The origin of dust polarization in the Orion Bar

The linear polarization of thermal dust emission provides a powerful tool to probe interstellar and circumstellar magnetic fields, because aspherical grains tend to align themselves with magnetic fields lines. However, while the Radiative Alignment Torque (RAT) theory provides a quantitative framework for the understanding of this phenomenon, some aspects of this grain alignment mechanism still need to be quantitatively tested. One such aspect is the possibility that the reference direction for the alignment may change from the magnetic field ("brat") to the radiation field k-vector ("krat") in areas of strong radiation fields, such as the regions affected by massive star formations feedback mechanisms. Currently, the poor understanding of the B- to k-RAT transition precludes the opportunity of making reliable measurements of the magnetic fields, and thus magnetic field support, toward HII regions or PDRs, for example. In order to provide a well-characterized prototypical system to compare to ab initio RAT theory, our work focuses on investigating this grain alignment transition toward the Orion Bar that undergoes intense irradiation from the trapezium cluster, the most massive O-type star in the cluster, with multi-wavelength SOFIA HAWC+ chop-nod and scan-pol dust polarization observations. The aim is to quantify to what extent the k-RAT mechanism could contribute to the polarization, and to extrapolate our conclusions to other regions of similar conditions. From our estimation of the radiation field and volume density, we can predict the grain size above which this alignment transition can occur. However, we also discuss the rotational grain disruption of grains, that potentially takes place on the irradiated edge of the bar. We conclude that most grains should be rotationally disrupted before they could reach the typical size after which the alignment shifts from "brat" to "krat".