Discovery of a widely separated UCD-WD binary

TL;DR

This paper reports the discovery of the widest known ultracool dwarf - white dwarf binary system, providing valuable benchmarks for studying ultracool dwarfs and white dwarfs.

Contribution

First spectroscopically confirmed widely separated ultracool dwarf - white dwarf binary, expanding understanding of such systems and their use as benchmarks.

Findings

Separation of 3650-5250AU at 41-59pc distance

White dwarf: Teff=7600K, log(g)=7.79-8.09, M=0.48-0.65Msun

Ultracool dwarf: M9+/-1, Teff=2000-2400K, M=0.07-0.08Msun

Abstract

We present the discovery of the widest known ultracool dwarf - white dwarf binary. This binary is the first spectroscopically confirmed widely separated system from our target sample. We have used the 2MASS and SuperCOSMOS archives in the southern hemisphere, searching for very widely separated ultracool dwarf - white dwarf dwarf binaries, and find one common proper motion system, with a separation of 3650-5250AU at an estimated distance of 41-59pc, making it the widest known system of this type. Spectroscopy reveals 2MASS J0030-3740 is a DA white dwarf with Teff=7600+/-100K, log(g)=7.79-8.09 and M(WD)=0.48-0.65Msun. We spectroscopically type the ultracool dwarf companion (2MASS J0030-3739) as M9+/-1 and estimate a mass of 0.07-0.08Msun, Teff=2000-2400K and log(g)=5.30-5.35, placing it near the mass limit for brown dwarfs. We estimate the age of the system to be >1.94Gyrs (from the white dwarf cooling age and the likely length of the main sequence lifetime of the progenitor) and suggest that this system and other such wide binaries can be used as benchmark ultracool dwarfs.