Fr.: naine Y
A type of ultra-cool → brown dwarf with an → effective temperature lower than 500 K. → Near infrared spectra of these objects show deep absorption bands of H2O and CH4. So far only seven brown dwarf candidates belonging to this class have been found, all Y0 subtypes. These objects are very dim, with H magnitudes 19-23. See Cushing et al. 2011 (arXiv:1108.4678). The precise definition of the Y class requires new findings in the future about these objects.
For the choice of the letter Y, see Kirkpatrick et al. 1993, ApJ 406, 701; → dwarf.