An Etymological Dictionary of Astronomy and Astrophysics
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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.