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

TL;DR

This work integrates multi-wavelength ALMA polarimetry to map dust polarization from envelope to disk scales around two nearby Taurus YSOs, K04166 and K04169. By modeling Stokes I with disk+envelope components and analyzing polarization across 3 mm and 1.4 mm data, the authors distinguish magnetically aligned-grain polarization in envelopes from self-scattering in disks, revealing a transition in the dominant mechanism around $20$–$50$ au. They find K04166 to host a substantial envelope with a small disk and an hourglass magnetic field, while K04169 is disk-dominated with minimal envelope, yet both systems show evidence of large grains and early dust evolution, suggesting sequential star formation in the B213 filament. These results illuminate how magnetic fields and dust growth evolve concurrently during the earliest stages of star formation and provide a framework for interpreting polarization in deeply embedded protostars with upcoming multi-frequency ALMA observations.

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.

Figures (8)

• Figure 1: Overview of Stokes $I$ dust continuum emission at different scales. Panel (a): Previous observations toward B213. Green contours represent the 2 mm dust continuum emission derived from IRAM-30m/NIKA at the contour levels of $[2,5,10,20]\times0.068\,\mathrm{MJy\,sr^{-1}}$Bracco2017AA...604A..52B. Background grayscale represents the Herschel $\mathrm{H_2}$ column density map. The symbols point to the source positions previously identified with X-ray Gudel_2007AA...468..353G, near-IR Cutri_2003yCat.2246....0C, mid-IR Cutri_2012wise.rept....1C, and FIR Marsh_2014MNRAS.439.3683MMarsh_2016MNRAS.459..342M, along with a T-Tauri source identified based on NIR-MIR Davis_2010MNRAS.405..759D. Panels (b) and (d): Zoom-in images of each source at $\sim$3000 au scale. Background grayscale and blue contours represent the total intensity of the 1mm-combine continuum emission, with its contour levels of [3, 8, 30, 100, 300, 500, 700]$\times89\mathrm{\mu Jy\,beam^{-1}}$ in panel (b) and [3, 8, 30, 100, 300, 500, 700]$\times116\mathrm{\mu Jy\,beam^{-1}}$ in panel (d). The magenta contours represent the total intensity of the 3mm continuum, and the contour levels are [5, 10, 50, 200, 400, 600]$\times17\mathrm{\mu Jy\,beam^{-1}}$ in panel (b) and [5, 10, 50, 200, 400, 600]$\times18\mathrm{\mu Jy\,beam^{-1}}$ in panel (d). The blue and magenta ellipses at the bottom left correspond to the synthesized beam of the 1mm-combine and 3 mm maps in both panels. Panels (c) and (e): Zoom-in images of panels (b) and (d) at $\sim$900 au scale. Grayscale and the magenta contours represent the total intensity of the 3mm data, and the contour levels are the same as panels (b) and (d). Yellow contours represent the total intensity of the 1mm-high continuum, at [5, 10, 20, 100, 500, 1000, 1500, 2000, 2500, 3000]$\times33\mathrm{\mu Jy\,beam^{-1}}$ levels in panel (c) and [5, 100, 500, 1000, 1500, 2000, 2500, 3000]$\times28\mathrm{\mu Jy\,beam^{-1}}$ levels in panel (e). Red and blue arrows represent directions of red- and blue-shifted $\mathrm{^{12}CO}$ outflow lobes with position angle of 32$\degr$ for K04166 Narayanan_2012MNRAS.425.2641N and 64$\degr$ for K04169 Takakuwa_2018ApJ...865...51T. Magenta and yellow ellipses at the bottom left correspond to the synthesized beam of the 3 mm and 1mm-high maps in both panels. Root-mean-square (rms) values of the contours are listed in Table \ref{['tab:para-continuum']}.
• Figure 2: 1.4 mm dust continuum emission visibility amplitudes as a function of baseline length (circularly averaged in logarithmically spaced bins), for both K04166 (left) and K04169 (right). The visibility amplitudes were derived from 1mm-combine data and are normalized to the highest flux (at a baseline $5.6 \,\mathrm{k}\lambda$), for comparison purposes. A logarithmic scale is used for the uv-distance, as it encompasses several orders of magnitude, to enhance the visibility of all data points. The profiles of the two sources are shown, with associated error bars reflecting the dispersion of amplitudes in the bin and the best-fit models combining a Plummer envelope and a Gaussian disk are shown as the red curves on top of the data points.
