Gaia0007-1605: an old triple system with an inner brown dwarf-white dwarf binary and an outer white dwarf companion

TL;DR

This paper reports the discovery and analysis of Gaia0007-1605, a hierarchical triple system with an inner brown dwarf-white dwarf binary and an outer white dwarf, providing insights into its formation, evolution, and age.

Contribution

It presents the first detailed characterization of an inner brown dwarf-white dwarf binary within a triple system, including spectroscopic measurements and orbital analysis.

Findings

Inner binary consists of a brown dwarf and a white dwarf with a 1.045-day orbit.

System is approximately 10 billion years old based on kinematic analysis.

Detected irradiation-induced variability in the brown dwarf from TESS light curves.

Abstract

We identify Gaia0007-1605AC as the first inner brown dwarf-white dwarf binary of a hierarchical triple system in which the outer component is another white dwarf (Gaia0007-1605B). From optical/near-infrared spectroscopy obtained at the Very Large Telescope with the X-Shooter instrument and/or from Gaia photometry plus SED fitting, we determine the effective temperatures and masses of the two white dwarfs (12018+-68 K, 0.54+-0.01 Msun for Gaia0007-1605A and 4445+-116 K, 0.56+-0.05 Msun for Gaia0007-1605B) and the effective temperature of the brown dwarf (1850+-50 K; corresponding to a spectral type L3+-1). By analysing the available TESS light curves of Gaia0007-1605AC we detect a signal at 1.0446+-0.0015 days with an amplitude of 6.25 ppt, which we interpret as the orbital period modulated from irradiation effects of the white dwarf on the brown dwarf's surface. This drives us to speculate that the inner binary evolved through a common envelope phase in the past. Using the outer white dwarf as a cosmochronometer and analysing the kinematic properties of the system, we conclude that the triple system is about 10 Gyr old.