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

Astronomy and Astrophysics, vol. 714, pp. A135

Abstract

Optical-to-near-infrared interstellar polarisation, induced by aligned dust grains, generally follows a convex wavelength dependence, known as the Serkowski relation. However, observations in the ultraviolet (UV) and at [mid-]infrared wavelengths have indicated that some of the spectra do not follow this relation. Specifically, about 25% show an excess in the degree of polarisation at mid-UV wavelengths ( λ −1 > 3 m −1 ), referred to as the super-Serkowski polarisation. For this study, we re-examined both the Serkowski and super-Serkowski spectra based on the joint effect of paramagnetic relaxation, known as the Davis-Greenstein (DG) and radiative torque (RAT) alignment. We used the observational data for HD 30614, HD 204827, HD 37903 and HD 161056 to constrain our modelling. We examined two types of radiation fields: one derived from the scaled interstellar radiation field and the other originating from a B-type star. For the super-Serkowski spectra of HD 30614 and HD 204827, our model demonstrates that RAT alignment enhanced by radiation produced from a B-type star below the Lyman limit ( λ = 912Å) can reasonably explain the observations and that a combination with the DG alignments results in a better fit for λ −1 ≥ 5.5 m −1 . For the Serkowski spectra in HD 37903 and HD 161056, only the RAT alignment by itself under the typical interstellar radiation field above the Lyman limit, within a typical cold neutral medium, can account for the observed spectra, with a combination of a very inefficient DG alignment. The capacity of our model to predict the starlight polarisation spectrum from infrared to far-UV is thus a promising tool for interpreting future missions that observe spectrophotometry in the UV bands.

Authors 7

1. Le Ngoc Tram Aachen Leiden Observatory

   Leiden University · Leiden Observatory

   Affiliation as printed

   Leiden Observatory, Leiden University

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. B. G. Andersson

   McDonald Observatory · The University of Texas at Austin

   Affiliation as printed

   McDonald Observatory, University of Texas at Austin

4. A. Lazarian

   University of Wisconsin–Madison

   Affiliation as printed

   Department of Astronomy, University of Wisconsin-Madison

5. Lapo Fanciullo

   National Chung Hsing University

   Affiliation as printed

   National Chung Hsing University

6. Bao Truong

   University of Science and Technology · Korea University of Science and Technology

   Affiliation as printed

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

7. Archana Soam

   Indian Institute of Astrophysics

   Affiliation as printed

   Indian Institute of Astrophysics