The Coldest Known Y Dwarfs: Estimates of their Effective Temperatures
For a decade there has been a factor of 2.5 gap in luminosity between the 275K WISE J085510.83-071442.5 (Luhman 2014) and all other Y dwarfs, with Teff >= 350K. Recently three objects were found which may fall in this gap.
Two are companions to Y dwarfs: WISE J033605.05-014350.4B (Calissendorff et al. 2023) and CWISEP J193518.58-154620.3B (De Furio et al. 2025); the third is MEAD 62B, a candidate companion to a white dwarf (Albert et al. 2025). Evolutionary models calculate a tight relationship between luminosity and Teff for Y dwarfs.
I determine luminosities and hence Teff for three Y dwarfs (WISE J085510.83-071442.5, WISE J173835.53+273259.0, WISE J182831.08+265037.7). I derive relationships between Teff and mid-infrared colors using these together with 22 T and Y dwarfs from Beiler et al. (2024) with luminosity-based Teff values. These relationships are used to explore the Teff distribution for Y dwarfs.
A sample of 31 Y dwarfs within ~20 pc is presented with 275 < Teff K < 425. The JWST colors for WISE J053516.80-750024.9 and WISE J182831.08+265037.7 support previous suggestions that they are unresolved binaries, the former a 480K and 340K dwarf pair and the latter a pair of 387K dwarfs.
Five other dwarfs have unusual colors; two are likely high gravity and/or metal-poor (WISE J024714.52+372523.5, WISEA J215949.54-480855.2), two low gravity and/or metal-rich (CWISEP J104756.81+545741.6, WISE J150115.92-400418.4), and the fifth cannot be interpreted (WISE J043052.92+463331.6). An Appendix provides colors which can be used as a reference for searches for brown dwarfs in JWST data.
S. K. Leggett
Comments: Accepted for publication in ApJ, March 25, 2026
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2603.24740 [astro-ph.SR] (or arXiv:2603.24740v1 [astro-ph.SR] for this version)
https://doi.org/10.48550/arXiv.2603.24740
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Submission history
From: Sandy Leggett
[v1] Wed, 25 Mar 2026 19:01:34 UTC (330 KB)
https://arxiv.org/abs/2603.24740
Astrobiology,
