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

Abstract

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.

The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.