Detection of photometric variability in the very low-mass binary VHS J1256-1257AB using TESS and Spitzer

TL;DR

This study detects and models photometric variability in the ultra-cool dwarf binary VHS J1256-1257AB using TESS and Spitzer data, revealing nearly synchronized rotation periods and phase-alternating fluxes.

Contribution

It introduces a two-wave model explaining the periodic and stochastic variability, and determines the rotation periods of the binary components using space-based photometry.

Findings

Binary components rotate with periods of approximately 2.08 and 2.13 hours.

Fluxes of the binary components alternate between phase and anti-phase.

The spin axes are likely inclined at nearly 90 degrees.

Abstract

Aims: We investigate the photometric properties of the M7.5 equal-mass binary VHS J1256-1257AB, which, combined with the late-L dwarf VHS J1256-1257 b, forms one of the few young triple systems of ultra-cool dwarfs currently known. Methods: We analyzed two-minute TESS and two-second Spitzer archival data with total durations of about 25 days and 36 hours, respectively. Typical precision in the data is $\pm$1.5 % for TESS and $\pm$0.1 % (in 1 minute) for Spitzer. Results: The optical and infrared light curves periodically exhibit epochs of quasi-sinusoidal modulation followed by epochs of stochastic variability, which resembles the beat pattern created by two waves of similar frequencies that interfere with each other. Our two-wave model for the TESS data shows that the components of VHS J1256-1257AB rotate with periods of $2.0782\pm0.0004$ h and $2.1342\pm0.0003$ h, which is also supported by the Spitzer observations. As a result, the fluxes of the equally bright VHS J1256-1257A and B alternate between states of phase and anti-phase, explaining the observed photometric variability in their combined light. The projected spectroscopic velocity of VHS J1256-1257AB is remarkably similar to those obtained by combining the measured rotation periods and the expected radii, which indicates that the spin axes of VHS J1256-1257A and B are likely inclined at nearly 90 deg, as previously reported for VHS J1256-1257 b.