The Coldest Known Y Dwarfs: Estimates of Their Effective Temperatures

Abstract

For a decade there has been a factor of 2.5 gap in luminosity between the 275 K WISE J085510.83-071442.5 and all other Y dwarfs, with Teff ≳ 350 K. Recently, three objects were found that may fall in this gap. Two are companions to Y dwarfs: WISE J033605.05-014350.4B and CWISEP J193518.58-154620.3B; the third is MEAD 62B, a candidate companion to a white dwarf. 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 S. A. Beiler et al. 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 480 and 340 K dwarf pair and the latter a pair of 387 K 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 are 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 that can be used as a reference for searches for brown dwarfs in JWST data.