Discovery of a brown dwarf companion to the A3V star \beta{} Circini

TL;DR

This paper reports the discovery of a wide L1 brown dwarf companion to the A3V star eta{} Circini, providing a valuable benchmark for studying brown dwarf evolution at an age of 370-500 million years.

Contribution

The discovery of a wide L1 brown dwarf companion to eta{} Circini and the analysis of its spectral properties at an intermediate age.

Findings

The companion has a spectral type of L1.0±0.5.

It is at a projected separation of about 6656 AU.

Signs of low surface gravity are absent, indicating age-related spectral evolution.

Abstract

We report the discovery of an L dwarf companion to the A3V star \beta{} Circini. VVV J151721.49-585131.5, or \beta{} Cir B, was identified in a proper motion and parallax catalogue of the Vista Variables in the V\'{i}a L\'{a}ctea survey as having near infrared luminosity and colour indicative of an early L dwarf, and a proper motion and parallax consistent with that of \beta{} Cir. The projected separation of $\sim$3.6' corresponds to $6656$ au, which is unusually wide. The most recent published estimate of the age of the primary combined with our own estimate based on newer isochrones yields an age of $370-500$ Myr. The system therefore serves as a useful benchmark at an age greater than that of the Pleiades brown dwarfs and most other young L dwarf benchmarks. We have obtained a medium resolution echelle spectrum of the companion which indicates a spectral type of L1.0$\pm$0.5 and lacks the typical signatures of low surface gravity seen in younger brown dwarfs. This suggests that signs of low surface gravity disappear from the spectra of early L dwarfs by an age of $\sim370-500$ Myr, as expected from theoretical isochrones. The mass of \beta{} Cir B is estimated from the BHAC15 isochrones as $0.056\pm0.007$ M$_{\odot}$.