Imaging the outward motions of clumpy dust clouds around the red supergiant Antares with VLT/VISIR

We present a 0.5"-resolution diffraction-limited 17.7 micron image of the red supergiant Antares obtained with the VLT mid-infrared instrument VISIR. The VISIR image shows six clumpy dust clouds located at 0.8"--1.8" (43--96 stellar radii = 136--306 AU) away from the star. We also detected compact emission within a radius of 0.5" from the star. Comparison of our VISIR image taken in 2010 and the 20.8 micron image taken in 1998 with the Keck telescope reveals the outward motions of four dust clumps. The proper motions of these dust clumps amount to 0.2"--0.6" in 12 years. This translates into expansion velocities (projected onto the plane of the sky) of 13--40 km/s with an uncertainty of +/-7 km/s. The velocities of the dust clumps cannot be explained by a simple accelerating outflow, implying the possible random nature of the dust cloud ejection mechanism. The inner compact emission seen in the 2010 VISIR image is presumably newly formed dust, because it is not detected in the image taken in 1998. The mass of the dust clouds is estimated to be (3-6)x10^{-9} Msun. These values are lower by a factor of 3--7 than the amount of dust ejected in one year estimated from the (gas+dust) mass-loss rate of 2x10^{-6} Msun/yr, suggesting that the continuous mass loss is superimposed on the clumpy dust cloud ejection.