• Figure 3: Polarization properties for K04166; 3 mm at $\sim$230 au resolution (panels a and d), 1mm-combine at the $\sim$156 au resolution (panels b and e), and 1mm-high at $\sim$36 au resolution (panels c and f). Top row: Total intensity at [5, 10, 50, 200, 400, 600]$\times17\,\mathrm{\mu Jy\,beam}^{-1}$, [5, 8, 10, 30, 100, 200, 400, 700]$\times89\,\mathrm{\mu Jy\,beam}^{-1}$, and [5, 10, 20, 100, 500, 1000, 1500, 2000, 2500, 3000]$\times33\,\mathrm{\mu Jy\,beam}^{-1}$ in panels a, b, and c, respectively, shown as the color scale and white contours. The green segments denote the B-vectors. The 1$\sigma$ for the white contours are listed in Table \ref{['tab:para-continuum']}. The blue and red arrows in panel a indicate the direction of the blue- and red-shifted outflow lobes. The magenta square in panel b corresponds to the map size in panels c and f, respectively. The map size is the same between panels a, b, d, and e. Bottom row: Polarization fraction and polarized intensity, respectively, shown as the grayscale and green contours. The contour levels are [3, 5]$\times16\,\mathrm{\mu Jy\,beam}^{-1}$, [3, 5, 8]$\times47\mathrm{\mu Jy\,beam}^{-1}$, and [3, 5, 15, 30]$\times18\,\mathrm{\mu Jy\,beam}^{-1}$ in panels d, e, and f, respectively. The blue crosses point to the peak of total intensity of the same data as in the top row. The orange segments are the polarization vectors, perpendicular to the B-vectors in the top panel. The synthesized beam is presented at the bottom left side of each panel. We note that all segments in this paper are shown at fixed lengths, which do not relate to the polarization fraction value, as the following figures in this paper also present the segments with a fixed length.
• Figure 4: Polarization properties for K04169; 3 mm (panels a and d), 1mm-combine (panels b and e), and 1mm-high (panels c and f). Top row: Total intensity at [5,155,305,455,605]$\times18\,\mathrm{\mu Jy\,beam}^{-1}$, [5, 155, 305, 455, 605, 755]$\times116\,\mathrm{\mu Jy\,beam}^{-1}$, and [5, 505, 1005, 1505, 2005, 2505, 3005]$\times28\,\mathrm{\mu Jy\,beam}^{-1}$ in panels a, b, and c, respectively, shown as the color scale and white contours. The green segments denote the B-vectors. 1$\sigma$ for the white contours are listed in Table \ref{['tab:para-continuum']}. The 1$\sigma$ for the white contours are listed in Table \ref{['tab:para-continuum']}. The blue and red arrows in panel a indicate the direction of the blue- and red-shifted outflow lobes. The magenta square in panel b corresponds to the map size in panels c and f, respectively. The map size is the same between panels a, b, d, and e. Bottom row: the grayscale and green contours represent the polarization fraction and polarized intensity, respectively. The contour levels are [3, 4]$\times15.8\,\mathrm{\mu Jy\,beam}^{-1}$, [3, 5, 10]$\times35.0\,\mathrm{\mu Jy\,beam}^{-1}$, and [3, 5, 10]$\times20.1\,\mathrm{\mu Jy\,beam}^{-1}$ in panels d, e, and f, respectively. The blue crosses point to the peak of total intensity of the same data as in the top row. The orange segments are the polarization vectors, perpendicular to the B-vectors in the top panel. The synthesized beam is presented at the bottom left side of each panel.
• Figure 5: Histogram of position angles of the polarization vector around the disks, i.e., from the 1mm-high data. Left: Histogram for K04166 with a two-component Gaussian fitting denoted by the blue solid line. The gray-shaded bins were derived within a circle with a radius of 1.5$^{\prime\prime}$ from the peak position in the total intensity, covering the extended polarized emission around the source. The dashed yellow and green vertical lines point to the position angles at the fitting peaks. The dashed red vertical line corresponds to the disk's minor axis in the continuum data (Table \ref{['tab:imfit_results']}). Right: Histogram for K04169 with a one-component Gaussian fitting denoted by the blue solid line. The gray-shaded bins were derived within a circle with a radius of 0.5$^{\prime\prime}$ from the peak position in the total intensity covering the compact structure in polarized intensity associated with K04169. The dashed yellow and red vertical lines are the same as those in the left panel.