This Pair Of Brown Dwarfs Can't Get Enough Of Each Other

Binary stars are known to transfer mass to one another.

In extreme cases, mass transfer can even cause a supernova explosion.

That happens when a white dwarf draws matter from a companion. But astronomers have never seen a pair of brown dwarfs transferring mass. Brown dwarfs are stuck in a no man's land between planet and star.

They're more massive than gas giants, but less massive than the smallest main sequence stars, red dwarfs.

Brown dwarfs are sometimes called failed stars or substellar objects because they're simply not massive enough to trigger and sustain hydrogen fusion like main sequence stars do.

Instead, they emit some light and heat due to deuterium fusion. This artist's illustration shows the relative sizes of the Sun, a low mass star, a brown dwarf, Jupiter, and the Earth.

The image is to scale.

Image Credit: NASA, ESA, SDO, NASA-JPL, Caltech, A.Simon (NASA-GSFC); Designer: E.

Wheatley (STScI) Astronomers aren't certain how common brown dwarfs are because they're so dim and difficult to detect.

But estimates suggest that the Milky Way could contain up to 100 billion of them.

Like other stars, many of these billions of brown dwarfs are in binary pairs. New research in The Astrophysical Journal Letters focuses on ZTF J1239+8347, a binary brown dwarf pair in an especially close orbit with one another.

The research is titled "A Mass Transferring Brown Dwarf Binary on a 57 Minute Orbit," and the lead author is Samuel Whitebook.

Whitebook is a grad student in the Division of Physics, Mathematics, and Astronomy, at Caltech. The pair of brown stars has an orbital period of 57.41 minutes.

That's an extremely tight orbit, and observations with NASA's Swift Observatory and other facilities show that the two brown dwarfs are in a stable mass-transferring relationship.

The researchers identified a hot spot on the surface of the donor brown dwarf that moves as the pair orbits each other. There are two possible outcomes for this arrangement. In one scenario, the accreting BD will continue to gain mass until it becomes massive enough to fuse hydrogen.

It will then be a main sequence star. In the other scenario, the pair will eventually merge and become one.

This will also result in a more massive, main sequence star.

In both cases, there's an increase in luminosity. "The failed stars get a second chance," lead author Whitebook said in apress release.