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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 Rc, i¢, H, and/or K bands to AV 25 mag. The ratio of the polarization percentage at different wavelengths provides an estimate of lmax, 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 AV ~ 3 mag, where both the average and dispersion of P P R H c decrease. In addition, we found lmax ~ 0.6 0.9 – μm for AV > 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-AV regions. Our data also reveal that polarization efficiency (PE º P A l V ) decreases with AV as a power law in the Rc, 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 AV steeply (index of −0.95 ± 0.30) when AV < 2.88 0.67 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 AV ~ 20 mag. Furthermore, the breakpoint found in the H band is similar to that for AV, where we found the P P R H c dispersion significantly decreased. Therefore, the flat PE–AV in high-AV regions implies that the power-index changes result from additional grain growth.We acknowledge support from the Aryabhatta Research Institute of Observational Sciences and Lulin Observatory concerning the data collected using AIMPOL and TRIPOL. This research was conducted in part using the Mimir instrument, jointly developed at Boston University and Lowell Observatory and supported by NASA, NSF, and the W.M. Keck Foundation. This work and the analysis software for Mimir data were developed under NSF grants AST 06-07500, 09-07790, and 14-12269 to Boston University. We thank Brian Taylor, Tao-Chung Ching, and Lauren Cashman for their help in the smooth operations of Mimir observations. This research has made use of the Herchel Science Archive (HSA), and the data are based on the observations performed with the ESA Herschel. Space Observatory (Pilbratt et al. 2010). J.W.W., S.P.L., and C.E. are grateful for the support from the Ministry of Science and Technology (MOST) of Taiwan through grants NSC 99-2923-M-008-002-MY3, NSC 101-2119-M-007-004, MOST 102-2119-M-007-004-MY3, 105-2119-M-007-022-MY3, and 105-2119-M-007-024. (NASA; NSF; W.M. Keck Foundation; AST 06-07500 - NSF; 09-07790 - NSF; 14-12269 - NSF; NSC 99-2923-M-008-002-MY3 - Ministry of Science and Technology (MOST) of Taiwan; NSC 101-2119-M-007-004 - Ministry of Science and Technology (MOST) of Taiwan; MOST 102-2119-M-007-004-MY3 - Ministry of Science and Technology (MOST) of Taiwan; 105-2119-M-007-022-MY3 - Ministry of Science and Technology (MOST) of Taiwan; 105-2119-M-007-024 - Ministry of Science and Technology (MOST) of Taiwan

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Last time updated on 09/07/2019

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