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.
Highlights
VHS J1256-1257AB is a 0 . 1, equal-mass binary with a combined spectral type of M7.5 ± 0.5 and an estimated age in the range 150–300 Myr (Gauza et al 2015; Stone et al 2016). Dupuy et al (2020) reported a trigonometric parallax of 45.0 ± 2.4 mas, in agreement with the recent 47.27 ± 0.47 mas from Gaia Data Release 3 (DR3; Gaia Collaboration 2021) that results in a distance of 21.2 ± 0.2 pc
We investigate the photometric properties of the M7.5 equal-mass binary VHS J1256-1257AB, which, combined with the lateL dwarf VHS J1256-1257 b, forms one of the few young triple systems of ultra-cool dwarfs currently known
Our analysis shows that VHS J1256-1257A and B have almost the same rotation period
Summary
VHS J1256-1257AB is a 0 . 1, equal-mass binary with a combined spectral type of M7.5 ± 0.5 and an estimated age in the range 150–300 Myr (Gauza et al 2015; Stone et al 2016). Dupuy et al (2020) reported a trigonometric parallax of 45.0 ± 2.4 mas, in agreement with the recent 47.27 ± 0.47 mas from Gaia Data Release 3 (DR3; Gaia Collaboration 2021) that results in a distance of 21.2 ± 0.2 pc. We currently know of few triple systems composed of only young dwarfs with spectral types later than M7 (“ultra-cool dwarfs”; Triaud et al 2020; Sahlmann et al 2021). Their characterization is important for testing evolutionary models at early ages. The determination of their individual rotation periods and spectroscopic velocities, combined with Their radii, will allow us to learn about the spin-axis relative orientation of the system, which is crucial for putting constraints on their initial conditions of formation and providing insights into angular momentum evolution. In this work we report the detection of photometric variability in both components of VHS J1256-1257AB using archival data of the Transiting Exoplanet Survey Satellite (TESS) and the Spitzer Space Telescope
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