Monitoring H$\alpha$ Emission from the Wide-orbit Brown-dwarf Companion FU Tau B

TL;DR

This study presents the longest continuous Hα monitoring of a substellar companion near the deuterium-burning limit, revealing tentative variability and overall stable accretion, demonstrating the feasibility of small telescope monitoring.

Contribution

It provides new insights into accretion variability in a brown-dwarf companion using extended Hα observations with small telescopes.

Findings

Tentative Hα variability on hourly and daily timescales.

No burst-like accretion events detected.

Accretion appears stable over the observation period.

Abstract

Monitoring mass accretion onto substellar objects provides insights into the geometry of the accretion flows. We use the Lulin One-meter Telescope to monitor H$\alpha$ emission from FU Tau B, a $\sim$19 $M_{\rm Jup}$ brown-dwarf companion at 5.7" (719 au) from the host star, for six consecutive nights. This is the longest continuous H$\alpha$ monitoring for a substellar companion near the deuterium-burning limit. We aim to investigate if accretion near the planetary regime could be rotationally modulated as suggested by magnetospheric accretion models. We find tentative evidence that H$\alpha$ mildly varies on hourly and daily timescales, though our sensitivity is not sufficient to definitively establish any rotational modulation. No burst-like events are detected, implying that accretion onto FU Tau B is overall stable during the time baseline and sampling windows over which it was observed. The primary star FU Tau A also exhibits H$\alpha$ variations over timescales from minutes to days. This program highlights the potential of monitoring accretion onto substellar objects with small telescopes.