Distortion of Magnetic Fields in a Starless Core II: 3D Magnetic Field
  Structure of FeSt 1-457
  Three dimensional (3D) magnetic field information on molecular clouds and
cores is important for revealing their kinematical stability (magnetic support)
against gravity which is fundamental for studying the initial conditions of
star formation. In the present study, the 3D magnetic field structure of the
dense starless core FeSt 1-457 is determined based on the near-infrared
polarimetric observations of the dichroic polarization of background stars and
simple 3D modeling. With an obtained angle of line-of-sight magnetic
inclination axis $\theta_{\rm inc}$ of $45^{\circ}\pm10^{\circ}$ and previously
determined plane-of-sky magnetic field strength $B_{\rm pol}$ of $23.8\pm12.1$
$\mu{\rm G}$, the total magnetic field strength for FeSt 1-457 is derived to be
$33.7\pm18.0$ $\mu{\rm G}$. The critical mass of FeSt 1-457, evaluated using
both magnetic and thermal/turbulent support is ${M}_{\rm cr} = 3.70\pm0.92$
${\rm M}_{\odot}$, which is identical to the observed core mass, $M_{\rm
core}=3.55\pm0.75$ ${\rm M}_{\odot}$. We thus conclude that the stability of
FeSt 1-457 is in a condition close to the critical state. Without infalling gas
motion and no associated young stars, the core is regarded to be in the
earliest stage of star formation, i.e., the stage just before the onset of
dynamical collapse following the attainment of a supercritical condition. These
properties would make FeSt 1-457 one of the best starless cores for future
studies of the initial conditions of star formation.
