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Abstract The HH 111 protostellar disk has recently been found to host a pair of spiral arms. Here we report the dust polarization results in the disk as well as the inner envelope around it, obtained with the Atacama Large Millimeter/submillimeter Array in continuum atλ∼ 870μm and ∼0.″05 resolution. In the inner envelope, polarization is detected with a polarization degree of ∼6% and an orientation almost everywhere parallel to the minor axis of the disk and thus likely to be due to the dust grains magnetically aligned mainly by toroidal fields. In the disk, the polarization orientation is roughly azimuthal on the far side and becomes parallel to the minor axis on the near side, with a polarization gap in between on the far side near the central protostar. The disk polarization degree is ∼2%. The polarized intensity is higher on the near side than the far side, showing a near–far side asymmetry. More importantly, the polarized intensity and thus polarization degree are lower in the spiral arms but higher in between the arms, showing an anticorrelation of the polarized intensity with the spiral arms. Our modeling results indicate that this anticorrelation is useful for constraining the polarization mechanism and is consistent with the dust self-scattering by the grains that have grown to a size of ∼150μm. The interarms are sandwiched and illuminated by two brighter spiral arms and thus have higher polarized intensity. Our dust self-scattering model can also reproduce the observed polarization orientation parallel to the minor axis on the near side and the observed azimuthal polarization orientation at the two disk edges in the major axis. Further modeling work is needed to study how to reproduce the observed near–far side asymmetry in the polarized intensity and the observed azimuthal polarization orientation on the far side.
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Probing the temperature structure of optically thick discs using polarized emission of aligned grains
ABSTRACT Polarized continuum emission from aligned grains in discs around young stellar objects can be used to probe the magnetic field, radiation anisotropy, or drift between dust and gas, depending on whether the non-spherical grains are aligned magnetically, radiatively, or mechanically. We show that it can also be used to probe another key disc property – the temperature gradient – along sightlines that are optically thick, independent of the grain alignment mechanism. We first illustrate the technique analytically using a simple 1D slab model, which yields an approximate formula that relates the polarization fraction to the temperature gradient with respect to the optical depth τ at the τ = 1 surface. The formula is then validated using models of stellar irradiated discs with and without accretion heating. The promises and challenges of the technique are illustrated with a number of Class 0 and I discs with ALMA dust polarization data, including NGC 1333 IRAS4A1, IRAS 16293B, BHB 07-11, L1527, HH 212, and HH 111. We find, in particular, that the sightlines passing through the near-side of a highly inclined disc trace different temperature gradient directions than those through the far-side, which can lead to a polarization orientation on the near-side that is orthogonal to that on the far-side, and that the HH 111 disc may be such a case. Our technique for probing the disc temperature gradient through dust polarization can complement other methods, particularly those using molecular lines.
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- PAR ID:
- 10160696
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 493
- Issue:
- 4
- ISSN:
- 0035-8711
- Page Range / eLocation ID:
- 4868 to 4883
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1093/mnras/staa542
