Additional Ultracool White Dwarfs Found in the Sloan Digital Sky Survey

TL;DR

This paper reports the discovery of seven new ultracool white dwarfs in SDSS, including the first binary system of two CIA white dwarfs, with detailed photometry, spectra, and proper motions to understand their properties.

Contribution

It presents seven newly identified ultracool white dwarfs, including the first binary system of two CIA white dwarfs, expanding knowledge of these faint stellar remnants.

Findings

Seven new ultracool white dwarfs discovered in SDSS.

First binary system of two CIA white dwarfs identified.

Spectral and photometric data show wide color range and CIA features.

Abstract

We identify seven new ultracool white dwarfs discovered in the Sloan Digital Sky Survey (SDSS). The SDSS photometry, spectra, and proper motions are presented, and additional BVRI data are given for these and other previously discovered ultracool white dwarfs. The observed colors span a remarkably wide range, qualitatively similar to colors predicted by models for very cool white dwarfs. One of the new stars (SDSS J1251+44) exhibits strong collision-induced absorption (CIA) in its spectra, while the spectra and colors of the other six are consistent with mild CIA. Another of the new discoveries (SDSS J2239+00A) is part of a binary system -- its companion is also a cool white dwarf, and other data indicate that the companion exhibits an infrared flux deficiency, making this the first binary system composed of two CIA white dwarfs. A third discovery (SDSS J0310-00) has weak Balmer emission lines. The proper motions of all seven stars are consistent with membership in the disk or thick disk.