SPATIALLY RESOLVING THE HK Tau B EDGE-ON DISK FROM 1.2 TO 4.7 μm: A UNIQUE SCATTERED LIGHT DISK

We present spatially resolved scattered light images of the circumstellar disk around HK Tau B at 3.8 and 4.7 mu m taken with the Keck Telescope Laser Guide Star Adaptive Optics (AO) system, and 1.6-2.12 mu m images taken with the Very Large Telescope/NACO AO system. Combined with previously published optical Hubble Space Telescope data, we investigate the spatially resolved scattered light properties of this edge-on circumstellar disk and probe for the presence of large grains. The 0.6-3.8 mu m scattered light observations reveal strong, and in some cases, unusual, wavelength dependencies in the observed disk morphology. The separation between the two scattered light nebulae, which is directly proportional to the disk-mass-opacity product, decreases by 30% between 0.6 and 3.8 mu m. Over the same wavelength range, the FWHM of the disk nebulosity declines by a factor of two, while the flux ratio between the two nebulae increases by a factor of similar to 8. No other disk known to date shows a flux ratio that increases with wavelength. Both the FWHM and nebula flux ratio are affected by the scattering phase function and the observed behavior can most readily be explained by a phase function that becomes more forward throwing with wavelength. The multi-wavelength scattered light observations also confirm the asymmetric nature of the disk and show that the level of asymmetry is a function of wavelength. We use the MCFOST radiative transfer code to model the disk at four wavelengths, corresponding to the I, H, Ks, and L' bandpasses. A single power-law grain size distribution can recreate the observed disk properties simultaneously at all four wavelengths. Bayesian analysis of the dust parameters finds a 99% probability that the maximum grain size is 5.5 mu m or larger. We also find that the grain size distribution is steep, with a 99% probability of a power-law index of 4.2 or larger, suggesting that these large grains are a small fraction of the overall dust population. The best-fit dust asymmetry parameter for each individual wavelength shows an unusual behavior, increasing with wavelength from the optical through the near-infrared, peaking at similar to 0.8 between 2.2 and 3.8 mu m, then decreasing by a factor of two by similar to 12 mu m. Comparing the wavelength dependence of the asymmetry parameter for HK Tau B with those for the interstellar medium (ISM) and dark cloud dust models, we find considerable evolution from an ISM state and argue for the presence of grain growth within the disk. Further, comparing the wavelength dependence of the asymmetry parameter for GG Tau, HV Tau C, and HK Tau B, the three disks that have been spatially resolved in scattered light between 0.8 and 3.8 mu m, finds a diverse range of dust properties, indicating differing degrees of grain growth for disks at a similar age.