The Vega Debris Disk: A Surprise from Spitzer

We present high spatial resolution mid- and far-infrared images of the Vega debris disk obtained with the Multiband Imaging Photometer for Spitzer (MIPS). The disk is well resolved, and its angular size is much larger than found previously. The radius of the disk is at least 43'' (330 AU), 70'' (543 AU), and 105'' (815 AU) in extent at 24, 70, and 160 μm, respectively. The disk images are circular, smooth, and without clumpiness at all three wavelengths. The radial surface brightness profiles follow radial power laws of r-3 or r-4 and imply an inner boundary at a radius of 11′′ ± 2′′ (86 AU). Assuming an amalgam of amorphous silicate and carbonaceous grains, the disk can be modeled as an axially symmetric and geometrically thin disk, viewed face-on, with the surface particle number density following an inverse radial power law. The disk radiometric properties are consistent with a range of models using grains of sizes ~1 to ~50 μm. The exact minimum and maximum grain size limits depend on the adopted grain composition. However, all of these models require an r-1 surface number density profile and a total mass of × 10-3 M⊕ in grains. We find that a ring, containing grains larger than 180 μm and at radii of 86-200 AU from the star, can reproduce the observed 850 μm flux, while its emission does not violate the observed MIPS profiles. This ring could be associated with a population of larger asteroidal bodies analogous to our own Kuiper Belt. Cascades of collisions starting with encounters among these large bodies in the ring produce the small debris that is blown outward by radiation pressure to much larger distances, where we detect its thermal emission. The relatively short lifetime (<1000 yr) of these small grains and the observed total mass, ~3 × 10-3 M⊕, set a lower limit on the dust production rate, ~1015 g s-1. This rate would require a very massive asteroidal reservoir for the dust to be produced in a steady state throughout Vega's life. Instead, we suggest that the disk we imaged is ephemeral and that we are witnessing the aftermath of a large and relatively recent collisional event, and a subsequent collisional cascade.