Grain alignment and dust evolution physics with polarisation (GRADE-POL)

Astronomy and Astrophysics, vol. 703, pp. A192

Abstract

The polarisation of light induced by aligned interstellar dust serves as a significant tool in investigating cosmic magnetic fields and dust properties, while posing 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 must 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. Here, we aim to test the MRAT mechanism in starless cores using the multi-wavelength polarisation from optical to submillimetre. Our numerical modelling of dust polarisation using the MRAT theory demonstrates that the alignment efficiency of starlight polarisation ( p ext / A V ) and the degree of thermal dust polarisation ( p em ) first decrease slowly with increasing visual extinction ( A V ) and then fall steeply as ∝ A v -1 at large A 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 V loss ) increases with 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 in the R band (0.65 μm) and the H band (1.65 μm), as well as emission polarisation in the submillimetre (870 μm). Successful modelling of observational data requires perfect alignment of large grains, which serves as evidence for the MRAT mechanism, and an increased maximum grain size with higher elongation at higher A V . The latter reveals the first evidence for a new model of anisotropic grain growth induced by magnetic grain alignment. This paper introduces the framework for probing the fundamental physics of grain alignment and dust evolution using multi-wavelength dust polarisation (GRADE-POL), and it is the first of our GRADE-POL series.

Authors 8

1. Le Ngoc Tram Aachen Leiden Observatory

   Leiden University · Max Planck Institute for Radio Astronomy · Leiden Observatory

   Affiliation as printed

   Leiden Observatory, Leiden University

   Max-Planck-Institut für Radioastronomie

2. Thiem Hoang

   University of Science and Technology · Korea Astronomy and Space Science Institute · Korea University of Science and Technology

   Affiliation as printed

   Department of Astronomy and Space Science, University of Science and Technology

   Korea Astronomy and Space Science Institute

3. Alex Lazarian

   University of Wisconsin–Madison

   Affiliation as printed

   Department of Astronomy, University of Wisconsin-Madison

4. Daniel Seifried

   University of Cologne

   Affiliation as printed

   I. Physikalisches Institut, Universität zu Köln

5. B. G. Andersson

   McDonald Observatory · The University of Texas at Austin

   Affiliation as printed

   McDonald Observatory, University of Texas at Austin

6. T. Pillai

   Massachusetts Institute of Technology

   Affiliation as printed

   Haystack Observatory, Massachusetts Institute of Technology

7. Pham Ngoc Diep

   Graduate University of Science and Technology · Trung tâm Vũ trụ Việt Nam · Vietnam Academy of Science and Technology

   Affiliation as printed

   Department of Astrophysics, Vietnam National Space Center, Vietnam Academy of Science and Technology

   Graduate University of Science and Technology, Vietnam Academy of Science and Technology

8. Lapo Fanciullo

   National Chung Hsing University

   Affiliation as printed

   National Chung Hsing University