Dissipation of magnetic fields in very dense interstellar clouds - II. Final phases of star formation and the magnetic flux of a newborn star

The behaviour of magnetic fields is investigated in the final phases of star formation. In a cloud contracting isothermally before a quasi-hydrostatic core forms, magnetic fields are found decoupled from the gas only in the region of hydrogen number density $$n_\text H\gtrsim \text {several}\times10^{11}\enspace \text {cm}^{-3}$$. In an envelope contracting on to a stellar core, magnetic fields are also decoupled only in the region of $$n_\text H\gtrsim \text {several}\times10^{11}\enspace \text {cm}^{-3}$$. At such high densities Joule dissipation is efficient, and only nearly current-free fields can exist. At the temperature $$T\simeq1\times10^3\enspace K$$ a contracting gas recovers the coupling with magnetic fields because the ionization degree is enhanced by the thermal ionization. The magnetic flux brought into the initial stellar core is found at most of the order of the flux of a magnetic star. Capture of magnetic flux in the later accretion phases of the stellar core is investigated, and the flux of a newborn star is found at most about the flux of the most strongly magnetic star.