2017ApJ...849..157W

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, i^′ , H, and/or K bands to A_V 25 mag. The ratio of the polarization percentage at different wavelengths provides an estimate of λ_{max}, 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 A_V∼3 mag, where both the average and dispersion of P_Rc_/PH_ decrease. In addition, we found λ_{max}∼0.6–0.9µm for A_V 2.5 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 (PE≡P_λ/A_V) decreases with A_V as a power law in the R_c, i^′ , 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 A_V 2.88±0.67 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 A_V∼20 mag. Furthermore, the breakpoint found in the H band is similar to that for A_V, where we found the P_Rc_/PH_ 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.