The TESS space telescope helped scientists study the brown dwarf HIP 61637 b. It orbits a massive and old star. By analyzing its orbit, scientists were able to learn more about how such objects form.
A Rare Cosmic Dwarf
The website Phys.org reports that astronomers from Harvard University have presented an interesting study of the brown dwarf HIP 61637 b. The TESS space telescope helped them carry it out. With its help, they determined the orbital parameters of this object and used them to uncover something interesting about its past.
The star HIP 61637, around which the brown dwarf orbits, is three times more massive and brighter than the Sun and belongs to spectral class A, meaning it is among the larger stars. However, because it is so massive, its evolution has proceeded much faster than that of the Sun, and it has almost completely exhausted its reserves of hydrogen fuel. Soon it will begin to leave the main sequence and transform into a subgiant, and later into a red giant.
This made it possible to determine the age of the entire system with sufficient accuracy. It turned out to be 396 million years, meaning that the star formed after life on Earth had already moved onto land. The brown dwarf, however, must be the same age, which gave scientists an excellent opportunity to determine what they still do not know about such objects.
How Brown Dwarfs Form
Brown dwarfs are objects that occupy an intermediate position between stars and planets. In the largest of them, thermonuclear reactions continue for some time after formation, while in smaller ones they do not. In any case, over time they all begin to shine only with residual light and, at an advanced age, become very similar to giant planets.
The problem is that scientists are still debating how such objects form. There are two possible pathways. They may collapse directly from a diffuse gas cloud together with their host star. The second pathway is their later formation in a gas-and-dust disk, in the same way as planets form.
In theory, the two different formation pathways should leave traces that can be observed. In particular, in the case of direct formation from a gas cloud, the planet’s orbit should be significantly more regular than when formation occurs in an accretion disk. In practice, however, the long-term evolution of the system erases these differences. In addition, determining the exact age of a brown dwarf is extremely difficult.
What the New Research Showed
The new study of HIP 61637 b plays an important role in addressing this question. First, this brown dwarf lies in the so-called “brown dwarf desert.” A mass of 48 Jupiter masses is relatively uncommon among these objects. More often, they are either significantly lighter or significantly heavier, which supports the theory that they may form through different pathways.
Second, HIP 61637 b is only the seventh brown dwarf for which the age has been determined with reasonable accuracy. Third, HIP 61637 is one of the largest stars around which such an object has been discovered.
And because its relatively young and well-established age gives reason to believe that its orbit has not undergone major perturbations, this means that it is approximately the same as it was at the time of formation. Analysis of the orbit indicates that the brown dwarf most likely formed in a protoplanetary disk, rather than directly together with the star itself.
