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Grain Alignment and Polarization in the Envelope of IK Tau

Presentation #130.04 in the session “Evolved stars”.

Published onJun 18, 2021
Grain Alignment and Polarization in the Envelope of IK Tau

Dust grains in the interstellar medium (ISM) can align with the magnetic fields, which in turn can cause the light from background stars to become polarized. By understanding the physics behind how grains align with magnetic fields, we can better understand many astrophysical processes. It is believed that the mineralogy of dust grains influences how they are aligned. However, grain types such as carbon and silicate are thoroughly mixed in the ISM, making it near impossible to understand how specific pure grain compositions affect alignment. Asymptotic Giant Branch (AGB) stars have circumstellar envelopes (CSE) which contain a purer dust composition, making them the perfect site to study dust induced polarization. IK Tau, an oxygen rich AGB star, has a CSE that is composed of paramagnetic silicate oxides. IRC+10216 is a carbon rich AGB star with a CSE dominated by diamagnetic carbonaceous solids. Here, we contrast the behavior of the silicate dust in IK Tau with the carbonaceous dust of IRC+10216. We produced a temperature model of the CSE of IK Tau using observational data from the Herschel satellite and the HAWC+ instrument on SOFIA. This model indicates that the CSE of IK Tau is hotter than the CSE of IRC+10216. Previous research shows that polarization data from IRC+10216 is centro-symmetric, which could be due to a secondary radiative effect or a result of outflowing gas causing grains to tumble in a radial pattern. Polarization data from IK Tau does not follow this radial pattern. Being paramagnetic in nature, silicate grains can be aligned by magnetic fields. Preliminary analysis shows a possible dipole nature of the HAWC+ polarimetry in IK Tau. Current work includes utilizing a numerical minimization of parameters to find a best fit model for a dipole magnetic field.


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