Metallicity, temperature, and gravity scales of M subdwarfs

Lodieu, N., Allard, F., Rodrigo, C., Pavlenko, Y., Burgasser, A., Lybchik, Y., Kaminsky, B., Homeier, D. 2019. Metallicity, temperature, and gravity scales of M subdwarfs. Astronomy and Astrophysics 628 DOI: 10.1051/0004-6361/201935299

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 similar to 3300), optical (R similar to 5400), and near-infrared (R similar to 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 450-2500 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 +/- 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 +/- 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 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.

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