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

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 Rc, , 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 m for mag, which is larger than the ∼0.55 m in the general interstellar medium (ISM), suggesting that grain growth has already started in low-AV regions. Our data also reveal that polarization efficiency ( ) decreases with AV as a power law in the Rc, , 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 AV steeply (index of −0.95 0.30) when mag, but softly (index of −0.25 0.06) for greater AV values. The soft decay of PE in high-AV 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 AV, where we found the dispersion significantly decreased. Therefore, the flat PE-AV in high-AV regions implies that the power-index changes result from additional grain growth.