SCUBA polarization measurements of the magnetic field strengths in the L183, L1544, and L43 prestellar cores

We have mapped linearly polarized dust emission from L183 with the James Clerk Maxwell Telescope SCUBA polarimeter and have analyzed these and our previously published data for the prestellar cores L183, L1544, and L43, in order to estimate magnetic field strengths in the plane of the sky, B(pos). The analysis used the Chandrasekhar-Fermi technique, which relates the dispersion in polarization position angles to Bpos. We have used these estimates of the field strengths ( neglecting the unmeasured line-of-sight component) to find the mass-to - magnetic flux ratios lambda (in units of the critical ratio for magnetic support). Results are B(pos) approximate to 80 muG and lambda approximate to 2.6 for L183, B(pos) approximate to 140 muG and lambda approximate to 2.3 for L1544, and B(pos) approximate to 160 muG and lambda approximate to 1.9 for L43. Hence, without correction for geometrical biases, for all three cores the mass-to-flux ratios are supercritical by a factor of similar to2, and magnetic support cannot prevent collapse. However, a statistical mean correction for geometrical bias may be up to a factor of 3; this correction would reduce the individual lambda's to lambda(cor) approximate to 0.9, 0.8, and 0.6, respectively; these values are approximately critical or slightly subcritical. These data are consistent with models of star formation driven by ambipolar diffusion in a weakly turbulent medium but cannot rule out models of star formation driven by turbulence.