Distortion of Magnetic Fields in a Starless Core VI: Application of Flux Freezing Model and Core Formation of FeSt 1-457

Observational data for the hourglass-like magnetic field toward the starless dense core FeSt 1-457 were compared with a flux freezing magnetic field model. Fitting of the observed plane-of-sky magnetic field using the flux freezing model gave a residual angle dispersion comparable to the results based on a simple 3D parabolic model. The best-fit parameters for the flux freezing model were a line-of-sight magnetic inclination angle of gamma(mag) = 35 degrees 15 degrees and a core center to ambient (background) density contrast of rho(c)/rho(bkg) = 75. The initial density for core formation (rho(0)) was estimated to be rho(c)/75 = 4670 cm(-3), which is about one order of magnitude higher than the expected density (similar to 300 cm(-3)) for the interclump medium of the Pipe Nebula. FeSt 1-457 is likely to have been formed from the accumulation of relatively dense gas, and the relatively dense background column density of A(V) 5 mag supports this scenario. The initial radius (core formation radius) R-0 and the initial magnetic field strength B-0 were obtained to be 0.15 pc (1.64R) and 10.8-14.6 mu G, respectively. We found that the initial density rho(0) is consistent with the mean density of the nearly critical magnetized filament with magnetic field strength B-0 and radius R-0. The relatively dense initial condition for core formation can be naturally understood if the origin of the core is the fragmentation of magnetized filaments.