Out of eight companions [2] observed, the research team was able to highlight that five objects are brown dwarfs, little-known substellar objects halfway between planet and star [3] Brown dwarfs are very difficult to detect due to their low luminosity, and even more so when they are close to stars a thousand times brighter. In this study, they orbit their stars at distances equivalent to the distance between the Earth and the Sun. This proximity raises questions about the formation of these objects. What’s more, some of the luminous fluxes observed are weaker than those predicted by theoretical models. This could indicate that some of these brown dwarfs are binaries, with a smaller companion in their orbit.
These unprecedented observations were made possible by the combination of two observatories, Gaia from space and the VLTI (Very Large Telescope Interferometer) on Mount Paranal, Chile. The Gaia space mission has catalogued hundreds of thousands of multiple systems from space, characterizing their positions and movements. This enabled scientists to identify the eight stars to be targeted. On the ground, the VLTI’s GRAVITY instrument plays the role of a magnifying glass : it is capable of measuring the minutest details of stellar objects with unrivalled precision [4], but this instrument needs to be guided over precise zones. It was thus able to capture the light signals from the eight companions pre-identified by Gaia. Scientists then analyzed the luminosity and mass [5] of these objects, and were able to deduce that five of them were brown dwarfs. Until now, these "hidden companions" of stars have been invisible.
This result, to be published on June 20 in Astronomy & Astrophysics, is the first glimpse of the power of the Gaia-GRAVITY combination. The findings offer new insights into the formation of atypical celestial objects such as brown dwarfs, and could also shed light on the formation of massive exoplanets and planets in our solar system.
Bibliographie
Combining Gaia and GRAVITY : Characterising Five New Directly Detected Substellar Companions. T. O. Winterhalder, S. Lacour, N. Pourré, C. Babusiaux et al. Astronomy & Astrophysics, le 20 juin 2024.
[1] The French laboratories involved in this study are Laboratoire d’études spatiales et d’instrumentation en astrophysique (Observatoire de Paris - PSL/CNRS/Sorbonne Université/Université Paris Cité), Institut de Planétologie et d’Astrophysique de Grenoble (CNRS/Université Grenoble Alpes), Laboratoire d’Astrophysique de Marseille (Aix-Marseille Université/CNES/CNRS) and Laboratoire Lagrange (CNRS/Observatoire de la Côte d’Azur/Université Côte d’Azur).
[2] A companion is an object orbiting a star
[3] One of the differences between a star and a planet is the ability to generate energy, and therefore emit light. A star is massive enough to burn the hydrogen in its core, something a planet cannot do. In the case of the brown dwarf, it is not massive enough to burn hydrogen, but is still much more massive than a planet. For example, a brown dwarf is about thirty times more massive than the planet Jupiter, but about thirty times less massive than the Sun, the star of our solar system.
[4] GRAVITY uses a process known as interferometry : the simultaneous orientation of several telescopes on the ground pointing at a single star enables very fine resolution, making it possible to observe very small objects of low luminosity
[5] Luminosity and mass are two key indicators for understanding how an object has cooled over time, and thus for tracing its formation