An article published in the journal “Astronomy & Astrophysics” reports a study on 5 objects on the boundary between brown dwarf and star discovered thanks to NASA’s TESS space telescope. A team of researchers coordinated by a group of Swiss entities conducted follow-up observations of the 5 objects to better understand the nature of brown dwarfs, what distinguishes them from stars, and what is the boundary between these two classes of objects.
Brown dwarfs have been known for a little over twenty years, although their existence was theorized much earlier. They’re called failed stars because their mass is not enough to trigger hydrogen fusion. The consequence is that they have very low electromagnetic emissions which make it difficult to identify them when they’re not companions of normal stars and even more to study their characteristics.
It’s no coincidence that the best-known brown dwarfs are in the cosmic neighborhood. In the case of the objects at the center of this study, there was help from the TESS space telescope, launched on April 18, 2018, to search for new exoplanets. TESS identifies the passage of exoplanets in front of their star from the telescope’s point of view but in some cases finds a slightly different object that can be even more interesting. The 5 objects at the center of this study were defined as companions because they’re far more massive than planets and lie on the boundary between brown dwarfs and stars.
The 5 objects discovered by the TESS space telescope are TOI-148b, TOI-587b, TOI-681b, TOI-746b, and TOI-1213b, where TOI stands for “TESS Object of Interest”. The image (©UNIGE Thibaut Roger, CC BY-NC-SA 4.0) shows an artistic illustration of these objects together with Jupiter as a size comparison. Follow-up observations were conducted as part of the TESS Follow-up Observing Program (TFOP) which includes many telescopes around the world. In particular, spectroscopic examinations of the stars around the 5 objects orbit and measurements of the radial velocity were carried out.
The study was helped by the fact that the 5 objects studied are close to their stars, to the point that their year ranges from 5 to 27 Earth days. Examination of their size indicates that they have a radius ranging from 0.81 to 1.66 times Jupiter’s. There was a lot of interest in estimating the masses of these objects, which range from 77 to 98 times Jupiter’s. These are characteristics that place them on the boundary between brown dwarfs and stars and make them particularly interesting.
Brown dwarfs have enough mass to trigger deuterium fusion, which can have a limited duration because this isotope makes up a limited percentage of the hydrogen that forms them. The consequence is that the available deuterium is expected to drop significantly over time, leading to the brown dwarf’s cooling. The reduction of deuterium fusion also means a drop in the energy that counteracts gravity and this leads to a shrinking of a brown dwarf. TOI-148b and TOI-746b are the two objects with the highest estimated age and are also the two smallest while TOI-587b and TOI-681b are the two with the lowest estimated age and are also the largest.
The nature of the objects studied hasn’t yet been established with certainty precisely because they’re on the boundary between a brown dwarf and a star. Further studies will be needed to better understand their characteristics, including chemical composition, as it could be important. These objects aren’t the first with masses of that order discovered since, for example, an article published in “The Astrophysical Journal” in September 2018 described 2 similar objects. Being able to study 5 more will help better understand both brown dwarfs and stars.
