Multi-band ALMA Polarization Observations of BHB07-11 Reveal Aligned Dust Grains in Complex Spiral Arm Structures

TL;DR

This study uses multi-band ALMA polarization observations (Bands 3, 6, 7) of the Class I protobinary BHB07-11 to identify the dominant dust polarization mechanism in a complex spiral-arm disk. By comparing polarization morphologies across wavelengths and with high-resolution Band 6 continuum and H$_{2}CO$ kinematics, the authors show that polarization traces spiral-arm structure and cannot be explained by dust self-scattering alone. They derive a beam-averaged opacity index $\beta\approx1.53\pm0.07$ and infer grain sizes of $\sim10$–$50\,\mu\text{m}$, arguing against a dominant $a_{\max}\sim150\,\mu\text{m}$ scattering population; morphological and timescale analyses favor the Badminton Birdie mechanical alignment mechanism over magnetic alignment. Radiative-alignment (k-RAT) is disfavored by the observed angular residuals, though some degeneracy remains between prolate-and-flow versus oblate-and-flow scenarios. Overall, the work provides evidence that spiral-arm gas–dust flows play a key role in disk polarization and suggests limited grain growth relative to ISM benchmarks, with implications for interpreting magnetic-field signatures and dust evolution in young circumstellar disks.

Abstract

Polarization-mode observations from the Atacama Large Millimeter/submillimeter Array (ALMA) are powerful tools for studying the dust grain populations in circumstellar disks. Many sources exhibit polarization signatures consistent with aligned dust grains, yet the physical origin of this alignment remains uncertain. One such source is BHB07-11, a Class I protobinary object in the Pipe Nebula with complex spiral arm structures in its circumbinary disk. While magnetic fields are often invoked to explain grain alignment in the interstellar medium, the contrasting conditions in circumstellar disk environments demand further investigation into grain alignment mechanisms. To determine BHB07-11's dominant polarization mechanism, we leverage ALMA polarization-mode dust continuum observations in Bands 3 ($λ$=3.1 mm), 6 ($λ$=1.3 mm), and 7 ($λ$=0.87 mm), in combination with high-resolution dust continuum and spectral line observations in Band 6. Observed polarization vectors in each band are consistent with emission from aligned grains and follow the structure of the spiral arms as shown in the high-resolution observations. Given the relationship between the observed polarization vector orientation and the spiral arms, we find that the polarization morphology is most consistent with grains aligned through a relative velocity flow between gas and dust in the spiral arms, as envisioned in the recently developed badminton birdie-like alignment mechanism, rather than alignment with a magnetic field or other known alignment mechanisms.

Figures (21)

• Figure 1: ALMA Band 3 (left), 6 (middle), and 7 (right) dust continuum maps of BHB07-11. Stokes I emission above a 3$\sigma$ cutoff is shown in each panel. The beam size and position angle on the sky are shown in the bottom left corner of each image. All masked emission is shown in black as indicated by the colorbar arrow.
• Figure 2: A high-resolution ALMA Band 6 dust continuum map of BHB07-11. BHB07-11A and BHB07-11B are labelled. Stokes I emission is shown above a 3$\sigma$ cutoff. The beam size and position angle on the sky is shown in the bottom left corner of the map.
• Figure 3: ALMA Band 3 (left), 6 (middle), and 7 (right) polarization mode dust continuum maps of BHB07-11. Polarized emission is shown above a 3$\sigma_{P}$ cutoff. Polarization vectors are shown in each frame, with their length proportional to the polarization percentage. Vectors are shown where polarized emission is above 4$\sigma_{P}$ in Band 3, 10$\sigma_{P}$ in Band 6, and 20$\sigma_{P}$ in Band 7, which limits the locations of the polarization vectors to lie within the disk. Vectors are Nyquist sampled across the beam major and minor axis in each map. A representative $4\%$ polarization vector key is shown in the top right of each frame, and the beam size and position angle on the sky are shown in the bottom left.
• Figure 4: The high-resolution Band 6 dust continuum map shown in Fig. \ref{['fig:Band 6 HR']} with polarization vectors from the Band 7 image shown in Fig. \ref{['fig:Band 367 P']} overlaid. As with Figure \ref{['fig:Band 367 P']}, polarization vectors are cropped above 20$\sigma$ in polarized emission to restrict them to the disk. Vectors are shown at the same intervals as in Fig. \ref{['fig:Band 367 P']}. The Band 7 (white) and high-resolution Band 6 (purple) beam sizes and position angles on the sky are shown in the bottom left, and a green vector representing 4$\%$ polarization is shown in the top right.
• Figure 5: Polarization percentage maps constructed from the ALMA Band 3, 6, and 7 Stokes I and polarized intensity images. The same polarized emission cuts from Figure \ref{['fig:Band 367 P']} are used to spatially restrict the polarization percentage. White pixels reflect masked data. The beam size and position angle are shown in the bottom left corner of each map.