Grain alignment and dust evolution physics with polarisation (GRADE-POL). I. Dust polarisation modelling for isolated starless cores

TL;DR

This study models dust polarisation in starless cores using the MRAT alignment theory, explaining polarisation holes and providing evidence for magnetic grain growth, thereby enhancing understanding of cosmic magnetic fields and dust properties.

Contribution

It introduces a detailed dust polarisation model based on MRAT theory that reproduces observations and reveals magnetic grain growth in starless cores.

Findings

Polarisation efficiency decreases with visual extinction, explaining polarisation holes.

Successful modelling requires perfect alignment of large grains and larger maximum grain sizes.

Evidence for anisotropic grain growth induced by magnetic alignment is presented.

Abstract

The polarisation of light induced by aligned interstellar dust serves as a significant tool in investigating cosmic magnetic fields, dust properties, and poses a challenge in characterising the polarisation of the cosmic microwave background and other sources. To establish dust polarisation as a reliable tool, the physics of the grain alignment process needs to be studied thoroughly. The Magnetically enhanced Radiative Torque (MRAT) alignment is the only mechanism that can induce highly efficient alignment of grains with magnetic fields required by polarisation observations of the diffuse interstellar medium. Our numerical modelling of dust polarisation using the MRAT theory demonstrated that the alignment efficiency of starlight polarisation ($p_{\rm ext}/A_{\rm V}$) and the degree of thermal dust polarisation ($p_{\rm em}$) first decrease slowly with increasing visual extinction ($A_{\rm V}$) and then falls steeply as $\propto A^{-1}_{\rm V}$ at large $A_{\rm V}$ due to the loss of grain alignment, which explains the phenomenon known as polarisation holes. Visual extinction at the transition from shallow to steep slope ($A^{\rm loss}_{\rm V}$) increases with the maximum grain size. By applying physical profiles suitable for a starless core 109 in the Pipe Nebula (Pipe-109), our model successfully reproduces the existing observations of starlight polarisation at R-band ($0.65\,\mu$m) and H-band ($1.65\,\mu$m), as well as emission polarisation at submillimetre ($870\,\mu$m). Successful modelling of observational data requires perfect alignment of large grains as evidence of the MRAT mechanism, and larger maximum size with higher elongation at higher $A_{\rm V}$. The latter reveals the first evidence for the new model of anisotropic grain growth induced by magnetic grain alignment.