ≳ 1016 cm-3, because the magnetic field is recoupled with warm gas. Finally, protostars at their formation epoch (nc ≃ 1021 cm-3) have magnetic fields of ∼0.1 -1 kG, which is comparable to observations. The magnetic field strength of a protostar depends slightly on the angular momentum of the host cloud. A protostar formed from a 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 a collapsing cloud is removed by magnetic effects such as magnetic braking, outflow, and jets. The formed protostars have rotation periods of 0.1-2 days at their formation epoch, which is slightly shorter than observations. This indicates that a further removal mechanism for the angular momentum, such as interactions between the protostar and the disk, wind, or jets, is important in the further evolution of protostars.
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science