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

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 h, respectively. Typical precision in the data is ±1.5% for TESS and ±0.1% (in 1 min) 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 ± 0.0004 h and 2.1342 ± 0.0003 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.

Roaring Storms in the Planetary-mass Companion VHS 1256-1257 b: Hubble Space Telescope Multiepoch Monitoring Reveals Vigorous Evolution in an Ultracool Atmosphere

“…In time-resolved observations, VHS 1256 b has exhibited high amplitude brightness and spectral variability (e.g., Bowler et al 2020;Zhou et al 2020). The host binary stars show low-level (<0.3%) photometric modulations (Miles-Páez 2021).…”

Roaring Storms in the Planetary-Mass Companion VHS 1256-1257 b: Hubble Space Telescope Multi-epoch Monitoring Reveals Vigorous Evolution in an Ultra-cool Atmosphere

“…In time-resolved observations, VHS 1256 b has exhibited high-amplitude brightness and spectral variability (e.g., Bowler et al 2020;Zhou et al 2020). The host binary stars show low-levels (< 0.3%) photometric modulations (Miles-Páez 2021).…”

The photometric periods of rapidly rotating field ultra-cool dwarfs

“…Independent of the variability nature, the monitoring of these brightness changes (usually at red-optical and infrared wavelengths) allows us to derive the rotation periods of ultra-cool dwarfs. Numerous searches for photometric variability have been carried out from both the ground (e.g., Tinney & Tolley 1999;Koen 2005;Harding et al 2013;Radigan et al 2014;Miles-Páez et al 2017a) and space (e.g., Buenzli et al 2014;Metchev et al 2015;Cushing et al 2016;Miles-Páez et al 2019;Tannock et al 2021;Miles-Páez 2021;Vos et al 2022), covering from late-M to Y dwarfs of different ★ pamiles@cab.inta-csic.es ages. Understanding the distribution of rotation periods in ultra-cool dwarfs is crucial for investigating their angular momentum evolution, which seems different from the well-known spin-down for higher mass stars caused by angular momentum loss via magnetized stellar winds (Skumanich 1972).…”