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

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 , I^\prime , 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}/{P}_H decrease. In addition, we found {λ }_\max ∼ 0.6{--}0.9 μm 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}\equiv {P}_λ /{A}_V) decreases with A _V as a power law in the R _c , I^\prime , 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}/{P}_H 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.