Discovery of the substellar transition zone of transitional brown dwarfs between stars and degenerate brown dwarfs

The destiny of a star is depending on the status of hydrogen fusion that it can reach after its formation. Stars have steady hydrogen fusion and can maintain their luminosity. Degenerate brown dwarfs (D-BD) have no energy supply from hydrogen fusion thus cool continuously. Transitional brown dwarfs (T-BD) have long-lasting low-rate unsteady hydrogen fusion in their cores to replenish the dissipation of their initial thermal energy, thus have slower cooling rate than D-BDs. T-BDs form a substellar transition zone (STZ) between stars and D-BDs. The STZ covers a narrow mass range and very wide ranges of temperature and spectral type over time. The STZ has been shown by evolutionary models (Burrows & Liebert 1993) before the discovery of brown dwarfs in 1995. However, the STZ was ignored by observers. Because first observers was looking for a clear mass boundary (the hydrogen burning minimum mass, e.g. HBMM) between stars and brown dwarfs. Secondly, the precision of brown dwarf mass measurement is larger or similar to the mass range of the STZ. Thirdly, the temperature range of very low-mass stars, T-BD, and D-BD are overlapped because of mass/age degeneracy of brown dwarfs in the field. Therefore, the STZ could only be resolved in extremely rare metal-poor brown dwarfs population, which all have been cooled for ~10 Gyr. In this talk, I will discuss the discovery of T-BDs and STZ from a well characterized sample of metal-poor brown dwarfs. This talk is partially based on works published in a series titled 'Primeval very low-mass stars and brown dwarfs' ( https://ui.adsabs.harvard.edu/public-libraries/gVGomDWcQGyKPWw2CGg3dg ).