The Substellar Transition Zone: A Stretched Temperature Canyon in Brown Dwarf Population due to Unsteady Hydrogen Fusion

Abstract: I summarize results on transitional and degenerate brown dwarfs presented in a series titled Primeval very low-mass stars and brown dwarfs. I introduce an L subdwarf classification scheme, which classified L subdwarfs into three metal subclasses. I would also like to draw your attention to transitional brown dwarfs which have long-lasting unsteady hydrogen fusion in their cores to replenish the dissipation of their initial thermal energy. The mass range of transitional brown dwarfs with solar metallicity is between 0.065 and 0.079 solar mass according to the latest evolutionary models. The temperature distribution of transitional brown dwarfs are stretched to a wide range and formed a substellar transition zone. The substellar transition zone is most significant in the old halo population and ranges from 1000 K to 2200-3000 K depending on metallicity. The transition zone has impacts on our observational properties of field brown dwarf population. Because field L dwarfs are composed of very low-mass stars, transitional brown dwarfs, and relative younger electron-degenerate brown dwarfs.