SCExAO/CHARIS Near-Infrared Scattered-Light Imaging and Integral Field Spectropolarimetry of the AB Aurigae Protoplanetary System (2410.11939v1)

Abstract: We analyze near-infrared integral field spectropolarimetry of the AB Aurigae protoplanetary disk and protoplanet (AB Aur b), obtained with SCExAO/CHARIS in 22 wavelength channels covering the J, H, and K passbands ($\lambda_{\rm o}$ = 1.1--2.4 $\mu m$) over angular separations of $\rho$ $\approx$ 0.13" to 1.1" ($\sim$20--175 au). Our images resolve spiral structures in the disk in each CHARIS channel. At the longest wavelengths, the data may reveal an extension of the western spiral seen in previous polarimetric data at $\rho$ $<$ 0.3" out to larger distances clockwise from the protoplanet AB Aur b, coincident with the ALMA-detected $CO$ gas spiral. While AB Aur b is detectable in complementary total intensity data, it is a non-detection in polarized light at $\lambda$ $>$ 1.3 $\mu $m. While the observed disk color is extremely red across $JHK$, the disk has a blue intrinsic scattering color consistent with small dust grains. The disk's polarization spectrum is redder than AB Aur b's total intensity spectrum. The polarization fraction peaks at $\sim$ 0.6 along the major disk axis. Radiative transfer modeling of the CHARIS data shows that small, porous dust grains with a porosity of $p$ = 0.6--0.8 better reproduce the scattered-light appearance of the disk than more compact spheres ($p$ = 0.3), especially the polarization fraction. This work demonstrates the utility of integral field spectropolarimetry to characterize structures in protoplanetary disks and elucidate the properties of the disks' dust.

• The paper uses SCExAO/CHARIS integral field spectropolarimetry to resolve the AB Aurigae protoplanetary disk and companion AB Aur b across JHK bands, analyzing scattered light and polarization.
• The study resolves spiral disk structures and finds that while AB Aur b is detected in total intensity, it is undetected in polarized light for wavelengths greater than 1.3 µm.
• Radiative transfer modeling indicates the disk's scattered light properties are best reproduced by small, highly porous dust grains with a porosity of 0.6-0.8, providing insights into disk composition.

Insights into the AB Aurigae Protoplanetary System through SCExAO/CHARIS Observations

The paper presents a comprehensive analysis of the AB Aurigae protoplanetary system utilizing near-infrared scattered-light imaging and integral field spectropolarimetry (IFSP) carried out with the SCExAO/CHARIS instrument. The paper focuses on resolving the disk and protoplanet AB Aur b across 22 wavelength channels from 1.1-2.4 µm, covering the J, H, and K passbands, and exploring angular separations of approximately 0.13" to 1.1" (equivalent to 20–175 au).

Key Observations and Results

1. Disk Structure: The SCExAO/CHARIS data resolves spiral structures within the protoplanetary disk at all wavelength channels. Notably, the longest wavelengths hint at the possible extension of the western spiral structure, aligning with previous ALMA observations of $CO$ gas spirals and polarimetric data.
2. Protoplanet Detection: While AB Aur b is detected in total intensity data, it remains undetected in polarized light for wavelengths $\lambda > 1.3 \mu m$. This finding is critical, suggesting AB Aur b does not produce a significant polarization signature at longer wavelengths.
3. Scattering Properties: The observed disk has a red appearance across JHK bands; however, the intrinsic scattering color of the disk is blue, indicating a dominance of small dust grains. The polarization spectrum presented is redder than the AB Aur b's total intensity spectrum.
4. Polarization Fraction and Radiative Transfer Modeling: The paper finds a polarization fraction peaking at approximately 0.6 along the major disk axis. Through radiative transfer modeling, the data suggest that small, porous dust grains with a porosity of 0.6-0.8 replicate the observed scattered-light properties more accurately than compact, low-porosity dust grains.

Implications

• Understanding Disk Composition: The disk’s polarization characteristics provide insight into the dust grain composition and structure. The preference for highly porous grains supports models where small, fluffy aggregates dominate the scattering process, influencing the overall disk morphology seen in scattered light.
• Protoplanet Formation: The non-detection of significant polarized light from AB Aur b might imply characteristics about the young protoplanet's atmosphere or its surrounding environment, impacting theoretical models of protoplanetary atmospheres and their detectability.
• Technique Validation: This research highlights the utility of IFSP in resolving intricate structures within protoplanetary disks and characterizing dust properties, which are pivotal for advancing our understanding of planet formation processes.

Future Directions

The outcomes of this paper set the stage for further exploration of protoplanetary disks using advanced adaptive optics and polarization techniques. Extension of these methodologies to other similar systems could help establish broader patterns in disk morphology and dust properties, contributing to a universal model of planet formation conditions across different stellar environments. Enhanced sensitivity through continued technological advancements will also facilitate more detailed investigations of faint features like protoplanetary companions and subtle disk structures. This research underscores the importance of combining total and polarized intensity data to unravel the complexities of circumstellar environments.