Stellar and planetary formation is a very active field, full of uncertainty on how these objects form. There’s a lot of focus on brown dwarfs, a class of failed stars, and a new discovery might help consolidate our understanding of these objects.
A team led by Justin Crepp was able to constrain the mass, age and composition of the brown dwarf HD 4747 B, as well as photograph it for the first time. These breakthrough measurements can be used as a benchmark to better understand these types of objects in the future.
The preliminary results indicate that the object has a mass of about 60 Jupiters and an age of 3.3 billion years. It’s in a very eccentric orbit around its companion star and located 61 light-years from Earth. The results have been submitted for publication in the Astrophysical Journal and are available online.
Brown dwarfs are a stellar missing link – objects between stars and exoplanets. They start off as stars, shining brightly due to the gravitational collapse, but then don’t have enough mass to ignite nuclear fusion in their core, so they eventually cool off. This limited brightness makes them difficult objects to study.
HD 4747 B as seen by Keck telescopes in Hawaii. Crepp et al.
The precise measurements of HD 4747 B were possible thanks to 18 years of detailed measurements of its companion star, HD 4747 A, a yellow star slightly small than our Sun (0.82 solar masses). This presented astronomers with a unique opportunity.
"We suspect that these companions form at the same time and from the same material," Crepp said in a statement. "As such, you can infer physical properties of the brown dwarf from its parent star, like age and composition. There are no other objects for which we know the mass, age and the metallicity simultaneously and also independent of the light that the companion gives off. We can therefore use HD 4747 B as a test-bed to study brown dwarfs, enabling precision astrophysics studies for a directly imaged substellar object."
Brown dwarf masses are usually estimated by comparing the light of an observed object with theoretical evolutionary models. The precision of this current work will likely help to improve the predictions of models when next applied to other objects.
"This field is transitioning from 'Hey, I found something neat' to 'Hey, I know the mass to within a few percent.' Now, we can test theoretical models," concluded Crepp.