The VAST Survey - IV. A wide brown dwarf companion to the A3V star ζ Delphini

We report the discovery of a wide comoving substellar companion to the nearby (D = 67.5 +/- 1.1 pc) A3V star zeta Delphini based on imaging and follow-up spectroscopic observations obtained during the course of our Volume-limited A-Star (VAST) multiplicity survey. zeta Del was observed over a five-year baseline with adaptive optics, revealing the presence of a previously unresolved companion with a proper motion consistent with that of the A-type primary. The age of the zeta Del system was estimated as 525 +/- 125 Myr based on the position of the primary on the colour magnitude and temperature luminosity diagrams. Using intermediate-resolution near-infrared spectroscopy, the spectrum of zeta Del B is shown to be consistent with a mid-L dwarf (L5 +/- 2), at a temperature of 1650 200 K. Combining the measured near-infrared magnitude of Del B with the estimated temperature leads to a model-dependent mass estimate of 50 15 Mkup, corresponding to a mass ratio of q= 0.019 +/- 0.006. At a projected separation of 910 +/- 14 au, zeta Del B is among the most widely separated and extreme-mass ratio substellar companions to a main-sequence star resolved to date, providing a rare empirical constraint of the formation of low-mass ratio companions at extremely wide separations.