2019A&A...628A..61L

Aims. The aim of the project is to define metallicity/gravity/temperature scales for different spectral types of metal-poor M dwarfs.
Methods. We obtained intermediate-resolution ultraviolet (R∼3300), optical (R∼5400), and near-infrared (R∼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 atmospheric models. Results. The BT-Settl models accurately reproduce the observed spectra across the 450-2500nm 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±0.5dex and effective temperatures of 3700-2600K, 3800-2900K, and 3700-2900K for subdwarfs, extreme subdwarfs, and ultra-subdwarfs, respectively. Metal-poor M dwarfs tend to be warmer by about 200±100K and exhibit higher gravity than their solar-metallicity counterparts. We derive abundances of several elements (Fe, Na, K, Ca, Ti) for our sample but cannot describe their atmospheres with a single metallicity parameter. Our metallicity scale expands the current scales available for mildly metal-poor planet-host low-mass stars. Our compendium of moderate-resolution spectra covering the 0.45-2.5 micron range represents an important resource for large-scale surveys and space missions to come.