| Ultra-cool dwarfs (spectral types M6 and later) represent a key population for studying both stellar and substellar objects, including brown dwarfs and exoplanets. However, their spectral characterization presents significant challenges due to their intrinsic faintness, the complexity of their atmospheres, and the limitations of current models. In this talk, I will explore the difficulties in deriving reliable parameters such as effective temperature (Teff), surface gravity (log g), and metallicity ([M/H]) for late-type M dwarfs. I will highlight how obtaining precise measurements can provide critical benchmarks for understanding the atmospheres of cool worlds, including brown dwarfs and giant exoplanets. Additionally, I will discuss the role of high-resolution spectroscopy, photometric surveys, and synthetic spectral libraries in overcoming these challenges, leading to a deeper understanding of ultra-cool atmospheres and advancing our knowledge of substellar environments. |
