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1. A survey of the linear polarization of directly imaged exoplanets and brown dwarf companions with SPHERE-IRDIS: First polarimetric detections revealing disks around DH Tau B and GSC 6214-210 B

   https://doi.org/10.1051/0004-6361/202039290  

   van Holstein, R.G. ; Stolker, T. ; Jensen-Clem, R. ; Ginski, C. ; Milli, J. ; de Boer, J. ; Girard, J.H. ; Wahhaj, Z. ; Bohn, A.J. ; Millar-Blanchaer, M.A. ; et al ( March 2021 , Astronomy & Astrophysics)

   null (Ed.)

   Context. Young giant planets and brown dwarf companions emit near-infrared radiation that can be linearly polarized up to several percent. This polarization can reveal the presence of an (unresolved) circumsubstellar accretion disk, rotation-induced oblateness of the atmosphere, or an inhomogeneous distribution of atmospheric dust clouds. Aims. We aim to measure the near-infrared linear polarization of 20 known directly imaged exoplanets and brown dwarf companions. Methods. We observed the companions with the high-contrast imaging polarimeter SPHERE-IRDIS at the Very Large Telescope. We reduced the data using the IRDAP pipeline to correct for the instrumental polarization and crosstalk of the optical system with an absolute polarimetric accuracy <0.1% in the degree of polarization. We employed aperture photometry, angular differential imaging, and point-spread-function fitting to retrieve the polarization of the companions. Results. We report the first detection of polarization originating from substellar companions, with a polarization of several tenths of a percent for DH Tau B and GSC 6214-210 B in H -band. By comparing the measured polarization with that of nearby stars, we find that the polarization is unlikely to be caused by interstellar dust. Because the companions have previously measured hydrogen emission lines and red colors, the polarization most likely originates from circumsubstellar disks. Through radiative transfer modeling, we constrain the position angles of the disks and find that the disks must have high inclinations. For the 18 other companions, we do not detect significant polarization and place subpercent upper limits on their degree of polarization. We also present images of the circumstellar disks of DH Tau, GQ Lup, PDS 70, β Pic, and HD 106906. We detect a highly asymmetric disk around GQ Lup and find evidence for multiple scattering in the disk of PDS 70. Both disks show spiral-like features that are potentially induced by GQ Lup B and PDS 70 b, respectively. Conclusions. The presence of the disks around DH Tau B and GSC 6214-210 B as well as the misalignment of the disk of DH Tau B with the disk around its primary star suggest in situ formation of the companions. The non-detections of polarization for the other companions may indicate the absence of circumsubstellar disks, a slow rotation rate of young companions, the upper atmospheres containing primarily submicron-sized dust grains, and/or limited cloud inhomogeneity. 

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2. Thermal emission and scattering by aligned grains: Plane-parallel model and application to multiwavelength polarization of the HL Tau disc

   https://doi.org/10.1093/mnras/stac753  

   Lin, Zhe-Yu Daniel ; Li, Zhi-Yun ; Yang, Haifeng ; Stephens, Ian ; Looney, Leslie ; Harrison, Rachel ; Fernández-López, Manuel ( March 2022 , Monthly Notices of the Royal Astronomical Society)

   Telescopes are now able to resolve dust polarization across circumstellar discs at multiple wavelengths, allowing the study of the polarization spectrum. Most discs show clear evidence of dust scattering through their unidirectional polarization pattern typically at the shorter wavelength of $\sim 870 \, \mu$m. However, certain discs show an elliptical pattern at ∼3 mm, which is likely due to aligned grains. With HL Tau, its polarization pattern at ∼1.3 mm shows a transition between the two patterns making it the first example to reveal such transition. We use the T-matrix method to model elongated dust grains and properly treat scattering of aligned non-spherical grains with a plane-parallel slab model. We demonstrate that a change in optical depth can naturally explain the polarization transition of HL Tau. At low optical depths, the thermal polarization dominates, while at high optical depths, dichroic extinction effectively takes out the thermal polarization and scattering polarization dominates. Motivated by results from the plane-parallel slab, we develop a simple technique to disentangle thermal polarization of the aligned grains T0 and polarization due to scattering S using the azimuthal variation of the polarization fraction. We find that, with increasing wavelength, the fractional polarization spectrum of the scattering component S decreases, while the thermal component T0 increases, which is expected since the optical depth decreases. We find several other sources similar to HL Tau that can be explained by azimuthally aligned scattering prolate grains when including optical depth effects. In addition, we explore how spirally aligned grains with scattering can appear in polarization images.

    

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3. General Strong Polarization

   https://doi.org/10.1145/3491390  

   Błasiok, Jarosław ; Guruswami, Venkatesan ; Nakkiran, Preetum ; Rudra, Atri ; Sudan, Madhu ( April 2022 , Journal of the ACM)

