A New Wide Pair of Cool White Dwarfs in the Solar Neighborhood

Abstract

We report the discovery of a wide pair (93'' angular separation) of extremely cool (Teff < 4000 K) white dwarfs with a very large common proper motion (~19 yr-1). The objects were discovered in a high proper motion survey in the poorly investigated southern sky region with δ < -60° using SuperCOSMOS Sky Survey (SSS) data. Both objects, SSSPM J2231-7514 and SSSPM J2231-7515, show featureless optical spectra. Fits of blackbody models to the spectra yield effective temperatures of 3810 and 3600 K, respectively, for the bright (V = 16.60) and faint (V = 16.87) components. Both degenerates are much brighter than other recent discoveries of cool white dwarfs with comparable effective temperatures and/or BJ-R colors. Therefore, they should be relatively nearby objects. The comparison with other cool white dwarfs and a photometric distance determination yield distance estimates between 9 and 14 pc. The latter seems to be more realistic since the good agreement of the proper motion of both components within the errors of about 8 mas yr-1 and the angular separation between the two stars support a distance of about 15 pc, with relatively small masses of the components. With a smaller distance we should be able to measure a differential proper motion due to orbital motion if the orbital plane is not strongly inclined and if the present orbital velocity vector is not close to the line of sight. The space velocity based on that distance and assumptions on radial velocity make the new pair of extremely cool white dwarfs probably some of the oldest members of the Galactic disk population, although the possibility that these objects are part of a Galactic halo dark matter component also cannot yet be ruled out.