The Gray Needle: Large Grains in the HD 15115 Debris Disk from LBT/PISCES/Ks and LBTI/LMIRcam/L' Adaptive Optics Imaging

We present diffraction-limited Ks band and L' adaptive optics images of the edge-on debris disk around the nearby F2 star HD 15115, obtained with a single 8.4 m primary mirror at the Large Binocular Telescope. At the Ks band, the disk is detected at signal-to-noise per resolution element (SNRE) ~ 3-8 from ~1 to 25 (45-113 AU) on the western side and from ~12 to 21 (63-90 AU) on the east. At L' the disk is detected at SNRE ~ 2.5 from ~1 to 145 (45-90 AU) on both sides, implying more symmetric disk structure at 3.8 μm. At both wavelengths the disk has a bow-like shape and is offset from the star to the north by a few AU. A surface brightness asymmetry exists between the two sides of the disk at the Ks band, but not at L'. The surface brightness at the Ks band declines inside 1'' (~45 AU), which may be indicative of a gap in the disk near 1''. The Ks – L' disk color, after removal of the stellar color, is mostly gray for both sides of the disk. This suggests that scattered light is coming from large dust grains, with 3-10 μm sized grains on the east side and 1-10 μm dust grains on the west. This may suggest that the west side is composed of smaller dust grains than the east side, which would support the interpretation that the disk is being dynamically affected by interactions with the local interstellar medium.