A determination of the low mass end of the main sequence and the hydrogen burning limit

Brown-Dwarf
Image credit: P. Marenfeld & NOAO/AURA/NSF. The ordinate is the stellar radius.

In research accepted for publication in the Astronomical Journal, the RECONS (Research Consortium On Nearby Stars) group from Georgia State University has found clear observational evidence for the theoretically predicted break between very low mass stars and brown dwarfs. The data came from the SOAR (SOuthern Astrophysical Research) 4.1-m telescope and the SMARTS (Small and Moderate Aperture Research Telescope System) 0.9-m telescope at the Cerro Tololo Inter-American Observatory (CTIO) in Chile.

For most of their lives, stars obey a relationship referred to as the main sequence, a relation between luminosity and temperature – which is also a relationship between luminosity and radius. Stars behave like balloons in the sense that adding material to the star causes its radius to increase: in a star the material is the element hydrogen, rather than air which is added to a balloon. Brown dwarfs, on the other hand, are described by different physical laws (referred to as electron degeneracy pressure) than stars and have the opposite behavior. The inner layers of a brown dwarf work much like a spring mattress: adding additional weight on them causes them to shrink. Therefore brown dwarfs actually decrease in size with increasing mass.

As Dr. Sergio Dieterich, the lead author, explained, “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 (see the attached illustration, based on a figure in the publication). 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. “

Dr. Todd Henry, another author, said: “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.”

The new paper is S. Dieterich, et al., 2014, in the Astronomical Journal, 147, 94: doi:10.1088/0004-6256/147/5/94. From the abstract:

We find evidence for the local minimum in the radius-temperature and radius-luminosity trends that signals the end of the stellar main sequence and the start of the brown dwarf sequence at T eff ~ 2075 K, log (L/L ☉) ~ –3.9, and (R/R ☉) ~ 0.086. The existence of this local minimum is predicted by evolutionary models, but at temperatures ~400 K cooler. The minimum radius happens near the locus of 2MASS J0523–1403, an L2.5 dwarf with V – K = 9.42.

m44franke1600
Praesepe, an open cluster, where brown dwarfs have long been sought observationally. Image credit & copyright: Bob Franke

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