ALMA Discovery of a Disk around the Planetary-mass Companion SR 12 c

Abstract

We report an Atacama Large Millimeter/submillimeter Array 0.88 mm (Band 7) continuum detection of the accretion disk around SR 12 c, an \(\sim\)11 \(M_{\rm Jup}\) planetary-mass companion (PMC) orbiting its host binary at 980 au. This is the first submillimeter detection of a circumplanetary disk around a wide PMC. The disk has a flux density of \(127 \pm14~\mu\)Jy and is not resolved by the \(\sim\)0.1" beam, so the dust disk radius is likely less than 5 au and can be much smaller if the dust continuum is optically thick. If, however, the dust emission is optically thin, then the SR 12 c disk has a comparable dust mass to the circumplanetary disk around PDS 70 c but is about five times lower than that of the \(\sim\)12 \(M_{\rm Jup}\) free-floating OTS 44. This suggests that disks around bound and unbound planetary-mass objects can span a wide range of masses. The gas mass estimated with an accretion rate of \(10^{-11}~M_\odot\) yr\(^{-1}\) implies a gas-to-dust ratio higher than 100. If cloud absorption is not significant, a nondetection of \({}^{12}\)CO(3-2) implies a compact gas disk around SR 12 c. Future sensitive observations may detect more PMC disks at 0.88 mm flux densities of \(\lesssim\)100 \(\mu\)Jy.