An ALMA Constraint on the GSC 6214-210 B Circum-substellar Accretion Disk Mass

We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of GSC 6214-210 A and B, a solar-mass member of the 5-10 Myr Upper Scorpius association with a 15 ± 2 M_Jup companion orbiting at ≈330 AU (2.″2). Previous photometry and spectroscopy spanning 0.3-5 μm revealed optical and thermal excess as well as strong H and Pa β emission originating from a circum-substellar accretion disk around GSC 6214-210 B, making it the lowest-mass companion with unambiguous evidence of a subdisk. Despite ALMA's unprecedented sensitivity and angular resolution, neither component was detected in our 880 μm (341 GHz) continuum observations down to a 3σ limit of 0.22 mJy/beam. The corresponding constraints on the dust mass and total mass are <0.15 M_⊕ and <0.05 M_Jup, respectively, or <0.003% and <0.3% of the mass of GSC 6214-210 B itself assuming a 100:1 gas-to-dust ratio and characteristic dust temperature of 10-20 K. If the host star possesses a putative circum-stellar disk then at most it is a meager 0.0015% of the primary mass, implying that giant planet formation has certainly ceased in this system. Considering these limits and its current accretion rate, GSC 6214-210 B appears to be at the end stages of assembly and is not expected to gain any appreciable mass over the next few megayears.