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

TL;DR

This paper reports the discovery of a wide, low-mass brown dwarf companion to the nearby A3V star $$ Delphini, providing insights into the formation of such substellar objects at large separations.

Contribution

The study presents the first detection and characterization of a wide, low-mass brown dwarf companion to an A-type star, using imaging and spectroscopy over a five-year baseline.

Findings

Companion is a mid-L dwarf with a temperature of 1650 K.

Mass estimate of the companion is 50±15 Jupiter masses.

Projected separation is approximately 910 au, making it one of the most widely-separated low-mass companions.

Abstract

We report the discovery of a wide co-moving substellar companion to the nearby ($D=67.5\pm1.1$ 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 $525\pm125$ 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 (L$5\pm2$), at a temperature of $1650\pm200$ K. Combining the measured near-infrared magnitude of $\zeta$ Del B with the estimated temperature leads to a model-dependent mass estimate of $50\pm15$ M$_{\rm Jup}$, corresponding to a mass ratio of $q=0.019\pm0.006$. At a projected separation of $910\pm14$ 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.