The First Science Results from SPHERE: Disproving the Predicted Brown Dwarf around V471 Tau

TL;DR

This study used the SPHERE instrument to search for a brown dwarf around V471 Tau to explain eclipse timing variations, but the non-detection challenges the circumbinary object hypothesis and supports the Applegate mechanism.

Contribution

First direct imaging attempt with SPHERE to detect a hypothesized brown dwarf in V471 Tau, providing evidence against its presence and the circumbinary explanation.

Findings

No brown dwarf detected in V471 Tau at the expected brightness and separation.

Results challenge the circumbinary object hypothesis for eclipse timing variations.

Supports the alternative Applegate mechanism as the cause of timing variations.

Abstract

Variations of eclipse arrival times have recently been detected in several post common envelope binaries consisting of a white dwarf and a main sequence companion star. The generally favoured explanation for these timing variations is the gravitational pull of one or more circumbinary substellar objects periodically moving the center of mass of the host binary. Using the new extreme-AO instrument SPHERE, we image the prototype eclipsing post-common envelope binary V471 Tau in search of the brown dwarf that is believed to be responsible for variations in its eclipse arrival times. We report that an unprecedented contrast of 12.1 magnitudes in the H band at a separation of 260 mas was achieved, but resulted in a non-detection. This implies that there is no brown dwarf present in the system unless it is three magnitudes fainter than predicted by evolutionary track models, and provides damaging evidence against the circumbinary interpretation of eclipse timing variations. In the case of V471 Tau, a more consistent explanation is offered with the Applegate mechanism, in which these variations are prescribed to changes in the quadrupole moment within the main-sequence star