Brown Dwarf Stars
An optical image of the sky near the Centaurus association of young stars. Astronomers have used the DECam survey of this region to identify an estimated 80-100 new brown dwarf stars in this region, and to conclude from their properties that current models are missing some key physical processes.
Low mass stellar objects include both low-mass stars (less than about one half of the Sun's mass) and substellar brown dwarfs which, having less than about eighty Jupiter-masses, are unable to sustain the nuclear fusion of hydrogen but instead burn deuterium. Astronomers are working to compile well-characterized samples of young low-mass objects to better understand their early evolution including how their circumstellar discs evolve, constraining the distribution of masses, and determining whether or not a they form under any different mechanisms than normal stars.
While some spectroscopic indicators are able identify young low-mass stars, the most successful method to age date these objects is to establish their membership in a young cluster or stellar association where the age of the stellar members can be used to estimate their ages. CfA astronomer David Bell was a member of a team of eight scientists who used the Dark Energy Camera (DECam) survey of objects in the Upper Centaurus Lupus and Lower Centaurus Crux associations of young stars (the groups are named for the constellations in which they reside). They obtain a catalog of over eleven million point sources with 466 candidate members of these associations including an estimated 80 – 100 brown dwarfs, adding significantly to the known population. The astronomers conclude that models of stars early in their lifetimes, including brown dwarf stars, are missing key physical processes with the result that models under-predict their luminosities, with the subsequent result that the estimated ages are too young. The team also reports finding excess infrared emission indicative of circumstellar disks around these small objects at a rate similar to that in regular higher mass stars. Spectral studies are now needed to confirm and refine these conclusions.
"DECam Survey for Low-Mass Stars and Substellar Objects in the UCL and LCC Subgroups of the Sco-Cen OB Association (SCOCENSUS)," Fred E. Moolekamp ,Eric. E. Mamajek, David J. James, Kevin L. Luhman, Mark J. Pecaut, Stanmir A. Metchev, Cameron P. M. Bell, and Sara R. Denbo, MNRAS 484, 5049 2019.