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Register a new userValidating Scattering-Induced (Sub)millimeter Disk Polarization through the Spectral Index, Wavelength-Dependent Polarization Pattern, and Polarization Spectrum: The Case of HD 163296
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An increasing number of young circumstellar disks show strikingly ordered (sub)millimeter polarization orientations along the minor axis, which is strong evidence for polarization due to scattering by ~0.1 mm sized grains. To test this mechanism further, we model the dust continuum and polarization data of HD 163296, one of the best observed disks with prominent rings and gaps, using the RADMC-3D radiative transfer code. We find that scattering by grains with a maximum size of 90$mu$m can simultaneously reproduce the polarization observed at 0.87 mm (ALMA Band 7) and the unusually low spectral index of $alpha$ ~ 1.5 between 0.87 and 1.25 mm (ALMA Band 6) in the optically thick inner disk as a result of more efficient scattering at a shorter wavelength. The relatively low spectral index of $alpha$ ~ 2.5 inferred for the optically thin gaps is also reproduced by the same (relatively small) grains, as a result of telescope beam averaging of the gaps (with an intrinsic $alpha$ ~ 4) and their adjacent optically thick rings (where $alpha$ << 2). In this case, the long-standing tension between the grain sizes inferred from polarization and spectral index disappears because the relatively low $alpha$ values are illusory and do not require large mm-sized grains. In addition, the polarization fraction has a unique pattern of azimuthal variation: higher along the major axis than the minor axis in the gaps but higher along the minor axis than the major axis in the rings. We find a rapidly declining polarization spectrum (with the fraction $p propto lambda^{-3}$ approximately) in the gaps, which becomes flattened or even inverted towards short wavelengths in the optically thick rings. These contrasting behaviors in the rings and gaps provide further tests of scattering-induced polarization that can be tested via multi-wavelength observations that resolve the disk substructure.
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The mechanism for producing polarized emission from protostellar disks at (sub)millimeter wavelengths is currently uncertain. Classically, polarization is expected from non-spherical grains aligned with the magnetic field. Recently, two alternatives have been suggested. One polarization mechanism is caused by self-scattering from dust grains of sizes comparable to the wavelength while the other mechanism is due to grains aligned with their short axes along the direction of radiation anisotropy. The latter has recently been shown as a likely mechanism for causing the dust polarization detected in HL Tau at 3.1 mm. In this paper, we present ALMA polarization observations of HL Tau for two more wavelengths: 870 $mu$m and 1.3 mm. The morphology at 870 $mu$m matches the expectation for self-scattering, while that at 1.3 mm shows a mix between self-scattering and grains aligned with the radiation anisotropy. The observations cast doubt on the ability of (sub)millimeter continuum polarization to probe disk magnetic fields for at least HL Tau. By showing two distinct polarization morphologies at 870 $mu$m and 3.1 mm and a transition between the two at 1.3 mm, this paper provides definitive evidence that the dominant (sub)millimeter polarization mechanism transitions with wavelength. In addition, if the polarization at 870 $mu$m is due to scattering, the lack of polarization asymmetry along the minor axis of the inclined disk implies that the large grains responsible for the scattering have already settled into a geometrically thin layer, and the presence of asymmetry along the major axis indicates that the HL Tau disk is not completely axisymmetric.
Dust polarization in millimeter (and centimeter) has been mapped in disks around an increasing number of young stellar objects. It is usually thought to come from emission by magnetically aligned (non-spherical) grains, but can also be produced by dust scattering. We present a semi-analytic theory of disk polarization that includes both the direction emission and scattering, with an emphasis on their relative importance and how they are affected by the disk inclination. For face-on disks, both emission and scattering tend to produce polarization in the radial direction, making them difficult to distinguish, although the scattering-induced polarization can switch to the azimuthal direction if the incident radiation is beamed strongly enough in the radial direction in the disk plane. Disk inclination affects the polarizations from emission and scattering differently, especially on the major axis where, in the edge-on limit, the former vanishes while the latter reaches a polarization fraction as large as 1/3. The polarizations from the two competing mechanisms tend to cancel each other on the major axis, producing two low polarization holes (one on each side of the center) under certain conditions. We find tantalizing evidence for at least one such hole in NGC1333 IRAS4A1, whose polarization observed at 8 mm on the 100 AU scale is indicative of a pattern dominated by scattering close to the center and by direction emission in the outer region. If true, it would imply not only that a magnetic field exists on the disk scale, but that it is strong enough to align large, possibly mm-sized, grains.
We propose a robust spectral beam combining scheme using wavelength dependent polarisation rotators and polarization beam combiners. We successfully demonstrated the concept for two Yb-doped fiber lasers at 1064nm and 1092nm up to a total input power of 90W. The results reveal a very good combining efficiency and the potential for scaling to high power operations in this method of beam combining.
We present a technique to determine the polarization properties of a telescope through observations of spectral lines that have no intrinsic linear polarization signals. For such spectral lines, any observed linear polarization must be induced by the telescope optics. We apply the technique to observations taken with the SPINOR at the DST and demonstrate that we can retrieve the characteristic polarization properties of the DST at three wavelengths of 459, 526, and 615 nm. We determine the amount of crosstalk between the intensity Stokes I and the linear and circular polarization states Stokes Q, U, and V, and between Stokes V and Stokes Q and U. We fit a set of parameters that describe the polarization properties of the DST to the observed crosstalk values. The values for the ratio of reflectivities X and the retardance tau match those derived with the telescope calibration unit within the error bars. Residual crosstalk after applying a correction for the telescope polarization stays at a level of 3-10%. We find that it is possible to derive the parameters that describe the polarization properties of a telescope from observations of spectral lines without intrinsic linear polarization signal. Such spectral lines have a dense coverage (about 50 nm separation) in the visible part of the spectrum (400-615 nm), but none were found at longer wavelengths. Using spectral lines without intrinsic linear polarization is a promising tool for the polarimetric calibration of current or future solar telescopes such as DKIST.
The mechanisms causing millimeter-wave polarization in protoplanetary disks are under debate. To disentangle the polarization mechanisms, we observe the protoplanetary disk around HL Tau at 3.1 mm with the Atacama Large Millimeter/submillimeter Array (ALMA), which had polarization detected with CARMA at 1.3 mm. We successfully detect the ring-like azimuthal polarized emission at 3.1 mm. This indicates that dust grains are aligned with the major axis being in the azimuthal direction, which is consistent with the theory of radiative alignment of elongated dust grains, where the major axis of dust grains is perpendicular to the radiation flux. Furthermore, the morphology of the polarization vectors at 3.1 mm is completely different from those at 1.3 mm. We interpret that the polarization at 3.1 mm to be dominated by the grain alignment with the radiative flux producing azimuthal polarization vectors, while the self-scattering dominates at 1.3 mm and produces the polarization vectors parallel to the minor axis of the disk. By modeling the total polarization fraction with a single grain population model, the maximum grain size is constrained to be $100{rm~mu m}$, which is smaller than the previous predictions based on the spectral index between ALMA at 3 mm and VLA at 7 mm.
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