Discovery of a Possible Cool White Dwarf Companion from the AllWISE Motion Survey

TL;DR

This paper reports the discovery of a high-motion object likely to be an unresolved binary system consisting of an M7 dwarf and a cool white dwarf, based on its unusual optical and infrared spectral features.

Contribution

It introduces a new candidate binary system with unique spectral characteristics, expanding understanding of white dwarf companions and spectral signatures.

Findings

Spectral features suggest a binary system of an M7 dwarf and a cool white dwarf.

Unusual Na D doublet may indicate atypical white dwarf properties.

Object exhibits high proper motion in the AllWISE survey.

Abstract

We present optical and near-infrared spectroscopy of WISEA J061543.91$-$124726.8, which we rediscovered as a high motion object in the AllWISE survey. The spectra of this object are unusual; while the red optical ($\lambda >$ 7,000 \AA) and near-infrared spectra exhibit characteristic TiO, VO, and H$_{2}$O bands of a late-M dwarf, the blue portion of its optical spectrum shows a significant excess of emission relative to late-M type templates. The excess emission is relatively featureless, with the exception of a prominent and very broad Na I D doublet. We find that no single, ordinary star can reproduce these spectral characteristics. The most likely explanation is an unresolved binary system of an M7 dwarf and a cool white dwarf. The flux of a cool white dwarf drops in the optical red and near-infrared, due to collision-induced absorption, thus allowing the flux of a late-M dwarf to show through. This scenario, however, does not explain the Na D feature, which is unlike that of any known white dwarf, but which could perhaps be explained via unusual abundance or pressure conditions.