Polarimetry toward theIRASVela Shell. II. Extinction and Magnetic Fields

We explore correlations between visual extinction and polarization along thewestern side of the IRAS Vela Shell using a published polarimetric catalog ofseveral hundreds of objects. Our extinction maps along this ionization front(I-front) find evidence of clumpy structure with typical masses between 1.5 and6 solar masses and a mean length scale L ~ 0.47pc. The polarimetric dataallowed us to investigate the distribution of the local magnetic field in small(~pc) scales across the I-front. Using the dispersion of polarization positionangles, 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 pressuregenerally dominating over the turbulent motions. These findings suggest thatthe magnetic component has a significant contribution to the dynamical balanceof this region. Along the I-front, the mean magnetic field projected on the skyis [0.018 +/- 0.013]mG. The polarization efficiency seems to change along theI-front. We attribute high polarization efficiencies in regions of relativelylow extinction to an optimum degree of grain alignment. Analysis of themass-to-magnetic flux ratio shows that this quantity is consistent with thesubcritical regime (lambda < 1), showing that magnetic support is indeedimportant in the region. Our data extend the overall lambda-N(H2) relationtoward lower density values and show that such trend continues smoothly towardlow N(H2) values. This provides general support for the evolution of initiallysubcritical clouds to an eventual supercritical stage.Comment: 31 pages, 9 figures. Accepted for publication in Ap