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

TL;DR

ALMA observations of FW Tau C reveal its dynamical mass is around 0.1 solar masses, indicating it is a low-mass star, not a planet, and suggesting FW Tau is a triple star system.

Contribution

This study provides the first dynamical mass estimate of FW Tau C using ALMA, challenging its classification as a planetary-mass companion.

Findings

FW Tau C is a low-mass star, not a planet.

FW Tau is a triple system of ~0.1 solar mass stars.

The accretion disk shows Keplerian rotation.

Abstract

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 $M_{\rm Jup}$ 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 ${}^{12}$CO (2-1). By modeling the Keplerian rotation of gas, we derive a dynamical mass of ~0.1 $M_\odot$. 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 $M_\odot$ stars.