Multiwavelength Stellar Polarimetry of the Filamentary Cloud IC5146. I. Dust Properties

Abstract

Abstract We present optical and near-infrared stellar polarization observations toward the dark filamentary clouds associated with IC5146. The data allow us to investigate the dust properties (this paper) and the magnetic field structure (Paper II). A total of 2022 background stars were detected in the R c , , H, and/or K bands to mag. The ratio of the polarization percentage at different wavelengths provides an estimate of , the wavelength of the peak polarization, which is an indicator of the small-size cutoff of the grain size distribution. The grain size distribution seems to significantly change at mag, where both the average and dispersion of decrease. In addition, we found μm for mag, which is larger than the ∼0.55 μm in the general interstellar medium (ISM), suggesting that grain growth has already started in low-A V regions. Our data also reveal that polarization efficiency ( ) decreases with A V as a power law in the R c , , and K bands with indices of −0.71 ± 0.10, −1.23 ± 0.10, and −0.53 ± 0.09. However, H-band data show a power index change; the PE varies with A V steeply (index of −0.95 ± 0.30) when mag, but softly (index of −0.25 ± 0.06) for greater A V values. The soft decay of PE in high-A V regions is consistent with the radiative alignment torque model, suggesting that our data trace the magnetic field to mag. Furthermore, the breakpoint found in the H band is similar to that for A V , where we found the dispersion significantly decreased. Therefore, the flat PE–A V in high-A V regions implies that the power-index changes result from additional grain growth.