   Arıkan’s exciting discovery of polar codes has provided an altogether new way to efficiently achieve Shannon capacity. Given a (constant-sized) invertible matrix M , a family of polar codes can be associated with this matrix and its ability to approach capacity follows from the polarization of an associated [0, 1]-bounded martingale, namely its convergence in the limit to either 0 or 1 with probability 1. Arıkan showed appropriate polarization of the martingale associated with the matrix ( G 2 = ( 1 1 0 1) to get capacity achieving codes. His analysis was later extended to all matrices M that satisfy an obvious necessary condition for polarization. While Arıkan’s theorem does not guarantee that the codes achieve capacity at small blocklengths (specifically in length, which is a polynomial in ( 1ε ) where (ε) is the difference between the capacity of a channel and the rate of the code), it turns out that a “strong” analysis of the polarization of the underlying martingale would lead to such constructions. Indeed for the martingale associated with ( G 2 ) such a strong polarization was shown in two independent works (Guruswami and Xia (IEEE IT’15) and Hassani et al. (IEEE IT’14)), thereby resolving a major theoretical challenge associated with the efficient attainment of Shannon capacity. In this work we extend the result above to cover martingales associated with all matrices that satisfy the necessary condition for (weak) polarization. In addition to being vastly more general, our proofs of strong polarization are (in our view) also much simpler and modular. Key to our proof is a notion of local polarization that only depends on the evolution of the martingale in a single time step. We show that local polarization always implies strong polarization. We then apply relatively simple reasoning about conditional entropies to prove local polarization in very general settings. Specifically, our result shows strong polarization over all prime fields and leads to efficient capacity-achieving source codes for compressing arbitrary i.i.d. sources, and capacity-achieving channel codes for arbitrary symmetric memoryless channels. We show how to use our analyses to achieve exponentially small error probabilities at lengths inverse polynomial in the gap to capacity. Indeed we show that we can essentially match any error probability while maintaining lengths that are only inverse polynomial in the gap to capacity. 

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4. Multi-epoch spectropolarimetry for a sample of Type IIn Supernovae: persistent asymmetry in dusty circumstellar material

   https://doi.org/10.1093/mnras/stae380  

   Bilinski, Christopher ; Smith, Nathan ; Williams, G. Grant ; Smith, Paul S. ; Leonard, Douglas C. ; Hoffman, Jennifer L. ; Andrews, Jennifer E. ; Milne, Peter ( February 2024 , Monthly Notices of the Royal Astronomical Society)

   We present multi-epoch spectropolarimetry and spectra for a sample of 14 Type IIn supernovae (SNe IIn). We find that after correcting for likely interstellar polarization, SNe IIn commonly show intrinsic continuum polarization of 1–3 per cent at the time of peak optical luminosity, although a few show weaker or negligible polarization. While some SNe IIn have even stronger polarization at early times, their polarization tends to drop smoothly over several hundred days after peak. We find a tendency for the intrinsic polarization to be stronger at bluer wavelengths, especially at early times. While polarization from an electron scattering region is expected to be grey, scattering of SN light by dusty circumstellar material (CSM) may induce such a wavelength-dependent polarization. For most SNe IIn, changes in polarization degree and wavelength dependence are not accompanied by changes in the position angle, requiring that asymmetric pre-SN mass loss had a persistent geometry. While 2–3 per cent polarization is typical, about 30 per cent of SNe IIn have very low or undetected polarization. Under the simplifying assumption that all SN IIn progenitors have axisymmetric CSM (i.e. disc/torus/bipolar), then the distribution of polarization values we observe is consistent with similarly asymmetric CSM seen from a distribution of random viewing angles. This asymmetry has very important implications for understanding the origin of pre-SN mass loss in SNe IIn, suggesting that it was shaped by binary interaction.

    

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5. Characterizing the Accuracy of ALMA Linear-polarization Mosaics

   https://doi.org/10.1088/1538-3873/ab99cd  

   Hull, Charles L. H. ; Cortes, Paulo C. ; Gouellec, Valentin J. M. Le ; Girart, Josep M. ; Nagai, Hiroshi ; Nakanishi, Kouichiro ; Kameno, Seiji ; Fomalont, Edward B. ; Brogan, Crystal L. ; Moellenbrock, George A. ; et al ( July 2020 , Publications of the Astronomical Society of the Pacific)

   We characterize the accuracy of linear-polarization mosaics made using the Atacama Large Millimeter/submillimeter Array (ALMA). First, we observed the bright, highly linearly polarized blazar 3C 279 at Bands 3, 5, 6, and 7 (3 mm, 1.6 mm, 1.3 mm, and 0.87 mm, respectively). At each band, we measured the blazar’s polarization on an 11 × 11 grid of evenly spaced offset pointings covering the full-width at half-maximum (FWHM) area of the primary beam. After applying calibration solutions derived from the on-axis pointing of 3C 279 to all of the on- and off-axis data, we find that the residual polarization errors across the primary beam are similar at all frequencies: the residual errors in linear polarization fractionP_fracand polarization position angleχare ≲0.001 (≲0.1% of StokesI) and ≲ 1° near the center of the primary beam; the errors increase to ∼0.003–0.005 (∼0.3%–0.5% of StokesI) and ∼1°–5° near the FWHM as a result of the asymmetric beam patterns in the (linearly polarized)QandUmaps. We see the expected double-lobed “beam squint” pattern in the circular polarization (StokesV) maps. Second, to test the polarization accuracy in a typical ALMA project, we performed observations of continuum linear polarization toward the Kleinmann–Low nebula in Orion (Orion-KL) using several mosaic patterns at Bands 3 and 6. We show that after mosaicking, the residual off-axis errors decrease as a result of overlapping multiple pointings. Finally, we compare the ALMA mosaics with an archival 1.3 mm Combined Array for Research in Millimeter-wave Astronomy polarization mosaic of Orion-KL and find good consistency in the polarization patterns.

    

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