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

Artikel i vetenskaplig tidskrift, 2021

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, (CO)-C-12 (3-2), (CO)-C-13 (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 mu m), 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 similar to 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 (CO)-C-12 and (CO)-C-13 line emission at a similar to 10 sigma and similar to 5 sigma significance, respectively. The polarization was detected in the line wings, reaching a peak polarization fraction of similar to 5% and similar to 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 (CO)-C-12 and (CO)-C-13 emission, is found at a less than or similar to 3 sigma 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.