Modeling Polarization Signals from Cloudy Brown Dwarfs Luhman 16 A and B in Three Dimensions

The detection of disk-integrated polarization from Luhman 16 A and B in the H band, and subsequent modeling, has been interpreted in the framework of zonal cloud bands on these bodies. Recently, Tan and Showman investigated the 3D atmospheric circulation and cloud structures of brown dwarfs with general circulation models (GCMs), and their simulations yielded complex cloud distributions showing some aspects of zonal jets, but also complex vortices that cannot be captured by a simple model. Here we use these 3D GCMs specific to Luhman 16 A and B, along with the 3D Monte Carlo radiative transfer code ARTES, to calculate their polarization signals. We adopt the 3D temperature–pressure and cloud profiles from the GCMs as our input atmospheric structures. Our polarization calculations at 1.6 μm agree well with the measured degree of linear polarization from both Luhman 16 A and B. Our calculations reproduce the measured polarization for both objects with cloud particle sizes between 0.5 and 1 μm for Luhman 16 A and of 5 μm for Luhman 16 B. We find that the degree of linear polarization can vary on hour-long timescales over the course of a rotation period. We also show that models with azimuthally symmetric band-like cloud geometries, typically used for interpreting polarimetry observations of brown dwarfs, overpredict the polarization signal if the cloud patterns do not include complex vortices within these bands. This exploratory work shows that GCMs are promising for modeling and interpreting polarization signals of brown dwarfs.

Discrete optical Zeno effect for polarization of light

Quantum Zeno effect concerns deterministic dynamics of a quantum system induced by a series of projective quantum measurements. Applying this effect in optics, one can achieve an arbitrary lossless transformation of linear polarization of light with help of linear polarizers. However, to demonstrate this effect in practice, we have to take into account unavoidable losses in each polarizer that limits probability of successful transformations. In this work, we theoretically study a realistic quantum Zeno effect with an optimal discrete set of polarizers and find the maximum success probability

A wideband dual-circularly polarized traveling wave antenna array

A dual-circularly polarized traveling wave antenna based on linear polarization array with wideband response is proposed. The circular polarization is realized by the sequential rotation arrangement of four linear polarization elements and series feeding with a phase difference 90°. Rotating four of such subarrays sequentially with a phase difference of 90° not only broadens the axial ratio (AR) bandwidth but also corrects the pattern beam deflection. Left-handed circular polarization (LHCP) and right-handed circular polarization (RHCP) are respectively achieved by exciting two different ports. The measured impedance bandwidth of the developed antenna array is 27.5 %. The 3-dB AR bandwidths of RHCP and LHCP are respectively 20.7 % and 22.4 % with the peak gains of about 8.7 and 8.3 dBic.

3D printing of metasurface-based dual-linear polarization converter

3D printing using digital light processing (DLP) technology has been studied in various fields because of its ability to create complex shapes through a simple process. In this study, DLP 3D printing was employed in the implementation of the metasurface-based dual-linear polarization converter (DLPC). The unit cell of the metasurface-based DLPC for linear polarization conversion was designed consisting of the upper and lower dipole-pair antennas connected through vias and a shielding layer that electrically shields the antennas from each other, and its fabrication was based on the characterization results of the dielectric properties of the photocurable substrate materials and electrical properties of the conductive materials used for synthesizing the metasurface. The printability evaluation of dipole pairs, vias, and a shielding layer was carried out to implement the detailed structures of the DLPC in 3D printing. The electromagnetic wave transmission characteristics of the 3D-printed 8×8 array DLPC demonstrated an orthogonal polarization conversion, as predicted by the simulation results. This study confirmed that the DLP-based 3D printing technology can go beyond the existing functions of manufacturing objects and can be applied to the implementation of various electronics based on different meta-structures.

A Revised View of the Linear Polarization in the Subparsec Core of M87 at 7 mm

The linear polarization images of the jet in the giant elliptical galaxy M87 have previously been observed with Very Long Baseline Array at 7 mm. They exhibit a complex polarization structure surrounding the optically thick and compact subparsec-scale core. However, given the low level of linear polarization in the core, it is required to verify that this complex structure does not originate from residual instrumental polarization signals in the data. We have performed a new analysis of the same data sets observed in four epochs by using the Generalized Polarization CALibration pipeline (GPCAL). This novel instrumental polarization calibration pipeline overcomes the limitations of LPCAL, a conventional calibration tool used in the previous M87 studies. The resulting images show a compact linear polarization structure with its peak nearly coincident with the total intensity peak, which is significantly different from the results of previous studies. The core linear polarization is characterized as fractional polarization of ∼0.2%–0.6% and polarization angles of ∼66°–92°, showing moderate variability. We demonstrate that, based on tests with synthetic data sets, LPCAL using calibrators having complex polarization structures cannot achieve sufficient calibration accuracy to obtain the true polarization image of M87 due to a breakdown of the “similarity approximation.” We find that GPCAL obtains more accurate D-terms than LPCAL by using observed closure traces of calibrators that are insensitive to both antenna gain and polarization leakage corruptions. This study suggests that polarization imaging of very weakly polarized sources has become possible with the advanced instrumental polarization calibration techniques.

Discovery of Molecular-line Polarization in the Disk of TW Hya

We report observations of polarized line and continuum emission from the disk of TW Hya using the Atacama Large Millimeter/submillimeter Array. We target three emission lines, 12CO (3–2), 13CO (3–2), and CS (7–6), to search for linear polarization due to the Goldreich–Kylafis effect, while simultaneously tracing the continuum polarization morphology at 332 GHz (900 μm), achieving a spatial resolution of 0.″5 (30 au). We detect linear polarization in the dust continuum emission; the polarization position angles show an azimuthal morphology, and the median polarization fraction is ∼0.2%, comparable to previous, lower frequency observations. Adopting a “shift-and-stack” technique to boost the sensitivity of the data, combined with a linear combination of the Q and U components to account for their azimuthal dependence, we detect weak linear polarization of 12CO and 13CO line emission at a ∼10σ and ∼5σ significance, respectively. The polarization was detected in the line wings, reaching a peak polarization fraction of ∼5% and ∼3% for the two molecules between disk radii of 0.″5 and 1″. The sign of the polarization was found to flip from the blueshifted side of the emission to the redshifted side, suggesting a complex, asymmetric polarization morphology. Polarization is not robustly detected for the CS emission; however, a tentative signal, comparable in morphology to that found for the 12CO and 13CO emission, is found at a ≲3σ significance. We are able to reconstruct a polarization morphology, consistent with the azimuthally averaged profiles, under the assumption that this is also azimuthally symmetric, which can be compared with future higher-sensitivity observations.