On the Age of the Widest Very Low Mass Binary

TL;DR

This paper investigates the age and nature of the widest very low-mass binary, using spectroscopy to determine whether it is a young brown dwarf pair or an older stellar system, with implications for understanding binary formation.

Contribution

The study provides the first spectroscopic age constraints on the widest very low-mass binary, ruling out its membership in a young association and clarifying its nature.

Findings

The binary is at least 200 Myr old, not part of the young Tucana/Horologium group.

Spectroscopic features indicate the system is likely composed of low-mass stars rather than brown dwarfs.

The system's age is estimated to be less than 2 Gyr based on its galactic lifetime.

Abstract

We have recently identified the widest very low-mass binary (2M0126AB), consisting of an M6.5V and an M8V dwarf with a separation of ~5100 AU, which is twice as large as that of the second widest known system and an order of magnitude larger than those of all other previously known wide very low-mass binaries. If this binary belongs to the field population, its constituents would have masses of ~0.09 Msun, at the lower end of the stellar regime. However, in the discovery paper we pointed out that its proper motion and position in the sky are both consistent with being a member of the young (30 Myr) Tucana/Horologium association, raising the possibility that the binary is a pair of ~0.02 Msun brown dwarfs. We obtained optical spectroscopy at the Gemini South Observatory in order to constrain the age of the pair and clarify its nature. The absence of lithium absorption at 671 nm, modest Halpha emission, and the strength of the gravity-sensitive Na doublet at 818 nm all point toward an age of at least 200 Myr, ruling out the possibility that the binary is a member of Tucana/Horologium. We further estimate that the binary is younger than 2 Gyr based on its expected lifetime in the galactic disk.