ultracool

Ultracool dwarfs are a class of very cool, low-mass objects whose spectra lie at the cool end of the stellar sequence. The term covers the spectral classes L, T, and Y. They have effective temperatures much lower than the Sun’s and, for the most part, do not sustain hydrogen fusion in their cores. As a result, many ultracool dwarfs are brown dwarfs, though some occupy the very low-mass end of the stellar regime.

Temperatures for ultracool dwarfs span from roughly 1,500–2,400 K for early L types down to well below 500 K for the coolest Y dwarfs. Their spectra are dominated by molecular absorption: L dwarfs show metal hydrides, oxides, and alkali metals with dusty atmospheres; T dwarfs exhibit strong methane bands; Y dwarfs display features such as ammonia. Clouds of condensates also shape their colors and variability.

Ultracool dwarfs are found throughout the Milky Way, and a substantial fraction lie within the solar neighborhood. They are intrinsically faint in visible light and are detected mainly through infrared surveys, including 2MASS, SDSS, and WISE, as well as spectroscopic follow-up. Distances are typically tens of parsecs, though some are farther.

Study of ultracool dwarfs informs models of cool, high-pressure atmospheres and condensate clouds, contributes to understanding the low-mass end of the initial mass function, and provides benchmarks for substellar evolution. Variability and weather phenomena have been observed in several objects, and a number host planetary-mass companions or free-floating planetary-mass objects.