Infrared spectroscopy of free-floating planet candidates in Upper Scorpius and Ophiuchus

International audienceContext. A rich population of low-mass brown dwarfs and isolated planetary mass objects has been reported in the Upper Scorpius and Ophiuchus star-forming complex. Aims: We investigate the membership, nature, and properties of 17 of these isolated planetary mass candidates using low-resolution near-infrared spectra. Methods: We investigated the membership by looking for evidence of their youth using four diagnostics: the slope of the continuum between the J and Ks band, the Hcont, and the TLI-g gravity-sensitive indices. In addition, we compared the spectra to young and field (old) M and L-dwarf standards. Results: All the targets but one are confirmed as young ultracool objects, with spectral types between L0 and L6 and masses in the range 0.004-0.013 M⊙, according to evolutionary models. The status of the last target is unclear at this point. Conclusions: Only one possible contaminant has been identified among the 17 targets, suggesting that the contamination level of the original sample must be low (≲6%). Full Table B.4 is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/664/A111