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Intrinsic polarisation of elongated porous dust grains

Kirchschlager, F; Bertrang, GH-M; Flock, M; (2019) Intrinsic polarisation of elongated porous dust grains. Monthly Notices of the Royal Astronomical Society , 488 (1) pp. 1211-1219. 10.1093/mnras/stz1763.

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Abstract

ALMA observations revealed recently polarised radiation of several protoplanetary disks in the (sub-)millimetre wavelength range. Besides self-scattering of large particles, thermal emission by elongated grains is a potential source for the detected polarisation signal. We calculate the wavelength dependent absorption and intrinsic polarisation of spheroidally shaped, micrometre and sub-millimetre sized dust grains using the discrete dipole approximation. In particular, we analyse the impact of dust grain porosity which appears to be present in disks when small grains coagulate to form larger aggregates. For the first time our results show that (a) the intrinsic polarisation decreases for increasing grain porosity and (b) the polarisation orientation flips by 90 degree for certain ratios of wavelength to grain size. We present a new method to constrain grain porosity and the grain size in protoplanetary disks using multi-wavelength polarisation observations in the far-infrared to millimetre wavelengths. Finally, we find that moderate grain porosities ($\mathcal{P}\lesssim0.7$) potentially explain the observed polarisation fraction in the system HD 142527 while highly porous grains ($\mathcal{P}>0.7$) fail unless the grain's axis ratio is extraordinarily large.

Type: Article Intrinsic polarisation of elongated porous dust grains An open access version is available from UCL Discovery 10.1093/mnras/stz1763 http://dx.doi.org/10.1093/mnras/stz1763 English This version is the version of record. For information on re-use, please refer to the publisher’s terms and conditions. polarization, interplanetary medium, circumstellar matter, stars: pre-main-sequence, dust, extinction, infrared: planetary systems UCLUCL > Provost and Vice Provost OfficesUCL > Provost and Vice Provost Offices > UCL BEAMSUCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical SciencesUCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Physics and Astronomy https://discovery.ucl.ac.uk/id/eprint/10078129