We used the Magellan adaptive optics system to image the 11 Myr substellar companion 1RXS 1609 B at the bluest wavelengths to date (z′ and Ys). Comparison with synthetic spectra yields a higher temperature than previous studies of Teff = 2000 ± 100 K and significant dust extinction of AV = 4.5+0.5-0.7 mag. Mass estimates based on the DUSTY tracks gives 0.012-0.015 M⊙, making the companion likely a low-mass brown dwarf surrounded by a dusty disk. Our study suggests that 1RXS 1609 B is one of the ∼25% of Upper Scorpius low-mass members harboring disks, and it may have formed like a star and not a planet out at ∼320 AU.
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