On the nature of the tertiary companion to FW Tau: ALMA CO observations and SED modeling

TL;DR

This study uses ALMA observations and SED modeling to investigate the nature of the third object in the FW Tau system, providing evidence for a gas disk and exploring whether it is a brown dwarf or a planet.

Contribution

First ALMA detection of a gas disk around the FW Tau third object, with modeling suggesting it could be a brown dwarf or a planet.

Findings

Detection of a dust disk around the third object.

First clear detection of a $^{12}$CO (2-1) line indicating a gas disk.

Current data are consistent with both a brown dwarf or a planet, requiring further observations.

Abstract

It is thought that planetary mass companions may form through gravitational disk instabilities or core accretion. Identifying such objects in the process of formation would provide the most direct test for the competing formation theories. One of the most promising candidates for a planetary mass object still in formation is the third object in the FW Tau system. We here present ALMA cycle 1 observations confirming the recently published 1.3 mm detection of a dust disk around this third object and present for the first time a clear detection of a single peak $^{12}$CO (2-1) line, providing direct evidence for the simultaneous existence of a gas disk. We perform radiative transfer modeling of the third object in FW Tau and find that current observations are consistent with either a brown dwarf embedded in an edge-on disk or a planet embedded in a low inclination disk, which is externally irradiated by the binary companion. Further observations with ALMA, aiming for high SNR detections of non contaminated gas lines, are required to conclusively unveil the nature of the third object in FW Tau.