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 of BHB07-11 to demonstrate that dust grains in its spiral arms are aligned via a velocity flow mechanism rather than magnetic fields, revealing insights into grain alignment processes.

Contribution

It provides the first evidence supporting a velocity flow alignment mechanism for dust grains in a complex circumstellar disk with spiral structures.

Findings

Polarization vectors follow spiral arm structures across multiple bands.

Alignment mechanism is consistent with velocity flow, not magnetic fields.

Supports the badminton birdie-like alignment model in circumstellar environments.

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 ($\lambda$=3.1 mm), 6 ($\lambda$=1.3 mm), and 7 ($\lambda$=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.