A model of dusty envelope around Vega.

A simplified evolutionary model is proposed for the dust shell around Alpha Lyr, assuming that the grains retain the same size distribution as they proceed to coalesce. The infrared continuum predicted by the model will best fit the airborne and IRAS data for Vega if it has shell mass 0.0002 solar mass, radius 100 AU, and age 100 Myr. Comparison with a similar model calculated for the circumsolar dust nebula implies that A-star dust shells should have a much larger optical depth, and thus be more readily detectable in the infrared, than the shells around later-type stars.