The First Detection of Spatially Resolved Mid-Infrared Scattered Light from a Protoplanetary Disk

We report spatially resolved 11.8 μm images, obtained at the W. M. Keck 10 m telescope, of the protoplanetary disk around the pre-main-sequence star HK Tau B. The mid-infrared morphology and astrometry of HK Tau B with respect to HK Tau A indicate that the flux observed in the mid-infrared from HK Tau B has been scattered off the upper surface of its nearly edge-on disk. This is the first example of a protoplanetary disk observed in scattered light at mid-infrared wavelengths. Monte Carlo simulations of this disk show that the extent (FWHM ~ 0.″5 or ~70 AU) of the scattered light nebula in the mid-infrared is very sensitive to the dust size distribution. The 11.8 μm measurement can be best modeled by a dust grain population that contains grains on the order of 1.5-3 μm in size; grain populations with exclusively submicron grain sizes or power-law size distributions that extend beyond 5 μm cannot reproduce the observed morphology. These grains are significantly larger than those expected in the interstellar medium, implying that grain growth has occurred; whether this growth is a result of dust evolution within the disk itself or had originally occurred within the dark cloud remains an open question.