Optical variability of the ultracool dwarf TVLM 513-46546: evidence for inhomogeneous dust clouds

TL;DR

This study uses multi-colour photometry to investigate optical variability in the ultracool dwarf TVLM 513, providing evidence that inhomogeneous dust clouds, rather than starspots, cause the observed sinusoidal brightness changes.

Contribution

It presents the first simultaneous multi-band observations showing anti-correlated optical variability, supporting dust clouds as the cause, and highlights the need for improved atmospheric models for ultracool dwarfs.

Findings

Anti-correlated Sloan-g' and Sloan-i' lightcurves observed.

Dust clouds likely cause the optical variability.

Current models underestimate dust presence in the photosphere.

Abstract

We present multi-colour photometry of the M8.5V ultracool dwarf "pulsar" TVLM 513-46546 (hereafter TVLM 513) obtained with the triple-beam photometer ULTRACAM. Data were obtained simultaneously in the Sloan-g' and Sloan-i' bands. The previously reported sinusoidal variability, with a period of 2-hrs, is recovered here. However, the Sloan-g' and Sloan-i' lightcurves are anti-correlated, a fact which is incompatible with the currently proposed starspot explanation for the optical variability. The anti-correlated nature and relative amplitudes of the optical lightcurves are consistent with the effects of persistent dust clouds rotating on the surface of the star. In the absence of other plausible explanations for the optical variability of TVLM 513, it seems likely that dust cloud coverage combined with the rapid rotation of TVLM 513 is responsible for the optical variability in this object. However, crude modelling of a photosphere with partial dust cloud coverage shows that the anti-correlation can only be reproduced using cooler models than the literature temperature of TVLM 513. We suggest this discrepancy can be removed if more dust is present within the photosphere of TVLM 513 than theoretical model atmospheres predict, though a definitive statement on this matter will require the development of self-consistent models of partially dusty atmospheres.