A study of the Chamaeleon I dark cloud and T-association – VI. Interstellar polarization, grain alignment and magnetic field

Abstract

We present new measurements of optical and near-infrared linear polarization towards 39 field stars reddened by dust in the Chamaeleon I dark cloud. New and previously published data are combined in a detailed investigation of the wavelength dependence of interstellar polarization in the cloud. The observations are well represented by the Serkowski formula, with values of the wavelength of maximum polarization (⁠|$\lambda_{\rm {max}}$|⁠) in the range |$0.47-0.75\mu\rm m$|⁠. The highest values of |$\lambda_{\rm {max}}$| are found in lines of sight that intercept the dense central region of the cloud. The ratio of total to selective extinction (⁠|$R_V = A_V/E_{B-V}$|⁠) is only weakly correlated with |$\lambda_{\rm {max}}$| suggesting a degree of independence among the populations of grains responsible for optical extinction and polarization. We show that the ratio of polarization to reddening is unusually high in Cha I, with |$p_{\rm {max}}/A_V \simeq 4.5$| per cent |${\rm {mag}}^{-1}$| in some lines of sight, indicating a remarkable degree of alignment efficiency in comparison to other dark clouds. The mean direction of the magnetic field in the plane of the sky is perpendicular to the long axis of the 6loud and parallel to the external magnetic field in the Galactic neighbourhood of Chamaeleon, suggesting that the cloud formed by uniform collapse along field lines. Our observations provide important constraints on theoretical models for both grain alignment and the relation between magnetic field, cloud collapse and star formation.