2005ApJ...630..381B

Result. of a comprehensive, new, ground-based mid-infrared imaging survey of the young stellar population of the ρ Ophiuchi cloud are presented. Data were acquired at the Palomar 5 m and at the Keck 10 m telescopes with the MIRLIN and LWS instruments, at 0".5 and 0".25 resolutions, respectively. Of 172 survey objects, 85 were detected. Among the 22 multiple systems observed, 15 were resolved and their individual component fluxes determined. A plot of the frequency distribution of the detected objects with SED spectral slope shows that YSOs spend ∼4x10⁵ yr in the flat-spectrum phase, clearing out their remnant infall envelopes. Mid-infrared variability is found among a significant fraction of the surveyed objects and is found to occur for all SED classes with optically thick disks. Large-amplitude near-infrared variability, also found for all SED classes with optically thick disks, seems to occur with somewhat higher frequency at the earlier evolutionary stages. Although a general trend of mid-infrared excess and near-infrared veiling exists progressing through SED classes, with Class I objects generally exhibiting r_K≥1, flat-spectrum objects with r_K≥0.58, and Class III objects with r_K=0, Class II objects exhibit the widest range of r_Kvalues, ranging from 0≤r_K≤4.5. However, the highly variable value of veiling that a single source can exhibit in any of the SED classes in which active disk accretion can take place is striking and is direct observational evidence for highly time-variable accretion activity in disks. Finally, by comparing mid-infrared versus near-infrared excesses in a subsample with well-determined effective temperatures and extinction values, disk-clearing mechanisms are explored. The results are consistent with disk clearing proceeding from the inside out.