Dissecting the dust distribution and polarization around two B213 young stellar objects with ALMA

Abstract

The earliest stages of disk formation and dust evolution during the protostellar phase remain poorly constrained. Millimeter dust emission and its polarization provide key insights into the physical processes and material distribution at the envelope-disk interface. We present ALMA polarimetric observations at 1.4 mm and 3 mm of two young stellar objects in Taurus, IRAS 04166+2706 (K04166) and IRAS 04169+2702 (K04169), probing scales from 25 au to 3000 au. We model the Stokes I emission to separate disk and envelope contributions and analyze the polarization properties to identify the dominant polarization mechanisms. K04166 shows extended Stokes I and polarized emission tracing a tentative hourglass magnetic field morphology in its envelope. In the inner envelope and disk (< 100 au), the properties of the polarized emission change, suggesting either a toroidal magnetic field or the presence of large grains. In contrast, K04169 exhibits compact Stokes I and polarized emission consistent with self-scattering from the disk. Both disks are extremely compact, but only K04166 retains a substantial envelope. Our multiscale ALMA polarimetric observations reveal a transition from magnetically aligned grains in envelopes to self-scattering in disks within the transition region of 20-50 au. These results provide important clues on dust grain growth and magnetic field morphology at the disk-envelope scales. Despite being embedded in the same filament, the two sources display striking differences, indicating that K04166 is a young embedded object with a substantial envelope threaded by relatively organized magnetic fields. Meanwhile, K04169 is more evolved, likely to be a young T-Tauri star. However, in both disks, the presence of large grains already suggests a scenario of early dust evolution in disks of the Class 0 stage.