Resolving the unresolved: Discovery and dynamical masses of the brown dwarf binary DE1756−45^★

¹ Main Astronomical Observatory, National Academy of Sciences of the Ukraine, Zabolotnogo 27, 03680 Kyiv, Ukraine
² European Space Agency (ESA), European Space Astronomy Centre (ESAC), Camino Bajo del Castillo s/n, 28692, Villanueva de la Cañada, Madrid, Spain
³ Department of Astronomy, University of Geneva, Chemin Pegasi 51, 1290 Versoix, Switzerland
⁴ Instituto de Astrofísica de Canarias, calle Víal Láctea, San Cristóbal de La Laguna, Spain
⁵ Centro de Astrobiologia, CSIS-INTA, Camino Bajo del Castillo, s/n 28692 Villanueva de la Canada, Madrid, Spain
⁶ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA

^★★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 27 August 2025
Accepted: 2 November 2025

Abstract

We present a method of resolving the geometric structure in unresolved CCD images of the two-component stellar objects with relative separations below the full width at half maximum (FWHM). The practical applicability of this method has been demonstrated on example tests of the newly discovered binary, DENIS-P J1756296-451822, with a relative separation of about 0.15″ (or 0.25 × FWHM). For this purpose, we used unresolved binary images obtained with the VLT/FORS2 camera, which provided precise astrometric positions of the system photocenter. Using the same images, we applied a new, tested method capable of resolving the geometry of the binary by taking into account the difference of the image shape of the binary and of single stars, adopting an effective elliptical point spread function (PSF). In this way, we derived independent additional information on the geometry of the binary system, which allowed us to estimate the mass ratio and improved the overall orbit fit. Also, we used a single series of adaptive optics observations with NACO. Combining these data, we derived a relative flux of the secondary in the I band of 0.66 ± 0.04 and a mass ratio of q ≃ 0.886 ± 0.049, along with dynamical masses of M₁ = 63.9_−2.1^+2.5M_Jup for the primary and M₂ = 56.6_−1.9^+2.7M_Jup for the secondary. We note that these values are below the substellar limit. Using theoretical cooling curves for brown dwarfs, we were able to estimate the age of this binary system at between 200 and 350 Myr.