An ALMA Disk Mass for the Candidate Protoplanetary Companion to FW Tau

TL;DR

This study uses ALMA observations to measure the disk mass of a faint, red companion to FW Tau, revealing a small but significant dust reservoir that could form terrestrial planets or satellites.

Contribution

First measurement of the disk mass around the candidate protoplanetary companion to FW Tau using ALMA, providing insights into its potential for planet formation.

Findings

Detected 1.3 mm continuum emission only from the companion.

Estimated dust mass of 1-2 Earth masses in the disk.

The disk mass is insufficient for giant planet formation but adequate for smaller planetary systems.

Abstract

We present ALMA observations of the FW Tau system, a close binary pair of M5 stars with a wide-orbit (300 AU projected separation) substellar companion. The companion is extremely faint and red in the optical and near-infrared, but boasts a weak far-infrared excess and optical/near-infrared emission lines indicative of a primordial accretion disk of gas and dust. The component-resolved 1.3 mm continuum emission is found to be associated only with the companion, with a flux (1.78 +/- 0.03 mJy) that indicates a dust mass of 1-2 M_Earth. While this mass reservoir is insufficient to form a giant planet, it is more than sufficient to produce an analog of the Kepler-42 exoplanetary system or the Galilean satellites. The mass and geometry of the disk-bearing FW Tau companion remains unclear. Near-infrared spectroscopy shows deep water bands that indicate a spectral type later than M5, but substantial veiling prevents a more accurate determination of the effective temperature (and hence mass). Both a disk-bearing "planetary-mass" companion seen in direct light or a brown dwarf tertiary viewed in light scattered by an edge-on disk or envelope remain possibilities.