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

TL;DR

This study uses ALMA observations to place upper limits on the dust and total mass of the circum-substellar disk around GSC 6214-210 B, suggesting it is in the final stages of formation with minimal remaining material.

Contribution

First ALMA constraints on the disk mass of a low-mass companion with confirmed accretion, indicating disk dissipation at the end of formation.

Findings

No detection of the disk in ALMA data, with a dust mass limit of <0.15 Earth masses.

The disk mass is less than 0.05 Jupiter masses, a tiny fraction of the companion's mass.

Giant planet formation likely ceased in this system.

Abstract

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 $\pm$ 2 Mjup companion orbiting at $\approx$330 AU (2.2"). Previous photometry and spectroscopy spanning 0.3-5 $\mu$m revealed optical and thermal excess as well as strong H$\alpha$ and Pa~$\beta$ 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 $\mu$m (341 GHz) continuum observations down to a 3-$\sigma$ limit of 0.22 mJy/beam. The corresponding constraints on the dust mass and total mass are <0.15 Mearth and <0.05 Mjup, 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 Myr.