Optical Coronagraphic Spectroscopy of AU Mic: Evidence of Time Variable Colors?

We present coronagraphic long slit spectra of AU Mic's debris disk taken with the STIS instrument aboard the Hubble Space Telescope. Our spectra are the first spatially-resolved, scattered light spectra of the system's disk, which we detect at projected distances between approximately 10 and 45 au. Our spectra cover a wavelength range between 5200 and 10200 angstrom. We find that the color of AU Mic's debris disk is bluest at small (12-17 au) projected separations. These results both confirm and quantify the findings qualitatively noted by Krist et al. and are different than IR observations that suggested a uniform blue or gray color as a function of projected separation in this region of the disk. Unlike previous literature, which reported that the color of AU Mic's disk became increasingly more blue as a function of projected separation beyond similar to 30 au, we find the disk's optical color between 35 and 45 au to be uniformly blue on the southeast side of the disk and decreasingly blue on the northwest side. We note that this apparent change in disk color at larger projected separations coincides with several fast, outward moving "features" that are passing through this region of the southeast side of the disk. We speculate that these phenomenon might be related and that the fast moving features could be changing the localized distribution of sub-micron-sized grains as they pass by, thereby reducing the blue color of the disk in the process. We encourage follow-up optical spectroscopic observations of AU Mic to both confirm this result and search for further modifications of the disk color caused by additional fast moving features propagating through the disk.