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

Abstract

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 σ 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 ${\it JHK}_{\rm s}$ and mid-infrared [3.6] and [4.5] IRAC/Spitzer photometry recently obtained for S Ori 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 $(H-K_{\rm s})$, $(J-K_{\rm s})$ and K s - [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 S Ori 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 σ Orionis cluster. Our measurement is consistent with the proper motion of the cluster within 1.5 σ the astrometric uncertainty. Conclusions. Taking into account both S Ori 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 S Ori 70's membership in σ Orionis, with an estimated mass in the interval 2–7 M Jup , in agreement with our previous derivation.