2025ApJ...981..128P

Polarization of starlight and thermal dust emission due to aligned non-spherical grains helps us to trace magnetic field (B-field) morphology in molecular clouds and to study grain alignment mechanisms. In this work, we study grain alignment and disruption mechanisms in a filamentary infrared dark cloud G34.43+0.24 using thermal dust polarization observations from JCMT/POL-2 at 850 μm. We study three regions: the North harboring the MM3 core, the Center harboring the MM1 and MM2 cores, and the South harboring no core. We find the decrease in polarization fraction P with increasing total intensity and gas column density, known as polarization hole. To disentangle the effect of magnetic field tangling on the polarization hole, we estimate the polarization angle dispersion function. We find depolarizations in the North and Center regions are due to a decrease in the net alignment efficiency of grains, but in the South region, the effect of magnetic field tangling is significant to cause depolarization. To test whether the radiative torque (RAT) mechanism can reproduce the observational data, we calculate minimum alignment and disruption sizes of grains using RAT theory, and our study finds that the RAT alignment (RAT-A) mechanism can explain the depolarizations in the North and Center regions where the B-field tangling effect is less important, except for core regions. We find hints of RAT disruption (RAT-D) in the core regions of MM3 in the North, and MM1 and MM2 in the Center. We also find that the high P value of around 8%-20% in the outer regions of the filament can potentially be explained by the magnetically enhanced RAT alignment mechanism.