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

We present near-infrared Large Binocular Telescope LMIRCam imagery of the disk around the Herbig Ae/Be star AB Aurigae. A comparison of the surface brightness at Ks (2.16 μm), H2O narrowband (3.08 μm), and L′ (3.7 μm) allows us to probe the presence of icy grains in this (pre)transitional disk environment. By applying reference differential imaging point-spread function subtraction, we detect the disk at high signal-to-noise ratios in all three bands. We find strong morphological differences between the bands, including asymmetries consistent with the observed spiral arms within 100 au in L′ . An apparent deficit of scattered light at 3.08 μm relative to the bracketing wavelengths (Ks and L′ ) is evocative of ice absorption at the disk surface layer. However, the Δ(Ks-H2O) color is consistent with grains with little to no ice (0%-5% by mass). The Δ(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 that the extremely red Δ(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 colocated, and that the optical depth effects in each wavelength result from probing the grain population at different disk surface depths. The morphological similarity between Ks and H2O suggests that their scattering surfaces are near one another, lending credence to the Δ(Ks-H2O) disk color constraint of <5% ice mass fraction for the outermost scattering disk layer.