Brown dwarfs are celestial bodies which are intermediate between a planet and a star. They have a mass less than 0.075 solar masses (the mass of our Sun), or approximately 75 Mjup (Jupiter masses).
The difference between brown dwarfs and stars is that brown dwarfs do not reach stable luminosities by thermonuclear fusion of hydrogen, unlike stars, and hence do not join the main sequence. Both stars and brown dwarfs produce energy by fusion of deuterium (an isotope of hydrogen). Astronomers draw the line between brown dwarfs and planets at the lower fusion boundary of 13 Mjup. According to the IAU (International Astronomical Union), if the astronomical object has a mass below 13 Mjup, it is not considered to be a star as it cannot sustain the burning of deuterium.
Brown dwarfs form via the fragmentation of dense molecular gas and are ejected from the dense environment before they are able to accrete to stellar mass. Hence, they can be referred to as ’failed stars’. There are four spectral types for brown dwarfs, M, L, T, Y.
Spectral type M is defined by the strong absorption features due to the diatomic molecule titanium oxide (TiO) at green and blue optical wavelengths. M-dwrafs comprise 70 % of all the stars in the galaxies and offer the best chance of finding habitable planets through sheer numbers and proximity to the Sun. They have rela- tively low temperatures, below 3000K, and luminosities, hence are less red compared to the other spectral types. M-dwarfs are extremely long-lived and eventually cool down to L and T dwarfs.
L-dwarf stars show weak absorption in CaH, KI and NaI absorption, and strong in VO bands and red TiO from the optical spectrum (0.6-1.0 μm). In terms of colours, L-dwarfs appear to be a dim-orange red with their spectrum peaking in the infrared. More luminous L-type stars can live for trillions of years, most massive stars undergo thermonuclear fusion for about 3-20 trillion years.
T-dwarfs are a class of low mass, low luminosity, and low temperature brown dwarfs and exhibit CH4 absorption bands between 1.0-2.2 μm. T-dwarfs have temperatures ranging between 500-1300K and are classified with numbers, T0-T8, the lower numbers correspond to hotter stars. Currently, there are roughly 60 known T-dwarfs, mostly identified by the 2MASS and SDSS surveys.
Y-dwarfs are are the coolest and least luminous substellar objects so far detected with temperatures below 500K. They are characterised by their deep methane(CH4) and water(H2O) absorption features. Modern observations with the JWST are evident of ammonia(NH3), carbon monoxide(CO), and carbon dioxide(CO2) in the atmosphere of Y-dwarfs. Brown dwarfs usually have a pressure–temperature (P–T) profile in an adiabatic form, pressure and temperature increase with depth. JWST spectroscopy proves that the P-T profiles for Y-dwarfs are not in the standard adiabatic form. Hence the upper layers of the atmosphere have a relatively higher temperature than the lower layers. This can be explained by rapid rotation of these celestial bodies.