We report on a deep photometric survey covering an area of 1.17 deg ^ { 2 } in the young Upper Scorpius stellar association using VIMOS Iz and UKIDSS ZJHK data taking several years apart .
The search for the least massive population of Upper Scorpius ( \sim 5–10 Myr , 145 pc ) is performed on the basis of various optical and infrared color-color and color-magnitude diagrams , including WISE photometry , in the magnitude interval J =14.5-19 mag ( completeness ) , which corresponds to substellar masses from 0.028 through 0.004 M _ { \odot } at the age and distance of Upper Scorpius .
We also present the proper motion analysis of the photometric candidates , finding that two objects successfully pass all photometric and astrometric criteria for membership in the young stellar association .
One of them , USco J155150.2 - 213457 , is a new discovery .
We obtained low resolution , near-infrared spectroscopy ( R \sim 450 , 0.85–2.35 \mu m ) of this new finding using the FIRE instrument .
We confirmed its low-gravity atmosphere expected for an Upper Scorpius member ( weak alkaline lines , strong VO absorption , peaked H -band pseudocontinuum ) .
By comparison with spectroscopic standards , we derive a spectral type of L6 \pm 1 , and estimate a mass of \approx 0.008–0.010 M _ { \odot } for USco J155150.2 - 213457 .
The colors and spectral slope of this object resemble those of other young , cool members of Upper Scorpius and \sigma Orionis ( \sim 3 Myr ) and field , high gravity dwarfs of related classification in contrast with the very red indices of field , low gravity , L-type dwarfs of intermediate age .
USco J155150.2 - 213457 , which does not show infrared flux excesses up to 4.5 \mu m , becomes one of the least massive and latest type objects known in the entire Upper Scorpius stellar association .