A mid-infrared study of very low mass stars and brown dwarfs in Upper Scorpius

Context. To investigate the formation of sub-stellar objects, we observed a sample of ultracool dwarf members of the Upper Scorpius OB association. Aims. The properties of disks, such as their composition, life-time, and frequency compared to the accretor frequency, provide important clues to the mechanisms responsible for the formation of stellar and sub-stellar objects. Methods. We report the results of mid-IR observations with VISIR at the VLT of 10 ultracool dwarf members of the nearby Upper Scorpius OB association in four filters ranging from 8.59 (PAH1) to 12.8 mu m (Ne II) and one brown dwarf obserbed with Spitzer between 3.6 and 24 mu m. Results. Seven of our targets were detected in at least one of the bands, and we derived upper limits on the fluxes of the remaining 4. These results combined with previous studies from the literature lead to an improved disk frequency of 50 +/- 12%. This frequency is significantly higher than for accretors (16.3% +/- 6.2%). Only one object showing mid-IR excess also has Ha emission at a level that indicates it must be accreting. Four of the detected targets are multiple system candidates. Conclusions. The observed disk frequency for sub-stellar objects in the Upper Scorpius association is similar to that of stars, consistent with a common formation scenario. It is also similar to the disk fractions observed in younger clusters, suggesting that the disk lifetimes might be longer for ultracool dwarfs than for higher-mass stars.