S Orionis 70: Just a Foreground Field Brown Dwarf?

We examine recent claims that the T-type brown dwarf S Ori 053810.1-203626 (S Ori 70) is a spectroscopically verified low-mass (3 MJup) member of the 1-8 Myr σ Orionis cluster. Comparative arguments by Martín & Zapatero Osorio asserting that S Ori 70 exhibits low surface gravity spectral features indicative of youth and low mass are invalidated by the fact that their comparison object was not the field T dwarf 2MASS 0559-1404, but rather a nearby background star. Instead, we find that the 1-2.5 μm spectra of S Ori 70 are well matched to older (age ~ few Gyr) field T6-T7 dwarfs. Moreover, we find that spectral model fits to late-type field T dwarf spectra tend to yield low surface gravities (log g = 3.0-3.5), and thus young ages (≲5 Myr) and low masses (≲3 MJup), inconsistent with expected and/or empirical values. Finally, we show that the identification of one T dwarf in the field imaged by Zapatero Osorio et al. is statistically consistent with the expected foreground contamination. Based on the reexamined evidence, we conclude that S Ori 70 may simply be an old, massive (30-60 MJup) field brown dwarf lying in the foreground of the σ Orionis cluster. This interpretation should be considered before presuming the existence of so-called "cluster planets."