Detection of Near-Infrared Water Ice at the Surface of the (pre)Transitional Disk of AB Aur: Informing Icy Grain Abundance, Composition, and Size

TL;DR

This study uses near-infrared imaging to detect water ice on the surface of the AB Aurigae disk, revealing complex grain compositions and surface structures that inform our understanding of icy grain abundance and disk morphology.

Contribution

It provides the first evidence of water ice detection on the surface of a (pre)transitional disk using near-infrared imaging and analyzes the implications for grain composition and disk surface properties.

Findings

Detection of water ice absorption features at 3.08 μm.

Discrepancy between ice mass fraction estimates from different wavelength colors.

Evidence of non-co-located scattering surfaces at different wavelengths.

Abstract

We present near-infrared Large Binocular Telescope Interferometer LMIRCam imagery of the disk around the Herbig Ae/Be star AB Aurigae. A comparison of surface brightness at Ks (2.16 ${\mu}$m), H2O narrowband (3.08 ${\mu}$m), and L' (3.7 ${\mu}$m) allows us to probe the presence of icy grains in this (pre)transitional disk environment. By applying Reference Differential Imaging PSF subtraction, we detect the disk at high signal to noise in all three bands. We find strong morphological differences between bands, including asymmetries consistent with observed spiral arms within 100 AU in L'. An apparent deficit of scattered light at 3.08 ${\mu}$m relative to bracketing wavelengths (Ks and L') is evocative of ice absorption at the disk surface layer. However, the ${\Delta}$(Ks-H2O) color is consistent with grains with little to no ice (0-5% by mass). The ${\Delta}$(H2O-L') color, conversely, suggests grains with a much higher ice mass fraction (~0.68), and the two colors cannot be reconciled under a single grain population model. Additionally, we find the extremely red ${\Delta}$(Ks-L') disk color cannot be reproduced under conventional scattered light modeling with any combination of grain parameters or reasonable local extinction values. We hypothesize that the scattering surfaces at the three wavelengths are not co-located, and optical depth effects result in each wavelength probing the grain population at different disk surface depths. The morphological similarity between Ks and H2O suggests their scattering surfaces are near one another, lending credence to the ${\Delta}$(Ks-H2O) disk color constraint of < 5% ice mass fraction for the outermost scattering disk layer.