Time-variable Scattered Light in Herbig Disks Observed with Subaru/SCExAO

Abstract

Using the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) instrument, we present near-infrared K-band polarimetric imaging of nine Herbig stars selected from a volume-limited sample within 200 pc. We detect the disks around MWC 480, HD 163296, and HD 143006 for the first time with SCExAO, and compare these observations with previous VLT/SPHERE datasets to identify surface-brightness variability. In MWC 480, we resolve two azimuthal brightness dips near the disk minor axis and find evidence that one of them shifted between 2021 and 2022. In HD 163296, we identify an apparent linear azimuthal motion of a localized peak in polarized intensity along the outer ring over a 15-month baseline. The rapid motion of these features relative to the local Keplerian velocity suggests that the observed variability is driven by changing illumination rather than physical material motion.Due to uncertainties in the underlying scattering background, however, we cannot determine the precise physical origin of the variability. No significant disk variability is detected in HD 143006 over a 10-month baseline. We also report the first detection of a protoplanetary disk using the fast-PDI mode on SCExAO, illustrating both the promise and current limitations of this observing mode. Finally, we report non-detections toward HD 144432, HD 56895, PDS 76, HIP 80425, HD 148352, and HIP 81474. All non-detections with Meeus classifications belong to Group II systems and are likely self-shadowed. For these six systems, we measure the system-integrated polarization fraction and angle of linear polarization, providing quantitative constraints on their unresolved circumstellar environments.

Figures (20)

• Figure 1: CHARIS PDI results showing $Q_\phi$(left) and $U_\phi$(middle) for the 3 previously-detected disks. The color bar is set such that red is positive and blue is negative. Each disk is detected as can be seen by the positive $Q_\phi$ and noise-like $U_\phi$. The 0$.\!\!^{\prime\prime}$1 coronagraphic IWA for MWC 480 is marked by the hatched region. The black points (indicated with arrows) in the left column show the approximate location of the kinematically predicted planets from Pinte2020 assuming CCW rotation (same as the disk; Barenfeld2016Teague2019) at Keplerian speed over a 5 year baseline. The right column shows radial SNR. Signal data was extracted from $Q_\phi$ and radial noise from $U_\phi$ using equation \ref{['eq:mad std']}. The central star is masked out and does not contribute to the calculation.
• Figure 2: CHARIS PDI results showing azimuthal Stokes for the six previously unobserved targets. The quadrupole pattern in $Q_\phi$ is consistent with non-detection of extended sources (disk) and polarized point-source (star) detection.
• Figure 3: The fast-PDI results for HD 143006. The units are normalized intensity, with the color-bar truncated to allow the disk to be seen. The inner IR ring is visible on the edges of the PSF but the outer ring is obscured by background scatter.
• Figure 4: Four epochs of MWC 480 observations: Kusakabe2012 HiCIAO 2010/01/24; Garufi2024 SPHERE 2018/11/27; Ren2023 SPHERE 2021/12/10; this work SCExAO/CHARIS 2022/1/24. All images are north-up, east-left. The images are normalized to the peak brightness. The original sign of the pixel values is conserved. The HiCIAO observation has no available $Q_\phi$ data so the $PI$ product is shown. We note two shadow-like brightness dips in our SCExAO observation. Major (143) and minor (53) axes -- determined by the disk model fitting (\ref{['subsec: mwc480']}) -- are indicated by the dotted lines. Note, all angles are measured here with north-up as 0.
• Figure 5: Azimuthal brightness profiles measured at a deprojected radius of 0$.\!\!^{\prime\prime}$19 for two epochs of MWC 480 PDI observations. The y-axis is in the same normalized intensity units as Figure \ref{['fig: MWC 480 4']}. The red curves are a gaussian fit (equation \ref{['eq: gauss']}) to the brightness dips on the east side of the image (Dip #1), while the blue curves are the gaussian fit to the west (Dip #2). The PSF FWHM bars represent the angular resolution at the position of the gaussian fit centers. The dotted lines show the centers of each brightness dip with the shading showing the $1\sigma$ errors.