An ALMA Dynamical Mass Estimate of the Proposed Planetary-mass Companion FW Tau C

Dynamical mass estimates down to the planet-mass regime can help to understand planet formation. We present Atacama Large Millimeter/submillimeter Array (ALMA) 1.3 mm observations of FW Tau C, a proposed ∼10 M _Jup planet-mass companion at ∼330 au from the host binary FW Tau AB. We spatially and spectrally resolve the accretion disk of FW Tau C in ¹²CO (2-1). By modeling the Keplerian rotation of gas, we derive a dynamical mass of ∼0.1 M _⊙. Therefore, FW Tau C is unlikely a planet, but rather a low-mass star with a highly inclined disk. This also suggests that FW Tau is a triple system consisting of three ∼0.1 M _⊙ stars.