Evidence for mass-dependent circumstellar disk evolution in the 5 Myr old Upper Scorpius OB association

We present 4.5, 8, and 16 mu m photometry from the Spitzer Space Telescope for 204 stars in the Upper Scorpius OB association. The data are used to investigate the frequency and properties of circumstellar disks around stars with masses between similar to 0.1 and 20 at an age of similar to 5 Myr. We identify 35 stars that have emission at 8 or 16 mu m in excess of the stellar photosphere. The lower mass stars (similar to 0.1 - 1.2) appear surrounded by primordial optically thick disks based on the excess emission characteristics. Stars more massive than similar to 1.8 M-circle dot have lower fractional excess luminosities suggesting that the inner similar to 10 AU of the disk has been largely depleted of primordial material. None of the G and F stars (similar to 1.2 - 1.8) in our sample have an infrared excess at wavelengths <= 16 mm. These results indicate that the mechanisms for dispersing primordial optically thick disks operate less efficiently, on average, for low-mass stars, and that longer timescales are available for the buildup of planetary systems in the terrestrial zone for stars with masses <= 1. M-circle dot.