ALMA Reveals Transition of Polarization Pattern with Wavelength in HL Tau’s Disk

Abstract

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 with 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 μm and 1.3 mm. The morphology at 870 μm 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 μm 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 μm 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.

ALMA Observations of Polarization from Dust Scattering in the IM Lup Protoplanetary Disk

“…This is the result of a slight azimuthal curvature in the polarization orientations furthest from the disk center (along the major axis), seen in the top panel of Figure 1. This is consistent with the pure-scattering models of Yang et al (2016aYang et al ( , 2017, as well as with the morphology of the polarization in the 870 μm observations of HLTau reported in Stephens et al (2017). Note, however, that while the slight azimuthal curvature can be explained by pure self-scattering, it could also be due to the superposition of polarized emission both from self-scattering and from dust grains aligned with the dust emission gradient (Tazaki et al 2017).…”

“…However, the longer-wavelength (3 mm) observations can be explained by alignment with the dust emission gradient (Tazaki et al 2017). This was clearly confirmed by Stephens et al (2017), who presented well resolved ALMA observations of polarization toward HLTau at 870 μm and 1.3 mm, in addition to the 3 mm data reported in Kataoka et al (2017). The polarization morphologies at each wavelength were dramatically different: the 870 μm map showed clear evidence of dust self-scattering, and the 1.3 mm data showed a roughly equal superposition of the patterns from self-scattering and from alignment with the dust emission gradient.…”

“…As mentioned in Section 2, these bands ranged in frequency from ∼336.5 to 350.5 GHz; the maximum difference of 14 GHz yields a fractional bandwidth difference of ∼4%. We found no significant changes in polarized intensity, polarization fraction, or polarization angle as a function of frequency, indicating that multi-wavelength studies of disk polarization (e.g., the work on HL Tau by Stephens et al 2017) will require observations at multiple distinct ALMA bands with wide frequency separations.…”

“…However, only HLTau's minor-axis profile peaks at the center; its majoraxis profile has a central depression in the polarization fraction. As shown in Stephens et al (2017), the polarization orientations at 870 μm are roughly along the minor axis, thus it is unlikely that the depression is due to plane-of-sky smearing of the polarization signal within the central synthesized beam (as opposed to the 3 mm ALMA map of HL Tau shown in Kataoka et al 2017 andStephens et al 2017, where the central beam is smeared as a result of the azimuthal polarization pattern). It is more likely that the depression along the major axis is caused by HLTau's high optical depth at 870 μm-1.3 mm wavelengths (ALMA Partnership et al 2015;CarrascoGonzález et al 2016;Jin et al 2016), which is predicted to reduce the polarization fraction (Kataoka et al 2015;Yang et al 2017).…”

“…However, substantial differences between the two sources are revealed in Figure 3, which shows one-dimensional cuts of the polarization fraction P frac across the major axis (left) and minor axis (right) of both IMLup and HLTau (the 870 μm HL Tau data are from Stephens et al 2017). Both the majorand minor-axis profiles of IMLup peak at the center.…”