Multi-wavelength Stellar Polarimetry of the Filamentary Cloud IC5146: I. Dust Properties

Jia-Wei Wang; Shih-Ping Lai; C. Eswaraiah; Dan P. Clemens; Wen-Ping Chen; A. K. Pandey

doi:10.3847/1538-4357/aa937f

paper

Multi-wavelength Stellar Polarimetry of the Filamentary Cloud IC5146: I. Dust Properties

arXiv:1710.03899 · doi:10.3847/1538-4357/aa937f

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 $R_{c}$ -, $i^{'}$ -, $H$ -, and/or $K$ -bands to $A_{V} ≲ 25$ mag. The ratio of the polarization percentage at different wavelengths provides an estimate of $λ_{ma x}$ , the wavelength of 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 $A_{V} \sim$ 3 mag, where both the average and dispersion of $P_{R_{c}} / P_{H}$ decrease. In addition, we found $λ_{ma x}$ $\sim$ 0.6-0.9 $μ$ m for $A_{V} > 2.5$ mag, which is larger than the $\sim$ 0.55 $μ$ m in the general 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 $R_{c}$ -, $i^{'}$ -, and $K$ -bands with indices of -0.71 $\pm$ 0.10, -1.23 $\pm$ 0.10 and -0.53 $\pm$ 0.09. However, $H$ -band data show a power index change; the PE varies with $A_{V}$ steeply (index of -0.95 $\pm$ 0.30) when $A_{V} < 2.88 \pm 0.67$ mag but softly (index of -0.25 $\pm$ 0.06) for greater $A_{V}$ values. The soft decay of PE in high $A_{V}$ regions is consistent with the Radiative Aligned Torque model, suggesting that our data trace the magnetic field to $A_{V} \sim 20$ mag. Furthermore, the breakpoint found in $H$ -band is similar to the $A_{V}$ where we found the $P_{R_{c}} / P_{H}$ 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.

31 pages, 17 figures, and 3 tables; accepted for publication in ApJ

Multi-wavelength Stellar Polarimetry of the Filamentary Cloud IC5146: I. Dust Properties

Abstract

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