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The Vega Debris Disk - A Surprise from Spitzer

Wednesday, March 9, 2005
Kate
Su (UA Steward Observatory)

One of the highlights of the IRAS mission was the discovery of a large infrared excess around Vega, which was attributed to thermal dust emission from a disk of debris orbiting the star. I will present the first high spatial resolution 24, 70 and 160 um images of Vega obtained with the Multiband Imaging Photometer for Spitzer (MIPS). The improved sensitivity and resolution of Spitzer allow us to detect fainter and more extended emission in the Vega debris disk system (roughly four times larger than found previously). The disk is well resolved, and the images are circular, smooth and without clumpiness at all three wavelengths. The radial surface brightness profiles follow radial power laws of r^-3 (inner disk) or r^-4 (outer disk), and imply an inner hole of radius 11"+/- 2" (~86 AU).

Assuming a mixture of amorphous silicate and carbonaceous grains, the disk can be modeled as an axially symmetric and geometrically thin disk, viewed face-on, with surface particle number density following an inverse radial power law. The disk radiometric properties are consistent with a power-law size distribution, n(a) ~ a^-3+/-0.5, with minimum and maximum size cut-offs ~1 um and ~40 um, respectively. We find that a ring, containing grains larger than 180 um and at a radius of 70 AU from the star, can reproduce the observed SCUBA 850 um flux, while its emission is negligible at the MIPS wavelengths. Cascades of collisions starting with encounters between 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 years) of these small grains and the observed total mass (~4xE-4 Mearth), set a lower limit on the dust production rate, ~1E15 g/s. This rate requires 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 collision involving a ~1000 km size asteroidal body.

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