New constraints on the membership of the T dwarf S Ori 70 in the sigma Orionis cluster

Aims. The nature of S Ori 70 (S Ori J053810.1-023626), a faint mid-T type object found towards the direction of the young sigma Orionis cluster, is still under debate. We intend to find out whether it is a field brown dwarf or a 3-Myr old planetary-mass member of the cluster. Methods. We report on near-infrared JHKs and mid-infrared [3.6] and [4.5] IRAC/ Spitzer photometry recently obtained for SOri 70. The new near-infrared images (taken 3.82 yr after the discovery data) allowed us to derive the first proper motion measurement for this object. Results. The colors (H - Ks), (J - Ks) and Ks - [ 3.6] appear discrepant when compared to T4-T7 dwarfs in the field. This behavior could be ascribed either to a low-gravity atmosphere or to an atmosphere with a metallicity that is significantly different than solar. The small proper motion of SOri 70 (11.0 +/- 5.9 mas yr(-1)) indicates that this object is farther away than expected if it were a single field T dwarf lying in the foreground of the sigma Orionis cluster. Our measurement is consistent with the proper motion of the cluster within 1.5 sigma the astrometric uncertainty. Conclusions. Taking into account both SOri 70' s proper motion and the new near-and mid-infrared colors, a low-gravity atmosphere remains as the most likely explanation for our observations. This supports SOri 70' s membership in sigma Orionis, with an estimated mass in the interval 2-7 M-Jup, in agreement with our previous derivation.