# Primeval very low-mass stars and brown dwarfs -- VII. The discovery of the first wide M + L extreme subdwarf binary

## TL;DR

This paper reports the discovery of the first wide M + L extreme subdwarf binary system Gaia J0452-36AB, analyzes its properties, tests classification schemes, and discusses implications for subdwarf populations and metallicity scales.

## Contribution

It presents the discovery and characterization of Gaia J0452-36AB, the first wide M + L extreme subdwarf binary, and evaluates metallicity classification schemes for late-type subdwarfs.

## Key findings

- Gaia J0452-36AB is a gravitationally bound wide binary with halo kinematics.
- Existing metallicity indices are inconsistent for late-type M subdwarfs.
- The metallicity scale overestimates metallicity for late-type M and L subdwarfs.

## Abstract

I present the discovery of the first wide M + L extreme subdwarf binary system Gaia J0452$-$36AB. The binary is located at a distance of 137.27$^{+0.68}_{-0.67}$ pc with a projected separation of 15828$\pm$78 au. I classified Gaia J0452$-$36AB as esdM1 and esdL0 subdwarfs, respectively. Gaia J0452$-$36AB have typical halo kinematics, metallicity of [Fe/H] $\approx -1.4$, and temperature of $\sim$ 3550 and 2600 K, respectively. Gaia J0452$-$36AB is a pair of very low-mass stars with masses of 0.151$^{+0.029}_{-0.019}$ and 0.0855$^{+0.0014}_{-0.0010}$ M$_{\odot}$, and is a gravitationally bound system. I tested the metallicity consistency of existing M subdwarf classification schemes with Gaia J0452$-$36AB and a sample of M and L subdwarfs with known metallicity. I found that the metallicity of each M subclass defined by the the metallicity index $\zeta_{\rm CaH/TiO}$ is not consistent from mid to late M subtypes. Because late-type M and L subdwarfs have dusty atmospheres and high surface gravity which have significant impacts on CaH and TiO indices that used in the classification. The metallicity scale of late-type M subdwarfs would be overestimated by the $\zeta_{\rm CaH/TiO}$ index. I discussed the mass range of M subdwarfs, and explained the lack of late-type M extreme and ultra subdwarfs, and decreasing binary fraction from sdM, to esdM, and usdM subclasses. The four M subclasses have different mass ranges. The comparison between M subclasses is between populations in different mass ranges. I also present the discovery of Ruiz 440-469B, an M8 dwarf wide companion to a cool DA white dwarf, Ruiz 440-469.