# 2MASS 0213+3648 C: A wide T3 benchmark companion to an an active, old M dwarf binary

## TL;DR

This paper reports the discovery of a wide T3 brown dwarf companion to an active, old M dwarf binary, serving as an important benchmark for studying the L/T transition in substellar objects.

## Contribution

The discovery of a wide T3 companion to an old M dwarf binary provides a valuable benchmark for understanding substellar evolution and the L/T transition.

## Key findings

- Companion has a mass of 0.063±0.009 M_sun.
- System is approximately 1-10 Gyr old.
- Companion is a key benchmark near the L/T transition.

## Abstract

We present the discovery of a 360 AU separation T3 companion to the tight (3.1 AU) M4.5+M6.5 binary 2MASS J02132062+3648506. This companion was identified using Pan-STARRS1 data and, despite its relative proximity to the Sun (22.2$_{-4.0}^{+6.4}$ pc; Pan-STARRS1 parallax) and brightness ($J$=15.3), appears to have been missed by previous studies due to its position near a diffraction spike in 2MASS. The close M~dwarf binary has active X-ray and H$\alpha$ emission and shows evidence for UV flares. The binary's weak {\it GALEX} UV emission and strong Na I 8200\AA Na absorption leads us to an age range of $\sim$1-10Gyr. Applying this age range to evolutionary models implies the wide companion has a mass of 0.063$\pm$0.009\,$M_{\odot}$. 2MASS J0213+3648 C provides a relatively old benchmark close to the L/T transition and acts as a key, older comparison to the much younger early-T companions HN~Peg~B and GU~Psc~b.