Cool Versus Ultracool White Dwarfs

A preliminary $BVRIJHK$ analysis of the white dwarfs SSSPM J2231$-$7514 and SSSPM J2231$-$7515 is presented. Although both stars were reported to have $T_{\mathrm{e}\mathrm{f}\mathrm{f}}<4000$ K, the analysis here indicates $T_{\mathrm{e}\mathrm{f}\mathrm{f}}\approx 4250$ K for both SSSPM J2231$-$7514 and SSSPM J2231$-$7515. Given substantial scientific interest in the coolest extant degenerate stars, it is necessary to distinguish sub 4000 K objects from the bulk of cool white dwarfs. This analysis reiterates the importance of near infrared observations in constraining the spectral energy distributions and effective temperatures of the coolest white dwarfs and briefly discusses their possible origins.