Where Brown Dwarfs End And Stars Begin
December 15, 2013

Evidence For Theoretically Predicted Break Between Very Low Mass Stars And Brown Dwarfs

John P. Millis, Ph.D. for redOrbit.com - Your Universe Online

Stars form as interstellar gas accumulates, increasing in density and temperature. Eventually, the environment at the core of the object reaches a state where hydrogen begins fusing at a high enough rate that the outward radiation pressure and the inward gravitational force reach a balance, and a star is born.

However, if the forming proto-star is not quite massive enough, the fusion process is not able to produce enough energy to ignite the core. Such objects are known as brown dwarfs. While massive compared to planets – at least 13 Jupiter masses or so – they are only a tiny fraction the size of the Sun.

Because these objects are not fusion dominant they have lower surface temperatures making them quite dim. Coupled with the fact that these objects are quite small, they are incredibly difficult to study. So, while astronomers have previously calculated the threshold at which an object will become a star instead of a brown dwarf, observational evidence has been elusive.

Now, a new study from researchers at the RECONS group from Georgia State University has found confirmation of the mass break between these two classes of objects. “We used the SOAR 4.1-m telescope to measure the visible light of faint stars and brown dwarfs, and the CTIO 0.9-m telescope to obtain precise measurements of their distances. We then combined these measurements with infrared data taken at the CTIO 1.3-m telescope and the WISE space telescope. Three out of four of these telescopes are public telescopes located at CTIO, and the fourth explores wavelengths that are only accessible from space,” explained lead author Sergio Dieterich in a recent statement.

Because of this, the team was finally able to observe objects on both sides of the boundary mass between where an object will either become a star or a brown dwarf. Ultimately, the team was able to confirm what theorists had long suggested, but the task was not easy.

“In order to distinguish stars from brown dwarfs we measured the light from each object thought to lie close to the stellar/brown dwarf boundary. We also carefully measured the distances to each object. We could then calculate their temperatures and radii using basic physical laws, and found the location of the smallest objects we observed. We see that radius decreases with decreasing temperature, as expected for stars, until we reach a temperature of about 2100K. There we see a gap with no objects, and then the radius starts to increase with decreasing temperature, as we expect for brown dwarfs," said Dieterich.

This new result, has allowed astronomers to specify the numerical boundaries between brown dwarfs and main sequence stars. “We can now point to a temperature (2100K), radius (8.7% that of our Sun), and luminosity (1/8000 of the Sun) and say ‘the main sequence ends there’ and we can identify a particular star (with the designation 2MASS J0513-1403) as a representative of the smallest stars,” noted Todd Henry, one of the co-authors on the study.

The research has been accepted for publication in The Astronomical Journal.

Image 2 (below): The relation between size and temperature at the point where stars end and brown dwarfs begin (based on a figure from the publication) Credit: P. Marenfeld & NOAO/AURA/NSF