Polarized (sub)millimetre emission from dust grains in circumstellar disks was initially thought to be because of grains aligned with the magnetic field1,2. However, higher-resolution multi-wavelength observations3,4,5 and improved models6,7,8,9,10 found that this polarization is dominated by self-scattering at shorter wavelengths (for example, 870 µm) and by grains aligned with something other than magnetic fields at longer wavelengths (for example, 3 mm). Nevertheless, the polarization signal is expected to depend on the underlying substructure11,12,13, and observations until now have been unable to resolve polarization in multiple rings and gaps. HL Tau, a protoplanetary disk located 147.3 ± 0.5 pc away14, is the brightest class I or class II disk at millimetre–submillimetre wavelengths. Here we show deep, high-resolution polarization observations of HL Tau at 870 µm, resolving polarization in both the rings and the gaps. We find that the gaps have polarization angles with a notable azimuthal component and a higher polarization fraction than the rings. Our models show that the disk polarization is due to both scattering and emission from the aligned effectively prolate grains. The intrinsic polarization of aligned dust grains is probably more than 10%, which is much higher than that expected in low-resolution observations (about 1%). Asymmetries and dust features that are not seen in non-polarimetric observations are seen in the polarization observations.
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This paper makes use of the following ALMA data: ADS/JAO.ALMA#2016.1.00115.S and ADS/JAO.ALMA#2019.1.01051. The observational data products generated and analysed during the current study are available in the Harvard Dataverse repository (https://dataverse.harvard.edu/dataverse/HLTau_Band7_Pol) under DOIs https://doi.org/10.7910/DVN/MM4V5M and https://doi.org/10.7910/DVN/7CQRBC. Raw ALMA data are available at ALMA Science (https://almascience.nrao.edu/aq/). Additional datasets (for example, modelling) are available from the corresponding author on request.
This research made use of APLpy, an open-source plotting package for Python57.
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ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan) and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under a cooperative agreement by Associated Universities. Z.-Y.D.L. acknowledges support from NASA 80NSSC18K1095, the Jefferson Scholars Foundation, the NRAO ALMA Student Observing Support (SOS) SOSPA8-003, the Achievements Rewards for College Scientists (ARCS) Foundation Washington Chapter, the Virginia Space Grant Consortium (VSGC) and the UVA research computing (RIVANNA). Z.-Y.L. is supported in part by NASA 80NSSC20K0533 and NSF AST-2307199. L.W.L. and R.H. acknowledge support from NSF AST-1910364 and NSF AST-2307844. C.C.-G. acknowledges support from UNAM DGAPA-PAPIIT (grant no. IG101321) and from CONACyT Ciencia de Frontera (project ID 86372). R.T. acknowledges financial support from the CNES fellowship.
The authors declare no competing interests.
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Extended data figures and tables
a, The colormap is the Stokes I image in mJy beam−1 and the line segments represent the polarization angle. The length of the segments are proportional to the polarization fraction with a 2% scale bar shown in the bottom. b, The linear polarization fraction image. c, The linear polarized intensity in mJy beam−1. The resolution is shown as a small white or black ellipse in the bottom right of each panel. The concentric ellipses on top of the disk mark the location of the gaps.
Extended Data Fig. 2 Comparisons of the profiles along the major axis between the observation and model.
The first three panels show the Stokes I (panel a), linear polarized intensity (panel b), and the linear polarization fraction (panel c). The observations are plotted in black solid lines with shaded areas showing the standard deviation. The dashed blue lines show the model profiles prior to beam convolution and the orange solid lines show the model profiles after beam convolution. Panel d shows the input optical depth of the model. The horizontal dotted line is where the optical depth is 1. The vertical solid lines mark the locations of the gaps.
These are plotted in a similar way as Extended Data Fig. 2. The additional panel d shows qp (see text for the definition). The positive offset is towards the northeast direction.
Figure caption is the same as Extended Data Fig. 1, but now with oblate grains. The color scales in panels b and c have been saturated at many locations so that the morphologies of the low-level polarization fractions and intensities are visible.
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Stephens, I.W., Lin, ZY.D., Fernández-López, M. et al. Aligned grains and scattered light found in gaps of planet-forming disk. Nature 623, 705–708 (2023). https://doi.org/10.1038/s41586-023-06648-7