The Transition of Polarized Dust Thermal Emission from the Protostellar Envelope to the Disk Scale

Abstract

Polarized dust continuum emission has been observed with ALMA in an increasing number of deeply embedded protostellar systems. It generally shows a sharp transition going from the protostellar envelope to the disk scale, with the polarization fraction typically dropping from $\sim 5\%$ to $\sim 1\%$ and the inferred magnetic field orientations becoming more aligned with the major axis of the system. We quantitatively investigate these observational trends using a sample of protostars in the Perseus molecular cloud and compare these features with a non-ideal MHD disk formation simulation. We find that the gas density increases faster than the magnetic field strength in the transition from the envelope to the disk scale, which makes it more difficult to magnetically align the grains on the disk scale. Specifically, to produce the observed $\sim 1\%$ polarization at $\sim 100\mathrm{au}$ scale via grains aligned with the B-field, even relatively small grains of $1\mu \mathrm{m}$ in size need to have their magnetic susceptibilities significantly enhanced (by a factor of $\sim 20$) over the standard value, potentially through superparamagnetic inclusions. This requirement is more stringent for larger grains, with the enhancement factor increasing linearly with the grain size, reaching $\sim 2\times 10^4$ for millimeter-sized grains. Even if the required enhancement can be achieved, the resulting inferred magnetic field orientation in the simulation does not show a preference for the major axis, which is inconsistent with the observed pattern. We thus conclude that the observed trends are best described by the model where the polarization on the envelope scale is dominated by magnetically aligned grains and that on the disk scale by scattering.

13 pages, 6 figures. To be published in MNRAS

References in corpus (14)

• Theory of Star Formation
• Athena: A New Code for Astrophysical MHD
• Planck intermediate results. XIX. An overview of the polarized thermal emission from Galactic dust
• Tracing Magnetic Fields with Aligned Grains
• A Large Catalog of Accurate Distances to Local Molecular Clouds: The Gaia DR2 Edition
• Interferometric observations of magnetic fields in forming stars
• ALMA observations of dust polarization and molecular line emission from the Class 0 protostellar source Serpens SMM1
• A Sub-arcsecond Survey Toward Class 0 Protostars in Perseus: Searching for Signatures of Protostellar Disks
• Alignment Between Protostellar Outflows and Filamentary Structure
• Dust Polarization Toward Embedded Protostars in Ophiuchus with ALMA. II. IRAS 16293-2422
• Synthetic Observations of Magnetic Fields in Protostellar Cores
• Size limit of superparamagnetic inclusions in dust grains and difficulty of magnetic grain alignment in protoplanetary disks
• Probing the Temperature Structure of Optically Thick Disks Using Polarized Emission of Aligned Grains
• Linear dust polarization during the embedded phase of protostar formation

Cited by in corpus (4)

• Physical Modeling of Dust Polarization from Magnetically Enhanced Radiative Torque (MRAT) Alignment in Protostellar Cores with POLARIS
• Is the mm/submm dust polarization a robust tracer of the magnetic field topology in protostellar envelopes? A model exploration
• Synthetic Modelling of Polarized Dust Emission in Intermediate-Mass YSOs: I: Constraining the Role of Iron Inclusions and Inelastic Relaxation on Grain Alignment with ALMA Polarization
• Modeling the Far-Infrared Polarization Spectrum of a High-Mass Star Forming Cloud