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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 IRAS Vela Shell using a published polarimetric catalog of several hundreds of objects. Our extinction maps along this ionization front (I-front) find evidence of clumpy structure with typical masses between 1.5 and 6 solar masses and a mean length scale L ~ 0.47pc. The polarimetric data allowed us to investigate the distribution of the local magnetic field in small (~pc) 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, one order of magnitude along the I-front, with the magnetic pressure generally dominating over the turbulent motions. These findings suggest that the magnetic component has 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(H2) relation toward lower density values and show that such trend continues smoothly toward low N(H2) values. This provides general support for the evolution of initially subcritical clouds to an eventual supercritical stage.