Discovery of a shell around Alpha Lyrae

Abstract

IRAS observations of Alpha Lyrae reveal a large infrared excess beyond 12 microns. The excess over an extrapolation of a 10,000 K blackbody is a factor of 1.3 at 25 microns, 7 at 60 microns, and 16 at 100 microns. The source of 60 microns emission has a diameter of about 20 arcsec. This is the first detection of a large infrared excess from a main-sequence star without significant mass loss. The most likely origin of the excess is thermal radiation from solid particles more than a millimeter in radius, located approximately 85 AU from Alpha Lyr and heated by the star to an equilibrium temperature of 85 K. These results provide the first direct evidence outside of the solar system for the growth of large particles from the residual of the prenatal cloud of gas and dust.