Context :

Aims : The aim of the project is to define a metallicity/gravity/temperature scale vs spectral types for metal-poor M dwarfs .

Methods : 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 .

Results : 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 3700–2600 K , 3800–2900 K , and 3700–2900 K for subdwarfs , extreme subdwarfs , and ultra-subdwarfs , respectively .
Metal-poor M dwarfs tend to be warmer by about 200 \pm 100 K and exhibit higher gravity than their solar-metallicity counterparts .
We derive abundances of several elements ( Fe , Na , K , Ca , Ti ) for our sample but can not describe their atmospheres with a single metallicity parameter .
Our metallicity scale expands the current scales available for midly metal-poor planet-host low-mass stars .
Our compendium of moderate-resolution spectra covering the 0.45–2.5 micron range represents an important legacy value for large-scale surveys and space missions to come .

Conclusions :