---
author:
- 'N. Lodieu [^1], F. Allard , C. Rodrigo , Y. Pavlenko , A. Burgasser , Y. Lyubchik , B. Kaminsky , D. Homeier'
bibliography:
- '../../AA/mnemonic.bib'
- '../../AA/biblio\_old.bib'
date: 'Received ; accepted '
title: 'Metallicity, temperature, and gravity scales of M subdwarfs'
---

[The aim of the project is to define a metallicity/gravity/temperature scale vs spectral types for metal-poor M dwarfs.]{} [We obtained intermediate-resolution ultraviolet (R$\sim$3300), optical (R$\sim$5400), and near-infrared (R$\sim$3900) spectra of 43 M subdwarfs (sdM), extreme subdwarfs (esdM), and ultra-subdwarfs (usdM) with the X-shooter spectrograph on the European Southern Observatory Very Large Telescope. We compared our atlas of spectra to the latest BT-Settl synthetic spectral energy distribution over a wide range of metallicities, gravities, and effective temperatures to infer the physical properties for the whole M dwarf sequence (M0–M9.5) at sub-solar metallicities and constrain the latest state-of-the-art atmospheric models. ]{} [The BT-Settl models reproduce well the observed spectra across the 450–2500 nm wavelength range except for a few regions. We find that the best fits are obtained for gravities of $\log$(g)=5.0–5.5 for the three metal classes. We infer metallicities of \[Fe/H\]=$-$0.5, $-$1.5, and $-$2.0$\pm$0.5 dex and effective temperatures of 3700–2600K, 3800–2900K, and 3700–2900K for subdwarfs, 