S Ori 70: Just a Foreground Field Brown Dwarf?

We examine recent claims that the T-type brown dwarf S Ori 053810.1-023626 (S Ori 70) is a spectroscopically verified low mass (3${}_{-1}^{+5}$ M${}_{Jup}$) member of the 1–8 Myr $\sigma$ Orionis cluster. Comparative arguments by Mart{'{i}}n & 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 $\text{micron}$ spectra of S Ori 70 are well-matched to older (age $\sim$ 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 ($\lesssim$ 5 Myr) and low masses ($\lesssim$ 3 M${}_{Jup}$), 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 re-examined evidence, we conclude that S Ori 70 may simply be an old, massive (30–60 M${}_{Jup}$) field brown dwarf lying in the foreground of the $\sigma$ Orionis cluster. This interpretation should be considered before presuming the existence of so-called ``cluster planets.‘’