Program: GN-2009A-DD-4

We propose to obtain follow-up JHK imaging and spectroscopy of a ~20 Mjup companion that we have identified around a 2.5 Msun star in the 5 Myr-old Upper Scorpius star-forming region. The candidate companion is located at a separation of 4.5", corresponding to ~600 AU at the distance of the primary. The proposed observations will be used to determine more precisely the Teff, log g and mass of the companion. Also, the spectrum obtained - with high S/N and high resolution - will arguably be the highest quality spectrum of a young very low mass object ever obtained and will be extremely useful to test and develop further the theoretical atmosphere and evolution models of young brown dwarfs and giant planets. This new companion is the first confirmed companion with such a low mass around a primary star of B spectral type. In particular, the large separation (~600 AU) and low mass ratio (~0.008) of this companion offer an interesting input for models of star formation.