S Ori 70: still a strong cluster planet candidate

Abstract

In this paper I show that the coolest $σ$Orionis cluster planet S Ori 70 is still a strong candidate member despite recent claims by Burgasser et al. that it could be a brown dwarf interloper. The main point of my argument is that the colors of S Ori 70 are significantly different to those of field dwarfs. This object has in fact the reddest $H-K$ color of all known T dwarfs, a clear indication of low gravity according to all published models. In a $J-H$ versus $H-K$ diagram, S Ori 70 lies in the region where models of ultracool dwarfs predict that low gravity objects should be located. I conclude that S Ori 70 is still a strong candidate member of the $σ$Orionis open cluster. I briefly discuss additional observational tests that can be carried out with existing facilities to verify the $σ$Orionis membership of this cluster planet candidate.