EVIDENCE FOR EVOLUTION AMONG PRIMORDIAL DISKS IN THE 5 Myr OLD UPPER SCORPIUS OB ASSOCIATION

Moderate-resolution, near-infrared spectra between 0.8 and 5.2 mu m were obtained for 12 late-type (K0-M3) disk-bearing members of the similar to 5 Myr old Upper Scorpius OB association using SpeX on the NASA Infrared Telescope Facility. For most sources, continuum excess emission first becomes apparent between similar to 2.2 and 4.5 mu m and is consistent with that produced by single-temperature blackbodies having characteristic temperatures ranging from similar to 500 to 1300 K. The near-infrared spectra for 5 of 12 Upper Scorpius sources exhibit Pa gamma, Pa beta, and Br gamma emission, indicators of disk accretion. Using a correlation between Pa beta and Br gamma emission line luminosity and accretion luminosity, mass accretion rates ((M) over dot) are derived for these sources that range from (M) over dot = 3.5 x 10(-10) to 1.5 x 10(-8) M-circle dot yr(-1). Merging the SpeX observations with Spitzer Space Telescope mid-infrared (5.4-37.0 mu m) spectroscopy and 24 and 70 mu m broadband photometry, the observed spectral energy distributions (SEDs) are compared with those predicted by two-dimensional, radiative transfer accretion disk models. Of the nine Upper Scorpius sources examined in this analysis, three exhibit SEDs that are most consistent with models having inner disk radii that substantially exceed their respective dust sublimation radii. The remaining Upper Scorpius members possess SEDs that either show significant dispersion among predicted inner disk radii or are best described by models having inner disk rims coincident with the dust sublimation radius.