Polarimetry toward the IRAS vela shell.: II.: Extinction and magnetic fields

We explore correlations between visual extinction and polarization along the western side of the Infrared Astronomical Satellite ( IRAS) Vela Shell using a published polarimetric catalog of several hundred objects. Our extinction maps along this ionization front ( I-front) find evidence of clumpy structure with typical masses between 1.5 and 6 M-circle dot and a mean length scale L similar to 0.47 pc. The polarimetric data allowed us to investigate the distribution of the local magnetic field in small ( similar to parsec) scales across the I-front. Using the dispersion of polarization position angles, we find variations in the kinetic-to-magnetic energy density ratio of at least 1 order of magnitude along the I-front, with the magnetic pressure generally dominating over the turbulent motions. These findings suggest that the magnetic component makes a significant contribution to the dynamical balance of this region. Along the I-front, the mean magnetic field projected on the sky is 0.018 +/- 0.013 mG. The polarization efficiency seems to change along the I-front. We attribute high polarization efficiencies in regions of relatively low extinction to an optimum degree of grain alignment. Analysis of the mass-to-magnetic flux ratio shows that this quantity is consistent with the subcritical regime ( lambda < 1), showing that magnetic support is indeed important in the region. Our data extend the overall lambda-N( H-2) relation toward lower density values and show that such a trend continues smoothly toward low N( H-2) values. This provides general support for the evolution of initially subcritical clouds to an eventual supercritical stage.