Evidence for Mass-dependent Circumstellar Disk Evolution in the 5 Myr Old Upper Scorpius OB Association

We present 4.5, 8, and 16um photometry from the Spitzer Space Telescope for204 stars in the Upper Scorpius OB Association. The data are used toinvestigate the frequency and properties of circumstellar disks around starswith masses between ~ 0.1 and 20 Msun at an age of ~ 5 Myr. We identify 35stars that have emission at 8um or 16um in excess of the stellar photosphere.The lower mass stars (~ 0.1-1.2 Msun) appear surrounded by primordial opticallythick disks based on the excess emission characteristics. Stars more massivethan ~ 1.8 Msun have lower fractional excess luminosities suggesting that theinner ~ 10 AU of the disk has been largely depleted of primordial material.None of the G and F stars (~ 1.2-1.8 Msun) in our sample have an infraredexcess at wavelengths <= 16um. These results indicate that the mechanisms fordispersing primordial optically thick disks operate less efficiently on averagefor low mass stars, and that longer time scales are available for the buildupof planetary systems in the terrestrial zone for stars with masses < 1 Msun.Comment: 19 pages, 2 figures, 1 table, accepted to ApJ Letter