P% vs. θ_P for the 62 sources (open circles) projected over an area of 0.3° × 0.3° around the protostar L1157-mm. The measurements are made in R_kc filter. The Planck polarization results (see Sect. 4.2) from within a 1° region around cloud L1157 are shown using filled gray circles. The Planck results from the region in which we carried out the optical polarization observations are shown using black squares. We also show the polarization values (filled triangles) of the sources distributed in a circular region of 5° radius about the protostar obtained from the Heiles (2000) catalog.