2020ApJ...905..157V

The optical and near-infrared (OIR) polarization of starlight is typically understood to arise from the dichroic extinction of that light by dust grains whose axes are aligned with respect to a local magnetic field. The size distribution of the aligned-grain population can be constrained by measurements of the wavelength dependence of the polarization. The leading physical model for producing the alignment is that of radiative alignment torques (RATs), which predicts that the most efficiently aligned grains are those with sizes larger than the wavelengths of light composing the local radiation field. Therefore, for a given grain-size distribution, the wavelength at which the polarization reaches a maximum (λ_{max}) should correlate with the characteristic reddening along the line of sight between the dust grains and the illumination source. A correlation between λ_{max} and reddening has been previously established for extinctions up to A_V~4 mag. We extend the study of this relationship to a larger sample of stars in the Taurus cloud complex, including extinctions A_V> 10 mag. We confirm the earlier results for A_V< 4 mag but find that the λ_{max} versus A_V relationship bifurcates above A_V~4 mag, with part of the sample continuing the previously observed relationship. The remaining sample exhibits a steeper rise in λ_{max} versus A_V. We propose that the data exhibiting the steep rise represent lines of sight of high-density "clumps," where grain coagulation has taken place. We present RAT-based modeling supporting these hypotheses. These results indicate that multiband OIR polarimetry is a powerful tool for tracing grain growth in molecular clouds, independent of uncertainties in the dust temperature and emissivity.