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Aligned grains and scattered light found in gaps of planet-forming disk

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Abstract

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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Fig. 1: The 870-µm polarization morphology overlaid on the total-intensity images of HL Tau.
Fig. 2: The 870-µm total-intensity (Stokes I) image of HL Tau.
Fig. 3: Polarization fraction and intensity maps of HL Tau.
Fig. 4: Stokes I, polarized intensity, and polarization fraction profiles along the major axis of HL Tau (southeast to northwest).

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Data availability

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.

Code availability

This research made use of APLpy, an open-source plotting package for Python57.

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Acknowledgements

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.

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Authors and Affiliations

  1. Department of Earth, Environment and Physics, Worcester State University, Worcester, MA, USA

    Ian W. Stephens

  2. Department of Astronomy, University of Virginia, Charlottesville, VA, USA

    Zhe-Yu Daniel Lin & Zhi-Yun Li

  3. Instituto Argentino de Radioastronomía, CCT-La Plata (CONICET), Villa Elisa, Argentina

    Manuel Fernández-López

  4. Department of Astronomy, University of Illinois, Urbana, IL, USA

    Leslie W. Looney & Rachel Harrison

  5. Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing, People’s Republic of China

    Haifeng Yang

  6. School of Physics and Astronomy, Monash University, Clayton, Victoria, Australia

    Rachel Harrison

  7. National Astronomical Observatory of Japan, Tokyo, Japan

    Akimasa Kataoka

  8. Instituto de Radioastronomía y Astrofísica (IRyA), Universidad Nacional Autónoma de México (UNAM), Morelia, Mexico

    Carlos Carrasco-Gonzalez

  9. Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo, Japan

    Satoshi Okuzumi

  10. Université Grenoble Alpes, CNRS, Institut de Planétologie et d’Astrophysique (IPAG), Grenoble, France

    Ryo Tazaki

  11. Astronomical Institute, Tohoku University, Sendai, Japan

    Ryo Tazaki

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  1. Ian W. Stephens
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Contributions

This project was led by I.W.S. Polarization modelling was performed by Z.-Y.D.L. Data reduction was performed by M.F.-L. All authors analysed and discussed the observations and contributed to the paper.

Corresponding author

Correspondence to Ian W. Stephens.

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Extended data figures and tables

Extended Data Fig. 1 Polarization images of the model.

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.

Extended Data Fig. 3 Profiles along the minor axis.

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.

Extended Data Fig. 4 Model using effectively oblate grains.

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.

Extended Data Table 1 The parameters used to produce the disk model and polarization image
Full size table

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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

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