Evidence of a substellar companion to AB Dor C

TL;DR

This study uses interferometry to suggest that AB Dor C is a binary system, which helps reconcile discrepancies between observed properties and stellar evolution models for low-mass objects near the brown dwarf and planet boundary.

Contribution

First interferometric detection of binarity in AB Dor C, providing refined mass estimates and insights into its nature as a brown dwarf or planetary-mass object.

Findings

AB Dor C is likely a binary brown dwarf system.

The components have masses near the hydrogen- and deuterium-burning limits.

Binarity explains previous discrepancies in mass-luminosity models.

Abstract

Studies of fundamental parameters of very low-mass objects are indispensable to provide tests of stellar evolution models that are used to derive theoretical masses of brown dwarfs and planets. However, only objects with dynamically determined masses and precise photometry can effectively evaluate the predictions of stellar models. AB Dor C (0.090 solar masses) has become a prime benchmark for calibration of theoretical evolutionary models of low-mass young stars. One of the ambiguities remaining in AB Dor C is the possible binary nature of this star. We observed AB Dor C with the VLTI/AMBER instrument in low-resolution mode at the J, H and K bands. The interferometric observables at the K-band are compatible with a binary brown dwarf system with tentative components AB Dor Ca/Cb with a K-band flux ratio of 5$\pm$1% and a separation of 38$\pm$1 mas. This implies theoretical masses of 0.072$\pm$0.013 M$_{\rm \odot}$ and 0.013$\pm$0.001 M$_{\rm \odot}$ for each component, near the hydrogen-burning limit for AB Dor Ca, and near the deuterium-burning limit, straddling the boundary between brown dwarfs and giant planets, for AB Dor Cb. The possible binarity of AB Dor C alleviates the disagreement between observed magnitudes and theoretical mass-luminosity relationships.