* Astronomy

Title: The VAST Survey - IV. A wide brown dwarf companion to the A3V star Zeta Delphini
Author: R. J. De Rosa, J. Patience, K. Ward-Duong, A. Vigan, C. Marois, I. Song, B. Macintosh, J. R. Graham, R. Doyon, M. S. Bessell, O. Lai, D. W. McCarthy, C. Kulesa

We report the discovery of a wide co-moving 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 Zeta Del B with the estimated temperature leads to a model-dependent mass estimate of 50±15 Jupiter masses, 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.

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